Emissions from Biomass Combustion

Jun 2024

This article is part of the IEA Bioenergy Annual Report 2023 and was developed by members of IEA Bioenergy Task 32, a group of international experts working to collect, analyse and share strategic, technical and non-technical information on biomass combustion applications.

The article can be found on page 18-29 of the Annual Report.

Biomass combustion is a cost effective and proven method to generate renewable heat and potentially power from biomass resources that are often locally available as a residue from e.g. agriculture, forest operations or wood processing industries. If harmful emissions – such as particulate matter, carbon monoxide (CO) or volative organic compounds (VOC) – are however not properly addressed, the impact of emissions on society may be significant. While biomass combustion technologies have been applied for many centuries, their performance in terms of efficiency, fuel flexibility, emissions profile and costs of application have improved tremendously over the last decades. The member countries of IEA Bioenergy Task 32 have been at the forefront of stimulating R&D in biomass combustion and accelerating their market application. This article provides insight in some of these developments, with a focus on reduction of emissions.

Some of the highlights of the article:

Role of biomass combustion

Biomass combustion technologies can be very effective in substituting fossil fuels and mitigating CO2 emissions. A shift away from traditional biomass use is crucial, as it allows the replacement of obsolete technologies by state-of-the art technologies that have a substantially lower emission profile and also consume less biomass for the same amount of heat production.

Biomass delivers 89% of renewable heat and 23% of all heat worldwide. About two-third of modern biomass heat concerns bioenergy use in industry, while the remainder concerns biomass combustion for heating buildings. The market for domestic boilers and stoves is predominantly a replacement market with little or no expansion, as alternatives for low temperature heating become increasingly available. Most of the anticipated growth in biomass heat is in larger scale industrial and utility scale biomass boilers.

Similarly, in the electricity market, biomass based global power generation is expected to triple from today’s contribution by 2050, representing 5% of power generation. This contribution is crucial to balance the grid which will have a high dominance of variable renewables like solar and wind energy; moreover, it provides opportunities for negative CO2 emissions in the power sector through the combination with capture and storage of biogenic CO2 emissions.

Emissions from biomass combustion

If combustion technologies applied are obsolete, used in a wrong manner or applied in combination with suboptimal flue gas cleaning technologies, its emissions may have an unnecessary high impact on air quality and public health in the area around the installation. This is particularly relevant in densely populated areas where many people are exposed. Measures ensuring low emissions from biomass combustion therefore need to be identified and enforced.

Flue gases from biomass combustion contain various pollutant emissions and can cause different health effects. Most emissions are related to incomplete combustion, particularly solid particles, volatile organic compounds and carbon monoxide. Furthermore, biomass combustion causes additional gaseous pollutants such as nitrogen oxides (NOx), sulphur oxides (SOx), and – in some cases – heavy metals and organic chlorine compounds, particularly when contaminated biomass is used.

The combination of advanced combustion technology, flue gas cleaning technology, and advanced control systems holds tremendous potential for emission reduction. These technologies work together to capture particulate matter, optimize combustion processes, diversify energy sources, and manage energy usage based on demand and weather conditions. By implementing these measures, emissions from residential biomass combustion can be significantly reduced, leading to improved air quality, mitigated environmental impact, and a more sustainable approach to residential heating.

System perspective

In addition to the impact of emissions at stack level on the local environment, it is necessary to consider the system perspective on emissions where biomass growth is included. This is important, since during growth of vegetable material, both CO2 and reactive nitrogen (NOx) are captured as part of the plant tissue. During a combustion process, in principle no more than the amount of carbon and reactive nitrogen contained can be released in the flue gas. Well designed and operated combustion systems release only 10-25% of the reactive nitrogen that was absorbed during the growth phase. Similarly, CO2 emission can be significantly reduced by applying BIOCCS or BIOCCU. On a system perspective, biomass combustion may therefore lead to beneficial environmental benefits mitigating climate change with negative emissions and fighting unwanted eutrophication of nature conservation areas.

Figure: Performance testing of wood stoves at TFZ Straubing, Germany [photo courtesy of Hans Hartmann, TFZ]