Gasification, thermal biomass feedstocks

As alluded to in Chapter 8, the ideal biomass feedstock for thermal conversion, whether it be combustion, gasification, or a combination of both, is one that contains low or zero levels of elements such as nitrogen, sulfur, or chlorine, which can form undesirable pollutants and acids that cause corrosion, and no mineral elements that can form inorganic ash and particulates. Ash formation, especially from alkali metals such as potassium and sodium, can lead to fouling of heat exchange surfaces and erosion of turbine blades, in the case of power production systems that use gas turbines, and cause efficiency losses and plant upsets. In addition to undesirable emissions that form acids (SOx), sulfur can... 

Hofbauer, H., et al., (1997) Gasification Feedstock Database. lEA Bioenergy, Task XU Thermal Gasification of Biomass, Vienna Institute of Technology, Vienna. 

Gasification coupled with water-gas shift is the most widely practiced process route for biomass to hydrogen. Thermal, steam, and partial oxidation gasification technologies are under development. Feedstocks include both dedicated crops and agricultural and forest product residues of hardwood, softwood, and herbaceous species. 

Most biomass gasification systems utilize air or oxygen in partial oxidation or combustion processes. These processes suffer from low thermal efficiencies and low Btu gas because of the energy required to evaporate the moisture typically inherent in the biomass and the oxidation of a portion of the feedstock to produce this energy. 

Thermal conversion involves the use of elevated temperature with or without the presence of oxygen to break down the structure of the feedstock. It includes torrefac-tion, pyrolysis, gasification, and combustion. Thermal conversion of biomass can also be carried out in a solvent (e.g. as in hydrothermal processing) [1], but in this chapter, only torrefaction fast pyrolysis gasification with air, oxygen, or steam and combustion in air will be considered. 

Biomass is similar to some coals with respect to total ash content as discussed in Chapter 3, but because of the diversity of biomass, several species and types have relatively low ash and also low sulfur contents. Woody biomass is one of the feedstocks of choice for thermal gasification processes. The ash contents are low compared to those of coal, and the sulfur contents are the lowest of almost all biomass species. Grasses and straws are relatively high in ash content compared to most other terrestrial biomass, and when used as feedstocks for thermal conversion systems, such biomass has been found to cause a few fouling problems. 

While the topic of this paper is Canada s PTGL (Pyrolysis Thermal Gasification and Liquefaction) research program it is useful to review the current status of conversion technologies for biomass. The goal is to describe the characteristics of each technology so that efficiencies, process steps and environmental factors are well quantified and under these circumstances for a known cost and nature of feedstock an economic or social decision can be taken as to whether or not to implement the technology. 

In recent years, several methods for hydrogen production starting from biomass materials have been investigated [60-62], and great efforts have been addressed in particular in selecting advanced solutions for optimization of the previously analysed thermal processes, such as SR or gasification, by substituting the fossil fuel feedstocks (coal or petroleum-derived fuels) with different types of biomass-derived fuels. 

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