Pressurised fluidised-bed gasification experiments with biomass, peat and coal at VTT in 1991-1994: Part 3. Gasification of Danish wheat straw and coal

Fluidised-bed air gasification of three different Danish straw feedstocks and Colombian bituminous coal was studied in the PDU-scale test facilities of VTT. The test programme was divided into two different modes of operation. Firstly, the usability of straw as the only feedstock was investigated by operating the gasifier at relatively low temperature normally used in biomass gasifiers. In this operation mode the main aim was to find out the limits for gasification temperatures, set by the sintering behaviour of the straw. Secondly, the use of straw as an additional feedstock in a fluidised-bed coal gasifier was examined by operating the gasifier at about 1 000 °C with different ratios of straw and coal feeding. The gasifier was operated at 5 bar pressure and at 800 - 990 °C. The product gas was cleaned by ceramic candle filters operated at 465 - 540 °C. Concen-trations of tars, nitrogen compounds, sulphur gases, vapour-phase alkali metals as well as chlorine were determined in different operating conditions. Total gasification of straw as the only feedstock in a pressurised fluidised-bed gasifier was very difficult due to the problematic sintering behaviour of straw ash. Carbon conversion of 90 - 92 % was reached for higher-quality straw under non-sintering operation conditions (i.e. below 850 °C). Cogasification of straw and coal did not result in sintering problems. The proportion of straw in the total fuel input was varied from 33 % to 54 % (wt%, dry ash-free basis), and Colombian bituminous coal was used as the additional feedstock. The applied gasification temperature of 970 - 990 °C turned out to be too low for this relatively unreactive hard coal. Thus, the carbon conversion effi-ciencies of the cogasification set points were lower than expected. A wide variety of different tar compounds was formed in low-temperature gasification of straw alone, and even at 850 °C benzene and heavier tars represented a significant part of the gas heating value and almost 5 % of the total carbon conversion. The concentrations of methane and light tars were somewhat higher in cogasification than in coal alone gasification. However, the concentration of heavy tars was almost negligible, indicating that the presence of coal particles in the bed and in the freeboard prevented the formation of high-molecular-mass tars, which may be problematic in biomass gasification. In straw gasification, 52 - 71 wt% of straw nitrogen was converted into ammonia, 4 - 14 % into HCN and less than 2 % into organic nitrogen containing compounds. The use of calcium-based bed additive under conditions where CaCO3 was calcined (at a sufficiently high temperature) resulted in much lower concentrations of HCN than those determined for the other set points carried out with the alumina bed. In cogasification, 43 - 51 % of the total input of fuel nitrogen was found in the product gas as ammonia, while the amounts of HCN and tar nitrogen were negligible. In straw gasification most of the sulphur was released as H2S to product gas, the concentration being in the range of 150 - 195 ppmv. The concentration of COS was of the order of 10 ppmv. In cogasification, part of the sulphur was captured by the limestone sorbent, but the residual H2S concentrations were still rather high (640 - 750 ppmv). This was due to the high steam feed rate, which resulted in a high H2O concentration in the product gas, which then had a strongly increasing effect on the equilibrium H2S concentration. The COS concentration ranged from 40 to 50 ppmv. In spite of the very high alkali content of straw the vapour-phase concentra-tion was reduced to below 0.1 ppm-wt when carrying out gas filtration at about 500 °C. Thus, it seemed to be possible to meet the alkali removal requirements of gas turbines in straw gasification, similarly as in coal or wood gasification, by effective barrier filters operated at about 500 °C.