Environmental optimisation of gas fired engines

摘要

Adjustments of natural gas fired engines leading to lower NO_x emissions normally lead to increased emissions of unburned hydrocarbons (UHC) and CO. This means that engine adjustment often is a trade-off between NO_x and other emissions. However, engines for combined heat and power (CHP) production are normally adjusted to meet the demands given by regulations and to obtain high efficiency. The engines meet the demands given by regulations, but that does not mean that the engines are adjusted to obtain the lowest environmental impact. One reason is that, so far, it has not been possible to specify what is actually the lowest environmental impact. The cost of harm caused by NO_X emissions is given as ? per kg NO_X emitted. Similar values are available for NMVOC (non-methane volatile organic compounds). But as the name indicates, NMVOC is a group of compounds, the composition of which depends on the source. Therefore, the general cost of NMVOC is not suitable for determining the environmental impact caused by emissions from natural gas fired engines. The overall aim of the project was to assess to which extent it is possible to reduce the emissions by adjusting the different engines examined and to determine the cost of the harm caused by emissions from natural gas combustion. However, only health and climate effects are included. External costs of the following chemical components present in flue from natural gas fired engines are determined: NO_x (nitrogen oxides), C_2H_4 (ethene) C_3H_6 (propene) and HCHO (formaldehyde). Methane, ethane and propane are not considered carcinogenic, and there is no mentioning in general literature of other chronic health effects. The emissions of NO_x, CO and UHC as well as the composition of the hydrocarbon emissions were measured for four different stationary lean-burn natural-gas fired engines installed at different CHP units in Denmark. On average, the NO_x reduction potential corresponds to a reduction of 40 % relative to the present level. Such a NO_x reduction will lead to an increase of unburned hydrocarbon emissions of 12 % and the CO emission will increase by 19 %. Furthermore, the natural gas consumption for the gas engines will increase by 1.5 % in order to keep the power production constant. This is due to reduced electrical efficiency as a consequence of the NO_x reduction. However, increased natural gas consumption leads to increased heat production. This is taken into account by assuming that the heat production from the boilers is reduced accordingly. The external costs related to emission will be decreased by 9 million ? per year as consequence of a NO_x reduction as described above. A welfare economic analysis was conducted. Besides the external costs, costs related to fuel consumption, operation as well tax yield are included in the analysis. The analysis shows a potential welfare economic gain of 10.4 million ? per year.