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Solid sources of bioenergy

Solid sources of bioenergy

At present, the most important bioenergy source is wood. Wood has been used for heating since the dawn of humankind. Each year millions of cubic metres of wood are burned for heating, in the form of firewood (typically logs), wood chips, pellets and briquettes. The most significant woodbased fuel in Germany is firewood from the forest - often sourced and collected by the end-users directly. Yet wood from one's own garden or from landscape conservation activity also merits mention, as do untreated used wood, remnants of logs from saw-mills, wooden briquettes and wood chips sourced from forest wood.

Heating with wood in private households

In private households, it is predominantly single-room fire units, such as stoves, masonry heaters and tile stoves etc. that are used to heat single domestic rooms or living areas. In most cases they supplement the central heating system and are often only used occasionally. A good one million households in Germany have a wood-burning central heating facility (log gasification boiler, pellet heating, wood-chip heating, etc.), supplying all rooms with heating via the water-operated central heating system, and usually simultaneously heating the water used for other domestic purposes.

Today, thanks to technical progress, modern biomass installations – such as pellet stoves/pellet boilers, log gasification boilers, and wood chip heating – achieve effciency grades that in many cases are already significantly over 90 percent. The technical development is remarkable: accordingly, modern installations are achieving effciency grades around 20 percent higher than woodburning boilers installed 20–30 years ago! By now, several manufacturers have also brought condensing technology to market readiness with regard to wood-burning heating units. Boilers with condensing technology use the fuel's energy content almost in its entirety, by also making use of the condensation heat of the water vapour in the flue gas. This makes them particularly effcient.
Modern woodburning systems are characterised by very low emissions of particulate matter, carbon monoxide and nitrogen oxides.

Stoves and boiler stoves

Stoves and boiler stoves, but also other singleroom fireplaces such as masonry heaters, clay stoves, tile stoves, wood-fuelled cookers etc. are highly popular in Germany; nowadays hardly any new house does not feature such a single-room firing installation. They lend the living space a certain ambience, a cosy radiant warmth, and convey a sense of security just in case the central heating system is ever out of action. With high fuel prices for oil and gas they can make a substantial reduction to heating costs.

Single-room fire units, such as stoves and boiler-stoves, are increasingly also offered and in demand as a water-operating model that supports the home's heating operation. In households of working people, stoves and boiler stoves are the first choice, because shortly after they are started up they provide a beautiful interplay of flames and also cosy warmth.
For houses occupied all day, e. g. homes occupied by two or more family generations, the spectrum on offer includes masonry heaters, clay stoves, heavy tile stoves and soapstone stoves, as well as other thermal-storage heating systems. Ignited in the morning and supplied with a good pile of wood, it takes a certain amount of time before the stove mass heats up, but then it exudes a pleasant radiant warmth the whole day.

Apart from pellet stoves, single-room fire units are fuelled by firewood (typically logs) or wood briquettes. 

Pellet boilers

Thanks to wood pellets, there has now been a new fuel on the market for about 15 years: it has given a powerful impetus to product development of highly effcient and particularly low-emission wood-fuelled heating units. There is a diverse product offering of pellet-fuelled heating units and of pellet stoves for households, operating on water-based and air-based systems. Cookers fuelled by wood pellets are available, as are stoves with pellet modules: according to choice, these can be fired by hand, using firewood and wooden briquettes or – e. g. if the home occupant is away during times of absence – automatically supplied with wood pellets. With the pellet heaters, wood-fuelled condensing boilers were created for wood fuels for the first time.

Pellet heaters are also offered at very low heat-capacity levels, making them the ideal heating option for modern passive houses or low-energy houses. Here in many instances it is enough to have water-operating pellet stoves – mostly used in combination with solar thermal installations – to completely take over heating the home and also heating the water used for the home's other needs.

Pellet stoves and pellet boilers are also equipped with electronic control and adjustment, managing the supply of heating in accordance with needs and providing optimum, low-emission combustion at every stage in the spectrum of capacity use. In most instances, the control elements are arranged so as to serve several heating circuits in a targeted way, as well as bringing other heat generators into the system, e. g. solar thermal units and buffer storage units. Also available are pelletheating units with low-temperature capability: these are for buildings with floor-heating systems and wall-surface heating systems. In a comparison among wood-fuelled heating systems, pellet-burning systems are characterised by a very high degree of comfort and low space requirements for heating and storage. Space-saving pellet heating systems are available installed on the wall, standing or hanging.

Pellet stoves have a supply container with 15–25 kg capacity. Depending on the season and on heating requirements, the fuel should be refilled by hand, at intervals ranging from daily to weekly. Wood pellets can also be obtained in sacks – handy 15 kg sacks – from regional fuel dealers or in do-it-yourself construction stores. Via shipping pellet manufacturers and dealers supply pellets in sacks, free house, based on a quantity of pallets, with one tonne of wood pellets per pallet. For private households, the storage units for pellet-fuelled boilers are typically dimensioned in such a way that they can house the whole year's wood-pellet requirement. In that case, wood pellets are supplied in tank trucks and the vehicle blows them into the storage areas. In the case of large pellet-heating installations, the storage capacities are arranged in such a way that they can take receipt of a truck's whole supply load. The wood pellets are sent to the pellet-fuelled heating boiler from the storage room or the silo container: this is done fully automatically by a conveyor spiral or a pneumatic conveyor system.


Log gasification boilers

In buildings currently in use, especially in rural areas, log gasification boilers are highly popular. For consumers with their own wood supply and favourable access to firewood – such as farmers and foresters, business owners with quantities of waste wood left in its natural state, or those directly obtaining the wood and working it in some way – heat can be provided, rather laboriously but at a low price. Operators of log gasification boilers also need to have the necessary storage area, because what one really should do is to maintain (and keep protected from rain) a stock of firewood (logs) to cover at least 2–3 years' need, for drying the wood off and for balancing out weather-related fluctuations in demand for wood.

By now, log gasification boilers are close behind pellet heaters in terms of effciency and emission levels, thanks to huge progress in development in recent years. They are absolutely not comparable anymore with the traditional boiler stoves for wood and coal, with their low levels of effectiveness and the nuisance caused to neighbours. There are modern log gasification boilers in the capacity range from 5 kW up to several 100 kW. The most modern firing and regulation technology provides effcient and clean combustion; the result is that many types of boilers already offer levels well below the stricter threshold values for emissions as required by the 2010 amendment to the Small Firing Installations Ordinance. By means of controlling the firing function via a smoke-gas temperature sensor and a lambda probe, the unit constantly provides the quantity of air or oxygen that is required for a complete combustion.

In the case of log gasification boilers, what is needed is to start them up in the mornings, putting plenty of wood in the filling space. The wood outgas over a period of many hours and is fully burned out in an afterburning chamber set up below or behind the grill. Control of output and adjustment of the firing operation guarantee best combustion and make it possible to operate the heating circuits directly. Surplus heat is stored in the buffer storage unit. Priority circuits take care of the supply of warm water for everyday domestic needs, but in many instances it is combined with solar heating.

Wood-chip heating units

Wood-chip heating units are available for single-family and multi-family residential buildings, from approx. 15 kW heat output. Yet wood-chip heating units are particularly worthwhile if the capacity level is above 50 kW or indeed 100 kW. In many cases, wood-chip heating units are used to supply heat to individual buildings or groups of buildings close to one another (micro heat networks). “Wood-chip heating plants” and “biomass heating plants” are the terms referring to larger wood-chip heating units, employing local heating networks or district heating networks to supply villages or whole streets and neighbourhoods of a city with heat sourced from biomass.

Many operators of wood-chip heating systems have wood available to them: they get it processed into wood chips, with their own chopping equipment or using an agricultural/forestry contractor. Yet there is also growth in the number of biomass stations specialising in the provision of wood fuels of all kinds; in addition, in many instances composting facilities and recycling companies are extending their field of operations to include solid biofuels, marketing wood chips and shredded wood. Thus the option is on offer, also for companies without their own wood resources (particularly those with high fuel consumption), to say goodbye to high and volatile costs for fossil fuels and switch over to a sustainable, stablepriced supply of heating.


Biomass combined heat and power (CHP) stations

If the firing output is of dimensions that make it worthwhile to generate electricity by a steam turbine, an ORC (organic-rankine cycle) turbine or a steam motor, then woodfuelled or respectively biomass-fuelled stations are set up as a combined heat and power (CHP) installation.

Wood-fuelled CHP plants are frequently in service at wood-processing industry sites – e. g. saw-mills and wood-pellet producers, at manufacturing sites for wood chips, OSB panels, parquet and laminate, as well as production sites for paper, mechanical wood pulp and cellulose. Wood-based residual material not usable in the production processes or better used elsewhere is placed in the biomass installations, to produce electricity, heat and process steam. Surplus electricity is fed into the public power grid. Energy supply companies, cities and municipalities or their respective municipal utility companies have set up numerous wood-fuelled CHP plants in recent years. It is mostly waste/used wood that is burned in the energy supply companies' installations, which in many cases are set up to have electrical capacity levels between 10 and 20 MW. Conversely, in cities' and municipalities' installations are set up for a capacity of < 5 MWel in most cases, what is used for energy is mostly assortments of forest waste wood and also wood from landscape conservation areas in the region, as well as wood material from municipal properties. Via local and district heating networks, the biomass installations' heat is made available to manufacturing, trade and service companies, as well as to housing cooperatives, private households and public buildings.


Straw-fired heating plants

In Denmark, straw-fired heating plants are widely spread. In agriculture for example, they generate heat for poultry and pig barns. In many instances, farmers also deliver surplus straw to large municipal heating plants that supply local and district heating networks. Even in a coal-fired power station, straw is burned to generate electricity for Copenhagen. By contrast, in Germany there are only a few straw-fired heating plants in service, but there is a growing level of interest in these installations. One manufacturer in Mecklenburg-Vorpommern is producing gasification boilers for straw bales, in which whole round bales can be fired; apart from that, there are straw-fired installations that bring large rectangular bales to a mechanism that breaks them down and chops them up; then the chopped straw is transported in the air stream for burning, in dosed amounts. Likewise, at capacities of approx. 400 kW–1,000 kW, several German strawfired heating plants use the surplus of straw available locally to provide the main input fuel for price-competitive heating: this heating is used in municipal heating plants or larger livestock facilities.

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