Coal ignition characteristics in CFB boiler
Introduction
The coal ignition temperature in CFB boilers, , 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 , 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 is safer but costs more startup time and fuel.
Laboratory scale fluidized beds are often used to measure the coal ignition temperature, , based on the criteria such as the flame and spark characteristics, bed temperature response and oxygen response [1], [2]. The coal ignition temperature, , 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, , 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 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 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 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)
- et al.
Fluidized bed combustion of wet brown coal
Fuel
(1980) - et al.
Devolatilization and ignition of coal particles in a two-dimensional fluidized bed
Combust Flame
(1989) - et al.
The ignition of coal particles
Fuel
(1991) - et al.
Experimental studies of ignition behaviour and combustion reactivity of pulverized fuel particles
Fuel
(1992)
Cited by (31)
A review on research and development of CFB combustion technology in China
2023, Powder TechnologyIgnition of water-coal fuel droplets during radiative-convective- conductive heating in relation to boilers operating on the technology of circulating fluidized bed
2022, Thermal Science and Engineering ProgressCitation Excerpt :At present, not many works have been published in world scientific periodicals describing the results of experiments to determine the values of the limiting ignition temperature of coal fuel particles in a fluidized bed. Several works can be noted (for example, [73,74]). In [73], the results of experimental studies of the ignition of lumpy coal in a fluidized bed are presented.
Attrition performance and morphology of limestone under different conditions in fluidized bed
2021, Fuel Processing TechnologyHeat transfer analysis of stationary bed materials in a CFB boiler after a sudden power failure
2021, Fuel Processing TechnologyExperimental study on the ignition characteristics of cellulose, hemicellulose, lignin and their mixtures
2019, Journal of the Energy InstituteCitation Excerpt :Ignition plays a crucial role in combustion, having a significant impact on boiler operation, energy efficiency, as well as on pollutant emissions [4]. Ignition temperature and ignition delay time are known as two important operational indicators to initialise combustion processes [5]. Most ignition studies were conducted aiming to control and minimise the self-ignition risks during transport, handling and processing biomass materials [6], but ignition mechanism with regard to the improvement of combustion performance are rarely reported.