District Heating - How is district heating produced?

District heating is usually produced in heating plants where water is heated by the direct combustion of a fuel. Alternatively, water may also be heated by an electric element through the use of waste heat from an industrial plant or a waste grid. These options are strengthened through the help of a heat pump, ant there are often several different types of production facilities in the same district heating grid. The plant that manages production under normal circumstances is called a base load plant.

In a base load plant cheaper fuels of less quality, such as municipal waste, can be used. When the heat demand increases, for example during cold winter days, peak load plants are connected. These most often use a more easily manageable and high-grade fuel, such as light fuel oil. In some district heating grids there are also reserve load plants that can be utilized if a base load furnace has operational problems.

Different types of furnaces
A variety of furnace types can be used for combustion. Fuel can be burned through burners, i.e., cyclone or rust, or by using a fluidization technique. In a burner or cyclone the fuel is sprayed together with air. The fuel can be oil, gas, or powder, such as powdered coal or peat for example. Rust burning is the oldest and so far the most common technique used for burning solid fuels in Sweden. The fuel is fed forward on a surface called rust and air is added from below. The fluidization technique involves air being blown through the fuel and a mixture of sand, ash, and lime or limestone causing the mixture to float when combustion takes place.

Flue-gas cleaning and heat recovery
The expansion of district heating and more efficient flue-gas scouring at the Swedish heating plants has contributed to a drastic decrease in pollutants released in the air. The pollutants with the largest decrease include sulfur, mercury, heavy metals, and acidified substances.

All fuels used in district heating furnaces still contain pollutants. Some pollutants are also formed during the burning process. In order to remove pollutants as much as possible different types of cleaning equipment is used. The type and extent of flue-gas cleaning equipment used depends on the purity of the fuel and which pollutants are formed during burning.

Fuel also contains moisture. This moisture evaporates when the fuel is burning and the resulting steam contains a lot of energy. This energy can be utilized in a flue-gas condensing plant. There the temperature of the flue-gas is reduced so that the steam is condensed, which releases energy. Through flue-gas condensation the efficiency of a plant can be increased between 10 and 40 percent.

Heat pumps
Heat pumps are common in district heating production. These can also be used in order to increase the temperature of industrial waste water, and also for using lake water as a heating source. The heat pump utilizes a process similar to one used in a refrigerator and consists of components such as a powerful compressor and supporting infrastructure. The compressor runs on electricity, and a common production condition is that a heat pump produces three units of heat for each unit of electricity used.

Source: Svensk Fjärrvärme, www.svenskfjarrvarme.se/