Emissions of nitrous oxide from combustion sources

Abstract Nitrous oxide (N 2 0) has recently become the subject of intense research and debate, because of its increasing concentrations in the atmosphere and its known ability to deplete the ozone layer and also to contribute to the greenhouse effect. There are both natural and anthropogenic sources for N 2 O; however, the man-made sources are increasing at a much higher rate than natural ones. Until very recently it was believed that the combustion of fossil fuels, especially coal, was the major contributing factor to these anthropogenic sources. For example, 30% of all N 2 0 released into the atmosphere was once attributed to combustion sources, with 83% of the combustion sources coming from coal combustion. Correction of a recently discovered sampling artifact, whereby SO 2 , H 2 O and NO in combustion gases react in a sampling vessel to produce N 2 O, has revealed that, in fact, less than 5 ppm of N 2 0 are found in most product gases from combustion systems. Fluidized bed coal combustors are the exception, though, yielding N 2 O levels of ca. 50ppm in their off-gases. The gas-phase reactions of N 2 0 in flames are reviewed first. It is clear that in most cases N 2 0 is a very reactive intermediate, which is quickly destroyed before being emitted from a flame. The important homogeneous reactions removing N 2 0 are thermal decomposition to N 2 and O 2 and also radical attack in e.g. N 2 O + H → N 2 + OH. Nitrous oxide is formed from nitrogen-containing species by NO reacting with a radical derived from either HCN or NH 3 ; the reactions are NCO + NO → N 2 0 + CO and NH + NO → N 2 0 + H. The levels of N 2 O observed are a balance betwen its rates of formation and destruction. It turns out that HCN is a more efficient precursor than NH 3 at producing N 2 0. The removal of N 2 O is fastest at high temperatures and in fuel-rich systems, where free hydrogen atoms are present in relatively large amounts. When coal burns in a fluidized bed, most of the N 2 O detected is produced during devolatilization, rather than in the subsequent stage of char combustion. It is clear that HCN and NH 3 are produced from nitrogenous material released during devolatilization; these two compounds give N 2 0 when the volatiles burn. The burning of char, on the other hand, involves the chemi-sorption of O 2 on to sites containing carbon or nitrogen atoms, followed by surface reaction, with one of the products being N 2 0, in addition to CO, CO 2 and NO. Fluidized coal combustors have temperatures around 900°C, which is low enough for the thermal decomposition of N 2 O to be relatively slow. In addition, the presence of the solid phase provides a large area for radical recombination, which in turn reduces the rate of removal of N 2 O by free radicals. Parametric studies of fluidized bed combustors have shown that factors such as: temperature, amount of excess air, carbon content and O/N ratio of the coal, all have a significant effect on N 2 O emissions. It is important to note that heterogeneous reactions with solids, such as CaO and char, can cause large decreases in the amount of N 2 O produced during the combustion of coal in a fluidized bed. In fact, there are several methods available for lowering the yields of N 2 O from fluidized bed combustors generally. Areas of uncertainty in the factors affecting N 2 O emissions from fluidized bed combustors are identified.