Coal, natural gas and oil are the most widely used carbonaceous energy at present. Due to composition and energy characteristics of fossil fuels and technical utilization efficiency, different fossil fuels have various CO
2 emission characteristics. Among fossil fuels, coal has the highest carbon content ranging from 0.024 kg /MJ to 0.026 kg /MJ [
13], which means that 0.08 kg ~ 0.1 kg CO
2 will be produced by direct combustion of coal with every 1 MJ energy released. CH
4 is the main component of natural gas with the carbon content about 0.015 kg/MJ, which is only half of that of coal. Obviously, natural gas is lower carbon than coal from the perspective of CO
2 emission characteristics. On the other hand, the average efficiency of coal-based power generation technology is only about 40%, while combined cycle burning natural gas can reach higher than 60%. Correspondingly, about 1 kg of CO
2 will be emitted into the atmosphere for every 1 kWh of electricity output by traditional coal power plants, and natural gas power plants adopting combined cycle only emit about 0.35 kg of CO
2 to the environment for every 1 kWh of electricity output, which is about 1/3 of that of coal power plants.
Figure 2 shows the carbon emission intensity of different fossil fuels with different power efficiency, and renewable energy. Peat and coking coal represent two typical coals with low and high carbon content per calorific value, respectively. When power generation efficiency (
η) equals to 100%, the value of carbon emission intensity equals to the carbon emissions per unit of fuel energy (44/12
C). It’s naturally that the net carbon emission intensity are both 0 gCO
2/kWh for carbon-neutral and carbon free renewable energy. Although biomass will produce a certain amount of CO
2 emissions during collection, processing (~ 20 gCO
2/kWh emissions) and transportation (about 3 gCO
2/kWh emissions for 50 km) processes [
15], the emission amount is very small and can be ignored. Thus, carbon emission reduction can be achieved by increasing the proportion of renewable energy in the power system.