Gasification of Biomass to Second Generation Biofuels: A Review

ABSTRACT Biomass gasification has gained significant attention in the last couple of decades for the production of heat, power and second generation biofuels. Biomass gasification processes are highly complex due to the large number of reactions involved in the overall process as well as the high sensitivity of the process to changes in the operational conditions. This report reviews the state-of-the-art of biomass gasification by evaluating key process parameters such as gasifying agent, temperature, pressure, particle size, etc., for fluidized bed and entrained flow gasifiers. The pros and cons of each technology and the remaining bottlenecks are also addressed. INTRODUCTION Biomass, the renewable source which stores energy in molecular carbon bond structures, is bound to play an important role in the current challenging energy scenario to provide the energy required to meet the continuous increase in energy demand and to mitigate climate change [1-2]. The large flexibility of biomass as a feedstock has been widely recognized as, besides heat and power, it can be converted into chemicals and transportation fuels. Biofuels can be used in recent infrastructures more or less directly, while other technologies, such as fuel cells and batteries, require changes in infrastructure and thus are considered as long-term solutions. Second generation biofuels can be grouped into biochemically or thermo-chemically produced, either route using non-food crops, purpose-grown perennial grasses, trees or residues. Among the different available thermo-chemical processes for the conversion of biomass to biofuels, gasification is perceived as one of the most attractive routes, as it converts feedstock very efficiently to the highest density fuels, i.e. synthetic, resulting in the most economical viable system [3]. The biomass gasification process produces synthesis gas through the chemical conversion of biomass under partial oxidation of the feedstock in reducing atmosphere in the presence of air, oxygen and/or steam [4]. The synthesis gas produced can be then converted to second generation biofuels. Various types of gasification reactor designs have been developed up to now. Fluidized bed and entrained flow gasifiers are currently the two main categories of gasification technologies for biofuels production. Fluid bed gasifiers operate below the biomass ash melting point in order to avoid fluid bed agglomeration and eventual collapse. This technology is attractive for its relatively low cost, ease of operation and good scale-up potential. However, it has associated relatively low energy efficiencies and poorer gas qualities; it requires intensive additional gas cleaning after the gasifier, namely tars handling and hydrocarbon reforming and is limited to small scale operations. On the other hand, entrained flow gasifiers operate above the melting point of the biomass ashes and produce a product gas that is essentially fully converted to synthesis gas with very low contents of residual tar components, resulting in high efficiencies and higher gas quality. However, the feeding is a challenge, it has higher investment and operating costs than fluidized beds and therefore it is only suitable for high capacities. Thus, although substantial progress has been achieved over the last years, none of the two technologies have become commercially available and therefore a significant amount of work is still needed in this field to enable the deployment of second generation biofuels production.