In times of nuclear and coal phase-out, the question of a safe and clean energy supply is increasingly in the focus of power producers. MAN Engines has been offering solutions in this sector for decades, supplying CHP manufacturers with robust, clean gas engines. The use of special gases such as biogas and sewage gas has proven to be particularly sustainable and popular. But with wood gasification, a lesser-known technology has recently come back into focus. Here, too, MAN Engines and its partners serve a still small but steadily growing market with powerful gas engines.
According to the Federal Ministry of Food and Agriculture, Germany currently has an annual forest growth of around 122 million m3 and a wood consumption of 76 million m3. Wood gasification is therefore definitely a technology that can be an interesting building block in the energy mix when using wood as a renewable raw material in the context of the energy transition. The principle of producing a combustible gas from the solid raw material and using it to operate combined heat and power plants (CHPs) is the most promising. To achieve this, the wood must first be transformed into gas in a so-called carburettor. In the process, it undergoes a thermochemical, multi-stage conversion process, at the end of which wood gas is used as fuel. This gas can then be used to operate a combustion engine for direct mechanical energy generation. MAN Engines currently offers special gas engines with outputs ranging from 54 kW to 320 kW for this purpose and has been pursuing the development of wood gasification technology with partners such as the Austrian boiler manufacturer Fröling for some time. Wolfgang Aichinger, authorised officer and sales manager at Fröling Heizkessel- und Behälterbau Ges.m.b.H., summarises the advantages of wood gasification: “The process is simpler and faster than gasifying biomass into biogas. On top of that, it takes place in the carburettor directly on site. The fuel doesn’t first have to be brought to the combustion engine via pipelines.” Thus, wood-fired power plants can make a contribution to decentralised energy supply, especially in rural areas.
The idea of using wood as an energy source is almost as old as mankind itself. Especially the heat released during combustion has been appreciated for thousands of years. But the use of wood gas as a fuel is not a modern invention either. Wood carburettors were the most widespread alternative propulsion systems in times of raw material scarcity, such as World War II or the oil crises. As early as the end of the 1930s, wood carburettors were installed in MAN trucks and tractors, for example. In the long run, however, wood gas was never competitive with cheaper fuels such as oil or natural gas. But thanks to improved production processes for wood gas and more efficient engines, the technology has now become an interesting alternative.
A trend that Fröling Ges.m.b.H. recognised early on. The company from Upper Austria is considered one of the pioneers of wood gasification technology and has continued to perfect the gasification process. It relies on gas engines from MAN Engines to convert the wood gas into electricity: “The engines used must be able to cope with the properties of weak gases, which include wood gas. They must be low-emission, easy to maintain and as efficient as possible. MAN engines meet these requirements and are the best fit for our system,” explains Aichinger.
The greatest challenge for the engines arises from the large component of carbon monoxide and hydrogen, which distinguishes the wood gas composition significantly from other fuel gases. Natural gas and biogas, for example, consist predominantly of methane. In addition, the gas quality of wood gasification varies more than that of natural gas and biogas, which creates difficult conditions for combustion engines. MAN Engines produces particularly robust engines that can also cope with special requirements, such as operation with gases with a high hydrogen content. Basically, the engines developed specifically for natural gas and special gas applications are designed in such a way that they can be used for almost any application without major adaptations. This is why the gensets from the MAN Engine Competence Centre in Nuremberg are not only used with natural gas and biogas, but are also found in more specialised applications such as wood gas CHP units.
The use of wood gas CHP units in wood-processing industries with a high heat demand is particularly interesting from an economic point of view. The Austrian wood chip producer “Hackgut Möslinger GmbH” operates a Fröling wood-fired power plant. Four MAN E0836 LE202 industrial gas engines – each with an output of 50 kW at 1,500 rpm – are installed on the company premises in Gaspoltshofen. Thanks to optimised combustion processes, the already high efficiency of the units is further increased by turbocharging the engines.
The electricity generated by the plant flows into the Austrian grid in return for a feed-in tariff, and the heat generated during electricity production is used directly on site for the company’s own operations. This heat is used to dry wood chips on a belt dryer, which is then used as fuel by wood gasification plants. “Theoretically, any type of wood can be used, but there are big differences in quality. Mixed wood with a high proportion of logs and willow is best. It is particularly important that the wood is as dry as possible,” says Aichinger. Wood fresh from the forest has a water content of 40 to 60 % and thus has a heating capacity of just under 2 kWh/kg. In contrast, dried wood chips with a water content of less than 15 % have a calorific value of about 4.5 kWh/kg.
“Hackgut Möslinger” is an ideal example of the efficient use of a wood gasification plant. But according to Wolfgang Aichinger, there are also new opportunities for other companies, such as sawmills: “Wood residues produced in the wood-processing industry – which could not be used before – get a whole new value as a fuel through wood gasification technology.” In addition, wood gasification plants with CHP units achieve peak overall efficiencies of over 90 % by means of cogeneration, of which about 30 % is electricity and 60 % heat.
In addition to their high efficiency, CHP units that run on wood gas are also convincing due to their positive eco-balance. The energy is produced on site, where it is needed. Furthermore, wood is a naturally renewable raw material that can be found almost everywhere. Long transport routes are unnecessary. In addition, some of the carbon dioxide released was initially absorbed from the air by the trees used in the wood gasification process. Aichinger sees good prospects for the technology in the medium term: “We expect a large market for decentralised systems due to the relatively simple technology. Here, wood gas seems predestined.”
Wood gasification is a thermodynamic process in which the solid biomass wood is converted into a gaseous fuel. The raw material wood is first dried at 100 to 200 °C and the macromolecules are pyrolytically decomposed at 200 to 600 °C before the chemical processes of oxidation up to 1,200 °C and reduction at approx. 900 °C take place. In the process, hydrocarbon compounds, pyrolysis oils and pyrolysis coke are produced at temperatures in the range of 150 to 500 °C. During oxidation, hydrocarbon compounds are split into smaller molecules by reaction with oxygen. Carbon monoxide, hydrogen, water, carbon dioxide and methane are produced. The subsequent reduction of carbon dioxide and water with carbon produces additional carbon monoxide and hydrogen, which forms the main part of the combustible components of the product gas.
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