PYROLYSIS OF STRAW: THE FIRST STAGE FOR THE PRODUCTION OF CHEMICALS AND FUELS FROM FAST GROWING BIOMASS; BASIC STUDIES

摘要

Biomass is expected to substitute gradually a part of the fossil fuels. The EU intends to double the biomass share in its energy mix until about 2010. Firewood contributes most, but biomass residues from agriculture, mainly surplus straw, can make a smaller but significant ca. 5% contribution to the world primary energy supply. Straw and strawlike agricultural residues, as all fast growing biomass, contain more ash, alkali and chlorine than wood and are more difficult to handle technically. Gasification is more flexible and superior to combustion; it allows a more economic coproduction of highly valuable chemicals, synfuel and electric power. Wood gasification in fixed or fluidised beds is commercial practice; gasification technologies for straw are not well developed. The common gasification technologies have been analysed and a new two-step pyrolysis/gasification concept for dry biomass has been developed, which considers the special straw properties. In the first stage thin-walled straw chops or any small biomass particles are pyrolysed by mixing with an excess of hot sand in a mixer reactor with two co-rotating screws. The sand is recycled in a closed loop via a sand heater. Small and dry particles with a characteristic length (= volume/surface-ratio) below 0.5 mm are rapidly pyrolysed in a few seconds. A large yield of condensable pyrolysis oil and little char and gas are obtained at 500 °C by fast vapour condensation. The char has a carbon content of ~ 65% and a heating value of ~24000 KJ/Kg. The low calorific pyrolysis gas can supply the energy for pyrolysis. A fraction of ≥ 20% weight of char powder can be suspended in the pyrolysis oil, to produce a pumpable slurry. Contrary to pyrolysis oils from fast or flash pyrolysis, the slurries are not used as a direct fuel for turbines or internal combustion engines. Therefore the purity requirements are low and permit some simplifications of the fast pyrolysis process. In the following stage from a number of regional pyrolysis plants the slurries are transported by rail to a large, complex central facility for syngas generation and use,. Slurry gasification at high temperature and pressure in a slagging entrained flow gasifier generates a tar-free syngas, simplifies gas cleaning and obviates gas compression prior to synthesis of methanol, CH-transport fuels or other chemicals.