Every year up to 13 million hectares of forest are cleared worldwide. But the trees are not just used to make paper, flooring or furniture. Increasingly, wood gasifiers are used to generate heat and electricity for homes, complete industrial areas, housing estates or public buildings such as swimming pools. But how is a tree in the forest turned into useful wood gas? The chippers required and even the wood gasification plants themselves are driven by MTU engines.
Nearly 15,000 people live in the picturesque Lower Bavarian town of Rohr. As part of a move away from fossil fuels since 2014, a retirement home, several apartment blocks and the trading estate have been supplied with wood gas. "We decided on a wood-fired generating plant so as not to be reliant on nuclear energy and fossil fuels. With this plant we can actively contribute to reducing climate change and conserving natural resources and play our part in the energy reforms," explains Heinz Högl, proprietor of the company Högl, who was responsible for implementation of the project in Rohr and operates the CHP module. The scheme is also supported by the German state of Bavaria through a subsidy from the Biosol program. But before the renewable energy can actually be used, a number of preparatory processes are required.
Wood waste for chips
The wood gas plant in Rohr only uses wood that would not be used for anything else. So, for example, forestry wastage. To make the available timber into small woodchips that are as evenly sized as possible, machines such as the DH 811 mobile precision chipper made by the German environmental equipment manufacturer Doppstadt are used. The DH 811 minces the tree trunks into the required rectangular chips, shaking and vibrating as it does so. Sections of trunk between 2 and 6 m long are used. The DH 811 also processes the leftover brush that falls on the mossy forest floor during felling or loading. And the chipper makes no distinction between softwoods such as pine and spruce and harder varieties such as beech and oak. "Our chipper swallows anything. That means any timber with a trunk thickness of up to 80 cm, which is about the depth of a standard desktop," explains Daniel Kürten, product manager at Doppstadt.
Not that sort of chips
If the first thing you think of when you hear the word "chips" is food, you are on the wrong track. What we are talking about are the small, usually oblong pieces of wood into which timber is chopped by special cutting tools. To produce the soughtafter fuel, the precision chipper waits just outside the forest. There, the large grab arm of a crane loads the pre-cut timber onto the chipper's infeed table. The crunching and whirring machine then draws the timber into its enclosed interior. Then everything happens very quickly. Inside the machine, the trunk meets the chipper drum. Five razor-sharp chipping cutters protrude 4.5 cm out of the chipper drum. The blades chop small lumps out of the trunk from above. "You have to imagine it like splitting a log with an axe," explains Daniel Kürten. The blades continually chop small chips out of the trunk. The large blade protrusion means that the woodchips are exactly the right size for burning in a wood gas plant. The wood gas facility in Rohr requires woodchips with an edge length of 50 to 70 mm. What is special about this machine is that the enclosed chipper drum means that the timber is chopped into very precisely sized chips. A chip sack collects the chipped timber before it is thrown with full force through a grading sieve by the rotation of the chipping rotor in the machine. The grading sieve ensures that the chips are of an even size. "We have very few rejects or oversized chips," recounts Daniel Kürten. "That is why this machine is called a Doppstadt precision chipper." From the grading sieve, the woodchips are carried to the machine discharge spout by an auger conveyor. The black discharge spout visible from the outside ejects the finished woodchips directly into a waiting truck or an adjacent woodchip store. The machine spits out as much as 300 m3 an hour of loosely piled chips in that way. That equates to a cube with side length of 7 m.
All power to the engine
"The processing capacity of the wood chipper is also determined by the power of the MTU engine. It is only the wide torque band that makes such a production rate possible," elucidates Daniel Kürten. The driving force for the machine is provided by an MTU Type 6R 1300 engine rated at 390 kW. Above all, the torque characteristics and the high inertia of the chipper drum have to be well matched in order to hold the rotor at its primary operating speed. "The more stable the rotation speed of the chipper drum is, the more evenly sized the woodchips produced are," Kürten expands. At 19 t, the Doppstadt DH 811 is a real heavyweight among chippers and is designed for continuous heavy duty. It is even capable of supplying several heat and power plants at once. The sturdily built machine is made entirely of steel and is extremely tough. "That is also the reason we chose an MTU engine," says Daniel Kürten. "There are very few diesel engines that can really stand up to such continuous loads and still comply with the EU Stage IV emission restrictions."
From chips to gas
So now we have a truck fully loaded with woodchips. But how are those chips turned into heat and electricity for Rohr? The answer is wood gas. It is obtained from the woodchips in a wood gasification plant operated by Holzenergie Wegscheid. "We produce electricity and heat from natural sources," explains Walter Schätzl, managing director of Wegscheid. To do that the company uses woodchips made of spruce, fir, pine, beech or birch. "Our machines do not require any other additional fuels and so do not burn any harmful substances," Schätzl elucidates. A lorry is just driving into the yard. The woodchip order has arrived. As yet, they are still too damp to be burned directly in the plant. "The best energy yield is obtained from woodchips with a residual moisture content of no more than 10 percent," Schätzl continues. Each plant has its own drying boxes or moving-bed dryers on site. This facility consumes around 5,430 m3 of woodchips a year.
Once the residual moisture in the woodchips has been reduced to the required level, they first have to pass through a grading system on their way into the wood gasifier. To obtain the optimum combustion results, the woodchips have to be as homogeneous as possible. To that end, the grading system filters out the chips that are too large or too small. They can then be sold for extra income. Woodchips with an edge length of 50 to 70 mm pass into the supply hopper. "In Rohr there is a constant demand for heat for the hot water supply, for example," points out Heinz Högl. "So that means our machines often run continuously for over 8,000 h a year. A large thermal store provides a continuous supply of heat." The electricity generated is not used by the plant itself but is fed into the local power grid.
The wood gasifier
The woodchips next travel from the storage hopper into the wood gasifier. "Our gasifiers produce gas with almost no tar content because environmental protection and ultrareliability are extremely important to us," explains Schätzl. In the wood gasifier, the woodchips are turned into wood gas. The generation of a flame is prevented by deoxygenating the atmosphere. So the woodchips smolder, giving off wood gas, which after being drawn off, filtered and cooled, can be combusted in the Series 400 gas engine from MTU Onsite Energy. The engine is connected to a generator for producing electric power. The recovered heat from the engine can also be reused for the drying processes, for example. "By utilizing the kinetic energy and recovered heat, an efficiency of 80 percent is achieved," Schätzl expounds. To prevent the engine from being contaminated, a filter developed specially for the purpose by Wegscheid is used. It cleans the gas dry. The heat produced goes directly into the connected district heating system, and the electricity generated is fed into the grid. "The main factors in our choice of MTU were the high-quality product and the international customer service," says Walter Schätzl. "And so far MTU has always been a reliable partner."
Fossil versus renewable
The Rohr plant achieves a high efficiency level of 32 percent electrical and 50 percent thermal. That equates to as much as 230 kW of heat and 125 kW of electrical energy per hour. And it is not only the production of wood gas that is highly sustainable. "The fuel supply system is very eco-friendly as well, because the cycle of tree growing, felling, chipping and combustion is a self-contained carbon-neutral cycle," said Schätzl. And that can be seen in the sales of wood gasification plants as more and more local district heating systems or industries are investing in renewable energies such as wood gas.
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