Esa Kurkela

Catalytic hot gas cleaning of gasification gas

Abstract Gasification gas containing dust can be efficiently purified from tars and ammonia with nickel monolith catalyst. Temperatures over 900°C and residence times of about 1 s (SV 2500 1/h) were needed at 5 bar pressure to obtain complete tar decomposition and 80% ammonia conversion with a feed gas from a pilot scale fluidized bed gasifier. At these conditions deactivation caused by carbon deposition or by H 2 S can be avoided. Decline of catalyst activity was not observed during 100 h long test runs. Dolomites and limestones can be used as effective tar decomposing catalysts at lower pressures and around 900°C temperatures, where they stay in calcined form. Low-cost iron containing materials can in turn be applied for catalytic ammonia removal.

Provisional protocol for the sampling and anlaysis of tar and particulates in the gas from large-scale biomass gasifiers. Version 1998

Abstract This paper presents tar sampling protocols for pressurised and atmospheric large scale gasification processes. Methods for constructing sampling lines either to on-line analysers or into sampling systems are described. The tar sampling system consists of a heated probe, a particulate filter and a series of impinger bottles. Dichloromethane is used as the tar abosrbing solvent. The solvent containing bottles are placed in a cold bath so that gradual cooling of the sampled gas from about 0°C to the final temperature −79°C takes place in them. Recommendations for suitable sampling gas flow rates and gas temperatures are given. Tar characterisation methods based on different garvimetric measurements and GC analysis are described.

Catalytic conversion of nitrogen compounds in gasification gas

Abstract New forms of energy production, such as combined-cycle power plants and fuel cells, can be introduced by gasification. When gasifying nitrogenous fuels, organic and inorganic nitrogen compounds form in the gas. In gas combustion these can form nitrogen oxides detrimental to the environment. Catalytic decomposition of the nitrogen compounds in the gasification gas is one alternative for reducing the formation of nitrogen oxides. In the research work under review, catalytic effects of various inexpensive materials on the nitrogen compounds of the gasification gas at high temperatures were studied. The materials were iron sinter, iron pellet, ferrous dolomite, dolomite and limestone, and, as reference materials, inert silicon carbide and a commercial nickel catalyst. The most significant nitrogen compounds formed in gasification are ammonia, hydrogen cyanide and organic nitrogen compounds of tar. The ferrous materials and the commercial nickel catalyst proved to be the most efficient agents for decomposing ammonia. Limestone and dolomite did not exhibit any essential catalytic capacity for decomposing ammonia, although they reduced the hydrogen cyanide content of the gas.

Tar-decomposing activity of carbonate rocks under high CO2 partial pressure

The tar-decomposing activities of dolomites and limestones in calcined and carbonated forms were compared. Tests were carried out in a fixed-bed tube reactor at 900°C under 2 MPa total pressure using an N2H2OCO2 gas mixture. Toluene was used as a tar model compound. The calcined rocks decomposed toluene efficiently, but activity was almost totally lost in the carbonated form. This decline of activity took place as soon as the CO2 partial pressure exceeded the equilibrium decomposition pressure of CaCO3. However, even in the carbonated form, the materials decomposed polyaromatics more efficiently than did an inert reference material. With a mixture of the main components of gasifier product gas and in the range 900–1000°C, the tar-decomposing activity was similarly lost under conditions in which the dolomite sample was carbonated.

Air gasification of peat, wood and brown coal in a pressurized fluidized-bed reactor. I. Carbon conversion, gas yields and tar formation

Abstract Fluidized bed air gasification of Finnish peat, sawdust and German brown coal were studied in a small pressurized fluidized-bed pilot plant operated at 0.4–1.0 MPa. The effects of air-to-fuel-ratio and other gasification variables on the conversion efficiencies and product yields are discussed in the article. Over 95% carbon conversion was achieved with all studied fuels under the best operating conditions. The lower heating value of the wet product gas ranged 3.5–5 MJ/Nm 3 . The highest heating value was achieved in low temperature gasification of wood, the light hydrocarbon gases representing nearly half of the gas heating value. The tar yields were dependent on the fuel type and on the freeboard temperature. The total yield of benzene and tars in wood gasification at 900°C was in the range of 3–4 wt% of dry ash-free fuel. In peat gasification the yield at the same freeboard temperature was only 0.7–1.2 wt% and in brown coal gasification below 0.5 wt%. The most stable components at higher freeboard temperatures were benzene and naphthalene. Toluene and phenol were almost completely decomposed when the temperature was raised above 850°C.

Behaviour of nitrogen compounds and tars in fluidized bed air gasification of peat

Abstract Formation of nitrogen compounds and tars, and the effect of process parameters on these were studied in a laboratory scale fluidized bed air gasifier under atmospheric pressure, using peat as fuel. Benzene was the main component of the tars, representing 50–75 wt% of the total tar. Addition of both secondary air and dolomite reduced the tar content of the gas. NH 3 concentration ranged from 3170 to 4600 ppm and HCN concentration from 520 to 930 ppm in the measurements. With no secondary air, 30% of fuel nitrogen formed NH 3 and 10% formed HCN. A rise in the freeboard temperature, caused by the addition of secondary air, had no significant effect on the concentration of nitrogen compounds. On the other hand, the presence of dolomite increased the NH 3 concentration and reduced the HCN concentration of the gas.

Development of simplified IGCC-processes for biofuels: Supporting gasification research at VTT

Abstract Gasification research-and-development activities in Finland are focused on the development of simplified integrated gasification combined-cycle power plants based on air-blown gasification and hot-gas cleaning. This research is mainly concentrated on the utilization of peat and different wood wastes. Effective utilization of these fuels, which are the only indigenous fuels in Finland, is crucially important for the Finnish energy economy. Pressurized-fluidized-bed gasification of biofuels and hot-gas cleaning from tars, particulates, and alkali metals are extensively studied at the Technical Research Centre of Finland. A brief review of the current experimental gasification research is presented in this paper.

A semi-empirical model for pressurised air-blown fluidised-bed gasification of biomass

A process model for pressurised fluidized-bed gasification of biomass was developed using Aspen Plus simulation software. Eight main blocks were used to model the fluidized-bed gasifier, complemented with FORTRAN subroutines nested in the programme to simulate hydrocarbon and NH(3) formation as well as carbon conversion. The model was validated with experimental data derived from a PDU-scale test rig operated with various types of biomass. The model was shown to be suitable for simulating the gasification of pine sawdust, pine and eucalyptus chips as well as forest residues, but not for pine bark or wheat straw.

Air gasification of peat, wood and brown coal in a pressurized fluidized-bed reactor. II. Formation of nitrogen compounds

Abstract The formation of nitrogen compounds in the air gasification of peat, wood and brown coal was studied in a small fluidized bed reactor, which was operated under pressure (0.4 MPa to 1.0 MPa) and at temperatures ranging from 800°C to 995°C. The fuel nitrogen of all fuels studied was converted mainly into ammonia. Smaller amounts of HCN and nitrogen-containing tar compounds were formed too. The ammonia concentration of the product gas was in the range of 0.56–1.13 vol% in peat gasification, representing 55–95 wt% of the peat nitrogen. Raising the freeboard temperature from 800 to 950°C resulted only in a small reduction in the conversion of peat nitrogen into ammonia. Similar conversions were also observed in brown coal and wood gasification, but since the nitrogen content of these fuels is lower, the ammonia concentrations were correspondingly lower and ranged between 0.05 and 0.09 vol% for wood and between 0.21 and 0.28 vol% for brown coal. The HCN concentration of the product gas in peat gasification increased with the rising temperature and ranged between 50 ppmv at 820°C and 200 ppmv at 920°C. In brown coal gasification the HCN concentration was in the range of 60–100 ppm at gasification temperatures of 850–910°C.

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.