Biomass gasification is an endothermic thermal process where solid biomass is converted into gaseous fuel. The gaseous fuel which results from biomass gasification process is called synthetic gas (syngas).
The produced fuel is a mixture of many gases like carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), methane (CH4), vapour (H2O), hydrocarbons (e.g. C2H6, C2H4) and nitrogen (N2). The last one is formed only when atmospheric air is used for the process and not when pure oxygen is used. Apart from the abovementioned compounds, various traces of pollutants may be found in the produced gas fuel (e.g. tar, ash, ammonia, acids, complex hydrocarbons). The gaseous fuel which results from biomass gasification process is called synthetic gas (syngas). When the process is made with atmospheric air utilization (the most common and less expensive choice), syngas has a net heating value of 4,6 MJ/m3, which is about 1/7 compared to natural gas. But in case that pure oxygen is used this value may be increased even three times. In both cases, whether is used atmospheric air or pure oxygen, syngas is appropriate to be burned as a fuel in combined heat and power system, in proper gas burners and turbines.
From a chemical point of view, biomass gasification process is quite complex and consists of the following main stages: organic biomass decomposition into a non condensable gas, vapour and tar, thermal cracking of gases into synthesis gas and tar, tar gasification and partial oxidation of syngas, vapour and tar. The required heat demands for biomass gasification are covered by the combustion of a part from the initial biomass feedstock material.
A crucial parameter in the biomass gasification process is the kind of biomass feedstock. Depending on its derivation it may has properties which differ significantly between each other, thus resulting in the process technological profile and the whole power plant viability. The properties which are in the epicenter of attention are the moisture content, the ash content, the elementary analysis, the heating value, the density and the size.
Regarding to the type and the design of the gasification reactor (or the “gasifier”), the number variations and categories, after a lot of decades research, is large. For example, gasifiers can be divided according to their gasification media (air, oxygen or steam), the way of required heat supply (autothermic or allothermic gasifiers), the operation pressure (atmospheric or under pressure reactors) and their design (fixed or fluidized bed).
It must be noticed that syngas shouldn’t be directly used in the power production engines. A pretreatment step is needed in order to cool the gas mixture and reduce the amounts of pollutants in it (e.g. tar, sulfur). Apart from syngas, the process produces also tar. Their proportion depends on various parameters like e.g. the type of biomass. Taking into account its high heating value, the best available practice of tar management is its energy exploitation inside the gasification plant.
Undoubtedly biomass gasification is a more complex process and with less commercial references than other conventional biomass to energy practices, like combustion. Nevertheless the benefits that arise from the process, mainly the higher energy efficiency, have led to a constant increase in the number of such ‘state of the art’ plants. To verify that progress, a CHP plant, with biomass gasification in Yamagata, Japan was awarded as the best renewable energy power plant, at the famous congress Power Gen Asia, in 2008. This plant has an installed capacity of 2 MWe and processes 60 tn of wood chips feedstock on a daily basis.
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