Further Developments
One current program is addressing some of these issues. It includes the use of membrane technology for bulk desulfurization, zinc oxide or similar chemisorption process for fine sulfur removal, a sodium carbonate-based sorbent for HC1 removal, and a high-temperature molecular sieve for ammonia removal (Gupta 2001). However, the use of polymer membrane technology is likely to limit its ability to operate at high temperature and thus any efficiency gains on this basis. Mercury capture is not specifically mentioned as being part of this program, although on the basis of currently published flowsheets it could possibly be incorporated as a separate stage.
Treating the syngas generated from biomass has special problems—particularly those associated with the presence of tars in the gas. Attempts have been made to reduce the tar content by cracking (Morris and Waldheim 2002; Bajohr etal. 2002). Other attempts have been made at using an oil wash (Boerrigter, den Uil, and Calis 2002; Hofbauer 2002). To date, success has been limited to achieving a quality suitable for power applications. Considerable work is still required to achieve a chemicals application syngas quality.
Besides having an important influence on the composition of the raw syngas from the gasifier itself, the CO shift reaction
C0 + H20^C02 + H2 -41M J/kmol (2-7)
can be and is operated as an additional and separate process from the gasifier at much lower temperatures in order to modify the H2/CO ratio of the syngas or maximize the total hydrogen production from the unit. As can be seen from the reaction 2-7, one mole of hydrogen can be produced from every mole of CO. The reaction itself is equimolar and is therefore largely independent of pressure. The equilibrium for hydrogen production is favored by low temperature.
The CO shift reaction will operate with a variety of catalysts between 200°C to 500°C. The types of catalyst are distinguished by their temperature range of operation and the quality (sulfur content) of the syngas to be treated.