Finding a clean woodstove – A 300-year quest

Abstract

This paper reports the results of an experimental study carried out over 30 years ago on the combustion and air pollution characteristics of biomass fuels when used with downdraft combustion. Such combustion involves the flow of fuel and air in the same direction (cocurrent) leading to the biomass to gasify first to a combustible mixture including carbon monoxide (CO) and hydrogen (H2) that will burn in a gas phase with high combustion efficiencies and low total suspended particulates (TSP). It is noteworthy that over 300 years ago, Justell published a report of a similar device that led to apparently very clean combustion.The study ascertained the extent to which complete combustion is possible in a cocurrent burner using primary air only or using both primary air and secondary air, exploring other variables such as dry or wet woodfuels and control of heat transfer from the combustion zone. The study has resulted in some generalized concepts for cocurrent combustion that link the oxygen requirements to the reactor conditions that impact upon the rate of pyrolysis or devolatilization of the biomass material. Clean combustion can be obtained with very little CO and TSP in the flue gas, with primary air only or a mixture of primary and secondary air. In the latter, the primary must be just enough to operate the device as a gasifier. The device seems to operate above the WHO PM2.5 Emissions Rate Target but meets the Intermediate Emissions Rate Target for cookstoves and easily meets the incoming USEPA emissions standard for woodheating stoves.