# Small Heat Source Used for Combustion of Wheat-Straw Pellets
## 2 Design and Method  
Direct biomass firing is the most common technique for every kind of biomass fuel conversion into energy [14]. Some heat sources for biomass burning should be able to handle differences of fuel quality and fluctuations. It is caused by seasonal changes or different available types of fuel during the year [14].  
There are different types of biomass boilers or furnace constructions, with different combustion processes. For example, grate furnaces, gasifying boilers or fire-through boilers, co-firing boilers, but also boilers for straw burning in bale form, where minimal handling effort is desired [15].  
Before designing a boiler, it is necessary to take into account possible emissions and heat output differences due to various burning conditions in cases of burning straw biomass [16]. Another problem is ash melting and deposition [17]. The construction of a new proposed heat source has to enable simple cleaning and maintaining.  
The design is focused on the problems of over burning the straw, the design of an arrangement to prevent the clogging of the exchanger, easy maintenance of the boiler, and the low cost of boiler.  
It is necessary to work with basic boundary conditions, such as maximum dimensions, a certain output, and also the type of fuel that will burn in the proposed boiler. The proposed heat power output of the boiler, which counts as its main target of use small farms and houses, is 20 kW. The type of fuel is chosen to be pellets of wheat straw. This type of fuel can have different properties of burning.  
Therefore, in our case, we decided for a tubular type exchanger. In comparison to the small heat source of Dongji Wang [18], which is used to burn rice pellets and consists of two combustion chambers and plate heat exchangers, our heat source is designed on a similar principle as a heat source designed by [PERSON] [15]. It uses a tubular heat exchanger and flue gas flows through the pipes. The main difference is in the connection between the two passes of the heat exchanger, which, in our case, are on top of the heat exchanger. This is due to the location of the combustion chamber. This type of construction allows more modifications, such as using turbulizators, introducing baffles to the combustion chamber, and on top of the boiler. The result will be better combustion conditions for the straw pellets.  
The suitable construction of the boiler is adapted to the spatial conditions of small boiler rooms, because it will be used for heating small agricultural buildings and family houses. After considering the insulation, which can reach a thickness of 75 mm even with the construction, and the minimum size of the installed boiler room entrance door, the maximum possible floor width has to be 400 mm. The maximum height of the device must not exceed 1.6 m. The scheme of the designed solution is shown in Figure 2. Proposed device used to combustion of wheat-straw pellets. The main idea of designing the boiler is to create a boiler that consists of modules. These modules can be used to build whole new equipment with different heat power levels. In addition, when it is used in construction, it consists of a two pass heat exchanger, so during failure or while fixing a damaged part of heat exchanger, the second part can be used to heat the building with reduced heat power.  
Figure 2: Proposed device used to combustion of wheat-straw pellets.  
The proposed device shown in Figure 2 consists of a rotary burner (1), an inlet chamber with an ashtrav (2), primary and secondary heat exchangers (3), a top chamber (5) and a chimney chamber.