Bio Fuels - Wood Heating

Lars Helbro, Energy Adviser, Himmerland's Energy Office

Wood has been used as heat source, as long as anybody remembers. And as subsistence on Earth requires a certain amount of vegetation, this possibility will always exist.

During the last years, other crops have become important for energy purposes - e.g. straw, elephant grass, and rape. There are many possible applications, but today they are limited, one of the reasons being that these fuels require more working up than available fossil fuels.

The use of straw for energy purposes is strongly increasing, partly for environmental reasons (according to a political decision it is prohibited to bum straw at the fields), and partly because it is often profitable even with a low efficiency. Wood is used to a less extent, and the wood used is often utilized with very low efficiency.

Today both wood and straw are waste products from corn production and forestry, windbreak belts, etc., - while other biomass products as rapeseed oil, elephant grass, and energy crops, are grown directly for energy purposes and therefore require larger supply of energy before the fuel is ready for use. Growing of bio fuels on large scale may also have adverse consequences, as exhausting of soil and other environmental drawbacks, if it is a monoculture production using pesticides and fertilizers.

Some energy crops may have other functions that save energy during their growth, e.g. as windbreaks or for waste water treatment.

The available annual resources can cover over 15% of Denmark's present energy consumption. With the planned wood planting and a more efficient utilization of both wood and straw, bio fuels are resources that absolutely should be taken seriously.

Vegetation - Combustion

During their growth plants absorb. solar energy, water (H2O), carbon dioxide (CO2), etc., - emit oxygen (O), and form carbon (C),- in the process named photosynthesis.

During the combustion of carbon the process is reversed, as oxygen (O) is absorbed, and heat (solar energy), water (H2O), and carbon dioxide (CO2) are emitted.

The amount of CO2 being absorbed and emitted is exactly the same. Therefore, biomass is a CO2 neutral fuel, wieved over the time span it takes new plants to reach a size that makes them useable as fuel. For straw it is normally less than one year and for ordinary trees about 15 years.

The CO2 relation is the same for oil, coal, and gas, but the time span needed to restore the CO2 balance is several millions years.

By a total combustion of biomass nitrogen oxides (NOX) form, besides CO2, water, and heat. NO: do not origin from the biomass, but from the nitrogen in the air which is consumed by the combustion together with oxygen. This consequently happens in all combustion processes where atmospheric air is added to have oxygen. NOX combine with the moisture in the air and form nitric acid.

The amount of NO' depends on combustion temperature and the amount of surplus air.

Energy Content

The energy content in 1 kg of totally dry wood is appr. 5.2 kWh. In practice it is not possible to dry wood that much. In Denmark wood is considered to have a moisture content of 20% weight of dry wood.

As it takes energy to boil away this water in the combustion process, it is not realistic to count on larger energy content than 4.5 kWh per kg wood. This corresponds to, that 2 kg wood is needed to replace 1 litre of fuel oil, if both are burned with the same efficiency. By increasing moisture content the energy content decreases, until the moisture content is so high that a total combustion is impossible, followed by a drastic decrease of combustion efficiency (see Proper/Improper Combustion).

In Denmark, it is possible to utilize appr. 1.2 mill. tonnes of wood for energy production annually, only half of this is used today and even with a very low efficiency (appr. 30%). The straw potential is about twice as big.

Environmental Aspects

Wood is completely decomposed in a proper combustion. It must be said to be our most environmentally compatible fuel (except hydrogen), as the only pollutants are NOX, which appear from every combustion where atmospheric air is used.

If the combustion, on the other hand, is incomplete then some harmful and malodorous substances appear, which can be of great inconvenience in the local environment. However, they are not comparable to the substances that occur from combustion of oil and coal and may do irreparable harm to the global environment.

The smoke shows, if the combustion is complete. The more black it is, the worse is the combustion. White smoke is not due to bad combustion, it is the moisture vaporized from the wood.

Proper/Improper Combustion

Wood and straw are gas containing fuels, which means they release gasses when burning. Roughly 80% of the energy is in these gasses which have different combustion temperatures. It is a very complicated fuel we are working with. Therefore it is often seen that in fact only part of it bums, while the rest leaves the fireplace as smoke.

A complete combustion of the gasses demands 3 things:

High Temperature

The heaviest gasses do first burn at temperatures around 900›C. This temperature is extremely difficult to achieve near a water or air cooled iron plate, therefore all fireplaces should be masoned.

The moisture in wood/straw needs heat for evaporation,- the more water the more heat is spent on this purpose and the lower the combustion temperature gets. Therefore it might be even more difficult to achieve an efficient combustion.

Creosote (a mixture of condensed water and unburned gasses) occurs easily, if the wood/straw isn't dried properly, as a lower combustion temperature also results in a lower chimney temperature.

Efficient Mixing with Air

As it is very difficult to mix cold air with very hot gasses, it is necessary to preheat the air strongly or to mix air and gasses under strong turbulence.

It must be possible to adjust the air quantity, while too much air will cool the gasses, and too little will be like driving with choke. In practice an air surplus of 100% will usually secure a good mixing.

If the chimney is too low, too leaky, or has too small inner diameter (e.g. because of soot), it will be very difficult to achieve good turbulence.

Space and Time

When temperature and air mixing are all right, the gasses burn. But they need space and time, without decrease of temperature, to burn completely. Therefore it is not enough, that the fireplace is masoned/well-insulated, - there must also be plenty of space for the burning gasses, before the heat is starting to be removed.

The 3 T'es

A good mnemonic rule is that the 3 Tes must be present: Temperature - Turbulence - Time. If just one of these conditions is missing, the combustion will be bad.

Performance- Consumption

The smaller a fire is, the harder it is to achieve an efficient combustion.

In systems with automatic stoking, where chipped fuel is often supplied (e.g. every 30 seconds), it is possible to go down to performances around 5 kW without getting a poor combustion. However, it will not be possible to achieve higher performances than 15 kW in the same system. In slightly bigger (more ordinary) systems the performance is between 10 and 25 kW.

In general it can be put that the larger nominal power the easier it will be to achieve an efficient combustion.

Unfortunately, this is not in harmony with the heat demand of an ordinary single-family house which is 3-4 kW on a standard day in January. It is therefore necessary to be able to store the extra heat for later use.

Combustion Technologies

District Heating

Both wood and straw can be utilized in district heating systems with automatic stoking.

Straw can be used as bales or chipped, whereas wood can almost solely be used as chips. The boiler efficiency will often be very high, but as the loss in the distribution system is seldom below 20%, district heating systems are not able to compete with efficient individual plants.

In district heating systems the need for storage can be diminished by using several smaller boilers, so the performance can be varied by taking one or more boilers out of operation.

Individual boilers

At the Danish market many boiler types are found, which are stated as designed for wood. Unfortunately only a few of them are able to meet the demands that must be considered in order to achieve an efficient combustion.

Amongst the types able to meet the demands, can be mentioned: automatic stokers designed for wood chips; manually stoked boilers for wood that operates after the principle adverse combustion. Boilers of the total combustion type and far the most under-combustion boilers do not achieve as high efficiency.

Automatic stokers are, on the whole, able to run without heat storage, if they are supplemented by solar heating during the summer. All other boiler types will work better and with higher comfort (less to take care of), if they are combined with a suitable heat storage. For the smallest boilers at the market (18-20 kW nominal power) a heat storage of minimum 1200 litres will be appropriate for an ordinary single-family house. Larger boilers need a larger storage, if the goal is one fire a day.

The possible combinations are countless. The ideal will depend on the heating system in the house (radiators, under-floor heating, etc.) and the possibilities for self circulation between boiler, storage, and any hot-water tank.

As straw release the latent gas quicker than wood, it is more difficult to construct a small boiler that in a decent way fulfil the technical demands on combustion, which should be required for resource and environmental reasons. At the same time, straw is at many farms a surplus product which the farmers have problems to get rid of, so efficient utilization does not play an important role as a sales argument. However, still more boilers, often with integrated heat storage tank, can show quite high efficiency (with proportional lower smoke problems and operation need).

Straw pressed to pills can be used in small automatic stokers with high performance.

Other applications for surplus straw gasification, fodder, in district heating plants, etc. will certainly raise the price of straw so boilers with low efficiency will disappear from the market.

Heat storage. In general the same applies to straw burning as to wood boilers, only the nominal boiler performance is mostly larger, therefore the heat store also ought to be larger.

Brick Stores

Brick stoves (masoned woodburning stoves) have been used for many generations in Finland and Russia, while they have gained access to other places only recently. This kind of stoves are often preferred in places where they fit the floor plan.

The brick stoves are characterized by combining qualities like high efficiency, minimum operation, good heat comfort (equalized heat supply all day), and high social value as the rallying point of the family.

Correctly designed the daily work will be reduced to one daily fire of 1 to 1.5 hours duration.

In brick stoves the extra heat production is stored in the stone mass. It is more difficult to adjust the store size to the consumption than in boiler systems, as the large stone mass around the fireplace will be able to store more heat, but at the same time releases the heat more slowly to the surroundings.

These stoves are extremely suitable for low energy houses with a reasonable open floor plan. The heat distribution has shown to be much better than for iron stoves and. At the same time, burning of dust in the room is avoided, which may be caused by the high surface temperatures of the iron stoves.

Hot water production in brick stoves has shown realistic without placing water cooled surfaces in the immediate vicinity of the flues (with the risk of condensation). An efficient household installation with very high user comfort may very well consist of a brick stove with hot water production to supplement a solar water heater.

Iron Stoves

Iron stoves are found in many versions, e.g. in Denmark and Norway, and are produced both in cast iron and sheet iron. Operationally it doesn't matter what kind of iron the stove is made of, but more work has often been done with the combustion technology in the sheet iron stoves. Thus some of these can prove decent efficiencies in a laboratory, but unfortunately often with heat performances that far exceed the actual demand in an ordinary home.

Here the only heat store is the building. Therefore it is important that the house is constructed in heavy materials which can reduce the temperature fluctuation, and that the residents of the house are prepared for stay in the transport way of surplus heat which gives relatively high temperature fluctuations.

The efficiency of wood use depends very much on the user, as an efficient combustion will often supply too much heat according to the actual demand. This is why many people prefer a lower efficiency (more smoke) and a more even heat supply.

It is a paradox that woodburning stoves with a small heating surface - e.g. without cooling of flue gasses - often operate best, with suitable heat emission, and without intense smoke generation. This is mostly due to the fact that the extra heat production from a sufficient combustion disappears through the chimney.

The flue gas temperature measured right before the chimney is often around 300 - 400›C which is far too much for a good burning system. To compare, the outlet temperature from masoned stoves is typically around 120›C.

An integrated water tank, together with a solar water heater, is capable of meeting the hot water demand, at the same time as part of the surplus heat (or unbumed flue gasses) is utilized. However, it is important to place the tank as far from the fire as possible, not to cool it.

Power Production

Both wood and straw may be used in large thermal power plants.

To avoid unnecessary transport demand, it will often be wise to look at the relation between the local straw/wood production and the energy demand at the same place.

An obvious possibility of generating power in connection with small stoves/boilers is the Stirling Engine (further described in chapter 12) which is characterized by being nearly unaffected by the purity of the fuel and very noise faint.

Gasification of biomass for engines may prove favourable for somewhat larger units (see the text on gasification).

Social Aspects

In addition to efficient combustion, easy operation, high security of supply, and low fuel costs, it is also important that the daily user feels responsible for and gets pleasure from functioning of the heating installation.

District heating has a serious drawback here as it will almost always be others fault, if there is no heat, as well as somebody must do something about it, if it appears that the system pollutes.

A family who is often away from home will be best served by an installation which only needs little operation and preferably at optional times. District heating will of course be perfect here, but also boilers with well-designed heat storage tanks can be used.

If the family is at home at regular hours every day, then brick stoves may be the right choice, if the house is suitable for it.

Iron stoves, on the other hand, need more operation, else the room temperature will deviate too much from the desired average. The stoves where it has been successful to lower the performance without compromising too much on efficiency need much care (frequently stoking). Therefore, these are sure to fit best in group families, or families with pensioners, etc.

As starting point ought to be found the solution where the heat production takes place as close to the consumption place as possible and where the operation is as easily seen through as possible, to avoid operation mistakes.

Possible Future Perspectives

As the moisture content of wood is essential for the degree of utilization and contributes often strongly to problems like smoke, chimney fire, tarry soot, etc. when used in households, then development of condensing boilers would better the possibilities for a reasonable utilization of biomass.

People, who already use dry wood (20% moisture), would achieve a reduction in consumption of more than 30%, as also the smoke temperature could be much reduced. Especially for chip burning large savings could be achieved.

The big advantage according these savings should probably not be evaluated so much on economy, as on the reduction in needed store space and workload which certainly will make more people utilize a completely local resource.