Residential solid biofuels combustion – continuous improvement over time
Residential solid biofuels combustion – continuous improvement over time | Download the Factsheet
Combustion of solid biofuels in stoves and boilers (<100 kW) contributes significantly to space and domestic water heating in residential dwellings in many countries over the globe. It provides a non-electrified source of energy for space and water heating which helps maintain the reliability of electrical grids that will face growing demands in future from different sectors. It can also support regional energy security for many communities through the use of locally and sustainably sourced biomass resources. It can be combined with other heating technologies, e.g. air-to-air heat pumps and solar heating, to meet the heat demand in an optimum way throughout the whole year. Types of solid biofuels and appliances commonly used in residential heating applications include firewood in wood stoves, inserts and wood log boilers and wood pellets in pellet stoves and pellet boilers (Figure 1).
Modern wood/pellet stoves and boilers have much better performance than older devices.
Figure 1: Pictures of selected technologies, a wood stove (left) and a pellet stove (right).
Residential solid biofuels combustion technologies have been the subject of continuous research and development for many decades, and today’s technologies offer high energy efficiencies and low emission levels (See Figure 2, for the case of small-scale wood boilers). These improvements have come largely as a result of better combustion concepts and control systems and improvements in the quality and consistency of solid biofuels.
Figure 2: Evolution of efficiency and CO emissions from type approval of small-scale wood boilers.
Firewood stoves and inserts are manually operated, while pellet stoves and boilers and often modern wood log boilers are automatically operated. Automatic operation means that the fuel is fed automatically depending on the heat demand and that the air needed to achieve good combustion conditions is supplied in the right amount, at the right place. Automatic combustion units offer higher energy efficiencies and lower emissions compared to manually fed systems, as their operations integrate continuous feeding of fuel and air supply control. Manual combustion appliances operate as batch, and operation of the appliance such as loading of fuel, ignition and setting up air controls is performed by user. As a result, the appliance performance is highly influenced by the user, i.e. user’s knowledge and experience.
The performance of wood stoves and inserts is improving, but still depends on proper handling of the user. Automatic control helps to ensure optimum operation.
Today’s wood stoves can come with automatically controlled air supply, ensuring optimum control to achieve higher energy efficiency and lower emissions. Other emission reduction measures, known as secondary measures, such as catalytic converters and electrostatic precipitators can be applied to reduce emission levels further. Old/poor technology or operating the combustion appliance with poor quality fuel (e.g. high moisture content) or in a wrong way can adversely impact environmental, health and climate performance of residential solid biofuels combustion appliances, especially wood log stoves and inserts. Today’s modern wood stoves and inserts operated with fuel that is of consistent quality and meeting manufacturer’s recommended fuel specifications and in the right way reduce harmful emissions resulting from incomplete combustion. Choosing a quality and robust stove helps maintain a good technical status over the years, without excessive leakage air intake. Manufacturer manuals contain instructions for proper operation. Key instructions that are commonly included in operational manuals include use of only seasoned or dry wood fuel, loading of combustion chamber with an amount of fuel suited for the appliance size and heating demand, and supplying proper amount of air.
In summary, to improve the efficiency and reduce the emissions of residential solid biofuels combustion there should be a continuous focus on:
- replacing old/poor technologies,
- educating the operators in the proper operation of their heating appliances and
- ensuring consistent fuel quality that meets the manufacturer’s specifications.
HIGHLIGHTS of Norway’s SusWoodStoves initiative
Results from the project SusWoodStoves show that modern wood stoves of today outperform the staged air combustion wood stoves introduced in the 1990s, which at that time outperformed the earlier stoves without staged air. This was the result of further continuous improvements and the introduction of new and improved designs.
Take home messages:
• Modern wood stoves have greatly improved during the last 25 years, resulting in large reductions in most emissions associated with incomplete combustion, and in increased energy efficiency.
- Environmental, climate and health impacts are reduced accordingly.
- Still, proper operation and wood fuel quality are key.
- Automation of the air supply contributes to proper operation.
- End-users should preferably operate their appliance at nominal load.
- Different wood species give mostly similar emissions, particularly the ones associated with incomplete combustion.
Figure 3: Relative comparison of emission factors for modern wood stoves. “Old” new stove is an average for the stoves produced from 1998 to 2016, while “New” new stove is an average for representative stove technologies anno 2022. In addition, emissions at nominal and part load operation are shown for these stoves.
Specifically, when comparing the average emission factors for modern wood stoves anno 1998-2016 with three modern wood stoves anno 2022, see Figure 3, it was found that the three modern wood stoves on average have from 50 to 80% lower emissions of PM2.5 (fine and most dangerous particles), CO (carbon monoxide), CH4 (methane), other relatively light gases (NMVOC) and the heaviest gas compounds that condense out as liquid particles in the atmosphere (OC). At the same time the stove efficiency has been improved, from an average of 75% to 80%, which gives further reduced emissions per kWh net heat output.
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Residential solid biofuels combustion – continuous improvement over time is the second in a series of factsheets prepared by IEA Bioenergy. These factsheets aim to inform and engage readers by addressing the key issues related to bioenergy, fostering greater awareness of its potential and challenges. Through these resources, IEA Bioenergy seeks to bridge the knowledge gap and promote the adoption of bioenergy solutions in the context of a global shift towards renewable energy.
Residential solid biofuels combustion – continuous improvement over time | Download the Factsheet