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.