NOx Formation
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.
NOx from Coal Combustion
  • About 95% NO, ~5% NO2, and < 1% of N2O,
  • Largely depend on combustion intensity
  • Uncontrolled NOx level of 175~3200ppm
NOx Sources
  • Thermal NOx
    • Formed by attack of O atom on N2 in combustion air
    • About 20% of total NOx emission from PC burners
    • Mainly affected by flame T & O2 concentration, with the former most important.
  • Fuel NOx
    • Formed by pyrolysis & oxidation of N compounds in coals
    • About 80% of total NOx emission from PC burners
    • Fuel N vs. fuel NOx relation is complex and unclear:
        • Coals with high N do not necessarily produce more NOx;
        • Increased N in coal will lead to a decreased conversion rate;
        • N in char and in volatiles have different conversion ways to NOx

      • O2 concentration plays an important role in fuel N conversion into NOx
    • Prompt NOx
      • Formed by capture of N2 by hydrocarbon radicals. Small contribution in coal combustion.

It is generally accepted that the level of NOxemissions resulting from the combustion of pulverised coal is influenced by three factors:
  • Coal properties.
  • Plant design, including furnace design and use of post combustion de-NOx plants.
  • Boiler operation, including load, excess air level, in-service mills/burner groups etc.
In this context, it is recognised that NOx 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 NOx 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 NOx.  These properties, together with the known NOx emissions of one coal, allow good estimates to be made on the likely NOx emissions of other coals under the same operating conditions.  But these rankings or NOx 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 NOxemissions.  This is the bases of several commercial NOx control software packages.
Currently, there is no simple means to accurately predict NOx when only limited plant operating or design data is available.  Boiler manufacturers continue to use in-house empirical correlations, not CFD models, to predict NOxformation.
The volatile yield and the early volatile nitrogen release of coals are important to the reduction of NOx 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 NOx see "Predicting NOx".