Encyclopedia of Renewable Energy. James G. Speight
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the biomass feedstock.
This gaseous product is usually a low-to-medium Btu gas (producer gas) which can be used to run internal combustion engines (both compression and spark ignition), can be used as substitute for furnace oil in direct heat applications, and can be used to produce, in an economically viable way, methanol – an extremely attractive chemical which is useful both as fuel for heat engines as well as chemical feedstock for industry.
Since any biomass material can undergo gasification, this process is much more attractive than ethanol production or biogas where only selected biomass materials can produce the fuel.
The preliminary gasification of biomass in (preferably) pressurized circulating fluidized bed reactor with secondary processing of the obtained product gas in a slagging entrained flow gasifier is another option. Usually, little or no pretreatment of the biomass is needed (particle size up to 5 cm is acceptable) and that a wide range of biomass feedstock can be processed.
Similar to pyrolysis, the availability of some bed material in the product gas, which is fed into the entrained flow gasifier, is not a concern either, as it improves the properties of molten slag.
Maintaining the stability of the feed flow necessary for the safe operation of entrained flow gasifiers could be a problem, because circulating fluidized bed gasifiers are characterized by some variations in the product gas flow. In order to keep the amount of nitrogen in the product gas under control, the first step circulating fluidized bed gasification is performed with steam, not with air.
After being pyrolyzed in a low-temperature gasifier, biomass pyrolysis gas and char are fed to an entrained flow gasifier and a tar-free gas with high content of carbon monoxide and hydrogen is obtained. The clean gas is cooled down to approximately 200°C (390°F) in a heat exchanger, increasing thereby the overall energy efficiency of the process by producing high-quality steam for power and/or heat generation. Next, the gas is cleaned from dust particles (in a de-duster) and from components, other than carbon monoxide and hydrogen (in a washer). At the end, clean synthesis gas, consisting of carbon monoxide, is obtained. Sufficient gas cleaning represents a key point in syngas and liquids production.
The catalysts for the synthesis of liquid fuels and chemicals are easily poisoned even by small amounts of alkali metals, halides, sulfur compounds, and carbon dioxide which therefore have to be removed to ppm and even ppb levels.
Besides torrefaction, pyrolysis, and pre-gasification, there is a fourth option to convert biomass into a semi-finished material. This is the hydrothermal upgrading process (HTU process), which, under certain conditions, could be considered also as a pre-treatment alternative for biomass conversion.
See also: Biomass – Gasification, Carbo-V Process, Hydrothermal Upgrading Process, Torrefaction.
Biomass – Gasification Reactors
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