Economic Report about Current Commercial Gasifiers
GE Energy Gasifier
GE Energy Gasifier uses the entrained flow, coal-water slurry feed, refractory lined slag and oxygen blown technology. The manufacturers at GE Energy have two variants, one with a radiant cooler and another full quench gasifier. Their products use a wide variety of fuels like low rank coals, pet coke, bituminous coal and other blends of pet coke. The design of GE Energy Gasifier emphasizes on operational eficiency. This is achieved through optimization of heat recovery and production of carbon monoxide. As espoused earlier, the GE Energy Gasifier predominantly uses the direct quench system. This design is not as efficient, nonetheless, is is also inexpensive compared to other methods. As such, losses in efficiency are compensated for by lower capital and operational costs. Additionally, the higher hydrogen to the carbon monoxide ratio that the manufacture embodies is ideal for the direct quench design. The GE Energy Gasifier that has a capacity of 1800 cubic feet is designed with a diameter of six meters and a height of between thirty to forty meters. The operational costs of the gasifier are 1250 dollars per kW. Because of carbon capture, the levelized cost of electricity is 4 cents per kWh.
Siemens Future Energy: SFG gasifier (e.g. SFG-500)
Siemens Future Energy: SFG gasifier SFG-500 is a gasifier that embodies economic efficiency. The dry feeding mechanism of the gasifier enhances high efficiency rates. The gasifier has high conversion rate of carbon at ninety eight percent. In terms of carbon conversion rate, the Siemens Future Energy: SFG gasifier SFG-500 is only rivaled by Shell Global Solutions: SCGP gasifier which has a higher conversion rate of ninety nine percent. From an environmental perspective, the low carbon emissions are a testament to the economic efficiency of the gasifier. Additionally, the Siemens Future Energy: SFG gasifier SFG-500 is a multi-fuel gasifier. It can accept various fuels from biomass, petcoke, liquid wastes, lignite, vacuum residues, sub-bituminous coal and bituminous coal. The wide variety of fuels allows operators to choose from inexpensive fuels in order to save on capital and operational costs. The diameter of the Siemens Future Energy: SFG gasifier SFG-500 is three meters and the height is eighteen meters. The maintenance costs of a Siemens Future Energy: SFG gasifier SFG-500 run into an annual amount of 4,554,000. Out of this amount, spare parts and materials take up 2,732,000 with the remaining going to labor costs.
Shell Global Solutions: SCGP gasifier
The economic efficiency of the Shell Global Solutions: SCGP gasifier is down to different design elements. For instance, the Shell Global Solutions: SCGP gasifier was designed with multiple burners. Due to this design element, there is a higher throughput per unit time compared to the other gasifiers. This increases the operational efficiency of the Shell Global Solutions: SCGP gasifier. Additionally, the gasifier has an efficient use of coal. This is because the gasifier completely converts any coal or coke resulting in lower emissions of carbon dioxide. The Shell Global Solutions: SCGP gasifier has the highest carbon conversion at ninety nine percent. This has a significant influence on the economic efficiency of the gasified because of the low operating expenditures. The optimized operation temperatures of the Shell Global Solutions: SCGP gasifier also speak about its operational efficiency. Due to the high cold gas efficiency, more syngas is produced using the same amount of raw material. This is because of the optimal operating temperatures of the gasifier. Finally, the Shell Global Solutions: SCGP gasifier has robust membrane walls. Additionally, the coal burners of the gasifier have a long life time. Due to this, the low maintenance costs, reduce the operational costs of the gasifier. The gasifier has a diameter of between five and six meters and a height of fifty meters. At a rate of 2000 tons a day, this gasifier uses fuel in the amount of 31MWe.
ConocoPhillips: E-Gas gasifier
Conoco Phillips E-Gas gasifier in its original design was designed by DOW Chemicals. The gasifier is oxygen blown and slurry fed. The operational costs of the equipment are reduced by the fact that there is a continuous removal of the slag. Additionally, the dry particulate matter is removed on a continual basis. This improves the operational efficiency of ConocoPhillips: E-Gas gasifier compared to the Prenflo gasifier. However, capital costs of the equipment are influenced by the fact that ConocoPhillips: E-Gas gasifier has processes that can work with a wide range of raw materials (coal). This coal ranges from powder river basin coal to pet coke to bituminous and other blends. This feature means that the gasifier can be fed with material that is inexpensive in the market. The Conoco Phillips E-Gas gasifier has a height of eight meters a diameter of six meters. At 80% capacity, the levelled electricity cost for this gasifier is 75 dollars per kWh. The operating costs at the same capacity are 5.8 dollars per kWh. The total plant costs are 43.3 dollars per kWh.
ThyssenKrupp Uhde: Prenflo gasifier
The Prenflo gasifier is a product developed by ThyssenKrupp Uhde technology giants. The operational efficiency of the gasifier is enhanced by the design of the feed characteristics and the conditions of the gasifier. The gasifier features a heating boiler that can handle integrated waste. Additionally, the quench gas that is used in the boiler is reticulated, hence reducing the capital costs of gas and operational costs. Additionally, the investment cost required for this gasifier is considerably low. Additionally, the construction and the supply schedule is short, hence reduced capital costs. However, it is noteworthy that operational costs on this gasifier emanate from the higher concentration of carbon dioxide in the raw synthesis gas and the lower thermal efficiency. Other reasons that result in losses of efficiency include increases in the ash content, the ratio of output of steam turbine, increase in nitrogen and oxygen and the rate of compression of the air. The Prenflo gasifier has a diameter of six meters and a height of seven meters. The internal pressure inside the gasifier range between thirty bar to sixty bar. Using the IGCC, the total costs incurred are 30.11 dollars per MWh. The fuel cost is between 2.56 dollars per MWh to 9.74 dollars per MWH depending on the material that is being gasified.
Mitsubishi Heavy Industries: MHI gasifier
The economic efficiency of all mechanical devices is paramount to their sustainability. Mitsubishi Heavy Industries took this into consideration when designing the MHI gasifier. This is because the intent behind the technology envisioned in the MHI gasifier was to maximize on gasifier efficiency. The design of the MHI gasifier minimizes capital and operational costs by blowing air into the compartments. In so doing, the design eliminates the operational costs emanating from power consumption and capital costs resulting from an oxygen plant. Additionally, the manufacturers design of the gasifier uses a dry feed, meaning that any form of gas can be used. This increases operational efficiency and reduces capital costs because users can source inexpensive gas alternatives in the market. The geometric data of the gasifier were designed in order to increase the efficiency of the equipment. For instance, the MHI gasifier has a diameter of three meters and a height of thirty five meters. The enclosed space, given the other factors is optimized for the gasification process. The levelled electricity costs for this gasifier is 155.25 dollars per MWh.
A Technical Description of Synthesis Gas Cooling Methods
After coal or biomass is gasified, the resulting synthesis gas has very high temperatures. The following cooling methods are used to cool it to the desired temperatures. This is a technical description of the methods used to cool the synthesis gas.
Water Quench
Water quench is one of the methods used for cooling synthesis gas during the gasification process. After the coal has been gasified, the resulting gases have extremely high temperatures. The water quench is a cooling system that uses a water bath to lower the temperatures to the desired level. This system uses a quench ring to direct the hot gases that exit the gasifier. Through the quench ring, the hot gases come into direct contact with water. It is important to note that the water bath is located on the lowest end of the gasifier vessel. The quench ring is immersed into this water bath.
The difference in temperatures between the water and the gases in the quench ring is exploited in this cooling system. The heat from the gases that exit the gasifier passed on into the water in the bath and the gases are cooled. Of note is the fact that gases that exit the gasifier come into the water quench at approximately one thousand five hundred degrees Celsius. The hot gases from the gasifier exit the water into the water scrubber at temperatures of three hundred degrees Celsius. The saturated and cooled gas is led into the scrubber where the particulate matter and soot are removed.
As discussed, the water quench reduces the temperatures of the hot gases dramatically. Nonetheless, the water quench method of cooling synthesis gas is not very efficient. The fact that the water quench cooling method is relatively cheap, the losses in efficiency means that this method is not appropriate for companies that are concerned with surging operational costs. The inefficiency of the water quench method emanates from the wastage of energy inherent in the synthesis gas. The inexpensive nature of the water quench cooling method does not outweigh the wastages in the heat energy, so that the overall, the method is still inefficient.
Gas Quench
As the name indicates, the gas quench method uses product gas to reduce the temperatures of the gases exiting the gasification vessel. The gases used in the gas quench are pressurized in order to increase their efficiency. Additionally, other measures are taken into consideration in the design process in order to ensure that the requirements for the cooling process are met. One of these elements of design is the fact that the product gas that is used during the cooling process flows at extremely high velocities. The high flow rate has implications on the process as the turbulence might result in separation of particulate material before it is filtered off the gas exiting the gasification vessel.
The heat transfer coefficients in the gas quench synthesis gas cooling method are affected by factors like pressure drop, velocity distribution, flow effects and uniformity. The same factors affect the uniformity of product that exits the gasification vessel. The efficiency of the gas quench method takes many more dimensions when compared to the water quench method of cooling synthesis gas. Firstly, the impact on the environment from this method of cooling gas is very minimal.
This is because the gas that is used in this method is the product gas that is cooled and recycled. As such, capital costs in the acquisition of coolants are minimized. Secondly, the gas quench synthesis gas cooling method uses pressurized gas that is injected into the hot gases emanating from the gasification chamber increases the contact between the cool gas particles and the hot gas from the gasification vessel. This makes the gas quench synthesis gas cooling method more efficient that the water quench cooling method.
Heat exchanger: The Radiant Syngas Cooler
The radiant syngas cooler is an enormous piece of equipment that requires an even enormous amount in capital costs to acquire. The radiant syngas cooler is designed to look like a ring of tubes that are joined together in a configuration that is known as a water wall. The hot synthesis gas exiting the gasification vessel flows through the middle of this configuration. High pressure steam at one thousand, six hundred and fifty psi is generated from within the tubes making the waterfall. This is done using boiler feedwater that is circulating. When the gasifier is at its operating temperatures, the heat transfer is done through radiation as the primary means.
As the gases exit the center of the water wall, the temperatures of the gas from the gasifier are already lowered through an exchange of heat from the gases exiting the gasifier to the pressurized steam in the tubes making the waterfall. Other radiant syngas cooler designs feature a built-in membrane inside the gasifier vessel. This membrane is covered with refractory material. Boiler feed water is left to flow into the membrane so that it maintains the refractory layer at cool temperatures. The radiant syngas cooler is more efficient that the water quench and gas quench synthesis cooling methods.
A significant amount of the sensible heat that is inherent in synthesis gas is recovered using the radiant syngas cooler. This improves the overall thermal efficiency of the gasification plant. However, the radiant syngas cooler is difficult to clean and is prone to fouling. The slag that is entrained in the synthesis gas often sticks on the radiant syngas cooler. The effect of this is deteriorating heat transfer, therefore affecting the cooling process. However, the operational reliability of the equipment is uncontested.
Chemical Quench Method
The chemical quench method is arguably the most effective method for cooling synthesis gas that is that is at high temperatures. The concept behind the chemical quench method is the free energy minimization, an approach conceived by Gibbs. Through thermodynamic analysis, the chemical quench process has been shown to work best at high temperatures. In using the chemical quench method in cooling synthesis gas, some considerations are required. The factors to be considered have an effect on the optimal feeding rate and the quench temperatures at which cooling occurs optimally. These factors include moisture content of the coal that is fed into the gasifier, the steam input and the temperature of the synthesis gas at the inlet.
Unlike the other three methods, the chemical quench method introduces a second step into the process. In the first stage of gasification, milled coal particles are heated in the presence of oxygen. The coal is milled in order to increase the surface area that is in contact with the oxygen. The gasification process at this stage produces a product gas, just like the other three methods. However, it is the second stage of the gasification process where the difference in the methods is highlighted. In the second stage of the gasification process, the product gas that exits from the gasification vessel is mixed with additional coal.
This is done in order to cool the product gas. To increase the operational efficiency, the coal particles that are put in at this stage are even smaller that those that were used in the first stage of the gasification process. This is in order to increase the surface area of the particles that is exposed to the heat from the product gas. The result is an endothermic reaction which cools down the product gas from the gasification vessel. The efficiency of the chemical quench method can be underscored through thermodynamic calculations from the equations of enthalpy changes resulting from reactions in the gasification vessel. For instance, the equation for the first stage of gasification yielding an enthalpy change of -283.1MJ/kmol in an exothermic reaction as shown in the equation below.
CO + 1/2O2 = 2CO2 = -283.1MJ/kmol
When fine particles of coal are added in the second stage of the gasification process, the endothermic reaction that ensues causes an enthalphy change the results in the cooling of the product gas from the gasification vessel.
C (s) + H2O = CO + H2 = +131.4MJ/kmol
This can be illustrated through calculations from the enthalpies of the two reactions from the two stages of the gasification process.
-283.1MJ/kmol + +131.4MJ/kmol = -151.7 MJ/kmol
This shows a decrease in the temperature of the product gas after the endothermic reaction of the second stage of the gasification process.
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