Burning wood emits more CO2 per unit energy than burning fossil fuels. This isn’t controversial – yet we are often asked why this is true.

There are two main parts to this calculation:

1. The CO2 emitted per unit energy inherent in the fuel (what would be emitted if the fuel were burned with 100% efficiency). IPCC data show that CO2 content of wood per unit energy is similar to coal, and much greater than gas.

2. The efficiency of the power plant as it converts energy inherent in the fuel to “useful” energy (megawatt-hours of electricity, or useful heat). Dividing the CO2 content of the fuel per unit energy by the power plant efficiency produces the CO2 emitted per megawatt-hour.

Here we do the calculations for standalone electricity plants.  Power plant efficiencies (for plants generating electricity only) vary greatly. The highest efficiency plants are combined cycle natural gas plants, which can reach up to 63% efficiency.

Biomass plants are among the most inefficient. Data from California puts average biomass efficiency at 22.7% using the US approach to efficiency calculation that employs the higher heating value and assumes use of a steam turbine and green wood chips, the most common form of biomass power conversion. Converting the US efficiency value using the EU approach that employs the lower heating value, the equivalent efficiency value is about 25%. However, this is an average. Small electricity-only plants may have efficiencies even lower than this, while gasification boilers may have somewhat higher efficiency values, although overall facility efficiency tends to be decreased due to “parasitic load” of energy required to run the plant.

Higher efficiencies may also be obtained when biomass is processed and pre-dried (as with wood pellets, which have a moisture content of around 10%, in contrast to green wood chips, which typically have moisture content of 45 – 50%).   Processing and drying wood pellets requires large energy inputs and emits large amounts of CO2 that is not associated with energy generation, so for the sake of simplicity, we do not include wood pellets in this example.

Putting these steps together in the table (using the US approach for calculating efficiency) shows the degree to which biomass CO2 emissions exceed emissions from burning fossil fuels, per megawatt-hour electricity produced.  At these power plant efficiencies, wood-burning power plants emit from 36% to 400% more CO2 per megawatt-hour than plants burning fossil fuels.  For combined heat and power plants, where the differences in efficiency between biomass and fossil fuels are not so large, the discrepancies are smaller.

Biomass plant CO2 emissions – an explanation
Tagged on:             

Leave a Reply