Small-scale combustion in stoves and boilers
The essential difference between stoves and boilers is the heat transfer medium. If the heat transfer medium is air, it is a stove and if its water, it is a boiler.
A stove is a space heater with a completely enclosed combustion chamber, with a fire door, which is normally closed. The heat is conveyed by radiation and/or convection. A stove is made of metal or fireproof ceramic. It is a unit in which heat is produced by combustion of solid, liquid or gaseous fuels or by electrical current. The heat is used for heating or cooking.
A boiler is the heat generating part of a central heating system and can be accomplished in different designs. Central heating boilers must be equipped with a water heat exchanger and connected to a heating water circuit. This ensures a controlled heat transport to the heating surfaces of the respective rooms.
The combustion technologies of available stoves and boilers are different and depend on: fuel, feeding technology, performance, etc. In the following, there is a description of stoves and boilers, which are fired by biomass and used typically in households.
There are many different kinds of stoves available. For the differentiate criteria please follow the link. The operation of a typical biomass-fired stove is described below.
A hand-fed stove for log wood has a combustion chamber, in which the combustion actually takes place. The primary air supply is required for optimum combustion, while the secondary air supply also keeps the viewing window clean. An exhaust gas stub enables an escape of exhausted gas.
The typical performance of a space heating stove is between 5 and 10kW. They are flat firing systems and need 1-3kg fuel for every recharging. Due to the low fuel quantity, the intervals of recharging are short.
An automatically-fed pellet stove consists of a fuel tank, a feeding system, an engine, an electrical ignition and a draught fan.
Control systems, like a controlled air supply or a temperature regulator etc. can be added to the stoves, to improve for example their efficiency.
Wood chips are not used for stoves. They are suitable for larger installations, like boilers.
A hand- fed boiler for log wood consists of a combustion chamber with a primary and a secondary air supply, a firebed and a gasification zone. There is also a secondary combustion chamber and a heat exchanger for a more efficient performance. The exhaust gas can escape trough the exhaust gas stub. A draught fan and devices for generating turbulences are also common. In order to store the generated heat and thus to use it more efficiently, a buffer is recommended.
Today, almost exclusively, the downdraft combustion principle is used for hand-fed central heating boilers. The typical performance of a boiler is between 20 and 50kW. Central heating boilers for log wood are fill firing systems and need approximately 30-50kg log wood per ignition.
Automatically-fed boilers for pellets are similar in structure, but also include a feeding system. Pellets are moved from the storage to the combustion chamber by a screw conveyor or a pneumatic system. There are three common types of feeding systems to the combustion chamber: underfeed, crosswise insertion and chute.
Another possibility is a boiler, which is heated with wood chips. These are used in various performance ranges. The lowest limit is about 10kW, but there are also installations with 60MW. As well as the pellet boiler, the fuel is automatically transported into the combustion chamber.
Pellet boilers have even lower emissions and an even higher efficiency than log wood and wood chips boilers, because of the homogeneous fuel. All of them can also be equipped with smart devices, like temperature and exhaust gas sensors or a controlled air and fuel supply or a solar collector to improve their performance and efficiency.