Learn to Run Your Vehicle on Wood Chips with Biomass Gasification
Department of Agricultural and Biological Engineering
The gasification of biomass is a process in which heat is applied to the feedstock in a low oxygen environment to convert the solid fuel into a gaseous mixture of carbon monoxide (CO) and hydrogen (H2). This process produces a low to medium-Btu gas (4-10 MJ/m3) which can be used to run gas powered devices for heat generation as well as internal combustion engines, gas turbines, and fuel cells. The gas derived from this process can contain up to 90% of the energy of the initial feedstock.
Gasification can proceed from any carbonaceous material with a presently common fuel source being coal. However, using biomass as the feedstock for the gasification process allows for a carbon neutral cycle meaning that it will not have a net affect on the existing greenhouse gas concentrations. Common sources of biomass feedstock for gasification include forest residues, agricultural waste, and municipal solid waste.
The six step process of biomass gasification is as follows:
The pretreatment process for biomass feedstock constitutes the steps that must be imposed on the raw material in preparation for the use in a gasification reactor. Raw material typically requires an initial drying before pulverization and screening to the desired size.
Biomass enters the gasification reactor into the drying zone whereby heat from the combustion section promotes evaporation of any remaining moisture in the feedstock. This section typically ranges in temperature from 70-200°C.
As the biomass nears the combustion region, the temperature increases until it reaches the point at which pyrolysis begins. Pyrolysis is the vaporization of the volatile components in the fuel caused by the high temperatures and absence of oxygen. Temperatures in this section typically range from 300-600°C. Pyrolysis drives tar and gases such as CO, H2, CH4, CO2, H20 and other light hydrocarbons from the solid fuel only leaving behind char.
Some of the combustible gas (CO, H2, CH4) and char generated from pyrolysis are oxidized in the combustion zone where a controlled amount of air is allowed to enter the reactor. The exothermic reactions of oxidation in this section provide the heat required for the endothermic reactions in the drying, pyrolysis, and reduction zones. This section typically ranges from 700-1200°C. The product gases from this section are CO2 and H2O.
In the reduction zone, char that has made it through the combustion zone from the pyrolysis zone is reacted with the CO2 and H2O from the previous steps. The CO2 and H2O react with the carbon in the char which causes these gases to reduce to CO, H2, and CH4 leaving behind the minerals of the char as ash Gas then exits the reactor typically in the range of 200-300°C.
Gas leaving the reactor contains ash and tar vapor. It is necessary to remove these contaminants using filters and scrubbers as well as cool the gas before it can be utilized in an internal combustion engine, gas turbine, or fuel cell.
Some benefits of biomass gasification include the ability to use waste products as fuel as well as the conversion of energy locked in solids to gas phase which then allow for the increased transportability of energy as gas. These benefits are can be realized all while biomass gasification is a carbon neutral process that can potentially be more efficient than direct combustion without the significant production of NOx compounds.
Current limitations of biomass gasification include a low heating value of product gases, a product gas contaminated with tar and particulate matter, a high energy demand for feedstock pretreatment, and not being a commercially widespread technology due to high capital and operational cost.