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• Mehdi Nakouri

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FRONT END ENGINEERING DESIGN

2012-09-05

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GENSER POWER GHANA

FRONT END ENGINEERING DESIGN

EMMA BENJAMINSON 7/9/2012

FRONT END ENGINEERING DESIGN

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Table of Contents Table of Figures ......................................................................................................................................... 4 A. COAL GASIFICATION SYSTEM ................................................................................................................. 5 List of Acronyms ........................................................................................................................................ 5 Phase 1 ...................................................................................................................................................... 6 Introduction........................................................................................................................................... 6 A1 Coal Delivery System ........................................................................................................................ 9 A1.1 80T Coal Storage Hopper ........................................................................................................... 9 A1.2 20T Coal Storage Container ..................................................................................................... 11 A1.3 Coal Feeding Hopper ............................................................................................................... 12 A2 Gasifier and Blast Medium Supply .................................................................................................. 13 A2.1 Air Blower ................................................................................................................................ 13 A2.2 Blending Chamber for Blast Medium ....................................................................................... 13 A2.3 Steam Tank (3 Bar)................................................................................................................... 14 A2.4 Steam Tank (0.5 Bar)................................................................................................................ 14 A2.5 Gasifier .................................................................................................................................... 16 A2.6 Hydraulic Ash Handling System................................................................................................ 19 A3 Gas Cleaning Equipment ................................................................................................................. 20 A3.1 Cyclone .................................................................................................................................... 21 A3.2 Heat Exchanger ........................................................................................................................ 23 A3.3 Air Cooler................................................................................................................................. 25 A3.4 Electrostatic Precipitator 1 ...................................................................................................... 27 A3.5 Indirect Cooler ......................................................................................................................... 28 A3.6 Electrostatic Precipitator 2 ...................................................................................................... 30 A3.7 Pressure Adder ........................................................................................................................ 31 A4 Phenolic Water System................................................................................................................... 31 A4.1 Phenolic Water Supply Pool ..................................................................................................... 31 A5 Tar Disposal System ........................................................................................................................ 32 A6 Waste Water System ...................................................................................................................... 32

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A7 Soft Water System .......................................................................................................................... 32 A7.1 Sodium Ion Exchanger ............................................................................................................. 32 Phase 2 .................................................................................................................................................... 33 Introduction......................................................................................................................................... 33 Bibliography ............................................................................................................................................ 34

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Table of Figures Figure 1: Gasification System Layout ......................................................................................................... 8 Figure 2: Engineering Drawing of Gasifier ................................................................................................ 15 Figure 3: Cyclone ..................................................................................................................................... 21 Figure 4: Heat Exchanger ......................................................................................................................... 23 Figure 5: Engineering Drawing of Air Cooler ............................................................................................ 25 Figure 6: C-60 Electrostatic Precipitator 1................................................................................................ 27 Figure 7: Indirect Cooler .......................................................................................................................... 28 Figure 8: C-97 Electrostatic Precipitator 2................................................................................................ 30

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A. COAL GASIFICATION SYSTEM List of Acronyms SP D CF CS CSC CFH H AB BB SD1 SD2 G A

Suction Pump Dust Plant 80T Coal Feeder Coal Sifter 20T Coal Storage Container Coal Feeding Hopper Hoister Air Blower Blending Bin for Blast Medium 3 Bar Steam Drum 0.5 Bar Steam Drum Coal Gasifier Hydraulic Ash Handling System

C

Cyclone

H

Heat Exchanger

AC

Air Cooler

ESP1 IC ESP2 PA PW

C-60 Electrical De-Tarrer Indirect Cooler C-97 Electrical De-Oiler Pressure Adder Phenolic Water Pool

PWP I TT

Phenolic Water Pump Phenolic Water Incinerator Tar Tank

TP CWT CWP SWT SWP SIE

Tar Pump Cold Circulation Water Tank Circulating Water Pump Soft Water Tank Soft Water Pump Sodium Ion Exchanger

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Phase 1 Introduction The coal gasification system takes coal as an input fuel and outputs synthetic gas (referred to as “syngas”) to be fired by the supplementary burners that produce steam for the steam turbine. Coal gasification is the first step in power generation. It involves taking the coal from the tipper trucks and delivering it to the gasifiers, burning the coal to produce syngas, and then removing tar and dust from the syngas and sending it on to the supplementary burners. The coal gasification system consists of a coal delivery sub-system, a gasifier, and a gas cleaning subsystem. During Phase 1 four gasifiers will be in operation, supplied by one central coal delivery subsystem and soft water, waste water and phenolic water sub-system. However, each gasifier will have its own set of gas cleaning equipment. The coal delivery sub-system uses an 80 ton coal storage hopper to collect coal from tipper trucks. The tipper trucks will deposit the coal on the ground near the hopper, and a pay-loader will transfer the coal into the hopper. The coal is deposited onto a conveyer belt and sent to a coal sifter that separates coal dust from the optimum-sized briquettes. The acceptable coal is then sent to a 20 ton storage hopper which deposits coal into a coal feeding bucket. The bucket is hoisted to the top of the gasifier tower and deposits coal into the gasifier. As coal enters the gasifier, it is goes through four separate processes. These processes occur at gradually higher temperatures as the coal descends to the bottom of the gasifier where complete combustion takes place. First, the coal is dried near the top of the gasifier chamber and then it undergoes pyrolysis, where the coal is converted to coke. Pyrolysis will produce some upstage gas which is piped out of the upper part of the gasifier; the flow will carry a high percentage of tar vapors out with the syngas. The coke next will undergo incomplete combustion, or gasification. The rest of the gas is produced at this stage, and it will have a high dust content as it is piped out of the gasifier. The remaining coal particulates will drop to the bottom of the gasifier and burn on top of a fire grate, which produces the heat for the gasifier’s reactions. The upstage and downstage gases are cleaned separately because they carry different contaminants. The upstage gas passes through an electrostatic precipitator which cleans the gas of tar vapors. The downstage gas passes through a cyclone to remove the dust particles, and then a heat exchanger and an air cooler to cool the gas so that it will be at about 100°C when it enters the indirect cooler and mixes with the upstage gas, which will also enter the cooler at 100°C. From the indirect cooler, the mixed gas passes through a second electrostatic precipitator to better clean the gas. Finally, the gas passes through a series of pressure adders which raise the pressure of the syngas entering the supplementary burners.

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Below is a conceptual layout of the gasification system.

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Figure 1: Gasification System Layout

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A1 Coal Delivery System The coal delivery system takes coal from the tipper trucks and conveys it to the top of the gasifiers. Tipper trucks offload the coal onto the ground near the coal storage shed, and a pay-loader transfers it into an 80T coal hopper which deposits the coal onto a conveyer belt that carries it to a coal sifter. The sifter removes coal powder from the supply of coal that is passed on to a 20T coal storage hopper. The storage hopper can be periodically opened to fill the coal feeding bucket, which is hoisted up to the top floor of the gasification tower. There is a piping system installed alongside this system which is connected to a suction pump; the pipes suck up coal dust that is generated and store it in a sealed dust plant. The piping system has outlets at the dispenser where coal from the feeder drops onto a rising conveyer belt, at the coal sorter and at the coal storage hopper. The dust plant (as well as the coal powder from the sifter) is periodically emptied into a truck and taken to the Kojokrom coal storage facility to be disposed of. A1.1 80T Coal Storage Hopper The pay-loaders load coal into the 80T coal storage hopper which dispenses the coal onto a horizontal conveyer belt. The hopper is manually opened and closed to dispense coal onto the conveyer. One tipper truck can carry 40T of coal, so two trucks can fill the hopper to capacity. One gasifier consumes 3.2T of coal per hour, so the total quantity of coal consumed in a day is:

Where: = coal consumption rate for 1 gasifier = number of gasifiers = total coal consumption rate = operational period (i.e. 24 hours) = total amount of coal consumed in operational period For this situation, the total amount of coal required for a 24 hour operational period is 307.2T. The total number of trucks needed to deliver 307.2T of coal every day is:

Where: = mass of coal carried by 1 truck = number of trucks

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From this calculation, 8 truckloads of coal need to be delivered to the plant every day, and this will fill the hopper to capacity about 4 times. Coal Feeders There are two main types of coal feeders in industry: volumetric and gravimetric flow feeders. Volumetric feeders control the flow of the coal by the volume that passes through the feeder. There are four main types of volumetric feeders: screw, belt, rotary valve and vibrating pan feeders. Gravimetric feeders control the mass flow rate of the coal, in two ways: continuously, by modulating the mass flow over time, or in batches, by depositing a certain mass of coal and then shutting off. Gravimetric feeders can be either loss-in-weight or weigh belt feeders. Coal is heterogeneous in its properties; the density and heating value of different pieces of coal within a sample can vary significantly, which can introduce variation in the performance of the gasifier. For example, denser coal with a higher heating value will produce more gas and create higher internal temperatures and pressures than lighter coal with a lower heating value. Volumetric flow control will allow both the variations in density and heating value to affect the performance of the gasifier, because the flow of coal is controlled by volume, not by weight. Conversely, gravimetric flow control eliminates the error due to density variation in the coal, because it sends a constant mass of coal to the gasifier. As a result, gravimetric flow control is more accurate than volumetric flow control, and facilitates more efficient operation of the gasifier. Other advantages of gravimetric flow control include: - Greater accuracy in controlling combustion reaction - Improved efficiency - Improved pressure control - Reduced fuel consumption - Less coal slag, which reduces clogging in the gasifier - Less NOx produced, which reduces environmental impact - Less corrosion of equipment - Improved stability and rapidity of response from combustion controls - Reduced O&M costs - Improved overall performance - Safer operation Advantages and Disadvantages There are some advantages and disadvantages to Genser’s choice to use a manually-controlled coal feeding system with the 80T hopper:

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Low maintenance:- The manual feeder has fewer moving parts and is a simpler design so it will require less maintenance than an automated feeder system. Inexpensive:- Again, the manual feeder’s simple design reduces the capital and installation costs for this system. Easier installation:- The manual feeder will not need to be connected to the main control system because it is not automated. In addition, it will have fewer parts and will be more compact than an automated feeder. Requires operator attendance:- Since the feeder is not automatic, an operator will have to open and close it during operation; this could add personnel costs and time delays in comparison with an automated system. Less precise coal feeding:- The manual feeder has no mechanism for measuring the mass or volume of the coal that is being sent to the gasifier, so the operator will not be able to precisely control the flow of coal into the gasification system.

A1.2 20T Coal Storage Container This 20T coal storage container is used to store the coal from the sifter, and to load coal into the feeding bucket. The coal delivery system works as a batch process. An operator will run the delivery system until the coal storage container has been filled up to a certain level, then the delivery system will be switched off. The container can be opened manually to allow coal to fill the feeding hopper. When the storage container is nearly empty, an operator can restart the coal delivery system to refill the container. The 20T coal storage hopper has an electromagnetic vibrator screen to filter dust out of the coal supply before it enters the gasifier, and to improve the movement of coal from the container to the hopper. It is important to remove the dust before it enters the gasifier because it can clog the gasifier and prevent gas from circulating in the gasifier and leaving it. Advantages and Disadvantages The vibrating pan feeder that is used with the 20T coal storage container is a volumetric flow control device, which has some disadvantages as well as advantages:  Volumetric flow control is less precise:- As described in the previous section, volumetric flow control allows both the density and the heating value of the coal to vary, which causes greater variations in gasifier performance than would be obtained using a gravimetric flow control device. However, since the 20T coal storage container does not feed directly into the gasifier, the error that the volumetric feeder introduces can be mitigated by further flow controls down the line.  Material can pack instead of flow:- A vibrating pan feeder can sometimes cause the material in the hopper to pack, instead of flow outwards.

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Robust design:- The vibrating pan has a simple, rugged design which is ideal for this equipment since it will be outside for several years. The feeder can also be enclosed so that coal dust does not escape into the atmosphere. Continuous coal discharge:- The vibrating pan feeder provides continuous coal discharge into the feeding hopper while it’s operating.

A1.3 Coal Feeding Hopper The feeding hopper receives coal that has been sorted and delivers it to the gasifier. The feeding bucket has a diameter of about 1.5m and is about 1.1m high, so its total capacity is:

Where : = diameter of the feeding bucket = height of the feeding bucket = volume of the feeding bucket This feeding bucket’s total capacity is 6.9 m3. Bituminous coal has an approximate density of 830kg/m3, so a capacity of 6.9m3 equates to a total mass of: Where: = density of bituminous coal = total mass of coal that feeding bucket can contain According to these calculations, each bucket of coal can therefore deliver about 5.7T of coal to the gasifier. The hoister for the bucket is electrically operated; an operator has to activate the hoister. There is a reel system that carries the bucket up to the top of the gasification tower and across to the coal bunker at the top of the gasifier. An operator has to then manually tip the bucket to offload the coal into the bunker. There are two buckets that service the four gasifiers, although one bucket is usually on standby. The hoister will lift the coal bucket to the top of the gasification tower and then a reel system can carry the bucket to all four gasifiers to fill the coal bunkers. If the gasifiers are operating at full capacity, then the second bucket can be added to the system to increase the rate of coal delivery. The system is organized this way because one bucket can deliver coal to all four gasifiers at a faster rate than the gasifiers can consume the coal. Therefore, the reel system is built so that one bucket can be hoisted up to feed one

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gasifier, and then refilled and hoisted up to feed a second gasifier, and so on. The bucket will refill the first coal bunker before it has exhausted its supply of coal. Advantages and Disadvantages  Low maintenance:- This system is simple because it uses at most two coal buckets and one hoister, so it will require less maintenance than if each gasifier had its own bucket and hoister.  Less expensive:- Again, the simple design reduces the capital cost of the bucket and hoister system.

A2 Gasifier and Blast Medium Supply The coal gasifier takes coal as its primary input, but it also requires a “blast medium” to fuel the combustion in the fire grate. The blast medium is composed of steam and air. A series of 3 air blowers sends air to a blending chamber to be mixed with 0.5 bar steam. The blast medium then flows from the blending chamber to the bottom of the gasifier. When coal is burned on the fire grate, it produces ash as a final product; this ash is collected in an ash tray at the base of the gasifier. A hydraulic ash handling system empties the tray of ash continuously to prevent the gasifier from clogging up. A2.1 Air Blower The air blowers supply compressed air to the blast medium for the combustion process in the gasifier. There are three air blowers in the system which feed all four gasifiers. They send air to the blending bins at a temperature of 30°C, at 12 m/s with a pressure of 11.776 kPa. Only two air blowers are operating at any time; the third is a standby in case another blower fails. Advantages and Disadvantages  Simple and robust design:- The engineering team decided to use three air blowers to supply the entire system (with two operating and one on standby) because this design was adequate for the needs of the system. The extra air blower on standby makes the system more robust because if one blower fails, the gasification system will still be able to run at full capacity. A2.2 Blending Chamber for Blast Medium The blending chamber collects both compressed air and steam and allows them to mix before they are passed on to the gasifier. The chamber is just a container that allows the air and steam to combine. It is important to control the proportion of steam in the blast medium. If too much steam is used, it can stifle the combustion process since water vapor does not burn.

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The blending chamber does not need to be insulated because the air and steam do not circulate inside the chamber for very long, so there is no significant heat transfer out of the system. The temperature of the gases inside the chamber is also monitored using several temperature transmitters. Advantages and Disadvantages  Effective mixing of blast medium:- The blending chamber ensures the blast medium meets the operating specifications of the gasifier so that combustion can be carried out smoothly. The blast medium supplied to the gasifier is the primary means of controlling the combustion process. The Genser engineers can modify the amount of blast medium (via the blending chamber) entering the gasifier to control the temperatures, pressures and gas outputs of the system. A2.3 Steam Tank (3 Bar) This steam tank holds steam at 3 bar (or 300kPa) which is used for purging the pipes throughout the gasification system. The 3 bar steam tank circulates soft water through a second water jacket that cools the gasifier. This water jacket is lower on the gasifier, closer to the fire grate; consequently the steam in this line is at 133°C, and circulates at 25m/s. Both steam drums are made of carbon steel and are insulated. The steam from this tank can be used to purge the pipes by blowing them out. The 3 bar steam tank is connected to the main header steam pipe for the gasification system (labeled as LS-DN100 on the P&ID); this header pipe is connected to four branches that serve each of the four gasifiers (the branches are labeled as LS-DN50 pipes). Each branch connects to all the pieces of equipment in each gasifier package (these branches are labeled as LS-DN25) so that any piece of equipment can be purged with 3 bar steam if necessary. Normally this steam line is full in order to supply steam to the base of the de-tarrer (ESP1). The steam prevents the tar from cooling, solidifying and blocking the pipeline coming out of the ESP1. Advantages and Disadvantages  Multipurpose design:- The 3 bar steam tank is able to supply steam to multiple lines both to blow them out and to heat the tar; this design reduces the cost of the entire system because one piece of equipment can serve two functions. A2.4 Steam Tank (0.5 Bar) This steam tank contains steam at a pressure of 0.5 bar (or 50 kPa), and circulates steam through a water jacket on the outside of the gasifier, as well as through the blending bin. The 0.5 bar steam is produced by passing soft water through a water jacket that covers the outside of the upper part of the gasifier. The upper part of the gasifier is cooler (because it is farther from where there is complete combustion in the fire grate) so the steam produced here is at a relatively low temperature and

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pressure. The steam contained in the tank is at a nominal temperature of 116°C, and it flows through the line at about 18m/s. Advantages and Disadvantages  Cools gasifier:- It is important to cool the gasifier using the water jackets because there should be a uniform heat distribution across the gasifier at every level. The uniform heat distribution ensures that all the coal entering the gasifier undergoes the same processes at the same time. Cooling the walls of the gasifier also prevents coal briquettes from sticking to it and creating hot points that could damage the walls or affect the heat distribution in the gasifier.

Coal Bunker

Surge Bin Stoking Valves

Upstage Gas Downstage Gas

Ash Tray

Figure 2: Engineering Drawing of Gasifier

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A2.5 Gasifier The gasifier takes coal as an input and produces two streams of syngas which will serve as fuel for the Power Generation System. The gasifier is 5 storeys high or about 25.7m tall. Coal is dropped in through the top of the gasifier and as it falls, it undergoes a series of chemical processes that produce syngas, which is sent out through a series of pipes to be cleaned. There is a fire grate in the base of the gasifier that burns the coal remnants and produces heat which drives the chemical processes. Coal is delivered to the gasifier by the feeding hopper, and drops into a 2-way double faucet stoking system. The first series of rotary valves is opened to allow the coal to fall into a surge bin and then they are closed again. The second series of rotary valves at the base of the surge bin will then open and allow coal to enter the gasifier chamber, and then they will close. The opening and closing of the valves happens at 6 second intervals. The stoking valves that control the flow of coal into the gasifier can be operated automatically or manually. However, in automatic mode, the valves can be programmed to operate at a certain rate, but this rate remains constant until the user changes the inputs to the program logic control (PLC). The valves do not change their operating rate automatically in response to variations in the gasification system. The coal is not fed directly into the gasifier because it would allow too much oxygen to enter the gasifier; the double faucet system and rotary valves limit the amount of air that can enter the gasifier. The chemical processes that take place inside the gasifier have to occur in the absence of oxygen, which is why it is important to control the flow of air into the gasifier chamber. The fire grate at the base of the gasifier is where complete combustion takes place. There the remnants of the coal burn, fuelled by the blast medium of hot air and steam. This is the hottest part of the gasifier and it supplies heat to the rest of the chamber; the temperature decreases near the top of the gasifier, which is at 150°C. When the coal enters the gasifier, it is first carbonated (dried), so that any moisture in the coal is removed. At this point the coal composition is: Compound CO H2 CH4 CnHm

Percent Composition 29-31% 17-19% 1-3% 0.2-0.4%

As it continues to fall downwards, the coal undergoes pyrolysis. Pyrolysis is an organic decomposition reaction which occurs in the absence of oxygen. The weaker chemical bonds in the coal are broken,

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releasing volatile gases; the remaining high molecular weight char continues to fall downwards to fuel other reactions in the gasifier. The reactions that occur during pyrolysis are: CnHm → CH4 + C + H + tars C + 2H2 → CH4 The methane gas (CH4) that is produced is drawn off at this stage; it is referred to as upstage gas. Upstage gas (as shown in the chemical equations above) contains tar vapors when it is drawn off from the gasifier, so it is sent to a separate line from the downstage gas that is designed specifically to remove these tar vapors. The upstage gas is at a temperature of 120°C, flowing at 8m/s and a pressure of 3.5kPa. Below pyrolysis, gasification takes place. Gasification is an incomplete combustion reaction in the absence of oxygen which produces more gas, as well as other by-products. At this stage the coal composition is: Compound Percent Composition CO 31-33% H2 9-10% CH4 0.4-0.5% The chemical reactions are:

Reaction Description Gasification with steam “Water-gas reaction” Gasification with carbon dioxide “Boudouard Reaction” Gasification with hydrogen “Methanation Reaction”

Chemical Equation C + H2O

Energy

H2 + CO

+131 MJ/kmol

C + CO2

2CO

+172 MJ/kmol

C + 2H2

CH4

-75 MJ/kmol

However, if there is a high rate of carbon conversion, these three reactions will be reduced down to two reactions:

Reaction Description

Chemical Equation

Energy

“Water-Gas-Shift Reaction”

CO + H2O

CO2 + H2

-41 MJ/kmol

“Steam-Methane-Reforming Reaction”

CH4 + H20

CO2 + 3H2

+206 MJ/kmol

The syngas produced at this stage contains dust as its main impurity, so when this downstage gas is drawn off from the gasifier, it is sent to a separate line that will remove the dust. The syngas comes out at a temperature of 600°C, at a speed of 6m/s and a pressure of 4.5kPa. Since the downstage gas is also

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at a much higher temperature than the upstage gas, it has to go through a series of cooling processes before it can be combined with the upstage gas. After passing through the gasification process, the remnants of the coal then undergo complete combustion in the fire grate, as described above. The ash that is produced falls through the grating to an ash tray. Blast medium is supplied continuously to maintain combustion in the fire grate. Gasifier Basic Design There are three main types of gasifiers in industry today. They are: - Fixed bed - Entrained flow - Fluidized bed Genser uses a fixed bed gasifier. Fixed bed gasifiers operate at atmospheric pressure; the coal is fed in through the top of the unit and the blast medium is fed through the bottom, underneath the grate. Combustion takes place on the fire grate, which is a ceramic inverted conical cup, where the blast medium is released from the top of the cone to fuel combustion. The fixed bed gasifier is simple in design and can operate with a variety of feedstock. It provides gas at low output temperatures, and is very efficient. Alternatively, entrained flow gasifiers introduce both the blast medium and the feedstock at the top of the gasifier at high temperature and pressure. This is difficult for Genser to do; the high temperatures also significantly reduce the lifetime of the components. And while gas is produced quickly at a high throughput, it is usually necessary to add fluxes or tightly control the feedstock so that the slag continuously flows out of the gasifier. The fluidized bed gasifier suspends feedstock particles in oxygen-rich gas, so that the mixture acts like a fluid. This facilitates a high heat transfer rate within the system, but the lower operating temperature means that this type of gasifier can only use highly reactive coal types. Advantages and Disadvantages  Operates at low pressure:- This one of the main reasons Genser chose a fixed bed gasifier, because Genser would have serious difficulties in providing the high pressure flows that an entrained flow gasifier requires.  Can use a variety of feedstock types:- This is advantageous because Genser will eventually introduce biomass into the fuel supply.  Double valve system in coal bunker acts as an airlock:- The double valve system prevents oxygen from entering the gasification chamber and increasing the combustion rate beyond

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design specifications. The rotary valves used in the coal bunker are designed to act as airlocks when feeding coal into high or low pressure chambers. Continuous supply of coal from bunker to gasifier:- The two sets of valves enables the coal bunker to alternate which set is releasing coal into the gasifier, so that there is a continuous stream of feedstock into the blast chamber. Can reach dangerous pressure levels:- The fixed bed gasifier can explode if the pressure within the chamber is not monitored. Syngas needs to be cleaned:- The syngas that the gasifier produces still needs to be cleaned to remove coal residue and sulphur, which adds cost to the overall system. Risk of clogging the system:- The feedstock supply has to be carefully controlled so that the slag continues to flow out of the base of the gasifier and does not clog the system. Rotary valves are volumetric flow devices:- The rate of combustion could vary because the coal that enters the chamber has a varying range of densities and heating values. In addition, rotary valves are not as good at handling bulk solids as other types of feeders. However, it is more important to prevent extra oxygen from disrupting the combustion process than to control the density of the coal entering the gasifier, which is why rotary valves are the best choice for this system.

A2.6 Hydraulic Ash Handling System The ash handling system removes ash from the bottom of the gasifier. There is a large ash tray at the base of the gasifier which catches all the coal ash produced during complete combustion. It is filled with waste water (as opposed to soft water) to dampen the ash and prevent it from flying out of the tray. The tray is rotated by a ratchet wheel connected to the tray on the underside of its base; a hydraulic motor connected to the ratchet spins the tray. As the tray rotates, the ash gathers against a “knife”, which is essentially a stationary wall inside the tray. A pile of ash will form against the knife and eventually spill out of the tray. The ash will fall down to the ground where there will be a receptacle (such as a wheelbarrow) that is ready to collect the ash. An operator will periodically empty the receptacle. The engineering team chose to fill the ash trays with waste water instead of soft water because soft water is expensive to produce and is not necessary in this application. The waste water flowing to the ash tray is not hot (so it is not likely to form scale at a rapid rate) and will be contaminated with ash at any rate, so there is little need to use soft water in this case. There is an ash storage facility that can store up to a month’s worth of ash produced in the four gasifiers. Genser is in the process of obtaining a permit to sell ash to construction companies that manufacture bricks, concrete or work in road construction, as well as cement companies.

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Types of Ash Handling Systems Ash can be handled either pneumatically or mechanically in a power plant. Pneumatic handling systems will use a series of blowers and suction pumps to collect ash in a piping system and store it in a dust silo. Pneumatic systems can use either a vacuum or high pressure to force ash through the system. Mechanical systems use a conveyer system to carry away coal ash from dust collectors and gasifiers. Usually mechanical systems are divided into two categories, for collecting bottom ash (from a gasifier) and fly ash (from dust collectors) separately. Bottom ash collectors are usually submerged underwater to wet the ash as it falls onto the conveyer belt, and then carry the ash up an incline to drain the water. Fly ash will either be transferred directly to an enclosed conveyer belt system, or mixed with water in a mixer before being discharged onto a conveyer system. Advantages and Disadvantages  Environmentally friendly:- The mechanical ash handling system that Genser uses dampens the bottom ash so that dust does not fly out into the atmosphere during disposal.  Low cost:- The mechanical system costs less than an equivalent pneumatic system because it uses fewer moving parts and has a simpler design.

A3 Gas Cleaning Equipment After upstage and downstage gas is funneled out of the gasifier, the two streams are cleaned separately before they are mixed together and sent to the supplementary burners. The downstage gas (which is mainly contaminated by coal dust) is sent first to a cyclone to eliminate most of the dust; then it is sent to a heat exchanger and an air cooler. The upstage gas (which mainly contains tar vapor) is sent to an electrostatic precipitator to remove the tar vapor, and then mixes with the downstage gas in the indirect cooler. From there the mixed gas passes through a second electrostatic precipitator before it is pressurized by a series of pressure adders, and sent to the supplementary burners.

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To heat exchanger

Downstage gas enters here

Gas swirls around central pipe

Water seal to prevent gas from escaping

Figure 3: Cyclone

A3.1 Cyclone The cyclone removes dust from the downstage gas flow. The downstage gas enters the cyclone at 6m/s, and is guided around a central pipe in a spiral motion. As the gas cycles around, the heavier dust particles drop out because they have a higher inertia and cannot follow the gas flow in a curve; the dust particles impact the cyclone wall and drop to the bottom of the cyclone. The cyclone narrows towards its base, so as the gas spirals around the smaller diameter at the base, even the lighter dust particles will drop out because they will not be able to follow the tight turns. The base of the cyclone is submerged in water to form a water seal that prevents gas from escaping from the cyclone.

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To collect fly ash in the dust cyclone, water is sprayed at the gas inlet, and the dust particles mix with the water to form slurry which collects at the bottom of the cyclone. The slurry is then manually removed from the cyclone as part of the regular maintenance schedule. The downstage gas flows from the cyclone to the heat exchanger. Types of Dust Collectors There are five main types of dust collectors: - Inertial separators - Fabric filters - Wet scrubbers - Electrostatic precipitators - Unit collectors A dust cyclone is an inertial separator – more specifically, it is a centrifugal separator. Industry uses both single and multiple centrifugal cyclones, and Genser uses a single cyclone design. Although multiple cyclones are more efficient because they are longer (so the gas circulates for a greater amount of time) and have a smaller diameter (so smaller particles are forced out), they cause a greater pressure drop as the gas passes through them. Fabric filters can also be up to 99% efficient and cost effective, but they would require more machinery or more maintenance to clean the filters on a regular basis. Wet scrubbers are too costly and more efficient than necessary for this application, since the gas passes through several cleaning devices, not just the dust collector. Electrostatic precipitators are used elsewhere in the gasification system, to augment the work done by the dust cyclone. Unit collectors are low cost and compact, but they require frequent maintenance to clean and empty them because they do not have much storage space, and cannot be emptied onto a conveyer system the same way a cyclone can. Advantages and Disadvantages  Reduced pressure drop:- Genser chose to use a single cyclone which has less impact on the gas pressure flowing through the device, but is still a very efficient dust collector design.

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Downstage gas enters from cyclone

To wind cooler

Pipes with soft water for cooling gas

Water seal to prevent gas from escaping

Figure 4: Heat Exchanger

A3.2 Heat Exchanger This is used to cool downstage gas. Gas enters the heat exchanger from the cyclone at 6m/s and passes through a series of vertical pipes. The pipes are surrounded by soft water at 30°C, which absorbs heat transferred from the gas and exits as 0.5 bar steam at 116°C. The gas cools to about 300°C; as it cools some water condenses and drops to the bottom of the heat exchanger, and dust can also drop out. The

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heat exchanger also has a water seal at the base which prevents gas from escaping. The heat exchanger is a shell and tube design. The downstage gas flows from the heat exchanger to the air cooler. Types of Heat Exchangers There are many different types of heat exchangers used in industry, and just within this gasification system, the heat exchanger, air cooler and indirect cooler are all different types of heat exchangers. There are two main categories of heat exchangers: parallel flow and counter flow exchangers. In parallel flow exchangers, both fluids flow in the same direction; in counter flow exchangers, the opposite is true. Counter flow exchangers, like the one used in this system, are more efficient because there is a greater average temperature difference across the length of the pipe, so the heat transfer rate is faster. Some different types of heat exchangers include: - Shell and tube - Plate - Plate and shell - Plate fin Shell and Tube Shell and tube exchangers, such as the one used in this system, are designed to operate at high temperatures and pressures. Shell and tube exchangers are also very robust because of their shape, which means this piece of equipment will have a long life span. Plate Plate heat exchangers have thin, slightly separated plates with a very large overall surface area, which facilitates a fast rate of heat transfer. While plate exchangers are therefore very efficient, they also require gaskets to seal them, and this exposes plate exchangers to leaks if the gaskets fail, therefore they were not chosen for this application, when the gas and fluid both flow at high pressure. Plate and Shell Plate and shell exchangers are a hybrid of the two designs, and consequently have a high heat transfer rate, can operate at high temperature and pressure, and are compact. This design also does not need gaskets, so it is not prone to leaks. Plate Fin Plate fin exchangers are plate exchangers with fins between the plates to guide the gas for more efficient cooling. The material used to make this design has a high heat transfer efficiency so plate fin exchangers generally operate at lower temperatures than exist in this gasification system, which is why they were not used.

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Advantages and Disadvantages  Robust design:- The shell and tube heat exchanger is designed to withstand high temperatures and pressures, so it can withstand the cooling water that is pumped at high pressure through the exchanger, and the syngas, which is also at a high temperature as it comes out of the cyclone.

Downstage gas enters here

Gas circulates through pipes to cool

Water seal to prevent gas from escaping Figure 5: Engineering Drawing of Air Cooler

A3.3 Air Cooler The air cooler is used to further cool the downstage gas. The air cooler is simply a series of pipes that circulates the downstage gas and exposes it to the outside air. As the air passes through the pipes, it transfers heat to the air, and cools from 300°C to 100°C. Fly ash from the air cooler is removed as a slurry the same way it is removed from the heat exchanger.

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It is necessary to have a dust cyclone, a heat exchanger and an air cooler in the gasification system – they cannot be consolidated into fewer pieces of equipment. The dust cyclone has to be in the system to remove the dust from the downstage gas. The heat exchanger is necessary because it cools the gas and also produces more 0.5 bar steam, which is supplied to the 0.5 bar steam drum. The air cooler both cools the gas and removes any remaining dust; although the dust cyclone and the heat exchanger also have these functions, if either piece of equipment fails, the air cooler will be able to take over their function and the system can continue to operate. In other words, having an air cooler in the system makes it more robust and less likely to shut down. The downstage gas from the air cooler passes to the indirect cooler to mix with the upstage gas. Advantages and Disadvantages  Low cost:- This design has all the advantages of a typical shell and tube exchanger, with the added benefit that, since the design is simpler, the capital and installation costs are lower because a second fluid does not need to be piped through the air cooler.

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To indirect cooler

Gas enters here

Positively charged pipes Negatively charged wires Figure 6: C-60 Electrostatic Precipitator 1

A3.4 Electrostatic Precipitator 1 This cleans the upstage gas by removing tar vapor. The electrostatic precipitator 1 (ESP1) is also referred to as the de-tarrer. It has a series of vertical negatively charged wires running through positively charged pipes. The ESP1 is supplied with 412VAC, which is converted to 60000 VDC in the electrodes. The electric field within these pipes ionizes the tar particles. The tar ions are negatively charged, so they are attracted to the inner surface of the pipes, where they condense and drip to the slanted base of the ESP1. The tar is then piped out to a tar tank. The upstage gas from the ESP1 flows to the indirect cooler to be mixed with the downstage gas. Advantages and Disadvantages  Appropriate type of dust collector for the tar vapor in syngas line:- An ESP was used here because it can remove tar vapor as well as dust, which is important since the gas flowing through the ESP1 mainly contains tar, not dust.

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

Energy efficient:- An ESP was used instead of a wet scrubber because wet scrubbers apply energy directly to the fluid flow, but ESP’s apply energy directly to the contaminant particles, which is more energy efficient.

Gas enters here

Phenolic water is sprayed out here

To ESP2

Soft water is circulated for cooling

Figure 7: Indirect Cooler

A3.5 Indirect Cooler The indirect cooler mixes the upstage and downstage gases, cools them to 35-45°C and scrubs the gases to remove tar vapor. The upstage and downstage gases flow into the indirect cooler at about 100°C, and flow through vertical pipes. Phenolic water is sprayed into the gas flow, and it mixes with the tar and dust particles in the gas and carries them out the bottom of the cooler into a phenolic water tank. There are pipes filled with soft water in between the syngas pipes, which cool the gas. The soft water moves as a counter current up the indirect cooler, and is heated from about 30°C at the inlet to 40°C at the outlet. As the syngas cools, any remaining water vapor or light oil fog condense and drain out with the phenolic water.

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The incoming phenolic water is simply waste water. The chemical name for the tar products carried with the syngas is phenol, C6H5OH, which is soluble in water. So when the phenolic water is sprayed into the syngas flow, it dissolves the phenol and carries it to a phenolic water tank. When the phenolic water reaches saturation, it is incinerated. The mixed syngas from the indirect cooler flows to the electrostatic precipitator 2. Indirect vs. Direct Cooling In direct cooling, the gas has to come in direct contact with the cooling medium, usually in a packed cooling tower; this process can be used to clean the gas as well as cool it. Indirect cooling, on the other hand, involves passing the gas counter-current to the cooling medium, while separating the two flows by a pipe wall. The indirect cooler is another type of heat exchanger called a fluid heat exchanger, because the gas to be cooled flows with a cleaning fluid spray in the same chamber. To clarify, the phenolic water’s purpose is to clean the gas, not to cool it, which is why this device is an indirect cooler, since the cooling fluid is contained in separate pipes from the gas. Advantages and Disadvantages  Multipurpose design:- This type of heat exchanger was chosen specifically because it cleaned the gas as well as cooled it.

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Gas enters here

To pressure adders

Figure 8: C-97 Electrostatic Precipitator 2

A3.6 Electrostatic Precipitator 2 This also removes any remaining tar from the mixed syngas. The electrostatic precipitator 2 (ESP2), also referred to as the de-oiler, is designed the same way as ESP1. It is called the de-oiler because at this stage, most of the remaining tar vapor in the syngas has the consistency of oil.

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Advantages and Disadvantages  Ensures syngas is clean:- A second electrostatic precipitator was included because the syngas coming out of the indirect cooler would still have some contaminants that needed to be removed. A3.7 Pressure Adder The pressure adders increase the pressure of the syngas flowing to the desulphurization system. The syngas flowing into the pressure adders has a pressure of 2kPa, and exits at a pressure of 39.2kPa. There are three pressure adders for the entire gasification system, but only two are ever in operation; the third is a backup compressor on standby. Advantages and Disadvantages  Maintain high pressure for desulphurization unit:- The pressure adders increase the pressure of the syngas to 39.2kPa so that the gas will travel the long distance to the desulphurization unit. If the gas were sent at 2kPa, it would take more time to reach the desulphurization unit. Over the entire distance, the pressure drops only 500-1000Pa.

A4 Phenolic Water System The phenolic water system supplies water to the indirect cooler to remove tar from the syngas. Waste water is stored in a tank and pumped through the indirect cooler where it dissolves the tar, forming phenolic water. This water is then re-circulated through the system until it reaches its saturation point; at that point, the water is incinerated in the incinerator, since the tar cannot be sold at this time. A4.1 Phenolic Water Supply Pool The phenolic water supply pool contains the phenolic water used to remove tar from the syngas. The tank is connected both to the indirect cooler to supply it with phenolic water, as well as to the incinerator. When the phenolic water is saturated, the valve connecting the tank to the incinerator will be opened and the phenolic water will be burned off to dispose of the tar. During the operational phase of the plant, the engineering team will test the phenolic water supply to determine when the water reaches saturation and needs to be incinerated. They will also determine how much water should be incinerated in a single pass, and how to incinerate the saturated phenolic water in the system while still supplying phenolic water to the indirect cooler.

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Advantages and Disadvantages  Stores tar before arriving at incinerator:- It would be inefficient to have the incinerator operating continuously; the water supply pool stores the tar until there is a significant quantity and then it is sent to the incinerator, operating in a batch process method.

A5 Tar Disposal System The tar disposal system collects tar from the electrostatic precipitators and collects it in a tar tank. Two pumps pump tar through the system.

A6 Waste Water System The waste water system supplies water to equipment that is not using steam. This includes the indirect cooler, the ash trays, and the water seals on the cyclone, the heat exchanger and the air cooler. The waste water is stored in a tank, and pumped throughout the gasification system by a series of three pumps.

A7 Soft Water System The soft water system supplies water to the equipment that uses steam, because soft water does not form the pipe-clogging scale that hard water deposits in hot environments. The system consists of one soft water tank, one sodium ion exchanger, and two pumps. A7.1 Sodium Ion Exchanger The sodium ion exchanger softens water so that it will not form scale on the inside of the steam pipes, which could clog them. It is important to soften the water flowing in the steam pipes because when hard water is heated, the mineral carbonates in the water can precipitate out and form a scale that clogs pipes. If there is a significant amount of scale in the pipes, it can also act as a thermal insulator, preventing heat transfer from the gasifier walls to the water in the pipes. This can cause the pipes to overheat and fail catastrophically. The sodium ion exchanger will attract the Ca2+ and Mg2+ ions that form this scale and bind them, replacing those ions with Na+ ions, forming soft water. The sodium ion exchanger and soft water tank are located in the water treatment plant (WTP) section of the power plant, but they are part of the gasification system. The ion exchanger and tank are located in the WTP to save space in the gas station; during the expansion from Phase 1 to Phase 2, there will need to be space for cranes to move around, and the ion exchanger and water tank could block their movement. The soft water is pumped over to the gas station for use in the steam pipes.

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Advantages and Disadvantages  Resin beads are low cost:- This sodium ion exchanger uses resin beads, not lime, which is more economical, because the resin beads can be regenerated by spraying them with the correct chemical. If the ion exchanger used lime, then Genser would have to purchase large amounts of lime to replenish the exchanger on a regular basis.

Phase 2 Introduction In Phase 2 four gas turbines will be added to the plant, so the gasification system will have to increase in size during Phase 2 to supply these four turbines with syngas. This is to say that two new gasification units and their accompanying parts will be added to the existing facilities in Phase 2. Three new air blowers and new soft water and phenolic water systems will also be added in Phase 2. Therefore, Phase 2 will be identical to Phase 1, except that it will be larger and supply more gas to the desulphurization system.

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[Accessed 10 July 2012.] “Mixer Unloader Systems.” On AshTech website. < http://ashtechcorp.com/mixer-systems> [Accessed 10 July 2012.]

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Morel, William C. “Economic Comparison of Coal Feeding Systems in Coal Gasification – Lock Hopper vs Slurry.” U.S. Department of the Interior, Bureau of Mines. On Argonne National Laboratory website. < http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/22_7_CHICAGO_0877_0155.pdf> [Accessed 9 July 2012.] “Pressure Pneumatic Ash Handling Systems.” On National Conveyers Company website. [Accessed 9 July 2012.] “Submerged Chain Conveyer Systems.” On AshTech website. < http://ashtechcorp.com/submerged-chain> [Accessed 10 July 2012.] “Vacuum Pneumatic Ash Handling Systems.” On National Conveyers Company website. [Accessed 9 July 2012.] “What is Ash Handling?” On National Conveyers Company website. < http://www.nationalconveyors.com/ash/ash-handling/overview.html> [Accessed 9 July 2012.]