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Author(s): Samuel Stucki, Alfred Waser (external authors)
Editorial office: WSL, Switzerland
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Wood – A versatile, renewable energy resource

At least 2 million additional cubic meters of wood could be sustainably harvested each year from Swiss forests. Wood is a CO2 neutral energy carrier, and the most important biomass resource in Switzerland for the foreseeable future. Since the amount of energy wood is limited, the focus should be on the most valuable energetic uses, without additional burdens on the environment. This task can most elegantly be solved when the wood is transformed into synthetic natural gas (SNG). Paul Scherrer Institute (PSI) has developed such a gasification process for the first time.

Buchenstamm

Fig. 1 - Wood provides about 2% of the primary energy supply in Switzerland today. Estimates suggest that two to three times as much could be used.
Photo: Thomas Reich (WSL)

 
Energy wood from Swiss forests
Fig. 2 - Energy wood potential from Swiss forests. (Source: BFE). Click on image to enlarge.

Energy wood from our forests can, for example, replace heating oil - and so provide a contribution to climate protection. But direct combustion in current, small heating systems without costly filters can create additional air pollution emissions - principally par ticulates and nitrous oxides.

Biomass wastes from household, agriculture and sewage treatment plants are already fermented into biogas in many locations in Switzerland. But for wood the process is not so simple, because it leaves such fermentation plants practically "undigested". Therefore PSI has developed a globally unique, new multi-stage process that gasifies wood and then catalytically transforms it into synthetic natural gas (fig. 3). Translating this process to industrial use can provide an ecologically and economically sensible alternative to the decentralized and direct burning of wood. SNG could be distributed through the natural gas pipe network, and used with high efficiency for a wide spectrum of applications, e.g. gas heating, fuel for gas vehicles or in gas-fired power plants for electricity and heat. What path is most economic depends upon political conditions.

The PSI technology for wood methanization is now being demonstrated in Austria at an existing wood gasification plant on an industrial scale (fig. 4). The basis for a first large plant in Switzerland has been established.

Innovative uses for wood energy

Wood provides about 2% of the primary energy supply in Switzerland today. Estimates by the Federal office of Energy (BFE) suggest that two to three times as much could be used to meet energy needs, above all from forest wood (fig. 2). In order for such an expansion to be sensible and sustainable, this wood must be prepared so that it is usable in ways that are practical, clean, flexible and affordable. this means that in the future wood should also be converted to high value electricity and fuel. Whenever possible the resulting heat should be used at the same time.

When biomass decomposes no more CO2 is set free than the plant has absorbed from the atmosphere during its growth. So the use of biomass is climate neutral, as long as no more is harvested than is regrown. In view of climate politics we should therefore increasingly use it, for example to replace oil and gas. In the short to medium term this is both technically and economically possible. However the avoided CO2 emissions should not be purchased at the cost of higher emissions of other air pollutants.

No more pollutants please

Until now, energy wood and waste from wood processing have almost exclusively been burned to provide heat. The forestry program passed by the Federal Office of the Environment (BAFU) in 2005 has now formulated a list of targets and measures for a sustainable forest economy and a contribution to the net reduction of CO2 emissions.

Among other goals, one target is to double energy wood use from forests over current levels by 2015. To avoid a parallel doubling of particulate and other air pollution emissions, technologies are needed that work with first class filter systems. This makes burning wood more expensive and provides a drive towards large plants, e.g. large cogenerators that produce electricity and can deliver the resulting heat to a district heating network. A cogeneration plant of this type is being built in Basel. It will convert 65'000 m3 of wood per year to 20 GWh of electricity (enough for 6700 households) and 100 GWh of heat (5500 households), and be the largest wood energy installation in Switzerland - a good thing. How many such plants are possible in Switzerland depends essentially on whether district heating networks can be found that need (as far as possible) a constant, year-round delivery of heat.

From wood to gas

To further avoid air pollution wood can also be transformed into gas, similar to the gas from known biogas plants. The process developed by PSI produces synthetic natural gas (SNG) that burns cleanly, e.g. in a gas combined cycle plant, a gas motor or in high temperature fuel cells. Wood gasification plants, as conceptualized for Switzerland, would be of a similar size as the wood cogeneration plant in Basel.

transformation from wood to SNG
Fig. 3 - The most important step of the transformation from wood to SNG
 

Wood in the tank and on tap

Biomassekraftwerk Güssing
Fig. 4 - The biomass power plant in Güssing, Austria.

The production of methane from wood is an environmentally friendly and economic alternative to the decentralized burning of firewood. Energy wood is transformed into a conventional energy carrier, and so opened up to a broad spectrum of convenient applications – independent of a heating network. Whether synthetic natural gas replaces fuel or produces electricity and heat depends upon the political and financial conditions.

Wood gasification has been used since the Second World War to provide transportation fuel. But due to its composition, this wood gas cannot be fed directly into the natural gas distribution network. It must be chemically changed so that it is composed predominantly of methane, the principal component of natural gas. PSI has researched the necessary basis for such a process, called catalytic methanization.

This new technology is being adapted to further upgrade a wood gasification process developed in Austria. Through a cooperative effort with the Technical University of Vienna, the biomass power plant in Güssing (fig. 4) and one plant construction company in both Switzerland and Austria, a complete technology for the production of SNG has been developed. It will now be tested on an industrial scale in the framework of an EU project with support from swisselectric research, thus preparing the way for a first commercial installation in Switzerland.

The high value SNG can be readily mixed with natural gas, and transported over the existing natural gas distribution network. Together with treated biogas from the fermentation of green biomass and agricultural waste, SNG can replace natural gas everywhere it is used today and in the future, preferably in transportation as a low emission fuel for gas vehicles. That also makes more ecological sense than use for heating, where there are already climate-friendly alternatives to heating oil. In the longer term, SNG can also be used for electricity generation in gas-fired power plants and cogeneration plants. An SNG plant that is conceivable in Switzerland in five years time would produce 18 million m3 of SNG annually from 50'000 tonnes of wood (with 10% moisture content), enough to drive a fleet of 13'000 gas cars each for 15'000 km. Twenty such plants are theoretically possible.

Costs

If wood is available for 70 CHF per cubic meter, then a plant of about 20 MW capacity can produce SNG for 8 to 10 Rp./kWh. That is three to four times more expensive than the price of imported natural gas from Siberia. The price of wood is critical in two regards. If it is too low, wood will not be harvested from the forests. If it doubles to 140 CHF/m3, the gas production costs will climb to 12 to 14 Rp/kWh. For such a biofuel to be successfully introduced into the market under these conditions it must be freed of the petroleum tax, a political position that has already been proposed. If the SNG is used in gas power plants or cogeneration plants, then higher efficiencies of 60% and low investment costs lead to interesting average generation costs. With a range of 15 to 20 Rp/kWh, these costs are comparable to those of a wood-fired power plant of similar size.

International Dimensions

The European Union has set ambitious goals for the use of biomass in the energy supply. Biofuels should supply 5.75% of the fuel demand by 2010, and even 8% by 2020. In the short term, this requirement can only be met by ethanol or plant oils from specifically grown crops. But synthetic fuels via gasification better exploit the biomass energy and bring a higher return. They therefore are in the center of technological and industrial interest. In the EU the concentration is on processes that follow gasification with a Fischer- Tropsch synthesis (so-called biomass to liquids, or BTL) to produce synthetic diesel fuel. This technology is very demanding, and only economical in large plants (500 to over 1000 MW).

There are no appropriate sites in Switzerland for such plants. In contrast, for SNG production the plants that are envisioned are approximately 20 times smaller and appropriate to Swiss geographic and economic conditions. But the production of SNG is also more attractive and economic when the logistics can be managed more rationally, i.e. when there is an abundance of wood and the transportation distances are short, as in the forested regions of northern Europe. There has already been interest expressed from Scandinavia in plants of the 100 MW class.

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