Gasification

• Organic materials go through Pyrolysis first***

• Liquid – 5%, Char – 10%, Gas – 85% (Mayhead)*

• Thermochemical decomposition*

• The limited presence of oxygen indicates gasification*

• Can accept any organic materials – dry and uniformly sized material is preferable*

• Equipment needed can be modular, so can be flexible siting in rural areas*

• Breaks material down into gas of mostly carbon monoixide and hydrogen. The feedstock is partially oxidized by introduction of air, which produces a syngas. The resulting syngas can be utilized in a similar fashion to the syngas that is resultant from pyrolysis*

• Low temperature gasification is around 700-1000 degrees C

    Product gas will have relatively high levels of hydrocarbons compared to high temp gasification.***

    It may be used directly to be burned for heat or electrical generation via steam turbine or IC engine for electrical generation if gas is cleaned properly***

• High temperature gasification is around 1200-1600 degrees C

    Fewer hydrocarbons and more CO and H2
    This is known as syngas and can be used to formulate other products***

Three different types

    Fixed Bed*
    Feedstock is placed on a static grate and heated while a gasification agent, such as nitrogen, is circulated through the gasifier. Variations of fixed bed gasification technologies employ different configurations of pressure and gas-flow direction.

    Examples include:*
    • downdraft co-current fixed bed gasifier
    • updraft co-current fixed bed gasifier
    • updraft counter-current fixed bed gasifier
    • cross-draft fixed bed gasifier
    • open core fixed bed gasifier

    Fluidized Bed*
    Feedstock is heated and suspended directly in the gasification agent, without a static grate. Fluidized bed gasification technologies may employ different configurations of pressure and gas flow.

    Examples include:*
    • pressurized circulating fluidized bed gasifier
    • atmospheric circulating fluidized bed gasifier

    Novel Designs*
    A category of miscellaneous gasifiers, including the following processes:
    • plasma arc gasification ie Adaptive Arc
    • 2-stage gasification
    • open-top gasification
    • aqueous phase reforming gasification

• Gasification is a thermochemical process that converts biomass into a combustible gas called producer gas. Producer gas contains carbon monoxide, hydrogen, water vapor, carbon dioxide, tar vapor and ash particles. Gasification produces a low-Btu or medium-Btu gas, depending on the process used.**

• Producer gas contains 70 percent to 80 percent of the energy originally present in the biomass feedstock. The gas can be burned directly for space heat or drying, or it can be burned in a boiler to produce steam. Medium-Btu producer gas can be converted into methanol, a liquid fuel. Electric power generation is possible by combining a gasifier with a gas turbine or fuel cell.**

• Filters and gas-scrubbers remove tars and particulate matter from producer gas. The clean gas is suitable for use in an internal combustion engine, gas turbine or other application requiring a high-quality gas. Use of producer gas in a fuel cell requires reforming clean gas into hydrogen ions and carbon monoxide. Fuel cells produce electricity and thermal energy from hydrogen through an electrochemical conversion process.**

• Gasification technology is in the development stage. There are a few demonstration projects that use varied gasifier designs and plant configurations. However, pretreatment of biomass feedstock is generally the first step in gasification. Pretreatment involves drying, pulverizing and screening. Optimal gasification requires dry fuels of uniform size, with moisture content no higher than 15 percent to 20 percent.**

• Biomass gasification is a two-stage process. In the first stage, called pyrolysis, heat vaporizes the volatile components of biomass in the absence of air at temperatures ranging from 450° to 600° C (842° to 1112° F). Pyrolysis vapor consists of carbon monoxide, hydrogen, methane, volatile tars, carbon dioxide and water. The residue, about 10 percent to 25 percent of the original fuel mass, is charcoal.**

• The final stage of gasification is called char conversion. This occurs at temperatures of 700° to 1200° C (1292° to 2192° F). The charcoal residue from the pyrolysis stage reacts with oxygen, producing carbon monoxide.**

• In the process of combustion, both stages of gasification occur. When wood burns, the heat of combustion produces pyrolytic vapors. Some gasification of these vapors also occurs. In combustion, however, the pyrolytic vapors are immediately burned at temperatures in the range of 1500° to 2000° C. In contrast, the process of gasification is controlled, allowing the volatile gases to be extracted at a lower temperature before combustion.**

• There are three principal types of gasification systems: updraft, downdraft and fluidized-bed. In an updraft (or "counterflow") gasifier, the biomass fuel enters the top of the reaction chamber while steam and air (or oxygen) enter from below a grate. The fuel flows downward, and upflowing hot gases pyrolyze it. Some of the resulting charcoal residue falls to the grate, where it burns, producing heat and giving off carbon dioxide (CO2) and water vapor (H2O). The CO2 and H2O react with other charcoal particles, producing carbon monoxide (CO) and hydrogen (H2) gases. The gases exit from the top of the chamber. Ashes fall through the grate.**

• The updraft design is relatively simple and can handle biomass fuels with high ash and moisture content. However, the producer gas contains 10 percent to 20 percent volatile oils (tar), making the gas unsuitable for use in engines or gas turbines.**

• Successful operation of a downdraft (or "co-flow") gasifier requires drying the biomass fuel to a moisture content of less than 20 percent. Fuel and air (or oxygen) enter the top of the reaction chamber. Downflowing fuel particles ignite, burning intensely and leaving a charcoal residue. The charcoal (which is about 5 to 15 percent of the original fuel mass) then reacts with the combustion gases, producing CO and H2 gases. These gases flow down and exit from the chamber below a grate. The producer gas leaving the gasifier is at a high temperature (around 700° C). Combustion ash falls through the grate. The advantage of the downdraft design is the very low tar content of the producer gas.**

• A fluidized-bed gasifier typically contains a bed of inert granular particles (usually silica or ceramic). Biomass fuel, reduced to particle size, enters at the bottom of the gasification chamber. A high velocity flow of air from below forces the fuel upward through the bed of heated particles. The heated bed is at a temperature sufficient to partially burn and gasify the fuel. The processes of pyrolysis and char conversion occur throughout the bed. Although fluidized-bed gasifiers can handle a wider range of biomass fuels, the fuel particles must be less than 10 centimeters in length and must have no more than 65-percent moisture content. The fluidized-bed design produces a gas with low tar content but a higher level of particulate compared with fixed-bed designs.**

• If the gasifier is pressurized, it produces gas at a pressure suitable for electric power generation using a gas turbine. High-pressure fuel-feed systems are in the development stage. Hot gas cleanup technology is also under development. Hot gas cleanup removes tars, chars and volatile alkalis to improve system efficiency.**

• Progress in the development of biomass-fired gas turbine technology may include combined-cycle electricity generation. In a combined-cycle facility, a gas-fired turbine generator produces primary power. Waste heat from the turbine exhaust is used to produce high-pressure steam, which then drives a steam turbine to generate secondary power.**


References Cited:
* http://www.epa.gov/sustainability/pdfs/Biomass%20Conversion.pdf
** http://www.oregon.gov/ENERGY/RENEW/Biomass/bioenergy.shtml
*** http://www.biomassenergycentre.org.uk/