EVC5: Small-scale combustion for residential heat

For provision of residential heat, wood stoves or wood boilers can be used. Stoves are primarily used for warm air supply to the installation room. The thermal heat output of stoves is typically in the range of 10 kW to 20 kW. In contrast, boilers are used as central heating systems for heat supply to all rooms of the building and also for domestic hot water production. Hence, boilers represent typically the primary heat source for the residential building whereas stoves are typically used as a secondary heat source. The thermal heat output of boilers for residential heat supply is typically in the range of 15 kW-20 kW (single family houses) up to < 500 kW (multifamily house with local district heating). For small-scale combustion typically wood fuel is used, i.e. wood pellets, wood chips, firewood and wood briquettes.

Combustion process

The principle of the combustion process of solid wood fuels is the same for stoves and boilers. The chemical energy of the fuel is converted into thermal energy of the flue gases via combustion. The most relevant steps or chemical processes of combustion are: Heating up and drying of the fuel (~ 150°C), pyrolytic degradation (~200°C-600°C, λ=0), gasification (~700°C-1000°C, 0<λ<1) and total oxidation (~700°C-1200°C, λ≥1). In most cases those phases proceed in parallel, especially when the fuel is continuously fed into the combustion chamber of the furnace.

Under assumption of a complete combustion, e.g. of wood pellets with ambient air, the main products of the hot flue gases are nitrogen (N₂), carbon dioxide (CO₂) and water (H₂O). Minor flue gas products of complete combustion are emissions which mainly depend on the chemical fuel composition (e.g. inorganic PM and NO_x emissions). Important parameters for complete combustion are high temperatures, sufficient air/oxygen supply, sufficient residence time of flue gases in the hot combustion zone and a good mixing of air/oxygen with the flue gases (turbulence).

In technical applications complete combustion cannot be guaranteed for the whole operating process. Especially during transient phases, like start-up and stop phases as well as load changes, incomplete combustion occurs. Therefore, further emissions which result from incomplete combustion processes are released via the flue gases (e.g. organic PM emissions like tar and soot and volatile organic compounds (VOC)).

Stove technologies

The main purpose of stoves is the provision of heat directly from the hot flue gases to the surroundings by convection and radiation via the envelope of the stove – either for room heating purposes or for cooking.

Stove technologies can be categorized in manually fuelled and automatically fuelled technologies. Manually fuelled stoves are so-called chimney stoves, insets, open fireplaces, tiled stoves, biomass cookers and slow heat release appliances. The principle of operation of those technologies is the manual fuel supply by one or several fuel charges of firewood or wood briquettes. The combustion air supply and the flue gas conveyance are driven by natural draft conditions. Typically, the user of the stove has to ignite the first fuel batch and recharge the single fuel batches manually. Furthermore, the user is also responsible for the damper settings of the stove for combustion air supply. A correct operation of the appliance is very important to enable a good combustion process and an efficient and low emission operation.

Pellet stoves represent the automatically fuelled stove technologies. In contrast to manually fuelled stove technologies (with firewood), pellet stoves are technically more sophisticated. The ignition, continuous fuel supply and adaption of combustion air supply proceed automatically and are supported by a flue gas ventilator. Compared to the previously mentioned manually operated wood stove technologies, pellet stoves achieve a better combustion performance, meaning higher thermal efficiency and lower emissions. Furthermore, the operating efforts for the users are lower and consequently the user comfort is higher when using automatically fuelled pellet stoves.

Boiler technologies

Boilers represent technologies which provide central heating and domestic hot water production. Boilers consist of a combustion system/furnace and the downstream boiler which technically represents a heat exchanger. Within the boiler the thermal heat of the hot flue gases is transferred to a fluid, i.e. water (e.g. in counterflow heat exchange). The thermal heat of the water downstream the heat exchanger is further transferred to a buffer storage tank, domestic hot water storage tank, radiators or floor heating system. For small-scale applications, typically fire-tube boilers (flue gas inside the tubes) are used and the temperature level of the water within the boiler/heat exchanger is below 100°C.

Boilers can be categorized in manually fuelled and automatically fuelled technologies, too. Manually fuelled boilers are firewood boilers, which are fuelled with one charge of firewood, which is subsequently combusted at a constant load. Typically, the thermal energy of the whole fuel charge is transferred to a buffer storage tank and then distributed in the central heating system according to the respective demand (room heating and domestic hot water).

Automatically fuelled boilers are pellet boilers and wood chip boilers. Those types of boilers can be further differentiated by their fuel supply system, e.g. underfed, side-fed or drop-fed boilers.

Compared to stoves, boilers are technically more sophisticated. For example, boilers are featured with specific controlling concepts, which start and stop or change load setting of the boilers automatically according to the heat demand of the building. Moreover, during operation the combustion quality is constantly controlled, e.g. by using lambda probes and several temperature sensors.