Fossil Fuels Combustion
Burning any fuel involves a reaction between oxygen and the constituents of the fuel. The main purpose of the process is to produce heat, and a packaged CHP unit converts some of this heat into power. However, burning any fuel gives off pollutant gases into the atmosphere, and these have damaging effects on the environment.
All fossil fuels contain carbon, which is the main combustible constituent. Hydrogen is the other significant energy source. The combustion of carbon and hydrogen produces large amounts of carbon dioxide and water vapour in the exhaust products. Combustion processes also produce other gaseous pollutants, depending on the composition of the fuel and the type of combustion process. It is important to consider the emissions from the following gases:
Carbon Dioxide
Carbon dioxide is the gas produced in the greatest quantity by combustion processes. The aim of any combustion process is to combine all the available carbon with atmospheric oxygen (O2) to release the maximum available energy, and this produces carbon dioxide gas.
Carbon dioxide is an inert gas that is present in the atmosphere as part of the earth’s natural carbon cycle. However, increasing CO2 levels enhances the ‘greenhouse effect’ and increases the risk of long-term climate change. There are now global agreements in place to reduce carbon dioxide emissions. The amount of carbon dioxide produced depends on the carbon content of the fuel concerned and the rate at which that fuel is used. Only by burning less carbon can carbon dioxide output be reduced. Any technique that improves the efficiency with which fuel is used reduces the level of emissions per unit of heat or power output.
Carbon Monoxide
Carbon monoxide is a toxic gas in high concentrations, and its presence in the atmosphere can contribute to local smog and respiratory problems.
Carbon monoxide is produced when there is insufficient oxygen available to completely burn all the carbon in the fuel. The quantities of carbon monoxide produced are usually small, but they can increase significantly if the CHP package is not correctly controlled and maintained. Emissions of carbon monoxide can be limited by proper control and operation of the CHP package.
Nitrogen Oxides
Within the high-temperature part of the fuel burning process, oxygen (O2) and nitrogen (N2) from the combustion air react together to form two oxides of nitrogen. These are nitrogen oxide (NO) and nitrogen dioxide (NO2),and they are generally referred to as NOx.
There are several adverse environmental impacts of NOx formation:
- Nitrogen dioxide can contribute to smog formation at ground level.
- Nitrogen dioxide is harmful to the respiratory system.
Atmospheric reactions involving nitrogen oxide result in the formation of nitric acid (HNO3), which contributes to acid rain.
Particulates & other compounds
In general, the use of clean gaseous fuel in a CHP package will not give rise to emissions of particles or other compounds. However, if the unit is not correctly controlled, soot or smoke may be produced in the exhaust, and excessive engine wear can result in some lubricating oil passing into the exhaust. Furthermore, the exhaust from gas engines tends to contain small quantities of the gaseous fuel – generally referred to as unburned hydrocarbon (UHC). Using waste gases as fuel can also result in other compounds being present in the exhaust, the nature of these compounds varying with the composition of the gas.
Other Topics
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