Hot gas cleaning

Integrated gasification combined cycle (IGCC) is a promising technology which is expected to contribute a considerable share of electricity in the next years. In theses systems, fuel gas is generated in a gasifier, purified in a gas cleanup system and sent to a gas turbine where it is burned. An important loss of energy is produced with the present cold desulfurisation systems which reduce the temperature below 300 °C with wet techniques or below 700 °C with regenerable sorbents. In situ desulfurization  by direct injection of calcium-based sorbents at high temperatures (800-1100 °C) would make a substantial contribution to the improvement of the thermal efficiency of electrical power generation in IGCC systems. The aims of group investigation are to analyse the sulphur capture effectiveness of different natural Ca-based sorbents (limestones, dolomites, calcium hydroxides) under atmospheric or pressurised gasification conditions, and to study the kinetics of sulphidation of Ca-based sorbents under calcining or non-calcining conditions. To improve the retention process, modified calcium hydroxides and novel sorbents as calcium acetate and calcium magnesium acetate, are also used. The different Ca-based sorbents for H2S removal are tested in thermogravimetric analysers, fluidised bed reactor, moving bed reactors and in a drop tube reactor. The direct combustion of coals with high sulphur contents, such as Spanish lignites, is problematic due to the high SO2 emissions. These SO2 emissions can be reduced (lowered) by addition of a Ca-based sorbent, like limestone or dolomite, into the boiler. So, the use of calcium based sorbents in pulverised coal boilers and fluidised bed combustors has been studied for our group. The injection of calcium based sorbents into the post-flame zone of a boiler for the purpose of controlling SO2 emissions has been recently selected as the lowest cost option to retrofit old power plants burning coals of low sulphur content. When a Ca-based sorbent is introduced in a pulverised coal boiler decomposes and the result is a porous solid which reacts with SO2 and O2 to form CaSO4. The sulphation of CaO is a thoroughly studied heterogeneous reaction due to its technological importance in connection with desulphurisation. In our group, the sulfation process is studied in a drop-tube reactor system at temperatures from 950 to 1250°C, by using similar conditions to those existing in the sorbent injection in pulverized coal boilers. The main variables affecting the sulfation process are analysed: Ca/S molar ratio, SO2 concentration, temperature, sorbent particle size and reaction time. Fluidized bed coal combustion allows in-bed sulphur retention by sorbent addition. The Ca-based sorbent feedrate needed to reach a fixed sulphur retention varies greatly depending on the characteristics of the sorbent used. So, it is necessary to know the sorbent behaviour before using it in a combustor. The effect of the principal variables affecting the sulphur retention (calcination conditions, sorbent particle size, temperature and SO2 concentration) are analysed in a thermogravimetric analyser and in a fluidized bed. In addition, mathematical models for the simulation and optimization of the sulphur retention in bubbling and circulating fluidised bed combustor have been developed. imagen-para-texto-linea-limpeza-caliente