Jukka Leppälahti

Nitrogen evolution from coal, peat and wood during gasification: Literature review

During gasification, fuel nitrogen is liberated mainly as ammonia, hydrogen cyanide, molecular nitrogen, or as heavy aromatic compounds, while a smaller part of the nitrogen is retained in solid char. Independent of the fuel gasified, more NH3 is formed than other nitrogenous compounds in most gasifiers. The NH3 content in the product gas seems to be dependent mainly on the nitrogen content of the fuel. The measured NH3 concentrations have varied between 150 and 10000 ppm, the lowest being for wood, which usually has a nitrogen content less than 1%, and the highest for peat and coal, which have nitrogen content varying from 0.5 to 3.0%. The fraction of the fuel nitrogen that is converted to NH3 is probably affected by several parameters, including gasification temperature, heating rate, pressure, residence time of fuel in the reactor, as well as the devolatilization rate of fuel and the nitrogen functionality in fuel structure. No detailed structural information on the nitrogen functionality of wood, peat or coal is available because no reliable methods for quantitative analysis of nitrogen groups in these complex mixtures of organic molecules have been developed. However, it is known that nitrogen in living organic tissues is mainly in proteins. In peat, nitrogen is bound in amino acids, peptides, proteins, amino sugars and probably in heterocyclic structures. The spectroscopic studies of coal indicate that most of the nitrogen is in pyrrolic and pyridinic form but also unidentified quarternary-type nitrogen has been found. The pyrolysis studies of model compounds have shown that pyridine and pyrrole release nitrogen as HCN. Proteins and aminoacids may also release NH3. The pyrolysis experiments of coal have shown that the measured NH3/HCN ratio is dependent on the heating rate. In rapid pyrolysis experiments more HCN than NH3 has been formed but in low heating rate conditions, the situation has been the reverse. It has been hypothesized that in coal pyrolysis, part of the NH3 is formed from HCN and hydrogen through secondary reactions. In fixed bed and fluidized bed reactors, longer gas-char contact time exists than in the entrained bed reactor, which may result in greater NH3/HCN ratio in the former types of reactors. The increase in temperature or decrease in pressure and residence time of fuel in the reactor will also decrease the extent of secondary reactions of char and gases, which could result in lower NH3 formation. In the gasification of peat, more NH3 is usually formed than in gasification of hard coals, which is probably partly due to direct liberation of NH3 from amino acids. In peat and wood gasification, most of the fuel nitrogen is liberated during the pyrolysis stage. During coal gasification at low temperatures (< 1200 K), most of the nitrogen is retained in char after pyrolysis and released during the char gasification stage. This means that the reactions of char nitrogen can contribute to the total NH3 conversion.

Catalytic purification of tarry fuel gas with carbonate rocks and ferrous materials

Abstract This study compared the capability of various low-cost materials to catalyse the decomposition of tarry constituents in fuel gas. Carbonate rocks (dolomite, limestone) and ferrous materials (iron sinter, pellet) were tested in a tube reactor. Sample gas flow from a peat-fired gasifier was used as feed. An increase in the treatment temperature and extension of the residence time decreased the tar content. With carbonate rocks almost complete tar decomposition was accomplished at 900 °C. The ferrous materials were not as effective, but the content of the most problematic tar fraction decreased significantly. The thermal value of dry gas increased, due to the increase of H 2 and CO contents by catalytic water-gas and water-gas shift reactions. The activity of the carbonate rocks increased with a greater Ca Mg ratio and smaller grain size of the original rock. Activity was also increased by the presence of iron in the carbonate rocks and by the mineral matter in the ferrous materials.

Formation of NH3 and HCN in slow-heating-rate inert pyrolysis of peat, coal and bark

In gasification processes, nitrogen compounds are formed in the gas. There are very few data available on the conversion of fuel nitrogen to nitrogen compounds in pyrolysis conditions that simulate fixed-bed or fluidized-bed gasification or other fuels than coal. In this study, relatively large fuel pieces of peats, bark and coal were pyrolysed in an inert gas atmosphere at a slow heating rate. All fuels produced more NH3 than HCN. The conversion of fuel nitrogen to NH3 ranged from 17 to 24 wt% for peat, 10 to 12 wt% for bark and 15 to 17 wt% for coal. The conversion of fuel nitrogen to HCN was 5–11 wt% for coals and considerably less for the other fuels. Increasing the tar cracking can increase the HCN conversion and NH3 conversion in peat pyrolysis. The pyrolysis of fuel pieces seems to produce the NH3 and HCN observed in the fixed-bed gasifiers.

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.

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.

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.

Nitrogen Compounds in Peat and Wood Gasification and Gas Combustion

Contents of nitrogen compounds in the product gas of peat and wood gasification in fixed bed were measured at two gasification heating stations. A significant amount of ammonia, NH3, formed in peat gasification, whilst in wood gasification its amount was insignificant. Part of the fuel nitrogen was converted to hydrogen cyanide, HCN, which formed in wood gasification nearly as much as in peat gasification despite the higher nitrogen content of peat. The proportion of nitrogen bound in the organic compounds of tar in the total nitrogen of the fuel was significant. The moisture content of peat had no essential effect on the amounts of nitrogen compounds formed in the product gas. The output of the plant has only an insignificant effect on the amount of gas nitrogen compounds, whilst the effect on NOX emissions was stronger.