Mekanika: Majalah Ilmiah Mekanika Design, Production Cost, and Air Flow Distribution of Biomass Pellet Furnace

Biomass attracts a great deal of attention because it is converted into green fuels in the form of pellets. The furnace is needed to burn pellets to generate heat of up to 300 kW. In addition to meeting the heat capacity needs of small and medium-sized industries, furnaces must also be competitive in terms of price. The purpose of this study is therefore to obtain details of the cost of manufacturing the furnace and the model of air flow that occurs in the furnace. This study employs a forward and reverse engineering approach, beginning with determining load and capacity, drawing, determining the bill of materials and bill of manufacturing, numerical modelling of airflow with ANSYS FLUENT, fabrication, and final testing. The outcome revealed that the cost of production of the furnace included manufacturing costs, assembly costs, machining and repair costs. The findings revealed that the key portion of the cost of the furnace was the material cost of 77%. The findings of the simulation showed that the total pressure difference of up to 850 Pa and had to be resolved by air-supplying blowers. The gas velocity ranged from 2 to 10 m/s and increased significantly near the exit to 42 m/s.


Introduction
There is a growing need for pollution-free energy sources. The use of fossil fuels as an energy source has adversely affected the cleanliness of the environment through exhaust emissions. In addition, the limited supply of fossil fuels [1] has sparked the growth of renewable energy technology and the quest for alternative energy sources, such as biomass. Biomass content continues to be renewed as long as the ecosystem in which it is preserved and can be used as a renewable source of energy production [2].
Woods, derived in particular from the Kaliandra (Calliandra calothyrsus) genus, have become a favourite to cultivate into pellets due to their superior properties. Kaliandra woods have a strong growth, sturdy, specific gravity of 0.5-0.8, easy to dry, easy to burn, and calorific value of 4200 kcal/kg [3]. Compared to dry wood, Kaliandra wood biomass pellets have a higher density [4].
In the meantime, biomass furnaces have been widely produced. Burning furnaces are a special device for converting energy from biomass. It is important to be able to convert biomass materials into usable energy. Wood pellet fuels are burned in the combustion chamber to generate heat energy. A fuel-matching combustion system is installed on the burner. The combustion chamber plays an important role in the process of converting biomass energy, thus requiring the design of a combustion chamber that is capable of efficient combustion [5]. A rocket type of pellet fuel stove has been designed [5] and has developed various furnace forms to obtain a common reference configuration for fuel pellet furnaces. Moreover, fourteen household pellet furnace experiments using the water boiling test (WBT) procedure have been conducted [6], which concluded that the furnaces were capable of improving fuel efficiency and lowering combustion exhaust emissions. Unfortunately, only a few researchers have reported the cost of manufacturing a furnace.
In addition to cost, the fuel burning process is a critical element in the furnace. The combustion mechanism is influenced by fuel as well as air. The supply of air must satisfy the needs of fuel combustion in order to achieve efficient combustion [7]. However, there is no precise information on the production cost and the gas distribution in the furnace, since each furnace is very specific, depending on the creativity of the designer and the manufacturer. Therefore, the study aims to obtain details on the cost of producing the furnace and the air flow model that exists in the designed furnace.

Materials
The primary feedstocks are wood pellets of the Kaliandra variety. The properties are shown in the Table 1. Kaliandra wood pellets have a diameter of 1-2 cm and a length of 2-3 cm.   The combustion furnace was designed using proximate and ultimate results. The sum of energy that reaches the furnace was calculated using calorific values. The furnace is capable of consuming the pellets at a capacity of 75 kilograms per hour. The fuel inlet (hopper), air inlet (blower), combustion chamber, and outlet are the four major components of the furnace. A perforated plate was used to direct air from the blower into the combustion chamber. Figure 1 depicts the preliminary design of the biomass pellet furnace.
From the initial model of biomass pellet furnaces, the price of materials was calculated, the production method and cost were determined, and the operating and maintenance process and cost were also determined. As a result, production cost information from the furnace can be generated during this design process. Improvements have been made to the initial model, especially the placement of the hopper, which is not on the top, as this would exacerbate the process of transporting feedstock content.

Air Flow Simulation
The next step is a cold simulation to assess the perforated plate's capacity to distribute air. ANSYS FLUENT software was used to conduct the air-flow simulation. Creating three-dimensional geometry, meshing, defining boundary conditions on geometry, checking mesh quality, determining the properties of the material used, determining the flow model used, determining the method of discretization, initializing, running, and post processing are just a few of the steps involved in air flow simulation. The mesh type of unstructured triangle was utilized in this investigation. The standard k-epsilon was used to set the viscosity model. The mass flow inlet for air and fuel was the boundary condition established. The pressure outlet was specified as a boundary condition on the outlet side. The SIMPLE schema served as the foundation for the simulation.
The following reaction equation is used to calculate the air specifications based on the air fuel ratio (AFR) needed to burn biomass pellets.
With a stoichiometric AFR, all fuel components can be converted to CO₂ and H₂O, leaving no residual fuel [8]. Cold simulation of airflow distribution at AFRs greater than AFR stoichiometry will also be performed in order to produce an impression of air distribution in the combustion chamber.

Volume 20 (2) 2021
The oxygen distribution in the furnace, air speed from the entrance, within the furnace, to the output side, and the pressure inside the furnace were all examined as key simulation factors. The contours of oxygen concentration, speed, and pressure were the output of the ANSYS FLUENT post processing procedure. The oxygen concentration, pressure, and velocity contour were the flow field for that iteration in the steady state.

Design, Manufacturing, and Assembly
The air requirement can be calculated based on the chemical reaction of combustion, i.e.,  The air from the blower is circulated via a perforated plate into the combustion chamber. A perforated plate, which is the main material of the fixed bed component, is made from a carbon steel plate with a thickness of 4 mm and a hole diameter of 8 mm.
Moreover, the hopper is used to feed biomass pellets into the combustion furnace. The hopper should be large enough to accommodate all of the fuel that would be used, so that the burner volume can be adjusted by the scale and shape of the hopper. The bottom of the hopper is narrowed and made of steel plate. The fuel delivery part of the combustion chamber is attached to the bottom hole. Figure 2 depicts a sketch of the modified furnace. The shape of the furnace is a cube with a combustion chamber size of 1 x 1 x 1 m. The outside of the combustion chamber is composed of a steel plate. There are three holes in the steel plate for air inlet, fuel inlet, and fire outlet. The refractory brick is then attached to the plate using refractory cement, which has been cut to size and shape. The coated plate is then welded on either side. The next step is to build a chimney where the fire comes out using a steel plate filled with refractory cement. The need for a cyclone-shaped smoke filter with two filtering steps using water as a filter medium. The hopper, the combustion chamber and the chimney were assembled together. The results of the design, manufacture and assembly of furnaces are summarized in Table 2. The actual results of production are shown in Figure 3. The furnace, made of carbon steel plate sheets, has a lifetime of 3-4 years [9]. The fire bricks, on the other hand, can survive for 20 years. This fire brick acts as heat insulation, preventing the carbon steel plate component from coming into direct touch with the fire. As a result, the fire brick will extend the lifetime of the furnace [6]. Furthermore, because there are no moving parts, the combustion furnace has a lifetime of more than ten years. Blowers, wet scrubbers, wet scrubber pumps, and spray nozzles are examples of important elements that require frequent maintenance and even replacement. The life of the furnace's wet scrubber and spray components will be extended if maintenance is performed once every three months and a replacement sprayer is installed once every two years. Previous research has shown that the fundamentals of stove life in general can be employed for up to five years. According to various studies, the minimum predicted lifetime of a furnace with proper maintenance is five years [10]. Even with improved material choices, the furnace's lifetime can be extended, but the investment expenses will rise. Enhance furnace life by up to two years, increase net present value (NPV), internal rate of return (IRR), and benefit-to-cost ratio. The suggested upgraded furnace with better quality steel cladding results in a 20% increase in investment costs, with a service life of up to 7 years.

Bill of Materials
The bill of materials (BOM) covers the costs of purchasing materials, manufacturing procedures, installations, and repairs organized in a sequence of production. The material requirement for a furnace capacity of 75 kg/h is shown in Table 3. The material cost is 77% of the overall cost of furnace production. In addition to working with machines, the method of producing parts often uses manual methods. Machining work, such as rolling and bending, can use machining facilities. Budget criteria for machining jobs, such as Table 4. The machining cost is 6% of the overall cost of furnace production.

Volume 20 (2) 2021
Assembly is the final stage in the sequence of production processes. The assembly stage is designed to integrate furnace parts and wet scrubber components. In addition to assembly, maintenance needs are also required in order to maintain good furnace efficiency. The assembly and maintenance requirements are shown in Table 5. The assembly and maintenance costs are 77% of the overall cost of furnace production.

Air Flow Distribution in the Furnace
Air composed of 21 per cent oxygen and 79 per cent nitrogen is used for oxidisers and flows through the inlet side of the air. Oxygen is distributed in the combustion chamber, as can be seen in Figure 4. Oxygen is most commonly distributed in the middle and at the bottom of the combustion chamber. The greater the amount of air added, which is marked by excess air, the more it is noted that much of the oxygen flows to the front portion of the perforated plate. This can lead to more vigorous combustion of the front and a lot of heat is applied directly to the outlet portion of the combustion chamber. The gas velocity in the combustion chamber and close to the perforated plate for different excess air can be seen in Figure 5 and Figure 6. The air flows at a speed of 7-11 m/s in the inlet chamber and then the speed in the combustion chamber decreases due to the wider area. In the surrounding portion of the outlet, the gas is experiencing a rise in velocity due to a decrease in density due to an increase in combustion temperature. The gas speed at the outlet is around 25-42 m/s, depending on the excess air. Figure 6 indicates that the perforated plate is very capable of spreading air to all the cross-sections on it. The speed difference above the perforated plate at the front and back of the air inlet path is 1-2 m/s.  Figure 7 shows the distribution of total pressure within the furnace. At 0 per cent air excess, the total pressure difference is 600 Pa, which does not accurately account for the loss of pressure due to pellet and ash piles in the combustion chamber. The higher the excess air causes the total pressure to rise. At 20% excess air, the difference in total pressure is 850 Pa. So, in the design step, using the head assumption for the blower of 2000 Pa was enough to overcome the pressure losses that occurred.

Conclusion
The furnace for biomass pellets with a capacity of 75 kg/h has been designed and installed. Production costs, material costs, machining costs, assembly costs and maintenance costs may have already been determined. Air supply at an air fuel ratio of 5.37 with excess air from 0 to 20 per cent has also been studied for oxygen distribution, velocity and total pressure that may occur in the combustion chamber. The total amount of pressure, up to 850 Pa, must be overcome by air-supplying blowers. The gas velocity varies from 2-10 m/s and drastically increases nearby the outlet, up to 42 m/s. Experimental tests and flow-heat simulations are necessary to learn more about the performance of the furnace.