Effect of air preheating and fuel moisture on combustion characteristics of corn straw in a fixed bed

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Abstract

Experiments were carried out on a one-dimensional bench combustion tests rig. The effect of air preheating and moisture level in the fuel on combustion characteristics of corn straw was investigated. The bed temperature distribution and the mass loss of fuel and gas components such as O2, CO, CO2 and NO were measured in the bed. The average burning rate and ignition front propagation velocity increased with increasing primary air preheating temperature. The total burning time was shorter under the higher primary air preheating temperature and the higher primary air preheating temperature produced a lower ignition front flame temperature in the bed. The variation of the flue gas O2, CO and CO2 concentrations with time was more intensive at a higher primary air preheating temperature during the ignition front propagation period and the char oxidation period. With the increase of the fuel moisture, average burning rate and ignition front propagation velocity decreased. As the fuel moisture was less than 30.71%, with the increase of the fuel moisture, residual mass loss rate decreased and ignition front flame temperature increased at a fixed air flow rate. Drier fuels resulted in fuel-rich combustion and higher CO concentration. The NO concentration decreased with increasing the moisture level in the fuel.

Introduction

Energy produced from biomass or its conversion products represents an important part among today’s energy sources [1]. Biomass is presently estimated to contribute of the order 10–14% of the world’s energy supply. According to the statistics from 1998 to 2003, biomass resources in China were equivalent to 700 million tons of standard coal, of which about half was from straw [2]. Combustion is a widely used energy conversion technology for biomass [3]. Fixed bed conversion of biomass on a grate is gaining growing attention.

The fuel properties and process conditions, such as fuel type, particle size, air flow rate, fuel moisture and air preheating affect the combustion characteristics, altering the heat generation, heat transfer and reaction rates in a complicated manner. There have been many researches into fixed bed combustion of biomass during last two decades. The propagation of a reaction front in a fixed bed for thermal conversion of municipal waste and biomass was investigated by Gort [4]. Saastanainen and Taipale [5] investigated the propagation of ignition front in bed of wood particles where the effect of air flow, moisture, particle size, density and wood species were considered. Van der Lans et al. [6] performed experiments on wheat straw combustion in a vertical reactor where both primary air flow rate and inlet primary air temperature were varied. Mass reduction during preheating, volatile matter combustion and char combustion process were measured and discussed of the waste material containing high moisture [7]. Friberg and Blasiak [8] measured the mass flux and stoichiometry of conversion gas from three different wood fuels as function of volume flux of primary air. Yang et al. [9] studied the influence of air flow rate and fuel moisture on the burning behaviours of biomass and simulated municipal solid in the packed bed. Channeling phenomena in fixed beds have been studies by Yang et al. [9] and Changkook et al. [10]. Zhou et al. [11] investigated the formation and reduction of nitric oxide in fixed bed combustion of wheat straw. Other important works were summarized by Ndiema et al. [12], Thunman and Leckner [13], Yang et al. [14], Zhou et al. [15] and Zhao et al. [16].

Though those studies provide important information on biomass combustion in a fixed bed, the obtained data were still very limited in most cases only ignition propagation speed and maximum ignition front temperature were reported. To understand the complicated combustion processes of corn straw, the effects of air preheating and moisture level in the fuel on the combustion characteristics of corn straw was investigated in a one-dimensional bench experiment rig. An improved understanding of such fuel parameter and process condition will help the clean and efficient use of corn straw as a source of energy.

Section snippets

Experimental rig

Fig. 1 shows a diagram of the one-dimensional bench combustion test rig. The reactor is a vertical cylindrical combustion chamber suspended from a weighing scale, with a mass measurement error of ±1%. The chamber is 1.3 m high with a diameter of 180 mm. The combustor is axis-symmetric, thermally insulated by a 50 mm thick refractory wall, and surrounded by a thick layer of insulating material and an external casing. The grate at the bottom of the chamber consists of a stainless steel perforated

Temperature, gas composition and mass loss

Fig. 2a shows the bed temperature profile at different measurement points. Once the fuel was ignited by the gas burner, the ignition front propagated downwards and reached the thermocouple, the bed temperature at each thermocouple increased rapidly from ambient level to a peak value as high as 780 °C. The average time span for the rising of local temperature form ambient level to the peak value was 188 s as the ignition front reached the thermocouple. After the ignition front left each

Conclusions

Experiments have been carried out for corn straw in a one-dimensional bench combustion test rig and the effects of air preheating and fuel moisture was investigated. In general, the following conclusions can be drawn from current study

  • 1.

    The total burning time was shorter under the higher primary air preheating temperature and the higher primary air preheating temperature produced a lower ignition front flame temperature in the bed.

  • 2.

    The average burning rate and ignition front propagation velocity

Acknowledgements

This work was sponsored by the Ministry of Education of China by the 2004 year New Century Excellent Talents in University (Contract No.: NCET-D4-0328), the Heilongjiang Provincial Natural Science Foundation of China (Contract No.:E200623).

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