Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 215

Biomass gasifiers can be classified in to three major groups which are (i) fixed bed reactors, comprising either updraft or downdraft mode, (ii) bubbling fluidized bed reactors, and (iii) circulating fluidized bed reactors.

The difference among each of the reactors is based on the means of supporting the biomass in the reactor vessel, the direction of flow of the biomass, the oxidant, and the way heat is supplied to the reactor. The fixed bed gasifiers are the oldest and simplest form of gasifiers. Although they are a simple, low-cost process, they are not suitable for large-scale production and limited to small-scale operations.

Large-scale biomass gasifiers employ one of two types of fluidized bed configurations: (i) the bubbling fluidized bed and (ii) the circulating fluidized bed or (iii) combination of both (indirectly fired) to maintain the bed temperature below the ash fusion temperature of the biomass ash.

Bubbling Fluidized Bed Gasifier

A bubbling fluidized bed consists of fine, inert particles of sand or alumina, which are selected based on their suitability of physical properties such as size, density, and thermal characteristics. The gas flow rate is chosen to maintain the bed in a fluidization condition, which enters at the bottom of the vessel. The dimension of the bed at some height above the distributor plate is increased to reduce the superficial gas velocity below the fluidization velocity to maintain the inventory of solids and to act as a disengaging zone. A cyclone is used to trap the smaller size particle that exit the fluidized bed, either return fines to the bed or to remove ash rich fines from the system.

Biomass is introduced either through a feed chute to the top of the bed or deep inside the bed. The deeper introduction of biomass into the bed of inert solids provides sufficient residence time for fines that would otherwise be entrained in the fluidizing gas. The biomass organics pyrolytically vaporize and are partially combusted in the bed. The exothermic combustion provides the heat to maintain the bed temperature and to volatilize additional biomass.

The bed needs to preheated to the startup temperature using hydrocarbon resources such as natural gas and fuel oil, either by direct firing or by indirect heating. After the bed reaches the biomass ignition temperature, biomass is slowly introduced into the bed to raise the bed temperature to the desired operating temperature which is normally in the range of 700 to 900°C (1,290 to 1,650°F). Bed temperature is governed by the desire to obtain complete devolatilization versus the need to maintain the bed temperature below the biomass ash fusion temperature. The advantages of the fluidized bed gasifiers are: (i) yield of a uniform product gas, (ii) able to accept a wide range of fuel particle sizes, including fines, (iii) exhibits a nearly uniform temperature distribution throughout the reactor, (iv) provides a high rate of heat transfer between inert material and biomass, aiding high conversion, with low tar. The disadvantage is formation of large bubbles at higher gas velocities, which bypass the bed reducing the high rate of heat/mass transfer significantly.

Circulating Fluidized Bed Gasifier

In a circulating fluid bed (CFB) the turbulent bed solids are collected, separated from the gas, and returned to the bed, forming a solids circulation loop. A circulating fluidized bed can be differentiated from a bubbling fluidized bed in that there is no distinct separation between the dense solids zone and the dilute solids zone. Lower bed density can be achieved with increase in gas flow rates in excess of transport velocity of the fluidized bed particles. The residence time of the solids in the circulating fluid bed is determined by the solids circulation rate, attrition of the solids, and the collection efficiency of the solids in the cyclones. The advantages of the circulating fluidized bed gasifiers are that they are (i) suitable for rapid reactions, (ii) high conversion rates possible with low tar and unconverted carbon, and (iii) high heat transport rates possible due to high heat capacity of bed material. The disadvantages are (i) temperature gradients occur in the direction of the solid flow, (ii) the size of fuel particles required to determine minimum transport velocity, (iii) high velocities may result in equipment erosion, and (iv) heat exchange less efficient than in the bubbling fluidized bed reactor.

Indirectly Heated Gasifier

The fluidizing media for the biomass gasification are either air and steam or pure oxygen and steam. Air-blown or directly heated gasifiers use the exothermic reaction between the oxygen and organics to provide the heat necessary to devolatilize biomass and to convert residual carbon rich chars. For directly heated gasifiers, the heat to drive the process is generated inside within the gasifier. When air is used, the product gas is diluted with nitrogen and typically has a dry basis calorific value of 110 to 160 Btu/ft³. The use of oxygen instead of air produces a medium heating value syngas, 265 to 400 Btu/ft³, suitable for combustion turbine application.

Oxygen production is expensive, and hence indirectly heated gasifiers which utilize twin bed concept similar to fluid catalytic crackers (FCC) that are being used in crude oil refining are being developed for generation of medium calorific syngas using air as fluidizing medium.

The syngas produced from the indirectly heated gasifiers is devoid of the nitrogen contamination, and hence they are also favored for other gas- to liquid-based technologies. The non-requirement of oxygen to generate medium heating value syngas, results in lower capital cost for the gasification plant.

If the gas turbines using fossil fuel (natural gas or diesel) which has heating value of approximately 885 Btu/ft³, needs to be interchangeable with the biomass syngas, higher heating values are preferred. In a natural-gas-fired turbine, the gas is only related to 2% v/v of the flow and the rest is air for dilution and combustion, while in contrast, syngas accounts for 14 to 16% v/v of the total gas.

The IGCC (Integrated Gasification Combined Cycle) uses air as the gasifying agent and requires modified turbine combustors to handle the low heating-value gas (110 to 190 Btu/ft³). The low calorific syngas up to 135 Btu/ft³ is only suitable for firing in the boiler or use in diesel engines.

Indirectly heated gasifiers in general generate syngas with a high percentage of hydrogen, methane, and C₂₊ compounds which directly contribute for the higher heating value of the syngas.

Similarly for the directly fired gasifiers, the higher range of hydrogen, methane, and C₂₊ components corresponds to the combustion using oxygen, leading to higher calorific value of the syngas. The absence of nitrogen (inert) in the system largely helps to concentrate the composition of syngas with higher heating value components. Production of tars, chars, and volatile alkalis are problems associated with gasification which requires further cleaning before it is utilized in turbines for power generation.

The product stream at high pressure and temperature needs to be cleaned under hot conditions not lower than 540°C (1005°F) (tar dew point) in order to maximize the energy conversion efficiency. Thus, a hot cleanup system is required, for which either catalytic tar crackers or a thermal tar cracker could be utilized. A catalytic tar reformer will operate at temperatures comparable to gasifier temperature of 825°C (1,515°F), while a thermal cracker will typically operate at 870 to 980°C (1600 to 1,795°F). After the tar reformer/cracker, the product gas will be partially cooled to minimize the amount of alkali vapors, typically to 350 to 650°C. The product will then pass through a filter to remove solids. As gas turbine application limits the alkali to less than 25 ppb, much of alkali as well needs to be removed.

See also: Synthesis Gas Quality.