Wood-burning stoves: arguably the world's cosiest source of heat – but also a sizable source of air pollution. A greener future might be in store, though: a new type of electronically controlled wood-burning stove limits the emission of smoke and particles while ensuring up to 40 per cent better utilisation of the firewood. (Image: Colourbox.)

Burning wood on autopilot

Monday 19 Jan 15


Weigang Lin
Associate Professor
DTU Chemical Engineering
+45 45 25 28 35
A more environmentally friendly wood-burning stove, with a remote control that lets you set the room temperature and tells you when it’s time to refuel. That’s the result of a Danish R&D collaboration to bring the first electronically controlled wood-burning stove on the market. (The article is retrived from Technologist Online, Jan. 14, 2015).

Winter has arrived, which means that in millions of European homes, people will once again be lighting their wood-burning stove. A bit of kindling, a few logs and a match is all you need to create the world’s cosiest source of heat. Unfortunately, it’s also a source of considerable air pollution.

Wood-burning stoves of an earlier date and poor firing methods are to blame for the emission of a series of air pollutants during combustion. Even modern stoves and sound knowledge of proper firing methods rarely ensure maximum energy use of the wood.

These problems have sparked public debate and raised the question as to whether wood-burning stoves should even exist, which poses a major challenge for producers of wood-burning stoves.

In Denmark, one of the country’s largest producers of wood-burning stoves turned this challenge into a green business opportunity by teaming up with researchers at Technical University of Denmark (DTU).

“All the talk about air pollution in the suburbs affected our business, so we decided to find a solution to the problems,” says Vagn Hvam, who founded HWAM in 1973. The business now exports around 80 per cent of its production to European customers.

“We came up with the idea of controlling the combustion process by means of electronics, but after a couple of years of developing the system, we realised that we needed more competencies – more knowledge of the complex combustion process – to move things forward. Therefore, we contacted DTU where Kim Dam-Johansen [the world’s most cited researcher within combustion in 1998-2008, according to Thomson Reuters] said our idea offered strong potential.”

The challenge: environmentally conscious combustion

It requires almost superhuman powers, or at least constant monitoring of the combustion process, to regulate the air supply and ensure precise combustion without emission of particles, volatile organic compounds and carbon monoxide (CO) – while at the same time ensuring sufficient utilisation of the firewood.

“Environmentally conscious combustion means ensuring a sufficient oxygen level. In case of oxygen deficiency, volatile organic compounds will not be combusted and can subsequently – even immediately – condense into hazardous particles in the chimney,” says the DTU project leader Jytte Boll Illerup, who has more than 25 years of experience researching combustion and emissions.

“Too much air, on the other hand, will reduce efficiency and thus result in poor utilisation of the firewood. Our studies show that optimal control of combustion in a wood-burning stove is virtually impossible if done manually. This is where HWAM’s automatic control system proves to be very helpful.”

The solution: automatically regulated air supply


In the test hall, the stoves are fired under controlled conditions using online measurements of the most essential process parameters as well as the O2, CO, CO2, NOx, SOx gas components and volatile organic compounds in the flue gas. Particle emissions are also measured. (L – R): DTU researcher Weigang Lin, HWAM founder Vagn Hvam Pedersen, and DTU project leader Jytte Boll Illerup. (Photo: Joachim Rode.)

In the DTU Chemical Engineering test hall, each stage of combustion in the wood-burning stove is carefully monitored – from arranging the firewood to removing the ashes.

This takes place using a unique particle collection system, which makes it possible to view and analyse the composition and size of the particles in the different stages of combustion: lighting stage, flame stage and coke stage.

“Time, temperature and turbulence – these are the three parameters that need to be controlled very carefully to ensure optimal combustion,” says Illerup. “Which definitely isn’t easy without any measuring instruments. It isn’t always possible to estimate whether oxygen and temperature levels are sufficient, and whether the correct amount of draught is supplied through the chimney.”

This is where HWAM’s electronically controlled stove comes into the picture. The three air intakes in the stove are regulated by motorised dampers which open and close by means of a computer program based on measurements of the room temperature as well as the temperature and level of oxygen in the flue gas. The user simply needs to set the required room temperature on the remote control, and the user will then be notified when it’s time to refuel the stove.

Autopilot reduces gas emissions and increases fuel efficiency

When the first version of the autopilot was ready in 2012, DTU arranged for five private users of wood-burning stoves to test it. Measuring equipment was installed in their homes, and the users were asked to use their stove in the same way as they normally did. After a week, they were given a stove with an integrated autopilot, and were asked to fire it in the way they normally did – for instance using the same kind of firewood.

“The measurements showed a significant improvement in combustion when using our system, including reduced emissions of uncombusted gases and reduced fuel consumption,” says Hvam. “But when the users were interviewed about the whole experience, they particularly underlined the comfort. It was so much easier to light the stove and maintain a stable room temperature.”

According to Jan Bünger, a project consultant from the Danish Energy Agency, the collaboration between the company and the university is benefiting the environment as well as helping to create jobs, laying the foundations for more green businesses. “Wood is a sustainable source of energy, but the energy efficiency of wood-burning stoves needs to be improved and particle pollution reduced. These are the challenges addressed by the project,” he says.

The smart stove: a win-win for the environment and the wallet. Data from sensors in the stove are used to calculate how much air is needed for optimal combustion – which varies greatly between the different stages of combustion. The smart control system ensures less release of harmful gases and particles into the environment and more economic use of the wood. (Illustration: Lasse G. Jensen.)

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