How Coal Properties Influence Emissions
AUTHOR: Robert M Davidson
DATE: February 2000
This report examines the influence of coal properties on the emissions of pollutants to the atmosphere. To illustrate this, four pollutants are considered. They are:
- sulphur oxides
- nitrogen oxides
- particulates and
- trace elements.
These pollutants are representative of the various impacts of the coal properties. Some pollutants, as represented by sulphur dioxide, are formed virtually stoichiometrically from their precursors in coal. However, even in these cases, other coal properties such as ash composition can affect the amounts emitted from the combustor. Other pollutants are affected by the coal quality in much more subtle ways.
Although the nitrogen in the coal is the precursor of most of the nitrogen oxides, the nitrogen content is not the only factor that includes how much is converted to NOx rather than inoffensive nitrogen. Partculate emissions reflect the ash content of the coal but their properties can also determine the effectiveness of the electrostatic precipitators used to control them. Finally, most trace elements, mercury and selenium excepted, are emitted on the particulate matter. Their properties and that of the coal and mineral hosts can affect how they are partitioned among the various size fractions of the ash. Various correlations and prediction equations are examined in this report. Coal property input into computerised modelling is considered but the details of the models are not. It is concluded that, even when emissions can be accurately predicted, default values should only be used with extreme caution.