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Optimising Combustion in a Portion-Fired Straw Boiler

Developing the

bio-boilers of the future

Straw-fired bio-boilers have undergone tremendous development since they took over much of the heat production in farming during the 1970s. But there is still a long way to go to catch up with ultra-modern wood pellet boilers. Shown here are Erik Fløjgaard Kristensen (left) and Jens Kristian Kristensen (right), a technician in the biomass heating laboratory. They are both working on optimising the straw-fired boilers of the future. (Photo: Anders Trærup)

Straw-fired boilers are used all over Denmark to provide heating for homes and farm buildings, but they create much more pollution than comparable wood pellet boilers. A new project will try to amend this because straw is a very inexpensive form of fuel, and the prospects are therefore considerable for farming and small heating plants.

The annual harvest is a recurring sign of the arrival of autumn in Denmark, where heavy combine harvesters slowly transform the golden-brown landscape into stubble fields dotted with large rectangular bales of straw. This is nature taking its course, and is just as certain as the leaves beginning to fade at this time of the year. But what actually happens to the bales of straw?

In the old days, straw was used as feed and bedding – for filling in beds and for thatching roofs. Nowadays, there are new application options such as straw insulation in experimental concrete buildings. In connection with the oil crisis in the 1970s, straw was also gradually used for fuel on many farms in simple straw-fired boilers. Just like wood, straw is regarded as a carbon dioxide-neutral fuel. And all over Denmark, especially in farming and small decentralised combined heat and power (CHP) plants, straw-fired boilers are used today to provide hot water in taps and radiators.

However, such boilers have developed significantly since the early 1970s. From being simple ovens that most of the time spewed out black and foul-smelling smoke from the chimneys, straw-fired boilers today are ultra-modern fuel boilers with a degree of efficiency close to 90 per cent, where it is hard to see by looking at the chimney whether the boiler is turned on or off.

Minimising emissions

However, this does not mean that the combustion cannot be improved and emissions reduced even more. It is actually a really good idea because low environmental impact from burning has become a very important competitive parameter since green energy and sustainability became buzzwords. In addition, strict emission requirements for solid fuel boilers, as described in the Executive Order on wood-burning stoves, will apply to all new straw-fired boilers after January 2018.

Alcon – a company in Central Jutland that develops, manufactures and sells straw-fired boilers – is therefore collaborating on a project with researchers at the Foulum Research Centre, Aarhus University, to make the company’s most efficient straw-fired boiler even better.

"The idea is to develop the most environmentally responsible portion-fired straw boiler on the market. With a starting point in the market’s best degree of efficiency, we now want to see how we can combine this with the lowest possible emission of environmentally hazardous substances such as carbon monoxide (CO), hydrocarbons and dust particles," says Erik Fløjgaard Kristensen.

And there is good use for this. Product development of the smaller, portion-fired straw boilers has actually been at a standstill ever since 2001 when the Danish Energy Agency abolished the subsidy scheme for small biofuel boilers that had been introduced in 1995 to ensure improved efficiency and lower emissions from precisely this type of boiler.

This generally means today that portion-fired straw boilers still pollute with more CO and other unburned flue gases than automatic straw stokers, and even more compared to wood pellet stokers. And because these emissions can contain carcinogenic hydrocarbons (PAHs), there is a need to improve combustion in these types of boilers.

The right temperature is critical

An important parameter as regards reducing CO emissions and unburned hydrocarbons is the temperature in the boiler’s combustion chamber. If the temperature is too low, the hydrocarbons in the flue gases are not burned well enough. However, if the temperature is too high, it can exceed the melting point of the ash, which can result in problems such as increased emissions of fly ash. Straw normally contains some salts and minerals that can evaporate from the melting ash and condense into a very fine-grained dust – fly ash – when the smoke is cooled on its way out of the boiler.

The temperature must therefore be highest at the top of the combustion chamber, where the flue gases accumulate, and lowest at the bottom, where the ash is deposited. There must also be the right mixing ratio between the amount of flue gases and the amount of combustion air supplied, which is achieved by an electronic control box automatically regulating the amount of primary and secondary air by increasing or decreasing it in relation to the oxygen content in the flue gas.

A standard boiler from Alcon was used in the first stage of the project and, based on experience with this boiler, a new one has been built with a number of improvements. The researchers expect that the new boiler will be able to reduce CO emissions so much that the boiler can comply with future limit values.


Optimising combustion in a portion-fired straw boiler

2016 – 2017

Financial framework
DKK 1.4 million, 
Innovation Fund Denmark

Alcon A/S
ChimneyLab Europe ApS
Eurofins Danmark A/S

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