Elsevier

Fuel

Volume 84, Issues 14–15, October 2005, Pages 1849-1853
Fuel

Coal ignition characteristics in CFB boiler

https://doi.org/10.1016/j.fuel.2005.03.029Get rights and content

Abstract

The coal ignition temperature in CFB boilers is an important parameter for designing the startup burner and for choosing operating method during the startup process. The combustion of seven different types of coal were measured in a laboratory scale fluidized bed with under-bed preheat system. The study analyzed the effects of particle size, furnace temperature, volatiles content and moisture content on the ignition temperature, TiF. TiF in the fluidized beds were measured at standard operating conditions. Comparison of the ignition temperatures and the feed temperatures in real CFB boilers shows that the TiF measured in fluidized beds can be used as a reference for the coal feeding process.

Introduction

The coal ignition temperature in CFB boilers, TiF, the lowest bed temperature required for static coal combustion, is an important parameter for designing the startup burner and for choosing the automatic control parameters during the startup process. Traditionally, the coal ignition temperature is estimated from experience with little theoretical basis and relatively large uncertainties in the estimates. These uncertainties result in operating difficulties and economic loses. For example, if the coal is fed into a furnace at a temperature lower than TiF, the coal will not burn and the furnace temperature will decrease even more. Once the fuel concentration and temperature in the furnace reach the critical conditions, the mixture will flash and the furnace temperature will suddenly increase which leads to control problems. Feeding the coal in the furnace at temperatures higher than TiF is safer but costs more startup time and fuel.

Laboratory scale fluidized beds are often used to measure the coal ignition temperature, TiF, based on the criteria such as the flame and spark characteristics, bed temperature response and oxygen response [1], [2]. The coal ignition temperature, TiF, in fluidized beds is not only a function of the physical and chemical properties of the coal, but is also influenced by the heating rate, the boiler structure, the fluidization velocity and the particle size as well. Therefore, a set of standard operating conditions was specified for the measurements of the coal ignition temperature, TiF, in a laboratory scale fluidized bed.

This article described a standard for measuring the coal ignition temperature with typical results in a laboratory scale fluidized bed with under-bed preheat system. The tests measured TiF for seven coal types were compared with the actual feed temperatures, TF, in real CFB boilers in China at various loads. The temperature differences between TiF measured in the laboratory fluidized bed and TF measured in the real CFB boilers for the various types of coal were plotted against the proximate analysis of the coal to estimate the lowest feed temperature in real CFB boilers.

Section snippets

Experimental facility and method

The ignition temperature was measured in a laboratory scale fluidized bed combustor with an inner diameter of 65 mm, which was heated with under-bed preheat system [3]. The inert bed material was quartz sand with a mean particles size of 280–300 μm. The static fluidized bed height was about 40 mm. The furnace temperature was controlled automatically and recorded by a data analyzer. The flame and sparks in the furnace were observed using a mirror fixed at the furnace top (Fig. 1).

Results and discussion

The seven tested coals are the feed coals fired in seven different CFB power plants in China. The coal analysis parameters are listed in Table 2.

Comparison with other methods

The TiF measured in the laboratory scale fluidized bed, the actual feed temperatures in real CFB boilers and the suggested feed temperatures provided by the foreign manufacturers are plotted as a function of volatile contents in Fig. 6. All the data show that ignition temperatures of coal particles decrease with increasing volatile content; these results are in agreement with measurements using pulverized coal particles [7], [8]. In addition, the large velocity difference between the gas flow

Conclusion

Coal ignition temperatures in a laboratory scale fluidized bed with under-bed preheat system were measured at a standardized operation conditions. The ignition temperatures of seven coal types measured in the test rig were compared with the feed temperatures in real CFB boilers in China for various volatile content. The results show that the ignition temperatures measured in the laboratory-fluidized bed accurately represent the real feed temperatures except for one anthracite coal type.

Acknowledgements

This project was financially supported by the National Science Fund Committee (No.50406002) and the National Basic Research Special Fund (ZZ02-06-01-02-4).

References (9)

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