Thermal Gasification of Biomass
Gasification is a complete thermal breakdown of the biomass particles into a combustible gas, volatiles and ash in an enclosed reactor (gasifier) in the presence of any externally supplied oxidizing agent (air, O2, H2O, CO2, etc.) when equivalent ratio (ER) is < 1. ER = 1 if the stoichiometric amount of oxidising agent is present. Stoichiometric amount is the theoretical amount of air or any other oxidizing agent required to burn the fuel completely.
Gasification is an intermediate step between pyrolysis and combustion. It is a two-step, endothermic process. During the first step the volatile components of the fuel are vaporized at temperatures below 600°C by a set of complex reactions. No oxygen is needed in this phase of the process.
Hydrocarbon gases, hydrogen, carbon monoxide, carbon dioxide, tar and water vapour are included in the volatile vapours. Char (fixed carbon) and ash are the by-products of the process which are not vaporized. In the second step, char is gasified through the reactions with oxygen, steam and hydrogen. Some of the unburned char is combusted to release the heat needed for the endothermic gasification reactions.
Main gasification products are gas, char, and tars. Gasification products, their composition and amount are strongly influenced by gasification agent, temperature, pressure, heating rate and fuel characteristics (composition, water content, granulometry). Gaseous products formed during the gasification may be further used for heating or electricity production. The main gas components are CO, CO2, H2O, H2, CH4 and other hydrocarbons.
Combustible gas, produced during gasification can be cleaned and used for the synthesis of special chemical products or for the generation of heat and/or electricity. Specific hydrogen – carbon monoxide mixtures (because of the production method or the final use) have the designation water gas, cracked gas, and methanol synthesis gas or oxo-synthesis gas.
The fundamental reactions of the synthesis gas chemistry are the methanol synthesis, Fischer – Tropsch synthesis, oxo-synthesis (hydroformylation) and methane synthesis; further synthesis gas reactions are being developed.
Gasification has the unique characteristic of being such a technology that can even convert waste (from MSW to agricultural or crop residues, like coconut shells, rice husks, straw, wood residues, bagasse, etc.) to a useful and high quality energy source. It is known how complicated the disposal of any kind of waste is, nowadays due to environmental regulations and legislations. Gasification gives the advantage of separating the noxious substances from the fuel gas prior to the combustion.