CO and PAH emissions from engines operating on producer gas

High carbon monoxide (CO) emission from gas engines fueled by producer gas is a concerning problem in the struggle to make biomass gasification for heat and power production a success. The standing regulations concerning CO emissions from gas engine based power plants in most EU countries are so low that no plant could uphold regulations without expensive equipment for after treatment of the emissions. CO emissions from engines operating on biomass producer gases are high, especially at very lean conditions where the emissions exceed the regulated value significantly. The high CO emissions are mainly due to the high content of CO in the fuel and can – in origin – be compared with the emission of unburned hydrocarbons (UHC), like the UHC emissions from natural gas engines, CO emissions producer gas engines are a measure of fuel passing unburned through the combustion. Measurements of the slip of the producer gas fuel components CO and CH4 showed that these are similar, the slip is a measure for the amount of a fuel component that passes unburned through the combustion process. The measurements show that the emission of CO from the engine is an emission of unburned fuel similar to the emission of UHC. When the environmental effect of the emissions is discussed, unburned hydrocarbons in the form of methane is a strong greenhouse gas (21 times higher than CO2) will CO only indirectly through photochemical reactions is involved in the production of the greenhouse gas ozone. The destruction of CO in the atmosphere may both lead to formation or destruction of ozone. Carbon monoxide concentrations are though short-lived in the atmosphere and are spatially variable. Regulations for CO emissions have been applied for solid fuel fired power plants for many years. The argument for regulating the CO emission has from the start been that a high CO emission is a measure for incomplete combustion of hydrocarbons and thus an indicator for the presents of organic micro emissions such as polynuclear aromatic hydrocarbons (PAH). Measurements from two operating gasification plants show that there is no correlation between the high CO emissions and PAH emissions from engines operating on producer gas. The measured PAH emissions were more than 20 times lower than the recommended emission limit for gas engines. Since unburned CO and UHC are similar in origin the reasonable regulated limit for CO emissions from engines operating on biomass producer gas should similar to the regulation for UHC.