The recent economic problems of Asian countries have slowed the anticipated growth in coal fired generating capacity.  This growth has been estimated to be 345 GW of new capacity in APEC countries by 2010.  The anticipated growth in coal fired generating capacity within Asia has encouraged the development of power plant technology to fire a wider range of fuels, to increase efficiency and lower capital costs.  In particular, the potential market for new power plants in China has led to newer burner technologies that achieves stable flames and reduced NOx emissions while firing coals that have lower volatiles than coals normally traded in the international coal market.

Increased global concerns on NOx emissions are reflected in the limits being set by regulating authorities of countries importing coal for electricity generation. This makes NOx emissions an important consideration in the design of new coal fired power plant. 

CoalTech' ACARP report reviewed the influence of coal properties on NOx formation. Coal properties, such as volatile and nitrogen content, impact the levels of NOx emissions, but there is no simple relationship between these coal properties and NOx emissions that fits all operational power plant.  The impact of coal properties is lessened through the use of low NOx burners, over-fired-air, fuel reburn and the post boiler technologies of  selective catalytic (SCR) and non-catalytic reduction (SNCR), though there may be cost disadvantages due to poorer burnout and higher operating costs.

NO_x from Coal Combustion

NO_x Sources

It is generally accepted that the level of NO_xemissions resulting from the combustion of pulverised coal is influenced by three factors:

In this context, it is recognised that NO_x generation is a  very complex process, and is dependent on localised temperatures, stoichiometry and chemical reactions, principally in the near-burner region.

To assist in the understanding of the interactions within this process several empirical models (fitting an arbitrary equation to data), simple reaction engineering models (combinations of CSTR and PFR reactors) using simplified chemical kinetics and complex computational fluid dynamics (CFD) models for the prediction of NOx emissions have been developed by organisations around the world.  All methods have their limitations and advantages.

To rank NO_x emissions of different coals from a power plant operating under the same conditions based on coal properties requires only two parameters -  the volatile yield at high temperature and the nitrogen content, as shown in the figure below.  The product of these parameters represents the volatile nitrogen release that can be converted to nitrogen within the flame or conversely the char nitrogen that can react to form NO_x.  These properties, together with the known NO_x emissions of one coal, allow good estimates to be made on the likely NO_x emissions of other coals under the same operating conditions.  But these rankings or NO_x emission estimates are most likely to change with changes in plant operation.

For a power plant, given sufficient NOx emission data on a wide range of coals and operating conditions, it is possible to formulate an empirical model to accurately predict NO_xemissions.  This is the bases of several commercial NO_x control software packages.

Currently, there is no simple means to accurately predict NO_x when only limited plant operating or design data is available.  Boiler manufacturers continue to use in-house empirical correlations, not CFD models, to predict NO_xformation.

The volatile yield and the early volatile nitrogen release of coals are important to the reduction of NO_x within the flame. The understanding of these high temperature processes is increasing with the research conducted in Australia and overseas, but generalized models that can be used with all coals has yet to be developed.

For details on the prediction of NO_x see "Predicting NO_x".