NOx Formation: Technology Review

The prime objective of project C9056 was to provide the coal industry with an understanding of the current state of knowledge about NOx formation during PF combustion. In doing this the study aimed to:

a) Clarify current understanding of the separate influences of coal properties, plant design and operational issues on NOx formation under PF combustion conditions, considering local and overseas plant designs and available emissions data.

b) Provide a concise review of current knowledge on NOx formation processes with particular emphasis on PF combustion conditions.

c) Identify (coal industry) research needs relevant to NOx formation from the combustion of Australian black coals

The review was conducted by a team of scientific and industry based personnel from CSIRO and Pacific Power. The methodology involved sourcing recent research results from the scientific literature, industrial researchers and equipment manufacturers. This information was reviewed to determine the present understanding of NOx formation mechanisms, relationships between coal properties and NOx production, current research activities on low NOx combustion systems and what the unknowns are. Emissions regulations in countries which utilise Australian thermal coals were reviewed to determine present NOx emissions limits and identify likely trends in legislation. The final component of the study was a review of where research into NOx emissions from PF combustion is heading and the research needs of Australian industry.

Emissions of NOx are subject to environmental controls that are becoming increasingly stringent. NOx emissions regulations depend considerably on a country's state or federal government policy. Factors which influence policy development include:

• Background levels of NOx and the degree of health related problems caused by NOx
• Degree of industrialisation of country
• Geographic locations of population centres relative to emissions sources
• Prevailing meteorological conditions
• Legislative approach of the country.

Legislative approaches to the control of NOx may take a number of forms, however most countries set limits on concentration of NOx in the stack flue gas. Economic measures are being adopted in a few countries an alternative to the setting of limits. Their aim is to provide economic incentives and encourage innovative approaches to pollutant abatement and control. However in countries such as the USA, the NOx emission legislation is expressed as a mass of NOx per unit energy output which places coal at a disadvantage when compared to alternative fuels such as natural gas.

The ever-tightening NOx emission limits have lead to a significant research effort on NOx production over the past decades. The complexity, heterogeneity and variability of thermal coals complicate the study of NOx production from coal combustion. There is presently a considerable research effort on the study of NOx emissions from pulverised coal combustion being conducted in Australia, Europe, Japan and the USA. In the USA over the past decade the total expenditure on NOx related research and demonstration projects on NOx emissions from coal combustion has been in excess of US$450 million.

The scientific community has focused on identification of nitrogen functionality and the relative importance of the various chemical routes to NO formation via volatile, char and soot species. Industrial research, in particular equipment manufacturers, have continued with the development of low NOx combustion methods which have lowered NOx emissions from plants not equipped with post combustion controls. The power industry whilst also being involved with the development of combustion systems has tended to seek relationships between coal properties and NOx emissions.

The considerable laboratory and empirical evidence gathered by scientific and industrial researchers has found that the principal factors influencing NOx emissions from pulverised coal fired plants in order of importance are:

1. Boiler design and firing configuration
2. Boiler operating conditions
3. Coal properties

The study concluded that although there has been a substantial research effort on NOx emissions over the past decade, the science of NOx production is still not completely understood. A number of industry research needs were identified to better understand the influences of coal properties on NOx production and to allow the coal industry to be able to respond effectively to trends in NOx control technologies. The research areas identified include:

• The partitioning of nitrogen between volatiles and char under combustion conditions to better understand NOx emissions under low NOx burner operating conditions.
• The continued development of models to allow the prediction of NOx emissions and better understanding of relationships between coal properties and NOx emissions.
• Investigation of the impacts of coal blending and co-firing with biomass fuels on NOx emissions to better manage fuel supplies at the power plant.
• The use of coal as a reburn fuel for NOx emissions reduction to assist in the development of more cost effective NOx reduction methods.
• Investigation of trade-offs between NOx emissions and residual carbon in flyash to improve the ability to optimise plant performance with individual coals