Coal Mineral Transformations - Effects on Boiler Ash Behaviour

Abstract:

The UK now imports more than 50% of the coal that is used for coal-fired power generation. UK generators are offered an increasingly wider range of world-traded coals for burning in boilers that were designed to burn a relatively narrow range of indigenous coals. This project was undertaken to provide UK boiler designers and operators with an improved knowledge of the combustion characteristics of coals for which they had little combustion experience. The study placed particular emphasis on the effects that a wider range of coal minerals and mineral matter distributions might have on the many aspects of boiler operation. These ranged from coal grinding for pulverised coal combustion, to combustion behaviour, levels of unburned carbon in ash, precipitator performance, gaseous and particulate emissions, and the slagging and fouling characteristics of the ash.

The coals were selected to reflect the wide range of world-traded coals that are now on offer and came from North and South America, Australia, South Africa, Indonesia, China, Russia and India. The coals were chosen on the basis of the ash content and ash chemistry that UK utilities might encounter. As a consequence of the varied geographical origins of the coals and the range of ash chemistry, the nature and distribution of the mineral matter in the coals was found to be significantly different from that of indigenous coals.

Coal and mineral matter characterisation was carried by Nottingham University and Imperial College London. Combustion studies were undertaken by E.ON, using the Combustion Test Facility (CTF) at the Ratcliffe Power Technology Centre and by Imperial College, using a high temperature Entrained Flow Reactor (EFR). In addition the EFR was used to study the mineral transformations of the minerals found in the suite of coals. The combustion facilities generated a range of samples for analysis and characterisation, including combustion ash and unburned char, cyclone ashes and deposits collected on ceramic probes and a slag panel. Characterisation of the samples enabled the combustion performance and slagging propensity of the coals to be assessed and ranked against that of a typical UK bituminous coal (Harworth).

Some of the coals would be unsuitable for UK boilers. Two coals from the US Powder River Basin had a high slagging and fouling potential, a high ash coal from India could give potential ash handling problems unless blended with a low ash coal, and a South African coal gave high NOx and high levels of unburned carbon. The remaining coals would be expected to give few operational problems.

The implications of burning a wider range of imported coals have been considered. Sales of boiler ashes to the construction market are an important consideration in the overall economics of coal-fired power generation. Several of the ashes with a high calcium content would be unlikely to meet current and anticipated specifications for use with cements and concrete.

Existing methods of coal and ash characterisation were found to be generally satisfactory in predicting the combustion performance of the coals burned at rig scale. The more advanced coal and ash characterisation techniques were found valuable in understanding the mineral transformations, the ash formation and ash deposition mechanisms.

1. Introduction
2. Project Overview - Aims and Objectives - Activities
3. Coals and Minerals Chosen for the Project
4. Coal Combustion Characterisation
5. Generation and Characterisation of Ashes from Coal and Mineral Combinations
6. Mineral Transformations and Ash Formation Processes
7. Ash Particle Interactions and Transformations
8. Implications of Findings for Power Station Performance
9. Ash Sales and Ash Disposal
10. Conclusions
11. Further Work
12. References

Publication Year:

2005

Publisher:

Department of Trade and Industry

DOI:

No DOI minted

Author(s):

Wigley, F. & Williamson, J.