4. Summary and Conclusion

Summary

Several technologies, both primary and secondary measures, can be leveraged to reduce emissions from Solid Fuel Local Space heaters (SFLSH). In this report the focus is on log wood stoves. It is important to operate and design wood stoves correctly to reduce these emissions. According to experts, both operation and design are equally important to reducing emissions. To optimally minimize emissions, the entire system, including the stove, chimney, and additional equipment, should preferably be optimized as one common unit and tested for correct functionality after installation.

It is essential to make sure the stove has proper draft (because most chimneys do not have a flue gas fan, which is used when stoves are certified) and uses appropriate fuel. The draft is affected by downstream equipment like particle filters and flue gas fans. However, the draft should neither be too low nor too high, as many times seen in real life

For the last two decades combustion control for wood stoves have made great leaps to prove that the technology works. ACC was initiated by a few industries and now many have followed, particularly the largest companies. Due to the cost of development, most systems are proprietary to each manufacturer, with each developing their own system. The main purpose of an AAC system is to adjust the combustion airflows during the different phases of the combustion process. While substantially improving the combustion, additional secondary measures can be added downstream of the combustion chamber to further reduce emissions. Both unburnt gases and particles can be catalytically oxidized, while remaining particles can be removed using ESPs. There are currently no practical secondary measures to address NO_x emissions from wood stoves.

Secondary measures require additional investment costs and have limited effect during the ignition, refilling, and the charcoal phases. Electric power is necessary for ESPs. In addition, they also impose higher maintenance costs and effort on end-users. Some secondary technologies also suffer from a lack of user acceptance due to, for example, additional costs, effort, noise, contamination by ﬂakes (ESPs), low availability, or short periods of operation between cleaning requirements. As mentioned in this report, installing separate secondary measures in an already existing stove-chimney system might affect its intended operation (e.g., reduced draft, etc.) if not installed by professionals and tuned specifically for each application.

Ideally, secondary measures should be most effective during operating phases when technical and non-technical primary measures are the least effective, such as in the cold start, start-up, refill, and charcoal phases. However, tests seem to imply that secondary measures primarily reduce emissions during the main combustion phase. Scientific studies have concluded that the highest emission reduction potential for both gaseous and particulate emissions combine primary measures with both a catalyst and an ESP. The studies show that extensive technical primary optimization based on appliance-speciﬁc best-practice user behavior, combined with a catalyst and an ESP, would represent a suitable system to achieve signiﬁcant emission reduction during real-life operation.

In Germany, around nine million wood stoves nationwide were at risk of being forced out of service at the end of 2024 due to the amendment to the German Federal Emission Control Act, which stipulates lower limits for particle emissions and CO. If an appliance does not meet these values, it must either be replaced, retrofitted, or shut down. This means that Germans who owned an old stove had to decide whether to exchange their stove for a modern replacement, shut it down, or retrofit it with an approved secondary technology. As ESPs only remove particles and yield gaseous emissions that are still too high, adding a complementary catalyst is necessary. At this point, the cost of the secondary measures and their effect on emissions and efficiency must be evaluated against exchanging the old stove for a new one.

For comparison, CoPilot Microsoft 365 GPT v5 was used, with references QA controlled by the author, to collect indicative emission reduction potentials for the three wood stove technologies of ACC, ESP, and catalyst. The reduction potentials for PM, OGC, and CO are shown in Figure 16. Note that actual performance varies with appliances, fuel, and operation. Sources: [1,9,11,14].

Figure 16: Indicative emissions reduction chart

A table is provided in Appendix A3 with additional information on PM reduction, CO reduction, OGC reduction, efficiency impact, maturity and availability, approximate cost, and notes on lifetime and maintenance. The additional information in Appendix A3, collected by Copilot GPT v5, as well as the emission reduction potentials vary slightly but are mostly in line with the information provided in this report.

Conclusion

Several technologies can be applied to reduce emissions from SFSHs, applying both primary and secondary measures. Most ACC solutions are tightly integrated into the stove, which is necessary for optimum functionality. If ACC, a primary measure, is integrated into the appliance, the cost versus a manually operated SFSH is expected to increase between €700–€2,000, depending on the level of sophistication. Sometimes sales prices are boosted due to the value of the novelty and to cover the expenses of development. A certain sales volume is also necessary to reduce production costs and retail prices. ACC reduces all emissions and improves efficiency (see details in the report). Adding an ESP, as a secondary measure, costs between €1,500 to €3,000 and reduces particle emissions by 40%–80% (not including condensed particles because the flue gas is not diluted upstream), especially emissions of larger particles, depending on each specific installation. A catalyst, as a secondary measure, can either be integrated into the appliance (preferred and most efficient way due to temperature requirements) or retrofitted close to the stove’s flue gas outlet. Prices for external products start at €250 and can reach up to €1,000, depending on the material. The reduction potential depends on the active material, mainly targeting CO and volatile organic compounds (VOCs), with reductions of approximately 50% and 25%, respectively. Catalysts typically have a limited lifespan and need to be replaced periodically (5–10 years). Both catalysts and ESPs require maintenance to ensure proper functioning.

The complementary application of technical primary and secondary measures is necessary to achieve (nearly) zero-emission technologies. Long-term ﬁeld evaluation under real-life end-user operation and natural draught conditions using different ﬁrewood room-heating technologies is essential. There are still open questions, such as user acceptance of ﬂakes around the chimney when using ESPs and the pros and cons of appliance-integrated ESPs versus ESPs integrated into rooftop chimneys. Further research and development are necessary for these aspects, as well as for maintenance efforts and costs between potential secondary emission abatement technologies. Ideally, secondary measures should be effective during transient combustion conditions, involve low maintenance efforts, be cost-efﬁcient, and enhance user comfort.

Due to increasingly stringent legal requirements regarding emissions and thermal efﬁciency, there is a trend towards (nearly) zero-emission technologies. Therefore, advanced combustion ideas combining both technical primary measures and complementary secondary emission abatement technologies strongly focusing on appliance-speciﬁc best-practice heating operation (ideal user behavior) are essential.

ESPs have become much better within the past five to 10 years. There are already ESPs on the market that are self-cleaning and can operate for several months with little degradation of capture efficiency. However, ESPs are designed with an upper limit in terms of rate of PM capture and flue gas throughput, which should not be exceeded. This requires knowledge of both the selected ESP and the specific appliance. Flue gas composition also plays a role in influencing capture efficiency. Smoke with high amounts of salts and carbon particles have a significantly higher capture rate than flue gas with high amounts of condensed organic components. Carbon particles are readily captured but might cause short circuits due to their high conductivity. Short circuits may lead to re-entrainment, in which the collected particles “catch fire” and thereby release a plume of secondary particles. High amounts of organic compounds in the flue gas, condensing as tar in the ESP, might also lead to short circuits and re-entrainment due to the low conductivity of tar. Furthermore, experimental tests seem to indicate that ESP technology increases the number of ultrafine particles emitted, produces “flaking” around the outlet, and alters the chemical and toxicological properties of the captured particles. Noise from the discharges has also been mentioned on several occasions. Based on the various studies and articles cited in this report, many measurements have been performed on ESPs, both under laboratory and real-life conditions, with a high degree of variation in the capture efficiencies reported. It is therefore difficult to assess the general reduction potential of ESPs as the reduction efficiency depends significantly on each specific installation. More development is recommended, especially for flue gas dilution before ESP and higher capture efficiencies of at least 99%. ESPs are mainly recommended for newer, less polluting stoves.

Regarding catalysts, their integration in domestic wood-burning appliances can be an efficient secondary measure to reduce emissions from wood burning and remedy potential odor nuisance. The highest emission reduction is usually achieved for carbon monoxide. The reduction of hydrocarbons and unburnt organic particles is also possible. The catalyst also works as a sort of filter, retaining some of the fine particles (fly ash). Usually, the investment and installation cost for a catalyst is lower than for an ESP. The highest reduction potential seems to be when the catalyst is integrated into the appliance itself, close to the combustion chamber. The catalyst will work best in a nominal combustion regime. On the downside, a catalyst may increase the flow resistance in case of bad maintenance, leading to poor combustion with higher emissions. Insufficient draught in the chimney also increases the risk of potentially dangerous flue gases leaking into the room, and an additional flue gas fan might be necessary. Catalysts are better suited for new stoves, and not that much if retrofitted in old ones, with high emissions of both particles and pollutant gases. Temperature peaks of at least 300 °C are required for proper operation of the catalyst, which is hard to comply with when retrofitted outside of the appliance.

As with all technology invented by humans, when the above-mentioned primary and secondary measures are used in real life, user errors occur. The emission reductions reported here are mostly taken from either the industry itself or from laboratory tests. To be certain of actual improvements, either a more true-to-life certification method should be developed or long-term field measurements should be gathered. All these technologies function properly only when using the correct fuel type, in the right amount, with the appropriate moisture content and log size. Additional technology may be necessary to monitor and guide the user in proper usage.