Design of Pyrolysis Reactor

1.0 OVERALL PROCESS This report summarises the root towards zero emissions in energy supply via biomass resources. Currently, biomass contribution to the total energy usage in the United Kingdom is minimal. 1.1 PROJECT OUTLINE In accordance to the design brief, the proposed objective was to design a fully operational electric power production system based in the United Kingdom, integrated with a biomass waste process centre. The required throughput is 4.5 MW of electricity by supplying approximately 5 tonnes of waste wood per hour to the designed power plant. In order to achieve this specified power production at high efficiency and keeping emissions minimal, pyrolysis and gasification techniques/technologies were discussed. 1.2 LOCATION As wood is the primary feedstock due to it being carbon neutral, selecting the precise location of the plant was majorly based on this factor, mainly due to availability and economic reasons. Three locations in the United Kingdom were explored, and each location, to a degree, had a coastline for easy access to water for cooling and cleanup requirements:   

Sussex – England, U.K. Powys – Wales, U.K. Dumfries and Galloway – Scotland, U.K.

After thorough research and considerations, Dumfries and Galloway (Scotland U.K.) was opted as a suitable location to site the plant due to relatively low land cost and abundance in forest, which will basically allow easy access to feedstock. Also, due to the presence of industrial development, there is potential for a Combined Heat and Power system (CHP), if the plant is successful. 1.3 PROCESS OVERVIEW FIGURE 1.3 (a): BASIC PROCESS FLOWSHEET

Author: P. D. Desai

Date: 02/01/11

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Pyrolysis is the central mechanism of the process. It emits products comprising combustible volatiles (bio-oils and synthetic gases – mainly CO & H), by the thermal decomposition of wood (in this case), in an oxygen free reactor. However, the ratios of products produced are affected by several factors including residence times, temperature and heating rate. This leads to the evolution of different types of pyrolysis, principally:  

Slow Pyrolysis: characterised by longer residence times and lower heating rates. Fast Pyrolysis: characterised by shorter residence times and higher heating rates.

Gasification, on the other hand, is essentially the conversion of carbonaceous matter to combustible gas (mainly H & CO). For this process, air can be used for combustion but this reduced the calorific value of the evolved syngas as the presence of nitrogen in air dilutes the end products. The major technologies employed in industrial pyrolytic and gasification processes include:   

Fluidised Beds Fixed Beds Rotary Kilns

From both pyrolysis and gasification, there will be the evolution of syngas (i.e. combustible gas) and by-products of tars and chars. The syngas produced is the main product to be used to generate electricity. However, a range of pollutants are incorporated within the syngas, created from the process and the quality of the process, which can hinder the power plant from minimal emissions and also damage equipment. For this reason, it is hence important to ‘clean’ the syngas before it can be sent to the downstream equipment. The clean up technologies reviewed included:   

Wet & dry scrubbing technologies Cyclones Electro-static precipitators

Ultimately, energy will have to be produced from the scrubbed syngas. Various technologies exist for the conversion of stored chemical energy to electric power, classified as either being engines or turbines. It was decided to opt for gas engines, as they usually operate under higher efficiencies. General pre-treatment will usually involve drying and pulverising. This basically aids to increase calorific value and decrease handling costs. 1.4 FLOWSHEET Due to the vast amount of technologies and different process routes available, it was decided to create two separate processes:  

The first based the predominant use of pyrolysis The second based on the predominant use of gasification

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FIGURE 1.4 (a): PROCESS DESIGN 1

Author: S. McCord

Date: 02/01/11

The first process is focused on staged pyrolysis (i.e. heating the biomass step by step, in a series of reactors). Indirectly fired rotary kilns were employed to improve syngas quality. The downdraft gasifying equipment was opted for gasification purposes due to its high thermal efficiency and low tar production. The main features include staged pyrolysis to reduce tar formation and an incorporated flow controller, which effectively sends a signal to the gasifier when syngas production from the reactor is low, to match it. FIURE 1.4 (b): PROCESS DESIGN 2

Author: P. D. Desai

Date: 02/01/11

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The second process design involves the predominant use of gasification technology. The main gasification technology employed was the downdraft gasifier due to its low tar production and wide spread industrial use. However, an optimum reactor was not decided upon, but the auger screw and rotary kilns have been found in industrial use. The major feature of this design process is the incorporation of a P.S.A (Pressure Swing Adsorption) system. This will help raise the calorific value of the produced syngas significantly to an acceptable level. 1.5 MASS / ENERGY BALANCES & COSTING NB: A mass/energy balance was not carried out for the overall process but rather for the two process designs respectively, however, the throughput was roughly the same. TABLE 1.5 (a): MASS AND ENERGY BALANCE Pyrolysis stage efficiency (%) 20 25 30 35 40 45 50 55 60 65 70 75 80 85 Author: N Driver

Mass of solids/liquids fed to gasifier (kg/sec)

Volume of gas produced (m3)

0.956 0.896 0.836 0.776 0.717 0.657 0.597 0.538 0.478 0.418 0.358 0.299 0.239 0.179

1.04 0.98 0.91 0.85 0.78 0.72 0.65 0.59 0.52 0.46 0.39 0.33 0.26 0.20

Energy available to power the process (MW) 14.6 13.7 12.8 11.8 10.9 10.0 9.1 8.2 7.3 6.4 5.5 4.6 3.6 2.7 Date: 02/01/11

The mass of dry wood entering the pyrolysis reactors is 1.194kg/sec, after moisture content in the original feed is reduced from 20% to 6 % via the dryer. From mass balances; assuming 20% pyrolysis stage efficiency and 40% gas engine efficiency, approximately 14.6 MJ of electricity will be produced from this process. From equipment energy balances, approximately 4 MW will be deducted implying around 5 MW will be in excess after electricity needs are met, which could be used to power up the process after start-up. The net profit of process design 1 was estimated to be 3.85 million/year implying the pay back on the build will be within 2 years. For process design 2, the net profit was estimated at 2.32 million/year, with a payback on installation within 15 years.

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1.6 SAFETY AND ENVIRONMENTAL CONSIDERATIONS In order to initiate commissioning of the discussed power plant designs, safety and environmental deliberations and hazards were researched. This aids in the prevention of major catastrophes and to ensure safe working environments. The table below summarises the risk and danger of some hazardous substances, during the power plant operation; HAZARDOUS SUBSTANCE RISK Hydrogen Leakage to atmosphere Methane Leakage to atmosphere Natural Gas Leakage to atmosphere Tar Leakage to atmosphere Carbon Monoxide Leakage to atmosphere Oxygen Leakage to atmosphere TABLE 1.6 (a): Risk and danger of some hazardous substances.

DANGER Flammable Asphyxiant/Flammable Explosion Irritant/Flammable Poisonous/Flammable Flammable/Explosive

Other potential hazards which exist are summarised below: HAZARD Electrical

EXAMPLES Generator, Insulation, Switchyard

RISK Malfunction, Failure

Thermal

Gasifier steam supply, Reactors Grinders, Engines, Turbines SOx, NOx, Ash

Excess pressure, Thermal runaway Malfunction, Failure

Device Other chemicals

Leakage to atmosphere

DANGER Insufficient power production, equipment damage Explosions Equipment damage, Explosions Contaminants, Carcinogenic

TABLE 1.6 (b): Other potential hazards. The major design rectifications which resulted are bulleted below:      

Equip vessels and/or reactors with material that will withstand the range of temperature and pressure in the process. Implement Calorimeters, ideally Scanning calorimeters to analyse thermal decomposition: Pyrolysis and Gasification for this process, while measuring the heat evolved. Appropriate environment - Well air-ventilated with fans to ensure ventilation as appropriate. Process devices need to be well grounded and all the lines bonded to avoid static charge build up. Keep flammable substances away from ignition sources. Use of electrical tools, in particular those with commutators and welding equipment, must be constrained to areas of the plant with a minimal explosion risk.

Also before commissioning, various laws governing power plants in the U.K were researched. The major standards any chemical power plant should meet include those published by:   

Renewable Energy Strategy (July 2009) Energy Bill (January 2008) Climate Change Bill (November 2007) 5

1.7 INDIVIDUAL DESIGN Upon completion of the group design project, various sections of either process designs were assigned to group members for design and evaluation. For my task, I opted to design an optimum reactor for process design 2. This will be designed for indirect firing taking into account mechanical considerations and chemical engineering design. Also, I was assigned the task of researching into pre-treatment techniques to maximise conversion efficiency of wood. Highlighted below in the flow sheet of the second process design, are the sections I have been assigned to research and design. FIGURE 1.7 (a): PROCESS DESIGN 2

Author: P. D. Desai

Date: 02/01/11

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2.0 DESIGN OF PYROLYSIS REACTOR FOR RENEWABLE ELECTRIC POWER PRODUCTION FROM BIOMASS WASTE

2.1 SUMMARY This report documents the design of an Auger Screw Reactor, for indirect firing, taking into account mechanical considerations and chemical engineering design, to aid in the achievement of a throughput 4.5MW, from the designed pyrolytic / gasification plant. This reactor to be designed specifically addresses [Process Flow Sheet 2], and it to be incorporated in the designed electric power production system. The following summarises the major specifications of the designed Auger Screw Reactor: TABLE 2.1 (a): AUGER SCREW REACTOR VESSEL GEOMETRY SHELL

Cylindrical shell with a configuration ratio of 4:1

HEAD

Flat Head

NOZZLE

Nozzles flanged to allow for connections, located at the top and bottom dish.

SUPPORT

Skirt support welded to the bottom head of the cylindrical vessel.

TABLE 2.1 (b): AUGER SCREW REACTOR VESSEL INTERNALS VESSEL AGITATOR

Helical Screw

BAFFLES

Helical

FLUIDISATION

Discarded

TABLE 2.1 (c): AUGER SCREW REACTOR VESSEL JACKET/HEATING MEDIUM VESSEL JACKET

Half pipe

HEAT TRANSFER MEDIUM

Syngas gas from gasification

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TABLE 2.1 (d): AUGER SCREW REACTOR FEEDER TYPE FEEDER TYPE

Screw conveyor

TABLE 2.1 (e): AUGER SCREW REACTOR MATERIAL OF CONSTRUCTION MATERIAL OF CONSTRUCTION

TYPE

Stainless Steel

316

THROUGHPUT    

The mass flow rate of the heating medium is approximately 0.3 kg/s. The energy content in the heating medium is approximately 90 kW. The average energy content in the product stream (comprising syngas +char/tar/bio oil) is approximately 1200 kW. Assuming a 40% conversion of the product stream into syngas, the energy content of syngas is approximately 500 kW, whereas char/tar/bio oil is approximately 700 kW.

SIZING The British Standards (PD 550:2009) was used in conjunction with the ASME code section VIII, Division 1, in order to aid in sizing the Auger Screw Reactor. The overall sizing of the Auger Screw Reactor is summarised below: AUGER SCREW REACTOR SPECIFICATION Volume Residence time Shell height Head height Shell diameter Helical screw diameter Baffle diameter Half pipe coil length Half pipe coil spacing Thickness of reactor vessel TABLE 2.1 (f): Overall sizing of auger screw reactor.

VALUE 0.2 m3 110 seconds 2m 0.1m 0.5m 0.2m 0.05m 0.7m 0.02m 10mm

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2.2 INTRODUCTION The chemical reactor is the most important equipment for the most part chemical processes. It is the vessel in which chemical reactions take place. Its function involves the conversion of raw materials into the useful product required. Many pyrolytic and gasification processes are based on the safe, economic and consistent operation of chemical reactors. Thermo-chemical conversion processes are often interrelated [Bridgwater, A.V (1999)]. Pyrolysis is known to be a precursor to both gasification and combustion. As a consequence, it is not necessary to develop or manufacture a reactor specifically for analysis of biomass pyrolytic reactions, as consideration has to be given to other possible reactions. Suitable reactors have been already outlined in the group report. Examples include:   

Fluidised beds Entrained Flow Rotary Kilns

However, Auger Screw Reactors have been in development and are considered to be in a ‘proof of concept’ phase, even though it has attained high success in lab-scale processes. It is therefore rather important to distinguish laboratory scale chemical reactors to industrial scale chemical reactors.  

Laboratory chemical reactors are used to obtain reaction characteristics. Industrial chemical reactors are designed for efficient production rather than gathering information

From the differences outlined above, the shape and mode of operation of laboratory scale chemical reactors are best designed to achieve well defined conditions of concentrations and temperature, so that a reaction model can be developed which will prove useful in the design of large scale / industrial scale reactor models. The largest example of this type of reactor used in an industrial process was a 200 kg/h unit constructed by Renewable Oil International [Bain, R. L. (2004)]. For the reason that the chemical reactor is the place in the pyrolytic/gasification process where the most value is added (i.e. low value feeds are converted to high value products), many aspects of reactor analysis and design must be considered carefully. This report documents the design of an Auger Screw Reactor, for indirect firing, taking into account mechanical considerations and chemical engineering design, to aid in the achievement of a throughput 4.5MW, from the designed pyrolytic / gasification plant. This reactor to be designed specifically addresses [Process Flow Sheet 2], and it to be incorporated in the designed electric power production system.

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2.3 MASS BALANCE The basis of this mass balance to be conducted typically involves calculations for the:   

Fuel feed rate (i.e. pre-treated wood) Flow rate of the heating medium Product flow rate

An illustration of the process schematic is depicted below: Heat Transfer Medium

System Boundary

Feed (Wood)

Product

FIGURE 2.3 (a): Reactor Process Schematic. The general assumptions to be incorporated in this mass balance for simplification include: 1. Continuous Process This is essentially a unit process which involves an uninterrupted sequence of operations, in which the feed material must be introduced in a schematic manner in order to maintain equilibrium conditions. 2. Steady State A system described to be at steady state implies all the variables occurring within the system are constant, in spite of the ongoing processes that strive to change them. This suggests accumulation can be ignored as there is no build up of materials. 2.3.1

FUEL FEED RATE

As described earlier in the group report, the feed of wood chips is approximately 5 tonnes per hour, with a moisture reduction from 20% to 6%. During drying (i.e. pre drying and post drying), the temperature of the feed is raised from 15°C to 150°C, reducing its feed rate to 4.4 tonnes per hour (i.e. 1.19 kilograms per second). 10

2.3.2

FLOW RATE OF HEATING MEDIUM

In order to proceed with a mass balance for this, the equivalence ratio must be deducted. The equivalence ratio (E.R) is basically the ratio of air-fuel to the stoichiometric air-fuel ratio. This term basically applies to air deficient systems, such as the reactor to be designed. Pyrolysis takes place in the absence of air, hence the E.R is zero. However, a completely inert environment is practically never achieved; the E.R will be greater than zero. The graph below depicts the effects of E.R and carbon conversion:



GRPAH 2.3.2(a): Equivalence ratio against carbon conversion efficiency [Basu,P (2010)].

A lower E.R value tends to increase tar production, but a higher E.R value tends to emit more products of complete combustion (i.e. CO2, etc). For this balance, an E.R value between 0.20 and 0.30 was employed. From this, the flow rate of the heating medium, according to [Basu,P (2010)], is given as:



EQUATION 2.3.2 (a). Where:

Mf(a): Flow rate of the heating medium (kg/s) E.R: Equivalence ratio Mf: Wood Feed Rate (kg/s) The table below shows the flow rate of the heating medium with varying equivalence ratio: 11

E.R

Mf(a) – kg/s

0.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3 TABLE 2.3.2 (a): Results for the flow rate of the heating medium 2.3.3

0.238 0.2499 0.2618 0.2737 0.2856 0.2975 0.3094 0.3213 0.3332 0.3451 0.357

PRODUCT FLOW RATE

The volume flow rate of the product gas, according to [Demirbas, A (2001)], from a desired net heating value is found by:



EQUATION 2.3.3 (a). Where:

V(g): volume flow rate of the gas produced (Nm3/s) Q: Reactor’s required power output (MW) LHV(g): Net heating value (MJ/m3) For the reason that the volume of gases change with temperature or pressure, it is necessary to specify the temperature and pressure the flow rate was measured at. However, [EQUATION 2.3.3 (a)] assumes standard conditions of temperature and pressure (i.e. 1 atmosphere and 0-20 degrees Celsius). For this balance, the LHV(g) is unknown and hence will be varied. According to [Demirbas, A (2001)], LHV(g) values for typical gasification systems can range from 5 MJ/Nm3 to 15 MJ/Nm3. Taking the reactor’s required output power as 5 MW (i.e. from design brief), and syngas density as 0.95 kg/s, the mass flow of the product gas can be resolved. Hence the mass flow of char/tar can be resolved from summing up the mass flow of the feed and heating medium and subtracting the mass flow of the product gas. The table below shoes the values for the volume flow rate of the gas produced and hence the mass flow rate of the gas produced and the char/tar/bio oil (M(c/t/b)) by-products with varying LHV(g) values:

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LHV(g) – MJ/m3

V(g) – Nm3/s

M(g) – kg/s

5 1 6 0.833333 7 0.714286 8 0.625 9 0.555556 10 0.5 11 0.454545 12 0.416667 13 0.384615 14 0.357143 15 0.333333 TABLE 2.3.3 (a): Results for the flow rate of the heating medium

M(c/t/b) – kg/s 0.95 0.791667 0.678571 0.59375 0.527778 0.475 0.431818 0.395833 0.365385 0.339286 0.316667

0.5375 0.695833 0.808929 0.89375 0.959722 1.0125 1.055682 1.091667 1.122115 1.148214 1.170833

2.4 ENERGY BALANCE Most pyrolytic/gasification reactions are predominantly endothermic. This Implies heat must be supplied to the reactor for these reactions to take place at the designed temperature. The amount of external heat supplied to the reactor depends on the heat requirements of the endothermic reactions as well as the pyrolysis temperature. The pyrolysis temperature is at 450 degrees Celsius, as stated in the group design project for process flow sheet 2. The general energy balance equation, according to [Brian Smith, E (2004)] is given by:



EQUATION 2.4 (a). Where:

Q: Energy m: mass flow rate Cp: kJ/kg K : Temperature Change (°C) The first step of this mass balance involves resolving the heat energy content of wood supplied to the reactor. Below are table of specific heats for different woods: TYPE OF WOOD SPECIFIC HEAT CAPACITY (kJ/kg K) Balsa 2.9 Oak 2 White Pine 2.5 Loose 1.26 Felt 1.38 TABLE 2.4 (a): Specific heats of different woods. Compiled from: [Engineering Toolbox (Unknown)] Taking an average specific heat value of 2 kJ and applying [EQUATION 2.4 (a)], the energy content of the wood after drying (from 15°C to 150°C) is approximately: 13

Heating requirements for the reactor are supplied via the heating medium (combustible gases). The specific heat and temperature change of the heating medium are known to be 1.017 KJ/kg K and 300K. Hence from application of the energy balance equation [EQUATION 2.4 (a)], the energy content of the heating medium, with varying mass flow rates are shown in the table below: Mf(a) – kg/s

Q heating - kW

Q products - kW

0.238 72.6138 0.2499 76.24449 0.2618 79.87518 0.2737 83.50587 0.2856 87.13656 0.2975 90.76725 0.3094 94.39794 0.3213 98.02863 0.3332 101.6593 0.3451 105.29 0.357 108.9207 TABLE 2.4 (b): Energy content of heating medium with varying flow rates

1072.614 1076.244 1079.875 1083.506 1087.137 1090.767 1094.398 1098.029 1101.659 1105.29 1108.921

The energy content in the product stream is basically the sum of energy content in wood and the heating medium. The energy content in the syngas produced is essentially a percentage of the energy content in the product stream. Taking the average value for the energy content of the product stream as 1199.844 kW, the energy content of syngas and char/tar/bio oil produced with varying percentage conversions are shown in the table below: Percentage Conversion (%)

Q Syngas (kW) Q Char/tar/bio oil (kW) 0 0 0.1 119.9844 0.2 239.9688 0.3 359.9532 0.4 479.9376 0.5 599.922 0.6 719.9064 0.7 839.8908 0.8 959.8752 0.9 1079.86 1 1199.844 TABLE 2.4 (c): Energy content of product compositions with varying percentage conversions

1199.844 1079.86 959.8752 839.8908 719.9064 599.922 479.9376 359.9532 239.9688 119.9844 0

For this reactor design, a 40% conversion in the product stream is assumed. 14

2.5 UNIT PIPING AND INSTRUMENTATION DIAGRAM NB: The drawing (PID) illustrated in this section is directly related to a HAZOP assessment. 2.5.2

(HAZOP) – DESCRIPTION OF PROCESS: PYROLYSIS CHAMBER

NB: L7 & L8 are not pipelines, but rather feeders (i.e. gravity chute and screw feeder respectively). Wood from pre-treatment is fed through L7, which is effectively a gravity chute feeder, to the reactor R1. The reaction occurring within R1 is a thermal decomposition (i.e. pyrolysis). The main product, syngas, is piped off to scrubbing via L6 and the by-products of char/tar are sent to the gasifier via L8, which is a screw feeder. In case of R1 malfunction (e.g. unwanted composition of syngas is evolved), VI9 can be closed to prevent syngas movement to downstream equipment. This in turn will increase the pressure within R1. To counteract this, a signal will be sent to control valve VC3 via pressure controller (PC). This open VC3 and relief pressure of R1 through L4, which is the exhaust stream. F2 is an induced draft fan, which removed flue gases from the reactor and forces exhaust through L4. In case of F2 failure, a back-up I.D fan F3 on L5 will be made operational by closing VI7 and VI8, in order to isolate F2. R1 is designed for indirect heating by air and syngas from the gasifier, fed into the C1, which supplies combustion gases to heat up the walls of R1. Air will be supplied through L1 to C1 via air handler F1, which acts to condition and circulate air as part of heating. Syngas from the gasifier will be supplied to C1 through L2. In order to ensure ratio balance of syngas and air, an implemented ratio controller (RC) monitors the ratio of syngas through VC2 on L2 and adjusts the ratio of air via VC1. Combustion gases from C1 will be supplied to the walls of R1 through L3. Temperature fluctuations of the combustion gases through L3 will be monitored by temperature controller (TC), which will send a signal to VC2 for adjustments. In case there is a fault with C1; VI6 will be closed to prevent further equipment damage. Increased pressure/ temperature within C1 is counteracted by closing VI5 on L2, which stops syngas from being supplied to C1. LEGEND        

C – Combustion Chamber F – Fan/blower VC & VI – Control Valve & Isolation Valve L – Pipe Line RC – Ratio Controller R - Reactor PC – Pressure Controller TC – Temperature Controller

A detailed Piping and Instrumentation diagram (PID), for the pyrolysis chamber in concern is shown below: 15

Drawing not to scale. For HAZOP purposes only.

FIGURE 2.5.2 (a): Pyrolysis/Gasification Renewable Energy Plant – PYROLYSIS CHAMBER

Wood from Pre-treatment

Exhaust

Syngas PC

L6

L7

F2

VI 9

VI 8

Air

L4

P-34

R1 VC 1

VI 7

VC 3 G1

F1

L1 F3

VI 3

VI 1 VI 2

L5

VI 4

L3 L8

RC

C1 VI 6

Char/Tar to gasifier

TC

L4 VI 5

VC 2

Syngas from gasification

Kwaku Asiamah. January 2011. Department of Chemical and Biological Engineering.

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2.6 CHEMICAL ENGINEERING DESIGN In order to establish and accomplish the chemical engineering design involved in the manufacture of chemical reactors, it is necessary to construct and illustrate some design principles, which can be applied at many size scales to many different types of chemically reacting systems. Chemical reactors may be operated in:  

Batch: this is where the reactants are initially charged, and the reaction proceeds with time, at a desired temperature and pressure, maintained until the end of the reaction cycle. Continuous: Reactant streams are continuously fed into the vessel and the product streams are withdrawn.

The reactor design under consideration in this project is operated continuously. 2.6.1

GENERAL CONSIDERATIONS

The successful operation of any chemical reactor largely depends on design, which relays to understanding fundamentals and establishing principles, which are to be adhered. This will typically involve critical evaluation of: 

Applicability:

This is basically considering the relevance of the chemical reactor to be designed, by virtue of its application, which in this case will be electric power production via pyrolysis and gasification techniques. 

Limitations:

These are principles that limit the extent of the chemical reactor application, and outlines restrictions in the mechanical and chemical design. The major design aspect associated with the design of chemical reactors is the design of the reactor vessel or process vessel. However, in order to initiate and commences designing, it is worth mentioning or delving into the general problem associated with reactor control, temperature. Temperature is a dominant variable and must be effectively controlled to achieve the desired:   

Compositions Conversions Yield

in the safe, economic and consistent operation of chemical reactors. Once temperature control has been achieved, providing base level stable operation and additional objectives for the control system can be specified. The reactor under consideration for design is the auger screw reactor. For this project, it will specifically function as a Continuous Stirred Tank Reactor (CSTR).

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Heat transfer is a major consideration in the design of a reactor to initiate pyrolysis. The diagram below illustrates how heat is transported to the fuel particle during pyrolysis:

Biomass pores

Heat transfer by radiation

Heat transfer by conduction Biomass inner surface Biomass outer surface Heat transfer by convection FIGURE 2.6.1 (a): Heat transfer to fuel particle. Concept from [Diebold, J.P & Bridgwater, A.V (1997)] Because of the relatively low thermal conductivity of wood which is the feed for this reactor, the interior of the wood particle is heated at a considerably lower rate since heat transfer to its interior is mainly by conduction. At the temperature range of 300°C-500°C, as in the operating conditions of the designed reactor in this project, heat/mass transfer is considered too high to offer any resistance to the overall rate of pyrolysis [Bridgwater, A.V (1999)]. However, at temperatures above this range, heat/mass transfer cannot be neglected as they influence the overall rate. The use of a jacket surrounding a reactor vessel is probably the most common method for providing heat transfer because it is relatively inexpensive in terms of equipment capital cost. Understanding the thermal design of jacketed vessels often involves specification of important parameters, which include:  

 

Vessel Geometry Internals:  Vessel Agitation  Baffling  Fluidisation Vessel Jacket Feeder

This is outlined in the following sections. 18

2.6.2

GENERAL SPECIFICATIONS

This section delves into a detailed description of the design criteria for the reactor to be implemented in the previously described process, to aid in efficient pyrolysis for renewable electric power generation. This will typically follow the process of converting theory into a set of:  

Constraints: aspects that are fixed Considerations: aspects that are flexible

Eventually, applications of these constraints and considerations will lead to the generation of a suitable reactor model. The reactor to be considered is essentially an ‘auger screw reactor’ with a continuous operation. 2.6.2.1 VESSEL GEOMETRY The normal configuration for a process vessel is a vertical cylindrical section closed by dished ends. [Bruce Nauman, E (2002)]. This section describes the main components of reactor vessels. This generally includes:    

Shell Head Nozzle Support 2.6.2.1.1

SHELL

The shell is the primary component of the reactor vessel that contains the pressure. They are typically welded together to form a structure that has a common rotational axis. Reactor vessels typically have cylindrical shells. Specifying the shell requires specifying the configuration ratio. This is usually in the order of 1:1. However, to maximise heat transfer through the jacket, a configuration ratio of 4:1 was chosen. 2.6.2.1.2

HEAD

All vessel shells must be closed at the ends. The end caps usually employed on cylindrically shaped vessels are referred to as Heads. These can be curved or flat. Since pyrolysis / gasification occur under pressure, it is advisable to opt for a bottom dish which can cope with this situation. Illustrated below are the two most common head configurations for process vessels:

19



ELLIPSOIDAL HEAD

FIGURE 2.6.2.1.2 (a): Ellipsoidal Head [Vickers. (2010)]. This is also known as a 2.1 Semi Elliptical Head. Due to their increased depth, are stronger but more difficult to form. The height of the head is just a quarter of the diameter. 

FLAT HEAD

FIGURE 2.6.2.1.2 (b): Flat Head [Vickers. (2010)]. This is basically a flat end with a knuckled outer edge. Relatively cheaper as its less difficult to form, but suffers from decreased strength as compared to the elliptical heads. These two head shapes can both be applied to pressure applications. However, the 2.1 Semi Elliptical Head is for very high pressure applications (<100 bar) and won’t be economical if employed. For this reactor vessel design, flat configuration was chosen as they are stronger and allow the heads to be thinner, lighter and less expensive, as compared to curved heads. They also allow for easy maintenance.

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2.6.2.1.3

NOZZLE

This is a cylindrical component that penetrates the shell or head of process vessels. The nozzle ends will be flanged to allow for the necessary connections and to permit disassembly. The nozzle will be used for the following applications:   

Attach piping for flow into and out of the vessel Attach instrumentation – E.g. Level gauges , etc Provide for the direct attachment of other equipment – E.g. Heat Exchangers, etc

Vessel nozzles are concentrated on the top disk. There will also be a centrally located nozzle at the bottom disk. 2.6.2.1.4

SUPPORT

The vessel support is intended to support the reactor process vessel on the support base. The type of support that is used depends primarily on the size and orientation of the reactor vessel. The support chosen must be adequate for the applied   

Weight Wind Earthquake loads

[Dedisumaha. (2010)] Various supports have been used in industrial process (reactor) vessels: 1. SADDLE SUPPORTS REACTOR VESSEL SHELL

REACTOR VESSEL HEAD SADDLE SUPPORTS

FIGURE 2.6.2.1.4 (a): Saddle Supports 21

2. LEG SUPPORT

REACTOR VESSEL HEAD

REACTOR VESSEL SHELL

LEG SUPPORT FIGURE 2.6.2.1.4 (b): Leg Supports

3. LUG SUPPORT

REACTOR VESSEL HEAD

REACTOR VESSEL SHELL

LUG SUPPORT

LUG SUPPORT

FIGURE 2.6.2.1.4 (c): Lug Supports 22

4. SKIRT SUPPORT

REACTOR VESSEL HEAD

REACTOR VESSEL SHELL

SKIRT SUPPORT FIGURE 2.6.2.1.4 (d): Skirt Supports FIGURES 2.6.2.1.4 (a-d) concepts compiled from :[Dedisumaha. (2010)] Since the reactor vessel to be designed is essentially tall, vertical and cylindrical, opting for the skirt support is the most appropriate. As can be seen from [FIGURE 2.6.2.1.4 (d)], the skirt support is a cylindrical shell section that is welded to the lower portion or bottom head of cylindrical vessels.

2.6.2.1.5

SUMMARY OF VESSEL GEOMETRY

SHELL

Cylindrical shell with a configuration ratio of 4:1

HEAD

Flat Head

NOZZLE

Nozzles flanged to allow for connections, located at the top and bottom dish.

SUPPORT

Skirt support welded to the bottom head of the cylindrical vessel.

23

2.6.2.2 VESSEL INTERNALS The reactor vessel internals will be designed to support and orient the biomass (wood) fuel assemblies and direct heat flow through the core. This will comprise of the design of:   

Agitator Baffles Fluidisation 2.6.2.2.1

AGITATOR

The primary function of the agitator to be designed is to promote mixing and also to promote heat transfer at the vessel wall. Vessel agitators are classed in relation to how close they are to the vessel wall. Vessel agitators may be either: 

Non-Proximity

These comprise turbines and propellers, typically mounted on a vertical shaft. 

Proximity

These comprise helical screws and anchors, and are usually employed for high viscosity processes. The choice on the type of agitator is made in accordance with the mixing requirements in the vessel. For the reactor to be designed, a helical screw (proximity) will be employed. 2.6.2.2.2

BAFFLES

Baffles are flow directing or obstructing vanes used in industrial process vessels. Implementation of baffles is decided on the basis of:   

Size Cost Ability to lend support

In this chemical reactor, baffles will be attached to the interior walls of the vessel to promote mixing and thus increase heat transfer and possibly the chemical reaction rate. The baffles used will also be helical in type (shape). 2.6.2.2.3

FLUIDISATION

Fluidisation is a unit operation and through this technique, a bed of particulate solids, supported over a fluid-distributing plate (referred to as a grid), is made to behave like a liquid by the passage of a fluid (gas, liquid, gas/liquid) at a flow rate above a certain critical value [Gupta, C.K & Sathiyamoorthy, D (1999)].

24

The concept of fluidisation is well understood in its application to pyrolytic / gasification processes. Employing this technology has been seen to give high reaction rates due to its turbulent nature and good temperature control. However, the major problem associated with fluidised bed technology has to deal with char / tar separation from the bed, which usually consist of sand / silica. For this reactor design, it has been opted to discard any form of fluidisation in view of the fact that separation is rather difficult, and if implemented will be rather costly as it would involve several optimisation processes for screening, separation and cleaning.

2.6.2.2.4

SUMMARY OF VESSEL INTERNALS

VESSEL AGITATOR

Helical Screw

BAFFLES

Helical

FLUIDISATION

Discarded

2.6.2.3 VESSEL JACKET The type of jacket and the heating medium to be supplied to the reactor will be resolved. 2.6.2.3.1 TYPE OF JACKET Several types of heating jackets are available. The vessel can also be fitted with an internal coil for heat transfer. However, the use of an internal coil is not necessary for this design. The style of the jacket to be used in a particular application and whether an internal coil will be used are determined by numerous factors, including:    

The rate of heat transfer required Critical cooling duties Lining in the process vessel Ease of cleaning of the process vessel

The main jackets resulting from these factors, according to [Integ. (2008)] include:    

Conventional Baffled conventional Half pipe Dimple

For this design, the half-pipe jacket was opted, as the rest of the listed jackets above usually apply when designing reactor vessels which deal primarily with liquids.

25

Half pipe give high heat transfer coefficients and are suitable for higher pressure operation. The space between adjacent coils is effective for heat transfer, with the overall effectiveness of heat transfer area averaging 95%. An illustration of half-pipe design is shown below:

FIGURE 2.6.2.3.1 (a): Half pipe design [Bulletin. (Unknown)] 2.6.2.3.2

TYPE OF JACKET

The heat transfer medium to be used could be either:    

Water Steam Hot Oils Dowtherm Vapours

[Integ. (2008)] However, for this pyrolytic process, the heat transfer medium will effectively by syngas, produced from the gasification process at 700 degrees Celsius. This will essentially be used to indirectly heat the walls of the reactor via the half pipes.

2.6.2.3.3 SUMMARY OF VESSEL JACKET VESSEL JACKET

Half pipe

HEAT TRANSFER MEDIUM

Syngas gas from gasification

26

2.6.2.4 FEEDER TYPE This is basically the flow handling system, into and out of the reactor. This is specifically important to ensure efficient and effective flow of pre-treated wood into the reactor and char/tar/bio oil out of the reactor to gasification. Many types of feeders are used in industrial processes when handling solids. These, according to [Enden, P.J & Lora, E.S (2004)] may include:      

Gravity Chute Screw conveyor Pneumatic injection Rotary spreader Moving-hole feeder Belt feeder

For this reactor design, two feeders will be looked into. These are discussed below: (A) GRAVITY CHUTE This is basically a simple device in which the fuel particles (i.e. pre-treated wood), are dropped into the bed, with the help of gravity. (B) SCREW FEEDER This is basically a positive displacement device, which moves the fuel particles from a high pressure zone to a low pressure zone. 2.6.2.4.1 DEDUCTIONS OF FEEDER TYPE Comparing these two feeder types, it was decided to opt for the screw feeder for this reactor design. This is mainly because the gravity chute feeder can neither control nor measure feed rate of the pretreated wood coming into the reactor, and the char/tar/bio oil coming out of the reactor to gasification. This might lead to uncertainty in the reactions occurring and possible damage to equipment in the likely hood of overloading. On the other hand, the screw feeder can easily control the feed rate. This is done via a drive, which can be used to vary the speed of the conveyor.

2.6.2.4.2 SUMMARY OF FEEDER TYPE FEEDER TYPE

Screw conveyor

27

2.6.3

MATERIALS OF CONSTRUCTION

Reactor process vessels may be constructed of a wide range of materials. The mechanical design of a reactor vessel can only proceed after the materials of construction have been specified. The main factors that influence selection are: 

Strength

This is basically the materials ability to withstand an imposed force. 

Corrosion resistance

Wear and tear of materials by chemical action, and influences its selection for a specific application. Alloys are typically used where increased corrosion resistance is required. 

Fracture Toughness

This is the ability of materials to withstand conditions that could lead to brittle fracture. Material selection should eliminate brittle fracture since this can be catastrophic to the equipment. 

Fabricability

This refers to the ease of construction. Since reactor vessels are typically welded, materials of construction must be able to be welded. Taking into account the discussed factors above, two materials of construction were researched: 1. Carbon Steel 2. Stainless Steel The two materials of construction listed above are essentially steels, which are alloys consisting of iron and carbon. Stainless steel differs from carbon steel by the amount of chromium present. 2.6.3.1

COMPARISON OF MATERIALS OF CONSTRUCTION

Carbon steel is the most inexpensive material of construction of the two, and has good strength and Fabricability. However, since corrosion is a major influence in the pyrolysis / gasification process, due to the presence of variable amounts of oxygen, stainless steel is opted as it offers better economics in relation to cost and efficiency. 2.6.3.2

STAINLESS STEEL.

Stainless Steel is a common name for metal alloys that consist of 10.5% or more Chromium (Cr) and more than 50% Iron (Fe) [United Performance Metals. (2006)]. Stainless steel is of various types. Typical mechanical properties as required by ASTM specification are shown in the Appendix. 28

For this process vessel design, Stainless Steel, Type: 316 will be chosen as the material of construction of the process vessel. This is because it offers relatively high tensile and yield strength, as seen from the mechanical properties above. Also, type 316 has a fairly high elongation, hence will be able to withstand more strain before failure in tensile testing. Typical properties are shown in the table below:

GRADE TYPE

TENSILE STRENGTH (MPa)

YIELD STRENGTH (MPa)

ELONGATION (% in 50 mm)

ROCKWELL B (HR B)

BRINELL (HB)

316

515

205

40

95

217

TABLE 2.6.3.2 (a): Properties of 316 SS. Compiled from [United Performance Metals. (2006)].

2.6.3.3

SUMMARY OF MATERIAL OF CONSTRUCTION

MATERIAL OF CONSTRUCTION

TYPE

Stainless Steel

316

29

2.6.4

SIZING

This section generally involves the geometric design, where the basic sizes (i.e. the geometric dimensions of critical components) of the reactor are determined. The geometric configuration and preliminary sizing of the reactor will be resolved. 2.6.4.1

ASME / BS CODES

Reactor vessels / pressure vessels are generally designed in accordance with the American Society of Mechanical Engineers, even for locations outside the United States of America [William, L. Luyben (2007)]. This is usually in accordance with the ASME code section VIII, typically Division 1, since it contains sufficient requirements for the majority of vessel applications . Also in this design report, the British Standard (PD 550:2009), will be used in conjunction with the ASME VIII, Division 1. The main objective of the ASME / BS Code is to establish minimum requirements that are necessary for the safe operation and construction of these process vessels. Detailed descriptions are given in the Appendix. 2.6.4.2

VOLUME OF REACTOR

In order for the volume to be resolved, the pyrolysis kinetics will have to be determined. This will involve determination of the wood pyrolysis reaction rate constant. The temperature dependence of the reaction rate constant, and hence the rate of the chemical reaction can be determined by the Arrhenius equation [Clark, J. (2002)]:

EQUATION 2.6.4.2 (a). Where: k = rate constant for chemical reactions (s-1) A = Pre-exponential factor (s-1) e = Exponential function Ea = Activation energy (J/mol) R = Gas constant (J/mol.K) T = Absolute Temperature (K) The pyrolysis reaction occurring within the reactor is at 450 degrees Celsius, implying the absolute temperature will be 723 Kelvin. The Gas constant is generally 8.314 J/mol K. A summary of the kinetic properties from wood pyrolysis are given below:

30

FUEL

Ea (J/mol)

A (s-1)

Wood

125400

1.0 × 108

TABLE 2.6.4.2 (a): Kinetic properties from wood pyrolysis [Basu,P (2010)] Inserting these kinetic data into the Arrhenius equation gives a rate constant of:

To determine the volume, a material balance will have to be carried out on the reactor. The general material balance for any individual reactant or product according to [Richard, M. Felder & Ronald, W. Rousseau (2005)] is:

Since the reactor in discussion is to be designed as a Continuous Stirred Tank Reactor (CSTR), the general assumptions are:   

Steady State No temperature / concentration gradients Continuous flow

Applying these assumptions to the general material balance above, term (1) becomes zero since operation is at steady state. Terms (2) and (3) are essentially flows in and out of the reactor and term (4) will just be the rate of the reaction. Substituting mathematical expressions and rearranging gives the volume of the reactor as:

[

]

Detailed algebra shown in Appendix section [Metcalfe, S. (2002)]. EQUATION 2.6.4.2 (b).Where: V = Volume of the reactor (m3) vt = volumetric flow rate (m3/s) k = rate constant (s-`) xa = percentage conversion The volumetric flow rate can be determined by dividing the mass flow rate of the feed (wood), by the density of wood. Taking the average density of wood as 670 kg/m3, the volumetric flow rate: 31

Applying the equation above, the table below shows lists of volumes with varying percentage conversion: V – m3

xa 0.1

0.002472

0.2

0.005563

0.3

0.009536

0.4

0.014833

0.5

0.02225

0.6

0.033375

0.7

0.051917

0.8

0.089

0.9

0.20025

TABLE 2.6.4.2 (b): Reactor volumes with varying conversions 2.6.4.3

REACTOR RESIDENCE TIME

This is basically the measure of time that wood particles will remain in the reactor, until they are converted into products and ejected. It is basically the ratio of the volume of the reactor to the volumetric flow rate [Metcalfe, S. (2002)]:

EQUATION 2.6.4.3 (a) Using the volumetric flow rate calculated (i.e. 1.78×10-3) and employing the above equation to [TABLE 2.6.4.2 (b)], a list of residence times (t), is shown in the table below:

32

V – m3

xa

t (s)

0.1

0.002472

1.388889

0.2

0.005563

3.125

0.3

0.009536

5.357143

0.4

0.014833

8.333333

0.5

0.02225

12.5

0.6

0.033375

18.75

0.7

0.051917

29.16667

0.8

0.089

50

0.9

0.20025

112.5

TABLE 2.6.4.3 (a): Reactor residence times For this reactor design, we shall assume a percentage conversion of wood into products as 90%. This implies the volume of the reactor to be designed is approximately 0.2 m3. This implies a residence time of approximately 112.5 seconds, roughly 110 seconds. 2.6.4.4

HEIGHT OF REACTOR & OTHER SPECIFICATIONS

SHELL The total height of the shell may be given by:

The height of the bed, in which the reaction takes place, according to [Basu,P (2010)], is given by:

EQUATION 2.6.4.4 (a). Where: Hbed = Height of the bed (m) V = Volume of the bed (m3) Ab = Cross sectional area of the bed (m2) The cross sectional area of the bed is given by:

EQUATION 2.6.4.4 (b). [Basu,P (2010)] Where: V(g) = volume flow rate of the gas produced (m3/s) 33

U(g) = Fluidisation velocity (m/s)  

Typical fluidisation velocity varies between (3-5) m/s. The volumetric flow rate of the gas produced is approximately 0.4 m3/s.

From this and applying the equation above, the table below shows variation of cross sectional areas with fluidisation velocity. Ab – m2

Ug – m/s 3

0.133333

3.2

0.125

3.4

0.117647

3.6

0.111111

3.8

0.105263

4

0.1

4.2

0.095238

4.4

0.090909

4.6

0.086957

4.8

0.083333

5

0.08

TABLE 2.6.4.4 (a): Variation of cross sectional area with fluidisation velocity Assuming the volume of the bed to be relatively proportional to that of the volume of the reactor, and applying [EQUATION 2.6.4.4 (a).], a range of calculated bed heights are shown in the table below: Ab – m2

Hbed - m 0.133333

1.501875

0.125

1.602

0.117647

1.702125

0.111111

1.80225

0.105263

1.902375

0.1

2.0025

0.095238

2.102625

0.090909

2.20275

0.086957

2.302875

0.083333

2.403

0.08

2.503125

TABLE 2.6.4.4 (b): Reactor bed heights For this reactor design, the fluidisation velocity was opted as 3 m/s giving a bed cross sectional area of 0.13 m2. This implies a bed height of 1.5m. Adding a freeboard height 0f 0.5m, gives a total height of shell as approximately: 34

SHELL DIAMETER The shell diameter can be approximated from the configuration ratio, which is basically the relationship between the height of the shell to the diameter of the shell. The configuration ratio for this reactor design was 4:1, implying a diameter of:

HEIGHT OF HEAD The height of the heads will be approximately a quarter of the diameter of the shell. This gives:

DIAMETER OF HELICAL SCREW The diameter of the helical screw will be approximately a third of the shell diameter. This gives:

BAFFLE DIAMETER Baffles are approximately a tenth of the diameter of the shell. This gives:

35

HALF PIPES The half pipes will basically be designed to be welded to the outer wall of the reactor vessel shell. For this design, the length of each pipe will be taken as half the circumference of the shell. This gives a length of:

Spacing between half pipes has been specified as 0.02m. [Bulletin. (Unknown)]

2.6.4.5

THICKNESS OF SHELL

The required wall thickness (tp) for internal pressure of a cylindrical shell is calculated using:

EQUATION 2.6.4.5 (a). [ASME code section VIII, Div 1] Where: P = Internal design pressure (N/m2) r = internal radius (m) S = Allowable stress (N/m2) E = Weld joint efficiency tp = Thickness of shell (m)

2.6.4.5.1

INTERNAL DESIGN PRESSURE

Generally, the internal design pressure is the maximum internal pressure that is used in the mechanical design of a reactor vessel. The reactor vessel to be designed for pyrolysis experiences both internal and external pressure. However, the mechanical design will be based on the conditions most severe, which in this case will be internal.

36

For this reactor design, it is assumed that the vessel is operated at full vacuum internal pressure, which is 103425 N/m2 [Eugene, F. Megyesy (Unknown)] 2.6.4.5.2

WELD JOINT EFFICIENCY

The weld joint efficiency is required to calculate the reactor vessel component thickness. This is the measure of the ratio of the joint strength to parent strength. Unless the welding is perfect, the joint between the plates is not as strong as the parent plate. A 100% joint factor essentially means a perfect weld. The value of the joint factor is therefore dependant on the type of weld, which is opted to withstand hydrostatic pressure. For this design, a single-welded butt joint with backing strip was chosen as the type of weld. The joint factor is also dependent on the degree of radiographic examination. A spot radiographic examination was selected as it will scan the weld for a section of the tank and relate it to the other sections, meaning it is relatively cheaper as compared to the full scan. With these suggestions, the joint factor was estimated to be 0.8. 2.6.4.5.3

ALLOWABLE STRESS

This is the maximum allowable stress based on the design material of reactor. This basically compensates for any uncertainty in the design methods, loading, quality of material and workmanship. According to the [British Standard, PD 500] for nominal design strengths, this material of construction (i.e. Stainless Steel 316),yields a design stress of 500 N/mm2. Applying all these factors discussed to [EQUATION 2.6.4.5 (a)] and a pressure factor of 1.1,

2.6.4.5.4

CORROSION ALLOWANCE

Corrosion, erosion and abrasion cause vessel components to thin during their operation life. To compensate for this thinning, the calculated thickness will be increased by 9mm. Hence the new resolved thickness of the shell is approximately 10mm. 2.6.4.6

THICKNESS OF HEAD

For this reactor design, the thickness of the head will be equivalent to the thickness of the shell as it allows for easier assembly, maintenance and reduces the cost of manufacture since specifications are not altered.

37

2.7 DETAILED DESIGN From the application of size calculations, a detailed design of the Auger screw reactor will be constructed in this section. This will typically involve three subsections of drawings:   

Concept design: this will basically aid in visualisation of the reactor and identification of various parts. Dimensions will not be included for this purpose. Mechanical drawings: this will entail two drawings showing the inner and outer components of the reactor with dimensions. Technical drawings: This will basically show sections of the reactor, with included dimensions.

These drawings listed above are shown in the following sub-sections:

38

2.7.1 AUGER SCREW REACTOR – CONCEPT DESIGN.

1 6 7 2

3

4

8 9

5

Drawings not to scale

1. 2. 3. 4. 5. 6. 7. 8. 9.

Kwaku Asiamah 02/02/11

NOZZLE BAFFLES HELICAL SCREW AGITATOR VESSEL INTERNALS SKIRT SUPPORT FLANGES HALF PIPE COILS VESSEL CYLINDRICAL SHELL FLAT HEAD

39

2.7.2 (A) MECHANICAL DRAWING OF INNER COMPONENTS OF AUGER SCREW REACTOR

SCALE: 3CM: 0.5M

0.5m

0.1m

0.15m

0.05m

1.3m 2m

0.1m

0.8 m

0.2m

Scale – 3cm:0.5m

0.01m 0.15m

Kwaku Asiamah 02/02/11

40

2.7.2 (B) MECHANICAL DRAWING OF OUTTER COMPONENTS OF AUGER SCREW REACTOR

SCALE: 3CM: 0.5M

0.5m

0.15m 0.02m

2.2m

Scale – 3cm:0.5m

0.7m

0.15m

Kwaku Asiamah 02/02/11

41

2.7.2 (B) TECHNICAL DRAWING OF AUGER SCREW REACTOR

SCALE: 1.5CM: 0.5M

Plan 0.5m

Scale – 1.5cm:0.5m 0.15m

0.5m

0.15m

2.2m

2.2m

0.1m 0.5m

Left End View

Front View

Kwaku Asiamah 02/02/11

42

2.8 HAZOP The HAZOP was carried out with the assumption that combustion gases will have to be produced in a combustion chamber to heat up the walls of the reactor. The combustion gases comprised of:  

Air Syngas

Conversely, it was discovered that syngas from the gasification process is essentially at a high enough temperature to heat up the walls of the reactor, hence this can be discarded. However, for the purpose of this HAZOP, the combustion chamber was included to show the relative changes made. 2.81 MODIFICATIONS MADE FROM HAZOP The major modifications made on the auger screw reactor unit piping and instrumentation diagram include: 

Incorporation of back-up equipment

This was basically implemented in order to keep the reactor functional in case of failure or maintenance. Isolation valves (ball valves) make it easy to isolate equipment. 

Flow controllers

These were added to measure and control the flow of fluids and gases 

Non return valves

These were implemented in some sections of the PID to prevent back flow which may give rise to problems. 

Bursting disc

This is essentially a non releasing pressure relief device and was basically incorporated to counteract over pressurisation.

The PID diagrams below basically depicts the modifications made after the HAZOP assessment.

43

2.82 BEFORE HAZOP PID

Drawing not to scale. For HAZOP purposes only.

FIGURE 2.8.2 (a): Pyrolysis/Gasification Renewable Energy Plant – PYROLYSIS CHAMBER

Wood from Pre-treatment

Exhaust

Syngas PC

L6

L7

F2

VI 9

VI 8

Air

L4

P-34

R1 VC 1

VI 7

VC 3 G1

F1

L1 F3

VI 1 VI 2

L5

VI 3

VI 4

L3 L8

RC

C1 VI 6

Char/Tar to gasifier

L4

TC

VI 5

Syngas from gasification

VC 2

Kwaku Asiamah. January 2011. Department of Chemical and Biological Engineering.

2.83 AFTER HAZOP PID FIGURE 2.8.3 (a): Pyrolysis/Gasification Renewable Energy Plant – PYROLYSIS CHAMBER

Drawing not to scale. For HAZOP purposes only.

Wood from Pre-treatment

Exhaust

Syngas PC

L7

L8

F3

VI 11

VI 8

L5

R1

Air FC

VC 1

VC 3

VI 7

F1

L1 VI 1

F4

VI 9

L6

VI 2

FC P-38

F3

VI 10

L4

L2

VC 4

VI 3 VI 4

L9

RC

C1

Char/Tar to gasifier

TC

L3

Syngas from gasification VI 5

VC 2

Kwaku Asiamah. January 2011. Department of Chemical and Biological Engineering.

44

2.9 START-UP/SHUT-DOWN PROCEDURES The auger screw reactor designed in this report is essentially continuously operated. For this reason, start-up can be rather tedious and complex. Generally, the most important controlled process variables to be considered during start-up include:  

Pressure Temperature

The start-up for this designed auger-screw reactor is relatively long, in view of the fact of heating requirements. A start-up burner will be supplied to heat the reactor walls initially. This will be to a temperature of around 800 degrees Celsius. Once a constant and uniform temperature has been achieved, pre-treated wood can be supplied to the reactor to initiate pyrolysis. The reactor vessel during start-up will not be pressurised. However, during the course of heat distribution the reactor walls via the start-up burner, gradual pressure will be added until vacuum. The reactor process vessel will be inerted before supplying heat via the start-up burner. This will be necessary to provide conditions adequate and necessary for the pyrolysis reaction to occur efficiently without disturbances. However, inerting medium required for this reactor is relatively low [Iowa State University. (2008)], which is a cost advantage. Once pyrolysis begins, the gasification system will supply the heating medium (i.e. syngas at 700 degrees Celsius), via the half-pipe coils which will heat up the walls of the reactor. This implies the start-up burner can be switched off as it is no longer required. The causes of many shut-downs are often related to failure of sime parts of the reactor. When this occurs, the gasification process will immediately be terminated; hence no heating medium will be supplied to the walls of the reactor. Ultimately, pyrolysis will be stopped, and the reactor can safely be checked to resolve the issues affecting it. Also to be noted is that the operating personnel and process engineers present during commissioning of the plant must be focused and made aware of:  

Safety constraints Controllability constraint ranges

to avoid upsets during start-up.

45

2.10

ESTIMATED COST

Economic evaluation of the auger screw reactor designed in this project will be based on estimations of: 

Capital Investment

This is basically the expenditure made to purchase capital assets, which in this case will be the reactor vessel incorporated with the half pipe coils and helical screw. 

Physical Plant Cost

This takes into account necessary factors in addition to the capital investment like instrumentation and piping. 

Fixed Capital Cost

This takes into account other indirect cost, to give a better value of the capital investment. The overall costing based on the above, are outlined below: 2.10.1 CAPITAL INVESTMENT Assessment of the capital cost can be estimated from pervious knowledge of the cost of an earlier project. This is the most common method of resolving the capital cost, as it related the capacity of a current project to the capacity of a previous project. However, since the auger screw reactor is a relatively new design idea, construction and operation has only been undertaken on lab-scale projects, hence capital cost estimation using this method cannot be achieved. For this reactor, the capital cost can be estimated from knowledge of geometry of the reactor. This is related to the equation:

EQUATION 2.10.1 (a). [William, L. Luyben (2007)] Where: Cost = Dollars ($) L = Length of the reactor vessel (m) D = Diameter of the reactor vessel (m) This equation above takes into account some assumptions. These are: 1. The reactor vessel is constructed of stainless steel. 2. The capital cost estimated is 10 times the calculated value using this equation because of heat transfer requirements (i.e. half pipe coils) and agitation (i.e. helical screw).

46

Applying equation [ref] above to the designed auger screw reactor in this project gives:

This gives the amount in dollars; hence an exchange rate is applied to estimate the cost in pounds. Using a conversion factor of $1=£0.628 [XE. (2011)], gives:

2.10.2 PHYSICAL PLANT COST The capital investment of the reactor design outlined above only gives an estimate of the cost of major equipments:   

The reactor process vessel Heat transfer equipment Agitation

However, to resolve a more accurate value, several other factors have to be considered. These may include:     

Piping Insulation and painting Instrumentation Electrical power and lighting Utilities (Services) – for example provision of water, air etc.

These contributions outlined above are estimated by multiplying the total capital cost by appropriate factors. Typical factors of the component capital cost required are listed in the table below:

TABLE 2.10.2 (a): Typical factors of the component capital cost [Sinnott, R.K (2005).p.252] 47

The physical plant cost can be estimated using the equation:

EQUATION 2.10.2 (a). Where: PPC = Physical Plant Cost PCE = Project Cost Estimation F = factors listed in table [ref] which apply The value obtained for the PPC has to be multiplied by the capital investment, in order to resolve a better value. The process type for this reactor is essentially solids and fluids. The factors to be considered for this reactor design are: ITEM PCE (f) Equipment erection 0.45 Piping 0.45 Instrumentation 0.15 Electrical 0.10 Utilities 0.45 TABLE 2.10.2 (b): Factors for component capital costs. Compiled from [Sinnott, R.K (2005).p.252] From this, the PPC is estimated at:

2.10.3 FIXED CAPITAL COST Also to be noted are indirect costs, typically: 

Design and Engineering cost

These are costs which cover design and engineering. This ranges from material purchasing to construction. 

Contractor fees

Since it is most likely that a contractor will be employed, the fees paid also have to be added to the total capital cost. 

Contingency allowance

In order to cover unanticipated circumstances (e.g. design errors), an additional allowance will have to be built into the capital cost estimate. The table below shows values for general indirect factors considered: 48

ITEM PCE - f Design and Engineering 0.25 Contractor Fee 0.05 Contingency 0.10 TABLE 2.10.3(a): Values for indirect factors considered. Compiled from [Sinnott, R.K (2005).p.252] The fixed capital cost can be estimated by multiplying the physical plant cost (PPC) by these factors above. This is given by the EQUATION 2.10.3 (a):

Applying equation [EQUATION 2.10.3 (a)] above, the fixed capital cost is estimated at:

From this, the overall costing required for the auger screw reactor designed will be approximately: £600,000

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3.0 CONSIDERATION OF WOOD PRE-TREATMENT TECHNIQUES 3.1

SUMMARY

Pre-treatment technologies were researched for wood feedstock, in order to alter structural and compositional impediments, to improve conversion rates and increase yields. The wood feedstock itself is a very attractive fuel option and aids in sustainable development due to it’s:    

Relative abundance Lower fuel cost Ease of maintenance Environmentally friendly - Carbon neutral

However, gasification/pyrolysis from wood often yields rather low energy densities due to its high oxygen to carbon ratio. Also, wood is thermally unstable and heating may lead to the formation of non condensable tars in gasifiers, thus creating problems in downstream equipment. Most common techniques of pre-treatment include drying and pulverising. Increase in energy density can be achieved by briquetting and pelletizing, however, not by significant amounts. A process that lowers the oxygen to carbon ratio of biomass is known as torrefaction. This implies increase in energy density as the O/C ratio is lowered, giving the wood fuel similar properties to coal. Torrefaction is essentially a mild thermal treatment, which is carried out on wood either before or after delivery to the power plant site. Carrying out torrefaction prior to delivery to the power plant site benefits from a greater increase in energy density, as briquetting or pelletizing can also be employed. A summary of the advantages of torrefied wood as opposed to untreated wood is shown in the table below:

UNTREATED WOOD      

Bulky Moist Fibrous Perishable Waste Expensive to transport

TORREFIED WOOD      

Dense, If Pelletized Dry and Water Resistant Easily Crushed Does not rot Valuable Fuel High Energy Density

50

The torrefaction reactor technology can be operated in two modes:  

Directly heated: biomass is brought into direct contact with the heat carrier Indirectly heated: biomass is in indirect contact with the heat carrier.

On the basis gas recycle, research shows that it is better to opt for the indirectly heated mode of operation. A summary of the mass and energy balances for the torrefaction reactor to be considered as pretreatment for wood feed stock is depicted below:

:

0.1 MWt

0.36 kg/s

Torrefaction gases

Wood (Untreated)

1 MWt

1.19 kg/s

TORREFACTION CHAMBER 200°C - 300°C

Torrefied Wood 0.9 MWt

0.84 kg/s

There are currently no commercially available torrefaction plants. However, there has been a growing interest in its implementation as a pre-treatment to improve biomass characteristics.

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3.2

BRIEF

Pre-treatment techniques are basically processing treatments raw materials receive, prior to major conversion. The goal of a pre-treatment is to alter or remove structural and compositional impediments to improve rates and increase yields. In this study, we primarily look in to new concepts in biomass (wood) pre-treatment, to aid in efficient pyrolysis. 3.3

INTRODUCTION

A major influence which encourages sustainability is the provision of energy that meets the needs of the present without compromising the ability of future generations. Concerning the increasing scarcity of fossil fuels worldwide and the increasing environmental pollution, numerous endeavours have been attempted to find other renewable and environmentally friendly energy sources and to advance the technologies. This will typically involve adopting technologies and researching new techniques to improve energy efficiency. As to power generation, harnessing renewable energy such as wind and solar is an appropriate first consideration, because apart from plant construction, there is no depletion of mineral sources and any direct air or water pollution. However, these energy resources have not been able to provide an economically viable solution for large scale applications. In recent years, there has been a growing interest in power generation via biomass gasification technology. One biomass energy based system, which has been proven reliable and had been extensively used in for transportation and on farm systems during World War II in several European countries was wood [Political Wag (2008)]. This proved successful as armies active in the war did not always have access to oil. Biomass gasification means incomplete combustion of biomass resulting in the production of combustible gases. Biomass gasification systems employed on large scale has major aspects including which can rectify issues concerning emissions and sustainable development. Under present conditions, economic factors seem to provide the strongest arguments of considering gasification. In many situations, where the price of petroleum fuels is high or where supplies are unreliable, the biomass gasification system can provide an economically feasible solution provided:    

The biomass feedstock is of high enough conversion to provide energy to meet population demands The biomass feedstock is easily handled The biomass feedstock can be processed at minimal cost The biomass feedstock is easily/readily available (as in the case of wood)

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3.4

FEED-STOCK (WOOD)

Wood is the hard fibrous liquefied substance under the bark of trees [Word Net Web (Unknown)]. It is typically yielded by trees, which increase in diameter by the formation between the existing wood and the inner bark of new woody layers. The resulting wood has various applications including:   

Construction: provides the base material for various purposes ranging from paper making to house/building construction. Dating: by carbon dating to make inferences about when a wooden object was created. Fuel: can be gasified to produce combustible volatiles of high enough calorific value for power generation. 3.41

COMPOSITION

Wood is essentially a composite material constructed from oxygen-containing compounds, making its pyrolytic chemistry different from traditional fossil fuel feeds. The major components, according to [Rowell, R.M. (1984)], consists of:    

Cellulose: comprise approximately 40% - 50% wt of mass of dry wood. These provide the wood strength. Hemicellulose: second major wood chemical constituent, accounting for 25% - 35% of the mass of dry wood. Lignin: the third major component of wood, comprising 16% - 25% of the mass of dry wood. It acts as a binder and shield of cellulosic fibres. Organic extractives and Inorganic minerals: Varying amounts. 3.42

WOOD HEATING

The facts that fossil fuels are non-renewable and the United Kingdom is heavily reliant on foreign sources are excellent incentives to adopt development of renewable energy sources. In addition, the burning of fossil fuels produces carbon dioxide (CO2), which has various environmental consequences. As there is currently no commercially practical way to offset the carbon dioxide added to the atmosphere (and the resultant greenhouse gas effect) that is a consequence from fossil fuel combustion, the use of wood provides significant environmental advantages. Wood is considered CO2 neutral. The increase of carbon dioxide from wood fuel combustion is counteracted by plant growth, needed to generate the feedstock. This is illustrated below: FIGURE 3.42 (a): Carbon Neutral [WoodFuelWales (Unknown)]

53

The wood feedstock can be in the form of chips or logs, or manufactured from industrial wood waste. The advantages of wood heating are illustrated below:  Lower Fuel Cost Wood is currently less expensive than oil and gas heating fuels and as a renewable source of fuel, will not be subject to environmental taxes such as the Climate Change Levi. Below is a table and graph of comparative cost of various fuels:

TABLE 3.42 (a): Comparative costs of various fuels [Teas Folláin Teo. (2007)]

GRAPH 3.42 (a): Comparative costs of various fuels [Teas Folláin Teo. (2007)] From TABLE 3.42 (a) and GRAPH 3.42 (a), chips are cheaper per unit energy than wood pellets. However, wood chips contain more moisture than wood pellets and they also have a lower bulk density hence have a lower calorific (heating) value.

54

 Ease of Maintenance In general, minimal attention is required for the up keep of wood heating systems, which will ultimately result in lower operational costs.  Environmentally Friendly The use of wood typically reduces the net carbon dioxide emissions by over 90% compared to fossil fuels, as it is considered CO2 neutral (as discussed previously). Also, emissions of other pollutants such as SOx and NOx are negligible. Below is a graph of carbon dioxide emissions which justify this case.

GRAPH 3.42 (b): CO2 Emissions of various types of fuels [Biomass Energy Centre. (Unknown)] The use of wood also reduces demand on land fill, hence backing the motive behind sustainable development.  Local Economy Sustainability is encouraged and the local economy is benefited from wood fuel activities by creating new opportunities for employment in fuel supply, production and servicing. Also biodiversity is promoted from renewed and expanded use of wood land.

55

3.43

WOOD PYROLYSIS

Wood pyrolysis is basically the breakdown of wood by heat, in an environment with controlled amount of oxygen. This produces a large number of chemical substances which can be substituted for conventional fuels. The table below summarises the temperatures at which thermal degradation of the major components of wood occurs: COMPONENTS OF WOOD THERMAL DEGREDATION °C Cellulose 240 – 350 Hemicellulose 200 – 260 Lignin 280 – 500 TABLE 3.43 (a): Thermal degradation of wood components. Compiled from [Rowell, R.M. (1984)]. Cellulose comprises majority of the mass of dry wood and is the major source for combustible volatiles, which can be used to produce electricity, via engine/turbine. 3.44

MAXIMISING EFFICIENCY

The need for supplying sustainable energy resources raises the urgency in finding optimised conversion technologies. Well established technologies like pyrolysis/gasification, provide adequate thermal out-put of wood feedstock. 3.5

PROBLEMS WITH WOOD GASIFICATION/PYROLYSIS

Although wood is a clean fuel with low nitrogen, sulphur and ash content, it is thermally unstable. This may lead to the formation of condensable tars in gasifiers, thus creating problems in downstream equipment such as choking and blockages on piping. Other disadvantages are low energy density of wood, typically 18 MJ/kg [Girard, P. & Shah, N. (1991)], in combination with high moisture content as a result of its hygroscopic character. Coal contains about 75% - 90% carbon, while wood’s carbon content is about 50% *Bourgois, J.P. & Doat, J. (1984)]. This means that the heating value of wood is lower. In general, higher gasification efficiencies are achieved from fuels with low Oxygen / Carbon ratios [Girard, P. & Shah, N. (1991)], such as coal as compared to wood with a high Oxygen / Carbon ratio. This is basically due to the high exergy of wood, which is not fully utilised when wood is gasified. The differences are explained by the Oxygen / Carbon and Hydrogen / Carbon ratios of each fuel, shown in the Van Krevelen diagram:

56

FIGURE 3.5 (a): O/C and H/C ratios of various fuels [Hustad, J. (2000)].

57

Hence rather than gasifying these fuels directly, it could be attractive/beneficial to modify their properties prior to gasification. On the other hand, pre-treatment is a step normally overlooked, which has significant influence on the final output (product) quality of wood, as it improves efficiency of the final conversion stage. Most commonly applied pre-treatments include drying and grinding/pulverising. Furthermore, enhancement of the energy density is advisable because a large amount of wood is required to replace an equivalent amount of coal in applications such as combustion/gasification. Common techniques to improve energy density include:  Briquetting This is basically a binding technique, in which pulverised fuel (wood) is united together under pressure and often with the aid of a binder. The binder is usually starch. It is also a common practice to include other additives like paraffin or petroleum solvents to aid ignition.  Pelletizing As the name suggests, this is the process which involves compressing or moulding of fuel (wood) into the shape of a pellet. Bio-solids are first stabilised and then later completely dried. They are then pressed into small pellets finally.

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3.6

TORREFACTION

‘A process that lowers the Oxygen/Carbon ratio of biomass is known as torrefaction’ - [Daey Ouwens, C. & Kupers, G. (2003)]. It is a mild thermal treatment which enhances the heating/burning characteristics of wood fuel for gasification or combustion and fuel properties which includes ease of grinding/pulverising. Even after drying, wood still has a tendency to absorb moisture but applying torrefaction as a thermal pre-treatment limits the uptake of moisture. Also, biological activity is eliminated in torrefied wood, which prevents decay/decomposition hence torrified wood can be stored for longer periods of time. It is also worth mentioning that the electricity requirement for size reduction of torrified wood is typically 50% - 85% smaller, depending on the torrefaction conditions, in comparison to fresh (untreated) wood A summary of the advantages of torrefied wood as opposed to untreated wood is shown in the table below: UNTREATED WOOD      

Bulky Moist Fibrous Perishable Waste Expensive to transport

TORREFIED WOOD      

Dense, If Pelletized Dry and Water Resistant Easily Crushed Does not rot Valuable Fuel High Energy Density

TABLE 3.6 (a): Advantages of torrefied wood as opposed to untreated wood. Compiled from [Panshin AJ, de Zeeuw C (1980)].

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3.61

TORREFACTION PROCESS

The use of torrefied wood was successfully tested in an air-blown down-draft gasifier. This resulted in an increase in product heating value from 5.3 MJ/m3 to 6.0 MJ/m3 [Girard, P. & Shah, N. (1991)]. Even though torrefaction has been successfully undertaken and shown to improve wood fuel properties, the process is not commercially available and considered to be in a ‘proof of concept’ phase. Torrefaction is a thermal process operated around 200°C to 300°C [The Engineer (2011)] in the absence of oxygen (O2) and at relatively long residence times. A basic block diagram of the process is illustrated below:

INITIAL HEATING

POST – DRYING

PRE – DRYING

SOLID COOLING

TORREFACTION

FIGURE 3.61 (a): Torrefaction process. Concept from [Bioenergy (2000)] 3.62

STAGES OF TORREFACTION

In relation to the block diagram above, the stages of torrefaction are discussed below: 1. Initial Heating: wood is heated initially, prior to the drying stage. Moisture evaporation begins at the end of this stage. 2. Pre-Drying: the temperature remains fairly constant at this stage, while moisture content is drastically reduced. 3. Post-Drying / Intermediate Heating: the temperature is increased and the biomass is practically free of moisture after this stage. Some mass loss can be expected. 4. Torrefaction: the stage of torrefaction contains a heating and cooling period at constant temperature. As soon as the temperature exceeds 200°C, torrefaction is initiated and ends when the temperature falls below 200°C again. Devolatilisation is initiated during the cooling period. 5. Solid Cooling: the end torrefied product is cooled to the desired final temperature. No further mass release occurs. 60

Stages in the heating of biomass (wood) from ambient temperature to the desired temperature and the subsequent cooling of the torrefied product are displayed in the graph below:

GRAPH 3.62 (a): Stages in heating of Biomass [Bourgois JP, Doat J (1984)].

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3.63

TORREFACTION OF WOOD OPTIONS

FIGURE 3.63(a)

Wood Production Location

ELECTRICITY

Torrefaction

Power Plant Site

Gasification/ Pyrolysis

Torrefied Wood

FIGURE 3.63(b)

Wood Production Location

Torrefaction ELECTRICITY Torrefied Wood

Power Plant Site

Gasification/ Pyrolysis

The block diagrams above basically shows two possibilities in the production of electricity via gasification/pyrolysis of torrefied wood. These possibilities differ in the way wood is transported to the power plant site: FIGURE 3.63(a): As wood itself (Untreated) FIGURE 3.63(b): As torrefied wood FIGURE 3.63(b) benefits from an increase in energy density. Furthermore, the bulk density can also be increased by densification by either briquetting or pelletizing, as discussed previously. 62

3.64

PROPERTIES OF TORREFIED WOOD

Below is a Van Krevelen diagram for: torrified wood (Tw) produced at different temperatures, untreated wood, coal & charcoal and peat samples:

FIGURE 3.64 (a): Torrefied wood (Tw) produced at different temperatures, untreated wood, coal & charcoal and peat samples [Hustad, J. (2000)].

63

Torrified wood (Tw) produced at different temperatures, untreated wood, coal & charcoal and peat samples [Bourgois JP, Doat J (1984)]. As can be seen from the diagram above, torrefied wood has a lower oxygen / carbon ratio than untreated wood. Also noticed is that the higher the torrefaction temperature, the lower the oxygen / carbon ratio becomes, making it have similar properties to coal. The same goes for an increase in time that wood is exposed to torrefaction. In reference to the Van Krevelen diagram above, it can be seen that wood looses more oxygen and hydrogen compared to carbon. The main consequence to this is an increase in the calorific (heating) value.

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3.7

REACTOR TECHNOLOGY

Atypical torrefaction reactor needs to combine two main process tasks: 1. Heating the biomass (wood) to the desired torrefaction temperature 2. Holding it at this temperature for a specific period of time The basis for suitable torrefaction reactor may come from three main fields known for biomass:  Drying  Pyrolysis  Gasification The reactor technology applied can be divided into two main modes of operation: 3.71

INDIRECTLY HEATED TECHNOLOG

The biomass is in indirect contact with the heat carrier (i.e. through the walls of the reactor). A typical schematic is illustrated below:

FIGURE 3.71 (a): Schematic of indirectly heated reaction [Bourgois J, Guyonnet R (1988)].

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3.72

DIRECTLY HEATED TECHNOLOGY

The biomass is brought in direct contact with the heat carrier. A typical schematic is illustrated below:

FIGRUE 3.72 (a): Schematic of directly heated reaction [Bourgois J, Guyonnet R (1988)].

3.63

COMPARISON

On the basis of the gas recycle, research shows that it is better to opt for the indirectly heated mode of operation for torrefaction reactors. In relation to the torrefaction gas, dust can lead to complications and heavier volatiles can condensate on cold surfaces and so foul equipment. This is similar to the tar produced by gasification. Also, the heat exchange in the directly heated concept is between gases, which is rather difficult and less effective as compared to the heat exchange between gas and oil in the indirectly heated concept.

66

3.8 MASS AND ENERGY BALANCES As described earlier in the group report, the feed of wood chips is approximately 5 tonnes per hour, with a moisture reduction from 20% to 6%. During drying (i.e. pre drying and post drying), the temperature of the feed is raised from 15°C to 150°C, reducing its feed rate to 4.4 tonnes per hour. After post drying, intermediate heating raises the temperature to 200°C. Assumptions   

Continuous process Steady-state (No Accumulation) Typically 70% of the mass is retained as a solid product, containing 90% of the initial energy content [Bio energy (2000)]. Below are table of specific heats for different woods: TYPE OF WOOD SPECIFIC HEAT CAPACITY (kJ/kg K) Balsa 2.9 Oak 2 White Pine 2.5 Loose 1.26 Felt 1.38 TABLE 3.8 (a): Specific heats of different woods. Compiled from: [Engineering Toolbox (Unknown)] Taking an average specific heat value of 2 kJ, and applying [EQUATION 2.4 (a)], the energy content of the wood after drying is approximately:

Taking into account the above assumptions, an illustration of the mass and energy balance is shown below: 0.1 MWt

0.36 kg/s

Torrefaction gases

Wood (Untreated)

1 MWt

1.19 kg/s

TORREFACTION CHAMBER 200°C - 300°C

Torrefied Wood

0.9 MWt

0.84 kg/s

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3.81

DEDUCTIONS FROM MASS AND ENERGY BALANCES

From the above mass and energy balance, it can be drawn that a fundamental advantage of the torrefaction process is the high transition of chemical energy (i.e. only 1% of the initial energy content is lost) from the feedstock to the torrified product, whilst the wood (fuel) properties are significantly enhanced, for instance its bulk energy density. 3.9

STATUS OF TORREFACTION TECHNOLOGY

Currently, commercially operated torrefaction plants are non-existent. However, over the recent years there has been a growing interest in its implementation as a pre-treatment to improve biomass characteristics (in this case, wood properties). 3.10 CONCEPT DESIGN OF TORREFACTION REACTOR TO BE IMPLEMENTED IN PROCESS The torrefaction reactor to be implemented into this pyrolysis/gasification process is essentially indirectly fired. Dried wood feed will be transferred to the torrefaction reactor via airlock hoppers. Heating takes place outside this reactor meaning the biomass being processed within this reactor is not in contact with combustion gases. Since products of combustion are completely isolated from the product being processed, two heat transfer mechanisms take place. Heat transfer is by conduction between the solids and the wetted portion of the hot shell. In most applications the shell is heated to fairly high temperatures, so radiation between shell and solids also prevails. Also, as these units are heated from the outside, the materials of construction must be capable of withstanding higher temperatures. A gas collection system, will aid in the isolation of volatiles and the transfer of combustible gases to the burner. To aid in efficient heat transfer, a screw will be incorporated into the reactor. This will cause mechanical movement of the dried wood chips through the torrefaction chamber. After the torrefaction process, torrefied biomass will then be sent to the pyrolysis reactor, designed in the main design. An illustration of the concept design of the torrefaction reactor is shown below:

68

FIGURE 3.10 (a): Concept design of torrefaction reactor. Cool Exhaust

Kwaku Asiamah. 2011-02-02. Department of Chemical & Biological Engineering.

Drier Wood Chips Heat Exchange Moist Wood Chips

Burner

Hot Exhaust

Hopper

Steam

Gas Collection System

VOC, CO, H2

Airlock Mechanically moving wood chips through torrefaction chamber

Hopper Combustion gases indirectly heating wood chips

Drawing not to scale Concept from [Lipinsky ES, Arcate JR, Reed TB (2002)].

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