Analysis of Wood Combustion Based on the First and Second Laws of Thermodynamics

Wood is a highly flexible fuel which can be used in a variety of ways for energy generation. Direct combustion of wood is the oldest method of utilization of this renewable energy source. This paper presents an analysis of wood combustion based on the first and second laws of thermodynamics, using a computer program developed for this study. Since the moisture content of the available wood fuels vary over a wide range, special attention is given to the effects of moisture content on the exergy and energy content of the wood. Because of the wide variation in particle size and moisture content of the wood fuels, different types of combustion systems have been designed and are utilized in industry. Each combustion system uses a certain amount of combustion air and is designed for a certain range of combustion air temperatures and wood fuel moisture contents. The effects of these variables on wood combustion and the efficiencies of the process have been studied and are presented in this paper. To point out the importance and differences between efficiencies that are commonly used, several efficiencies based on the first and the second laws of the thermodynamics are calculated and compared. Based on the results presented in this report, it is concluded that, over the range of variables studied, the most efficient conditions for wood combustion can be achieved by using the minimum amount of excess air at highest permissible temperature. It is also concluded that the lower moisture contents result in higher combustion efficiencies.