Energy-efficient boilers Aalborg Energie Technik

The Danish company Aalborg Energie Technik a.s. (AET), a leading engineering company and EPC contractor, can now offer fuel flexible biomass boilers with a boiler efficiency of 92 per cent and an reliability of 99 per cent. The company, whose mission is to develop, supply, erect, commission and service biomass fired boilers and CHP plants of 25–170MW thermal input, continues to supply its customers with advanced, economical solutions on a long-term basis.

ENERGY-EFFICIENT BOILERS

IN recent years, a number of Danish companies have begun to lead the way in several key renewable energy markets, and today they are the leading global and European players. For one such company, AET, innovation is the key to its international reputation. Its employees have more than 25 years of experience in the biomass boiler business, giving it a strong in-house knowledge base and allowing it to supply some of the most advanced biomass power and CHP plants in Europe.

The AET solutions

Good engineering is the key to the design and supply of reliable biomass power plants. In order to achieve optimal plant performance day after day, each of the components needs to be carefully engineered and integrated to function as a whole. This can, according to AET, be best achieved by creating a detailed design of the most essential parts in-house.

AET’s knowledge and experience means that it can offer clients biomass plants with a number of advantages. They have an availability of more than 99 per cent (more than 96 per cent with planned stops included) giving stable electricity, heat or steam production throughout the year, and a higher boiler efficiency (92 per cent) resulting in a higher net output for the same amount of fuel. They are fuel flexible and incorporate robust technology which avoids unnecessary shut-downs due to foreign objects that might be hidden in the fuel. Furthermore, they have low maintenance costs and require low in-house power consumption with reduced operating costs. Lastly, they are an environmentally friendly solution with low flue gas emissions.

The AET Combustion System

The AET Combustion System plays a crucial role in the supply of efficient and reliable biomass boiler plants. This concept, which is based on spreader stoker technology and a travelling grate, has been developed and refined, and today it is one of the most suitable technologies for biomass firing.

Burning the biomass efficiently depends on several factors, such as the fuel dosing method and the mixture with air in the combustion chamber. Within the AET Combustion System the fuel is pneumatically

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transported into the combustion zone using AET BioSpreaders for wood chips. The fuel is injected uniformly into the furnace and is spread on the grate by an injection-stoker. Around 50 per cent of the fuel is burned in a fluidised bed above the combustion chamber before it reaches the grate; the remaining half is burned in a fixed bed combustion on the grate. This gives a low grate load, high combustion efficiency and residence times of less than four seconds.

The grate, which forms a ‘floor’ in the furnace, is an AET-BioGrate travelling grate. The continuously moving grate carpet slowly transports the burning fuel layer towards the boiler front where the ash falls into the ash pit. The grate speed can be adjusted to account for variations in fuel quality and ash content.

One of the requirements for achieving good combustion is that air and fuel in the lower part of the furnace must be mixed thoroughly. This is achieved by placing the air nozzles at the right places and injecting the air through the nozzles with high velocity and by a jet layer. Another advantage is that the plant sends less heat throughout the chimney because the amount of flue gas is smaller, meaning less environmental pollution.

Technology for a sound economy

For investors, economy is often the determining factor when deciding to build a biomass power or cogeneration plant. Choosing the right EPC contractor or boiler supplier is in many cases determined by the price, but other aspects need to be taken into consideration. By choosing a boiler with a high efficiency and low in-house power consumption the accumulated income over 20 years can be increased by up to 20 per cent. For example, the in-house power consumption for the entire Western Wood Energy plant was only 1150 kW, equivalent to only 2.1 per cent of the fuel heat input or 6.7 per cent of the gross power output.

Plant availability is another factor to be considered. If this is increased by 400 hours, which is realistic with AET solutions, the accumulated income over 20 years can be improved by around 10 per cent.

Finally, fuel flexible boiler plants also have an influence on the overall economy. A 5 per cent change in the fuel price will result in a change of up to 10 per cent of the accumulated income over 20 years. Since AET boiler plants can be fired with various types of biomass, this enables customers to be more flexible to fuel price variations. Fuels fired in AET biomass power plants and combined heat and power plants include wood chips, bark demolition wood, wood waste, sander dust, agricultural waste and straw. n