Energy

District heating: an excellent solution

The excellent solution to exploit biomass, as well as the use of individual heating in pellet boilers (wooden balls) or nipper boilers, is the district heating with small-sized biomasses (10 megawatts), that supplies heat to households and/or business activities, and is located close to the biomass production site (wood, farm land, sawmill, etc). If the biomass is produced locally, the dimension of the plant has to be carefully decided, in order to allow sources to regenerate. Sizes bigger than 10-15 megawatts force to excessively increase the supply, therefore increasing economic and environmental costs of transports on the one hand, and going at the disadvantage of the wood sector (production, processing and the sale of products) on the other hand.
In Austria small rural district heating plants with biomass are more than 300, with a power included between hundreds of kilowatts and 8 megawatts. In Italy, instead, plants are only a few tens, although the sector seems to be very vital. One of the main reasons for the success of these plants is Austria is linked to the fact that the agricultural economy concentrates on forest activities. There is also a reduction in the wood demand; this led to a price drop and provoked problems to the agricultural sector, forcing to find a solution that could allow prices to go up to an acceptable level for local producers.
The majority of district heating plants was created in rural and economically depressed areas, but also many tourist sites have devoted attention to this technology, based on very low emissions. They saw this renewable source as an element for tourist promotion. Also in Italy these plants could be the answer to the depression of some areas, with the creation of new jobs for the maintenance of forests (economically and environmentally convenient activity), and the prevention of erosion, landslides, floods and fires. 
Biomass district heating is to be considered as a complementary technology, not be seen as an antagonist of domestic biomass boilers. District heating installations, with grill boilers, can burn all types of wood waste, although it is very wet and with a low calorific value.
Household wood boilers, instead, cannot use these types of waste. They burn dry and quality wood, of adequate size, or it is necessary to opt for pellet boilers in order to automatize the installation, avoiding to have to supply it continuously (even several times a day in winter). This wood, in fact, occupies less room with the same quantity of dry mass being burnt, guarantees a more regular combustion and a simpler transport/storage. This variability in the supply system of biomass installations allows to exploit all the products deriving from wood maintenance: the waste (branches, barks, roots, etc. also very wet) is used for district heating if there is an adequate number of users, while dry trunks and pelletized waste for isolated households.
The residues (of wood cleaning, agricultural farming, sawmills, etc), without a biomass-supplied installation, would be disposed of in a different way: if they were left in the air they would produce the same quantity of CO2 as was stocked during their growth. If fermentation occurred in the absence of oxygen, methane would be produced, whose contribution as a greenhouse gas is 21 times (in terms of weight) as high as the weight of CO2. If these residues are disposed of in factories (paper mills, etc), they often impose quite high transport costs (environmental and economic ones). 
In order to plan the construction of a biomass district heating installation, it is necessary to satisfy the following pre-requisites:

• the distance from the supply site cannot be too big, as the transport can significantly affect the cost of the raw material (and the quantity of CO2 released by the installation)
• the closeness to the supply source can allow to have a smaller storage volume inside the district-heating plant (allowing to build it also in narrow areas), having the chance to do the storage at the supplier’s place
• it is necessary to have an adequate area, close to the transport network and conveniently close to the households, where the installation and the storage warehouses will be built, without having too many problems due to the traffic.