Thermal Biomass Conversion

Another cause of activity loss is carbon deposition, which can be avoided if a high steam to carbon (S/C) ratio is employed [45, 46], However, economic evaluations indicate that the optimum S/ C ratio tends to be low. The presence of tars in the reforming reactor enhances coking and it is the main cause of carbon formation in reforming a gas from biomass thermal conversion [29]. 

TABLE 12-4. Biomass Thermal Conversion Process Comparison... 

Della Rocca, P.A. (1998) Study on biomass thermal conversion processes. DPhil thesis, Universidad de Buenos Aires. 

U.S. capacity for producing biofuels manufactured by biological or thermal conversion of biomass must be dramatically increased to approach the potential contributions based on biomass availabiUty. For example, an incremental EJ per year of methane requires about 210 times the biological methane production capacity that now exists, and an incremental EJ per year of fuel ethanol requires about 14 times existing ethanol fermentation plant capacity. 

J. L. Jones and S. B. Radding, eds.. Thermal Conversion of Solid Wastes and Biomass, ACS Symposium Series 130, American Chemical Society, Washington,... 

Thermal conversion of organic waste material, such as plastics, or of biomass under the influence of oxygen into crude synthesis gas yields a hydrogen gas. 

Uncertainties with the availability and suitability of biomass resources for energy production are primarily due to their varying moisture content, and to a lesser degree to their chemical composition and heating value. As the moisture content of biomass increases, the efficiency of thermal conversion process decreases. At some point more energy may have to be expended to dry the biomass than it contains. Uncertainties can be reduced by conducting a detailed chemical and physical analysis of the biomass sources. 

Stassen, H.E.M., Prins, W., Van Swaaij, W.P.M., Thermal conversion of biomass into secondary products the case of... 

Ensuring a constant feed supply is very important, because most of the biomass is only available on a seasonal basis. In such cases continuous operation of the conversion facility will require either extensive long-term storage of the feedstock or a feed reactor that is flexible enough to accommodate multiple feedstocks. Most thermal conversion processes demand a finely divided, substantially dry feed and therefore some pre-treatment is required to match the feedstock to the process. The main pre-treatment operations are [19] ... 

Gort R., On the Propagation of a Reaction Front in a Packed Bed - Thermal conversion of municipal solid waste and biomass, PhD Thesis, University Twente in Enschede, Dept of Environmental Sciences, Energy Research, and Process Innovation of TNO, the Netherlands, (1995). 

Converting biomass into fuel grade products such as gases, alcohols, and oils can be accomplished by processes such as fermentation, anaerobic digestion, thermal conversion, and oil extraction. The fuel grade products produced from these processes can be utilized directly as alternative fuel or, if appropriate, serve as components for blending into existing petroleum-based fuels. 

Pyrolysis, gasification, and combustion are typical thermal conversion processes for biomass. These processes are compared and contrasted in TABLE 12-4. 

In principle, biomass resources can be converted using any of the biochemical or thermal conversion processes. 

The first case covers for example flue-gas treatment, which requires the filtration of fly-ash and the reduction of NOx, or gasification processes, where particulates and high-boiling tars have to be removed. An example of the second case is that of combustion processes, where incomplete combustion leads to the emission of carbonaceous particulates. The most relevant topic in this category is the reduction of diesel particulate emissions ( diesel soot ) by catalytic filtration. A more exotic example is the reaction cyclone for the thermal conversion of biomass, which also combines chemical reactions and separation in one apparatus, though its separation mechanism is not filtration. 

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. 

Figure 2.3.2 Possible routes for thermal conversion of biomass. Temperature is given in °C.

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