Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 221

The properties of biomass that have a significant bearing on its thermal conversion are its relatively high moisture, oxygen, hydrogen, and volatile matter content, and low heating value. The high oxygen and hydrogen contents account for the high proportion of volatile matter and consequent high yields of gases and liquids on pyrolysis. A relatively high water yield results from the high oxygen concentration in biomass, and which consumes considerable hydrogen. Consequently, the advantages of the high H/C ratio associated with biomass are not reflected in the products to the extent that might be expected. In fact, pyrolysis gases can be deficient in pure hydrogen and pyrolysis liquids are highly oxygenated, viscous tars.

An additional and significant source of water vapor in biomass gases is the high moisture content of the source materials. In countercurrent flow schemes such as the Lurgi moving bed gasifier, this water is evolved in the relatively low temperature drying and pyrolysis zones and does not partake in gas phase or carbon-steam gasification reactions.

On the other hand, in fluidized bed systems, the moisture is evolved in the high- temperature well-mixed reaction zone and therefore does participate in the reactions. If the system is directly heated and air-blown, the additional heat required to evaporate the water will result in more nitrogen being introduced, and more carbon dioxide being produced, so reducing the calorific value of the product gas. As the gas from air-blown processes is, in any case, a low-calorific value product, this factor is probably of little consequence other than with very wet feedstock. In oxygen-blown systems, however, the additional pure oxygen required and higher carbon dioxide content of the medium calorific value off-gas may be of sufficient impact to dictate some degree of drying as a pretreatment.

Apart from drying, additional beneficiation may be undertaken to yield a resource of higher energy density. These operations will normally be undertaken at the source, so transport and subsequent storage costs may be reduced as well. Beneficiation steps include size reduction and densification. Waste heat, if available, may be used for drying, while size reduction and compression to form pellets or briquettes is estimated to require less than 2% of the energy in the dry biomass. Nevertheless, these operations are time consuming, and can be either labor or capital intensive.

Some advantages of biomass over conventional fossil fuels are the low sulfur content and highly reactive char. In addition, biomass materials do not cake and can therefore be easily handled in both fluidized and moving bed reactors. Finally, catalyst poisons are not present in biomass in significant concentrations. This can be important for the initial thermal processing as well as for subsequent upgrading operations.