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EPI fluidized bed gasifiers convert biomass waste products into a combustible gas that can be fired in a boiler, kiln, gas turbine or other energy load. EPI produced the first wood fired fluidized bed gasifier power plant in the US and we continue to provide innovative gasifier solutions to unique industry applications. We are currently introducing the gasifier approach as an add-on to utility coal fired power plants to provide a means to convert a portion of the fuel supply to clean, renewable biomass fuel.
In a fluidized bed gasifier, the bed material can either be sand or char, or some combination. The fluidizing medium is usually air; however, oxygen and/or steam are also used. The fuel is fed into the system either above-bed or directly into the bed, depending upon the size and density of the fuel and how it is affected by the bed velocities. During normal operation, the bed media is maintained at a temperature between 1000EF and 1800EF. When a fuel particle is introduced into this environment, its drying and pyrolyzing reactions proceed rapidly, driving off all gaseous portions of the fuel at relatively low temperatures. The remaining char is oxidized within the bed to provide the heat source for the drying and de-volatilizing reactions to continue. In those systems using inert bed material, the wood particles are subjected to an intense abrasion action from fluidized sand. This etching action tends to remove any surface deposits (ash, char, etc.) from the particle and expose a clean reaction surface to the surrounding gases. As a result, the residence time of a particle in this system is on the order of only a few minutes, as opposed to hours in other types of gasifiers. The large thermal capacity of inert bed material plus the intense mixing associated with the fluid bed enable this system to handle a much greater quantity and, normally, a much lower quality of fuel. Experience with EPI's fluidized bed gasifier has indicated the ability to utilize fuels with up to 55 percent moisture and high ash contents, in excess of 25 percent. Because the operating temperatures are lower in a fluid bed than other gasifiers the potential for slagging and ash fusion at high temperatures is reduced, thereby increasing the ability to utilize high slagging fuels. Energy densities in a fluid bed gasifier are dependent on the fuel characteristics and have been reported as high as four million BTU/hour/ft.2 Normally, the dryer the fuel the higher the energy density and the better the quality of low Btu gas produced. The reasons for this fuel dependence will be better understood from the discussion of the gasification process within the fluidized bed. Gasification Principle In principle, gasification is the thermal decomposition of organic matter in an oxygen deficient atmosphere producing a gas composition containing combustible gases, liquids and tars, charcoal, and air, or inert fluidizing gases. Typically, the term "gasification" refers to the production of gaseous components, whereas pyrolysis, or pyrolization, is used to describe the production of liquid residues and charcoal. The latter, normally, occurs in the total absence of oxygen, while most gasification reactions take place in an oxygen-starved environment. In a gasifier, the wood particle is exposed to high temperatures generated from the partial oxidation of the carbon, primarily. As the particle is heated, the moisture is driven off. This could range from below 10 percent to over 50 percent of the incoming fuel weight. Further heating of the particle begins to drive off the volatile gases. For wood, this volatile content could be as much as 75 to 80 percent of the total dry weight. Discharge of these volatiles will generate a wide spectrum of hydrocarbons ranging from CO and methane to long-chain hydrocarbons comprising tars, creosotes and heavy oils. After about 900EF, the wood particle is reduced to ash and char. In most of the early gasification processes, this was the desired by-product. In gas generation, however, the char provides the necessary energy to effect the heating and drying previously cited. Typically, the char is contacted with air or oxygen and steam to generate CO and CO2 and heat. The quality of gas generated in a system is influenced by fuel characteristics, gasifier configuration, and the amount of air, oxygen or steam introduced. The output and quality of the gas produced is determined by the equilibrium established when the heat of oxidation (combustion) balances the heat of vaporization and volatilization plus the sensible heat (temperature rise) of the exhaust gases. The quality of the outlet gas (BTU/ft.3) is determined by the amount of volatile gases (H2, CO, CH4, C2, etc.) in the flue gas stream. Considering the system equilibrium, it can easily be seen how the moisture content of the fuel can impact the gas quality. With the heat released by the char a fixed quantity (assuming a constant air flow), the more moisture in the fuel, the more heat consumed by evaporation. Less energy remains to for volatilization and sensible heat, so the fuel rate must be decreased. Consequently, less volatiles are produced and the combustible gas quality and quantity is reduced. As the system output increases, the operating temperature is reduced. This is explained by the fact that, again for a fixed heat (of oxidation) release due to the constant air flow, the more fuel fed into the system, either wet or dry, the more energy is required for both volatilization and evaporation, and the less energy available to raise system temperatures via sensible heat increases. In effect, the latent heat fraction increases at the expense of the sensible heat. The result of this is that as more volatilization occurs, the combustible content of the outlet gas is increased and the overall heat content is improved. Thus, the highest gas quality occurs at the lowest temperatures; however, when the temperatures drop too low, the char oxidation reaction is suppressed and the overall heat release diminishes. Essentially, the "lights" go out! Optimum gas yields are obtained at operating temperatures around 1100EF to 1200EF. Higher gas heat contents (BTU/ft.3) can be obtained at lower system temperatures; however, the overall yield of fuel-to-gas is reduced by the unburned char fraction. With this basic understanding of fluidization and gasification processes, it is possible to better understand the combined processes within a fluidized bed gasification system. The first design consideration is fluidizing velocity to the bed. This is determined by the size of the bed media used and establishes the air flow into the system. Upper air flowrates are limited by the entrainment velocities of the bed particles. Lower flowrates are determined by the minimum fluidizing velocities at which acceptable mixing occurs. These boundary conditions typically limit the fluidizing air flow to a 2-to-1 operating range. With a given fuel quality (moisture content and heat value), the output of the gasifier can be modulated to a 3-to-1 turndown ratio. At maximum output both the fuel feedrate and the air flowrate are at maximum. The gasifier operates around 1100EF to 1200EF. As fuel is reduced, the output is reduced and the system temperature increases (constant air flow). To compensate, air flow is reduced, thereby reducing total energy release from the oxidation of the carbon, dropping the temperatures back to the 1200EF range. This ratcheting effect can continue until the air flow has been reduced to the minimum velocities. Further turndown beyond that point allows for reduction in the fuel feed only with a corresponding increase in operating temperatures once again. Theoretically, this temperature could increase to the adiabatic flame temperature of the fuel, often as high as 3000EF. Other operating constraints become limiting, such as ash slagging temperatures below 2000EF, materials of construction, i.e., ducting, dampers, below 1800 EF, etc. Additional output modulation can be achieved by regulating the moisture content of the fuel. The wetter the fuel, the greater the fraction of available system heat required for evaporation. Thus, for a constant air flowrate, wetter fuel results in a lower energy output of the same sized unit. For comparison, the typical output of a gasifier on ten percent moisture fuel would approach 2.5 M BTU/hour per unit area of bed. With 45 percent moisture fuel, the output would be 1.3 M BTU/hour, or roughly half of the dryer fuel capacity. The outlet gas quality drops from over 175 BTU/ft.3 to around 100 BTU/ft.3. By adjusting the moisture of the inlet fuel, the output of the unit can be controlled from a dry-fuel maximum of 2.5 M Btu/hr/ft2 to a wet fuel minimum of only .45 M BTU/hr.-ft.2 thereby creating an operating range of almost six to one. With air-supplied systems, the outlet gas heat content is on the order of 100 to 200 BTU per standard cubic foot and is typically called low-Btu gas, or LBG. It is comprised of hydrogen, methane, carbon monoxide and dioxide, and nitrogen. With the high dilution from the nitrogen introduced with the air, the optimum LBG quality is only around 200-250 Btu/scf. In some instances, use of another medium to replace some of the fluidizing air could increase gas quality and expand the operating window. Steam, for instance, would provide added potential to support methane production from carbon dioxide ( water-gas shift reaction) and would be more readily removed from the output gases by cooling and condensing, thereby increasing the potential gas heat value. In some instances, the increased fuel gas quality will justify the use of steam in the process. In most instances, an air blown system can be simpler and more efficient to use. Application In general, it is probably acceptable to say that gasification systems could be used in nearly every application in which natural gas, oil, or pulverized coal are currently being used. Low BTU gases can be used to fire cement or lime kilns, rotary dryers, wood veneer dryers or dry kilns, air heaters, steam boilers, and turbine or diesel generator sets. Possibly, the simplest application for a fluidized bed gasifier is to fire or co-fire an existing steam boiler. This presents the most likely opportunity where the steam demand is located adjacent to a fuel source. In food processing, wood processing, textiles, paper, and numerous other industries, a boiler system is already in operation which could be retrofitted to LBG produced from fuels generated by the plant wastes or from external supply. In the utility industry, numerous pulverized coal (PC) fired power boilers represent serious opportunities for co-firing with alternate fuels via gasification. In a PC boiler, the coal burners release the combustion energy in the form of intense flame zones directly in the furnace. The design of the furnace utilizes the concentrated heat release to generate most of the steam production within the water-wall surfaces of the furnace. Much the same holds true for oil and gas fired boilers, also. Once out of the furnace, the high temperature exhaust gases continue to generate steam and superheat through the remaining boiler sections. In considering the replacement of the coal by an alternate fuel, the production capacity and steam superheat conditions of the boiler, both critical elements for the optimum plant performance, are intimately determined by the burner heat release rates and temperature profile. To maintain output conditions, any replacement of coal capacity must be accomplished by a suitable fuel which will burn in suspension within the furnace and at the burner levels established by the coal. In some instances, this can be accomplished by introducing some portion of the alternate fuel directly into the coal feed system, ahead of the pulverizers, and displace some of the coal feed directly into the burner unit. While this concept represents the simplest, lowest cost approach to this type of retrofit, it is restricted by the ability of the existing coal handling and pulverizer units to handle very high fractions of alternate fuels. For 5-10 percent co-firing rates, and possibly up to 20 percent in some specific instances, this approach is possible and is currently in operation. It does have limitations to the fuel characteristics, their wear potential or plugging impact on the pulverizers, and the effective quality of the fuel, per pound and per cubic foot, as compared to the coal. In addition, it may require modifications to some parts of the coal firing system which pre-empt the reversal of operation back to 100 percent coal firing, if the need or desire should arise. A fluidized bed gasification system approach to a boiler retrofit has the specific advantages of maintaining total independence of the coal handling and processing equipment from the storage system all the way into the boiler furnace, or the burners. Not only does this maintain complete capacity for 100 percent coal firing as a future option, it also provides additional reliability and redundancy to the overall firing system by providing a totally independent system of fuel delivery into the furnace. In addition, the fluid bed gasifier can use a variety of fuels having a range in size up to four inches, moisture contents as high as fifty percent and high in ash content. Having the gasification step prior to delivering the fuel into the boiler, most of the fuel variations are eliminated, and the boiler sees a constant and fairly uniform energy supply as LBG. In order to better understand the potential impact on the boiler when displacing a portion of the coal with LBG from an alternate fuel source, a comparison of the two fuels is helpful. From the accompanying table, it can be seen that the energy value for the coal is double that of the wood, Btu per pound; however, by the time the fuel (or fuel generated LBG) is converted to combustion by-products at 20 percent excess air, the wood fuel at 25 percent moisture, represents a higher value energy (note: adiabatic, or theoretical temperature) than does the coal. This is due to the fact that the wood requires less combustion air than the coal per equivalent energy unit because of the increased amount of oxygen already present within the wood. As a result, the combustion gases produced per unit of energy are actually lower for wood than coal, and the output of energy per unit is greater. It should be noted that this does not hold true for the wood at moisture levels around 45 percent. At these higher moisture levels, the wood energy value is below that of coal and a negative impact on the boiler would be possible. |
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Energy Products of Idaho
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Background