Pyrolytic Gasification

The primary products of biomass pyrolysis under conventional pyrolysis condi­tions are gas, oil, char, and water. As the reaction temperature increases, gas yields increase. It is important to note that pyrolysis may involve green or predried biomass, and that product water is formed in both cases. Water is released as the biomass dries in the gasifier and is also a product of the chemical reactions that occur, even with bone-dry biomass. Unless it is rapidly removed from the reactor, this water would be expected to participate in the process along with any added feedwater or steam. As will be shown later, the exothermic heat from the steam gasification of woody biomass under certain conditions appears to be sufficient to eliminate the need for an external heat source or the use of oxygen. Self-sustained steam gasification can effectively be carried out with biomass feedstocks, according to some investigators.

One of the more innovative pyrolytic gasification processes is an indirectly heated, fluid-bed system (cf. Alpert et al, 1972; Bailie, 1981; Paisley, Feldmann, and Appelbaum, 1984). This system uses two fluid-bed reactors containing sand as a heat transfer medium. Combustion of char formed in the pyrolysis reactor takes place with air within the combustion reactor. The heat released supplies the energy for pyrolysis of the combustible fraction in the pyrolysis reactor. Heat transfer is accomplished by flow of hot sand from the combustion reactor at 950°C to the pyrolysis reactor at 800°C and return of the sand to the combustion reactor (Fig. 9.5). This configuration separates the combustion

and pyrolysis reactions and keeps the nitrogen in air separated from the pyrolysis gas. It yields a pyrolysis gas that can be upgraded to a high-energy gas (substitute natural gas, SNG) by shifting, scrubbing, and methanating without regard to nitrogen separation. The pyrolysis gas with hybrid poplar feedstocks typically contains about 38 mol % carbon monoxide, 15 mol % carbon dioxide, 15 mol % methane, 26 mol % hydrogen, and 6 mol % C2’s. This is a medium-energy gas having a higher heating value of about 19.4 MJ/ m3 (n). The projected gas yields are about 670 m3 (n) of pyrolysis gas, or about 200 m3 (n) of methane per dry ton of feed if SNG is produced.