火行为对森林地表可燃物燃烧碳转化的影响

摘要

The present work is dedicated to theoretically exploring the effects of fire behavior on carbon conversion rates by burning surface fuels in the wildland.Starting from the energy and elemental mass conservation equations operated in the combustion of wildland fuels in the open,a mathematical model was developed to correlate carbon conversion rates parameters with fire behavior parameters,including flame length and temperature.The computations were performed with the typical surface fuels in Guangdong forest areas with their known property data.It was observed that with the increase in flame length or flame temperature,the combustion efficiency increases,which results in an increase in the conversion rate of consumed carbon to CO2 (ηCO2);for the surface fire with the flame temperature fluctuating between 1 000 K and 1 200 K,the conversion rate of the consumed carbon to CO2 varies between 0.44 and 0.95,whereas the emission ratio of CO to CO2 falls within the range between 0.02 and 0.40.Further analyses indicate that an increase in flame length or flame temperature implies a more complete combustion occurring in the flame zone,which causes a lower percentage of carbon emitted in the form of CO and carbon soot,and consequently leads to an increase in ηCO2.It has been confirmed that the carbon conversion rates in the fire spread over wildland fuels are in fact the reflection of their fire behavior,which well explains the variation patterns of the field experimental data reported in the literature.The mathematical model developed in this paper lays a foundation for accurately evaluating the amounts of carbon emitted in individual wildfires.%为了从理论上探索森林地表可燃物燃烧过程中碳转化率与宏观展现的火行为间内在联系,依据燃烧现象中能量平衡和元素质量守恒方程,建立起森林地表可燃物燃烧碳转化参数与其火行为参数相关联的数学模型.针对广东林区典型地表可燃物计算结果表明,随着火焰长度或火焰温度的增加,燃烧效率提高,消耗碳向CO2的转化率ηCO2增大;当火焰温度介于1 000～1 200K之间时,碳转化率ηCO2以及CO对CO2排放比的变化范围分别为0.44 ～ 0.95和0.02～ 0.40.进一步分析表明,火焰长度或火焰温度的提高意味着火焰区的燃烧反应更完全,消耗的碳向CO和碳烟颗粒的转化率相对减小,导致碳转化率ηCO2增加.从本质上来说,森林地表可燃物火蔓延过程中的碳转化率实际是火行为的一种表现,由此可以解释野外试验观察到的碳排放效率的变化规律.本项工作中建立的计算模型为可靠估算实际火灾现场碳排放量奠定了基础.