Effects on Boiler Efficiency Standards of Pulverizing Coal

Abstract

This paper examines the effects of particle size on the calorific value of hydrocarbons, shedding light on the thermodynamics of pulverizing coal in a commercial power plant. Both laboratory testing results and energy balances around an actual pulverizer are presented. Although tacitly known to any power plant engineer, efficient combustion is seen in two parts: preparation of the material’s surface/mass ratio, and then its combustion with the proper air/fuel mix and associated mechanics. This work attempts to put a thermodynamic face on the first part. A theory is presented which demonstrates that a hydrocarbon’s surface/mass ratio affects its potential to release its full chemical energy. This theory has been generally supported in this work by laboratory testing of pure substances; however this testing was not conclusive and should be repeated. If an optimum surface/mass is not achieved, unburned combustibles will result — and this regardless of subsequent air/fuel mixtures and/or burner sophistications. This work is suggests that a unique optimum surface/mass ratio exists for each hydrocarbon substance (and coal Rank); that once its full potential is reached, a higher ratio provides no further benefit. Since surface tension describes a material’s free energy, an aspect of surface tension, termed hydrogen bonding free energy, was shown to relate to the A¨calorific value penalty associated with non-optimum surface/mass ratio. A correlation was developed relating surface/mass ratio to observed an A¨calorific value penalty and hydrogen bonding free energy. This correlation’s form may be applied to coal if supported with additional research. The impetus for this work was the ASME Performance Test Code 4’s allowance of pulverizer shaft power to influence boiler efficiency’s “credit” term, thus affecting efficiency. It was demonstrated that surface/ mass affects calorific value and thus efficiency. However, there is no observable difference between grinding a hydrocarbon to a given surface/mass ratio, versus manufactured spheres. Although laboratory preparation of coal samples should emulate pulverizer action, this work suggests that a renewed and careful review of laboratory procedures is required. Recommendations are provided for critique and debate.