Chemistry of the Conversion of Biomass to Syngas
Synthesis gas, or syngas, is a gas mixture that contains hydrogen, carbon monoxide, and often some carbon dioxide as the major components. The term synthesis gas was given to this gas mixture since it can be used as a feedstock for the synthesis of various types of fuels and chemicals. This type of gas mixture can be prepared by multiple routes, which includes steam reforming of natural gas, liquid hydrocarbons, and gasification of coal or lignocellulosic biomass. Syngas is combustible, can be used directly as a fuel in internal combustion engines and gas turbines, or as an intermediate for the production of liquid fuels and other chemicals. A comprehensive overview of syngas production from biomass is found in two review papers published in 2009 [4,5]. Biomass gasification is the pyrolysis process which converts lignocellulosic biomass into a synthesis gas at high temperature. The heat required for heating the biomass and for the endothermic gasification reactions is supplied by the combustion of part of the biomass; this process is generally known as direct gasification. The other technique is to supply the heat from an external source, which is known as indirect or allothermal gasification. Lignocellulosic biomass is first chipped into smaller pieces to feed the reactor, and this will provide higher surface area and faster reaction rates.
Biomass material undergoes several different complex chemical processes during gasification. These processes in increasing order of temperature can be outlined as follows:
1. Dehydration — Typically, the moisture content of biomass feed ranges from 5% to 35%. Dehydration occurs at around 100°C, resulting in the loss of adsorbed water from the biomass. The moisture content in the biomass is reduced to below 5% at this initial stage.
The resulting steam is mixed into the gas flow and may be involved with subsequent chemical reactions.
2. Pyrolysis — This process occurs at around 200-300°C. During this step biomass undergoes a thermal decomposition in the absence of oxygen or air, and volatile matter in the biomass is reduced at this stage. This results in the release of hydrocarbon gases, reducing the biomass to solid charcoal. These hydrocarbon gases can condense at a sufficiently low temperature to generate liquid tars.
3. Combustion — In this step C, H, O in biomass and some of the char produced in the pyrolysis stage reacts with oxygen to produce carbon dioxide, water and carbon monoxide. The proportion of CO to CO2 produced depends on the amount of oxygen available. This is a reaction between solid carbonized biomass and oxygen in the air, resulting in formation of CO2. Hydrogen present in the biomass is also oxidized to generate water. A large amount of heat is released with the oxidation of carbon and hydrogen. If oxygen is present in sub-stoichiometric quantities, partial oxidation of carbon may occur, resulting in the generation of carbon
monoxide. Combustion or biomass oxidation reaction can be summarized by the following equation:
C, H,O + O2 ^ CO2 + H2O + CO (11.1)
4. Reduction — A number of reduction reactions take place at this stage in the absence (or sub-stoichiometric presence) of oxygen. The reactions which occur in the 800-1000°C temperature range are mostly endothermic, and the key reactions in this category are summarized below.
The gasification process occurs as the char reacts with steam to produce carbon monoxide and hydrogen.
C + H2O ^ H2 + CO — 131.4 kJ/mol (11.2)
Bounded reaction:
C + CO2 ^ 2CO — 172.6 kJ/mol (11.3)
Water gas shift reaction is a very important reversible reaction that occurs at high temperatures. This reaction reaches equilibrium very fast at high temperatures in a gasifier and controls the balance of carbon monoxide, steam, carbon dioxide and hydrogen.
Water gas shift reaction:
CO + H2O ^ CO2 + H2 — 42 kJ/mol (11.4)
Methane reaction:
C + 2H2 ^ CH4 — 75 kJ/mol (11.5)