Pekka Simell

Formation and Catalytic Decomposition of Tars from Fluidized-Bed Gasification

This paper describes studies concerning the formation of tars in a pressurized fluidized-bed gasifier and their catalytic decomposition in a separate reactor. In the gasification tests, the effects of fuel and freeboard temperature on tar yields and composition were measured. Finnish peat, saw dust and German brown coal were used as fuels. The tar yields decreased with increasing freeboard temperature. The total tar concentration in wood gasification at 900 °C was in the range of 10-18 g/m3n. In peat gasification the concentration at the same conditions was only 3-6 g/m3n and with brown coal less than 1.5 g/m3n. The main components of tar for all the fuels were benzene and naphthalene.

Formation and Removal of Nitrogen Compounds in Gasification Processes

When gasifying nitrogenious fuels, organic and inorganic nitrogen compounds form in the gas. The main nitrogen compound in gasification atmosphere is ammonia. The ammonia content of the product gas has ranged in pressurized peat gasification processes between 6 000-13 400 ppm and hydrogen cyanide content 10 - 500 ppm. Decomposition and removal of nitrogen compounds can be accelerated by catalysis. The effect of several catalytic materials on the nitrogen compounds from an operating peat gasifier was measured. Nickel catalyst and ferrous materials proved to be most efficient in decomposing ammonia at high temperature. Limestone and dolomite did not have any strong catalytic effect on the decomposition of ammonia in the atmospheres studied. The calcareous and ferrous materials can increase the ammonium content of the gas at low temperatures by converting part of the organic nitrogen into ammonia if gas contains high tar loading.