One important advantage of district heating (DH) compared with other heating methods is its flexibility regarding the utilisation of different fuels. A natural gas network is limited to the use of natural gas, and to change an individual oil burner to, for ex ample, solid fuel is as a rule very difficult if not impossible. But with a DH system neither the installations in the individual houses nor the network have to be replaced when the fuel is changed. This means that a DH system can be readily adjusted to changes of the supply fuel as prices change.
This is clearly illustrated by the development of the Danish DH sector during the last 20 years. In 1972 production of DH was based almost solely on heavy fuel oil, and obtained partly from combined heat and power (CHP) plants (25%) and partly from heatonly plants (72%), with the remaining 3% coming from incineration plants.
After the oil supply crisis in 1973/74 a rapid readjustment took place and the oil was replaced by coal in both CHP and DH plants. Since then there has been an increasing interest for other possibilities such as surplus heat from industry, biomass. and biogas. This development has continued during the 1980s and 1990s. Coal is still used as fuel in large CHP plants; natural gas has come into the picture primarily for medium and small decentral CHP plants and for peakload and reserve boilers. Furthermore, the importance of waste incineration and various renewable energy sources has increased gradually during the years.
Since 1990 it has been the policy to change all coal and oilfired DH plants with a capacity of 1 MW or more into CHP plants based on natural gas or natural gas in combination with waste incineration. Only DH plants situated outside the natural gas distribution system will continue as DH plants based on biofuels.
This trend is expected to continue in the years to come, but the DH plants will con tinuously adapt to the actual supply situation. If DH supply is to compete and thus secure its own survival, it must necessarily. seen over the years, use fuels that provide cheaper heat than either oil or natural gas in individual boilers. And DH has an advantage in being readily able to adjust production to changes of fuel prices and taxes.
Coal has been utilised in many Danish DH plants during the last 20 years This is a convenient and cheap fuel, especially for baseload boilers, and the technology of boilers and smoke purification equipment is known and welldeveloped. Because of increasing environmental demands concurrent with the introduction of environmental taxes, coal is now on the way out as a fuel in individual heating plants, but it is expected to last for a long period as a primary fuel in large CHP plants.
Oil is an easy fuel to handle, and the technology for its utilisation is welldeveloped. As mentioned above, oil was in fact the dominant energy supply for Danish district heating 20 years ago. Its importance has gradually decreased during the years and in the future oil is expected to be used for reserveload boilers only. This is primarily due to price conditions as the difference between fuel oil and heavy fuel oil has narrowed sufficiently to make it difficult for a heavy fueloilfired DH plant to compete with Individual oil heaters. Environmental taxes also play a role here.
Natural Gas, like oil, is easy to handle without residual waste and with minimal environmental impacts, but here again price relationships favour the use of individual natural gas heaters over DH gas heaters. However, higher taxes on coal or oil still make natural gas more attractive for use in DH compared with them. Within the next few years natural gas is expected to be utilised in small decentral CHP plants especially, and for peakload and reserveload boilers.
On a global basis large amounts of heat are lost In exhaust gas and waste water from industrial processes. Because of the development of district heating in Denmark it has been possible to utilise a part of this lost heat. Here also the low temperatures used in the Danish DH systems enable them to utilise surplus heat at relatively low temperatures.
An example of an industry supplying DH Is Aalborg Portland's cement works, which supplies approx. 80 TJ/year to the DH sys tem In Aalborg. This factory covers approx. 15% of the municipal heat demand in Aalborg and for the environment this means a reduc tion of emissions of approx.:
NOx 230 tonnes/yearSO2 3,900 tonnes/yearCO2 44,000 tonnes/yearParticles 29 tonnes/year
Along with these environmental advantages comes an economic advantage to Aalborg Portland and to the DH customers.
Incineration plants that supply heal by burning waste from households and industry have been in operation in Denmark for more than 50 years. These plants are usually con nected to a DH system, but In recent years wastefired CHP plants have also been built. often in combination with natural gas as combinedcycle plants.
The technology of waste incineration plants is well developed, this applies espe cially to the construction of furnaces and grids, which can be designed to cover a spec trum of fuels. i.e. household and industrial waste as well as waste wood, bark, and straw. Also, the technology of smoke purification is continually under development. and with modern smoke purification equipment waste incineration plants offer hardly any environmental problems. A major reason for this is that slurry and particles can be handled in a controlled way. Further more, incineration is close to CO; neutral. that is, most of the CO2 released to the atmosphere has been derived from the atmosphere.
The size of Danish waste incineration plants varies from small plants that handle 1520,000 tonnes of waste per year up to plants for 350.000 tonnes per year.
It is part of Danish environmental and energy policy to recycle as much as possible of the waste. The aim is that by year 2000 54% of the waste should be recycled. Of the remaining waste the combustible part 25% must be incinerated and the rest (21%) deposited. DH plays an essential role in this policy in utilising the heat produced. In this way, primary energy consumption and environmental loads can be reduced and the economy of the incineration plants is improved.
In Denmark there are approx. 4.2 mill. tonnes of surplus straw, 2.1 mill. tonnes of surplus wood, 2.5 mill. tonnes of organic waste from households and industry, and 40 mill. tonnes of animal manure produced per year, corresponding to an annual energy potential ofapprox. 150 PJ or about 25% of Danish energy consumed in producing electricity and district heat.
The utilisation of these energy sources has been intensified during the last few years to reach approx. 45 PJ. This development is expected to continue and accelerate in the years to come, supported by the Danish government. The political Interest in biofuels is due primarily to their environmental ad vantages, especially because the mentioned biofuels contribute no net CO; burden to the atmosphere, as noted above
The government has tried to encourage the use of biofuels by providing various sub sidies to develop technology and establish demonstration plants. Another way to en courage their use has been by the introduction of energy/environmental taxes on other energy supplies.
Straw is today utilised as fuel In more than 50 DH plants, which vary in size from 2 to 10 MW. The technology of straw incineration has been strongly developed dur ing recent years, this applies to stocking and feeding systems, boiler plants, and smoke purification equipment. Today Danish straw burners are fully commercial and without serious operational problems.
Normally the straw is delivered continuously throughout the year from the farmers in bales of approx. 500 kgs, but promising tests have also been made with cut straw, where the straw is cut directly from the combine harvester and stocked in the field.
At the plant the bales are burned directly or after they have been whittled. For smoke purification usually filter bags are used, and the ashes are brought back to the farm for distribution on its land.
Chips and other kinds of wood waste are burned today in 30 DH plants. In this field a considerable development of technology has also taken place during recent years, and chip burners are now commercially developed without operational problems.
The chip plants vary from 1.5 to 10 MW and because of the rather large content of water in the chips condensing smoke purification plants are mostly used.
Biogas from animal manure and organic waste is currently produced in I I major bio gas plants and 10 small farm plants. The largest of the plants processes approx. 300 tonnes of animal manure and 50 tonnes of industrial waste per day with a biogas production of 11,000 rn' corresponding to 7,100 m' natural gas per day.
The development of these plants is strongly supported by the Danish government, partly through direct subsidies to plant constructions and partly through technological followup programmes. The operational reliability of the plants has improved significantly during recent years, and biogas plants are today reaching the commercial stage. Their accelerated use is also based on their environmental advantages to both farmers and society.
A series of experiments has been started utilising the organic part of the household waste in biogas plants. In this way a production of gas is achieved and additionally the organic waste is brought back to the farms as fertiliser.
The biogas is generally utilised in gas motorgenerators for producing electricity and heat.
The generation of biogas from waste is now a common procedure at large dumping grounds. It is relatively free from problems and economically advantageous. As gas production is inevitable in any event, it ought to be collected rather than allowed to ooze into the surroundings where it can harm plant life and give rise to other environmental damage such as destroying the ozone layer. The gas is gathered through a series of drill holes in which a slight vacuum Is maintained.
Energy Crops may play an important role in supplying future energy needs. According to EC regulations 15% of Danish farmland should to be laid out as fallow land. If it were instead planted with energy crops, it would yield approx. 15 PJ per year.
This crop could, for instance, be willow, elephant grass, or corn (grain and straw). In order to utilise energy crops boilers have to be set up, and there must be a DH network available. Also here the Danish DH sector is well prepared, especially because of the experience gained from the many heating or CHP plants that incinerate straw and chips. Contributor 0jvind Toftgaard, Volund Energy Systems A/S Falkevej 2, DK6705 Esbjerg 0