PNNL

Abstract

A Renewable Fuel Heating Plant (RFHP) is being proposed for the U.S. Department of Energy (DOE) National Renewable Energy Laboratory (NREL). The current NREL heat plant uses natural gas and the RFHP will use a biomass fuel, wood chips. This paper addresses 1) why despite an increase in emission rate the conversion is considered an effective reduction in greenhouse gas emissions, and 2) how the proposed wood chip combustion process emissions compare with other means of disposing of or using wood chips. The 2001 and 2007 Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC) considers the evidence for global climate change and the potential consequences of such changes [IPCC, 2001; 2007]. Based on the result of worldwide research efforts, this paper concludes that the earth's climate has changed over the last century. It also notes that there is recent strong evidence that human activities have caused most of the warming observed in the last 50 years. The current computer models are predicting that this temperature rise should continue over this century. In terms of the net CO2 in the atmosphere, the argument is made based on current scientific understanding on climate change processes, that burning of wood chips is much more desirable than a fuel that contains carbon that has been sequestered underground. The CO2 from wood chip combustion has a “net zero” emission rate based on factors in the Environmental Protection Agency’s AP-42. The “net zero” emission rate is based on an assumption that CO2 from burning wood from forests represents no increase in the net amount of CO2. A cycling of carbon between the atmosphere and forests results in no net gain or loss of airborne CO2. On the other hand CO2 from burning natural gas represents an increase in the net amount of CO2 from the introduction of “new” carbon that has been previously sequestered underground. Thus argument for the proposed conversion is to stop the introduction of the new carbon into the current atmospheric carbon cycle. From the viewpoint of minimizing impacts on global climate change, the burning of wood chips also tends to be more desirable than the common alterative use of wood chips in composting activities. Although there is great variability and uncertainty in the published emission rates, the gaseous emission from both open burning and composting tend to have much larger emissions of greenhouse gases, and specifically larger fractions of gases such as methane (CH4) and ammonia (NH3) than the proposed process for burning the wood chips. The published source terms for open burning show the incineration option to be preferable from the viewpoint of having lower emissions. Of particular importance are mix of combustion products from these activities. For example because methane is currently thought to be many times more effective for inducing climate changes than CO2, the potentially higher methane levels from open burning and composting make these activities less desirable from the viewpoint of minimizing the potential impact of green house gas emissions. The paradox in the proposed conversion is that from an absolute quantity perspective the RFHP would emit more CO2 than is being currently emitted with natural gas firing. Although the main thrust in reducing atmospheric levels of greenhouse gases has been to reduce introduction of “new” carbon by the combustion of fossil fuels, some efforts have considered the possibility of combustion control strategies for agricultural and forestry products. A review was conducted of recent literature relevant to these issues. The predominance of current literature that points to a need to reduce greenhouse emissions and it is assumed for the purpose of this review to be a reasonable basis for proceeding with actions that will reduce those emissions. The results of this review are reported below.