Strojniški vestnik - Journal of Mechanical Engineering 51(2005)7-8, 426-430 UDK-UDC 536.2 Izvirni znanstveni članek - Original scientific paper (1.01) Investigations of combustion process in stove fired on biomass Dragoslava D. Stojiljkovič , Vladimir V. Jovanovič , Milan R. Radovanovič , Nebojša G. Manič , Ivo R. Radulovič , Slobodan V. Perišič Faculty of Mechanical Engineering, University of Belgrade, 27. marta 80, Belgrade, Serbia, Serbia and Montenegro, dstojiljkovic@mas.bg.ac.yu Milan Blagojevič a.d. Smederevo, Dure Strugara 20, Smederevo, Serbia, Serbia and Montenegro Abstract The aim of the investigation was to make some reconstructions on the existing stove used for cooking and baking and to obtain the combined cooker–boiler which will fulfill the demands of European standard EN 12815. Implementation of modern scientific achievements in the field of combustion on stoves and furnaces fired on biomass was used. During the investigations four various constructions were made with different fresh air inlet and secondary air supply with the intention to obtain more complete combustion with increased efficiency and reduced CO emission. Three different fuels were used: firewood, coal, and wood briquette. A numerous parameters were measured: fuel weight changes during the combustion process, temperature of inlet and outlet water, flue gas composition (O2, CO, SO2, CO2, NOx), flue gas temperature, ash quantity ect. The result of the investigations is the stove with the efficiency of more than 75% - boiler Class 1 (according EN 12815) and CO emission of about 1 %v/v. The results obtained during the measurements were used as parameters for modeling of combustion process. Introduction Basic aim of the investigations was development of original domestic appliance fired on solid fuel that could be simultaneously used for cooking and heating the water for central heating system and heating the space in the room where it is installed. Original design of combined cooker and water heater – MBS 90KV was used as a base for further improvements. During the realization of the investigations several prototypes of combined cooker-boiler were tested and, based on the obtained results, appropriate reconstructions were made. Afterwards, the modified constructions characteristics (thermal and environmental) were tested. Tests of the combined cooker-boiler MBS 90KV The tests were based on definition of test program and choice of different fuels. Definition of listed parameters was necessary to provide the same test conditions for different constructions of combined cooker-boiler. Test procedure The tests of combined cooker-boiler MBS 90KV were made according to the demands of regulation EN 12815. Typical test cycle for lower grill position is foreseen by this standard, that comprise: - startup and pre-test for reaching the steady state; - minimum 1 hour testing for the nominal heat power when the firewood is used as a fuel, and minimum 3 hours when the coal or other fossil fuel is used for determination: a. total efficiency b. heat output. - time required to reach the steady state. The test rig was designed and constructed according to the demands of regulation EN 12815 (Figure 1). Mepe y MHUHMeTpiiMa Figure 1: Test rig used for investigations A-heating appliance, B-flowmeter, C-throttling valve, D-valve for pressure reduction, E-stop valve, F-weighing balance, G-circulating pump, H-steel reservoir, isolated with 120 mm thick mineral wool insulation or with cork 426 Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 426-430 peaces, J-safety device, K-thermal safety overflow device, L-overpressure valve for pressurized systems, M-expansion vessel, N-flexibile junction, P-outflow The combined cooker-boiler prepared for the test is shown on Figure 2. Figure 2: Combined cooker-boiler MBS 90KV on the test rig Test fuels Three different fuels were used for tests: brown coal “Banoviči” (letter “u” in the test marking), firewood - common beech (no letter in the test marking), briquette made of wood residues from furniture production (letter “b” in the test marking). Proximate and ultimate analysis of all test fuels were made according to the regulation JUS B.H0.021. Constructions of combined cooker-boiler MBS 90KV During the investigations different constructions of combined cooker-boiler MBS 90KV were tested: CONSTRUCTION 0 - Basic construction made by modification of classic solid fuel cooker MBS 90. Modification was primarily made on the firebox, which was increased in depth providing the space for insertion of heat exchanger in the upper zone. Other construction elements of the original cooker were not altered. CONSTRUCTION 1 - The firebox grill was lowered related to the basic prototype. Air inlet from the front side was closed. Position and dimensions of the ashtray are such that in the working position it is completely blocking rear side air inlet (which is the only air inlet for both primary and secondary air). In this way the airflow is allowed only along the sidewalls of the ashtray providing the primary air to the grill. This construction is marked as Model 0. CONSTRUCTION 2 - Ashtray height was reduced for 15 and 25 mm respectively, to allow air entrance in the air plenum beneath the firebox. The first modification was named Model 2 and the second one Model 1. CONSTRUCTION 3 – The secondary air supply was modified in a such a way that the direct flow upwards was blocked and redirected under the firebox grill. This modification was named Model 1a. CONSTRUCTION 4 – Firebox grill was moved backwards, to the rear wall, and the secondary air grill was placed on the front firebox wall instead on the back. This modification was named Model 1b. Test results Results obtained during the experiments were classified in 4 groups: - fuel analysis, - thermal characteristics, - functional characteristics and - environmental characteristics. Fuel analysis Analysis of all test fuels were made according to the regulation JUS B.H0.021. Proximate analysis was made in Fuel & Combustion Lab (University of Belgrade, Faculty of Mechanical Engineering) and ultimate analysis in the Institute for chemistry, technology and metallurgy – Center for chemistry. The results of proximate and ultimate analysis (only for as received mass) for all test fuels are given in the Table 1. Table 1: Proximate and ultimate analysis of test fuels PROXIMATE wood ANALYS IS ette Total moisture (%m/m) 8,23 12,42 9,76 Combustibles (%m/m) 91,00 78,05 89,87 Ash (%m/m) 0,77 9,52 0,37 Volatiles (%m/m) 75,07 38,93 76,06 Fixed carbon (%m/m) 15,94 39,12 13,80 High heating value (kJ/kg) 18031 21143 17420 Low heating value (kJ/kg) 16624 19724 15782 ULTIMATE A NALYSIS Carbon (%m/m) 43,43 56,47 46,05 Hydrogen (%m/m) 5,34 4,73 6,19 Nitrogen (%m/m) 0,45 1,35 0,67 Sulphur (%m/m) - 1,40 0,21 Oxygenas the difference (%m/m) 41,79 14,10 36,73 Thermal characteristics Thermal characteristics that were determined during the tests were: - nominal total heat output; - heat output to the central heating water; - heat output to the ambient; - total efficiency of the combined cooker-boiler. Investigations of combustion process in stove fired on biomass 427 Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 426-430 Regulation EN 12815 doesn’t have strict limits for the nominal heat output and ratio of heat output to the central heating system and ambient. However, total efficiency is strictly defined and, depending on it, all appliances are classified in 4 classes given in Table 2. Table 2: Classes of appliances Appliance class Total efficiency (%) Class 1 Class 2 Class 3 Class 4 more than 75 from 70 to 75 from 65 to 70 from 60 to 65 Nominal heat output, heat output to the water for central heating system, heat output to the ambient air and total efficiency are given in Table 3. Experimental results from the tests of final model (adopted for production) for different fuel types are given on Figure 3 (nominal heat output) and Figure 4 (total efficiency). Based on the classification given in Table 2, combined cooker-boiler MBS 90KV is an appliance of Class 1 (efficiency grater than 75 %). Table 3: Nominal heat output and efficiency for different models Test Nominal heat output (kW) Heat output to the water (kW) Heat output to the ambient (kW) Total efficiency (%) Firewood 0 20,01 5,85 14,16 74,83 1 22,05 7,50 14,55 74,60 2 19,64 7,10 12,54 73,58 1a05 18,09 5,40 12,69 79,23 1a10 20,50 6,14 14,36 74,58 1b05 17,84 6,92 10,92 74,69 1b10 20,39 6,75 13,64 76,12 1b15 22,17 7,36 14,81 70,01 Coal 1bu10 16,13 5,87 10,26 80,32 Briquette 1bb05 15,70 5,79 9,91 80,85 1bb10 18,38 6,13 12,25 74,06 Note: first digit and first letter are model letter is mark of test fuel used (no letter – coal and b – briquette), last two digits draught in flue gas duct. mark, second firewood, u – are mark of 1 1 15 10 20,39 Nominal heat output Heat output to the water Heat output to the ambient 16,13 18,38 13,64 10,26 12,25 6,75 5,87 6,13 Firewood Coal Briquette Fuel type Figure 2: Heat outputs for all test fuels (final model) 100 80,32 76,12 70 60 74,06 Firewood Coal Fuel type Briquette Figure 3: Total efficiency for different fuel types (final model) Functional characteristics Functional characteristics are defining heating of the water for central heating system. Determination of these characteristics comprised measurement of: - water temperature at the outlet of the boiler, - temperature difference between outlet and inlet. Values for these temperatures defined in regulation EN 12815 are given in Table 4. Time period when those values are satisfied was just recorded as requested by this regulation. 428 StojiljkovičD.D. -Jovanovič V.V. -RadovanovičM.R. -ManičN.G. -RadulovičI.R. -Perišič Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 426-430 Table 4: Requested values for temperatures Requested water temperature at outlet (°C) 80+5 Requested temperature difference between outlet and inlet (°C) from 10 to 25 Measurement results for different models and all three fuels are given in Table 5. Table 5 Measurement results for the functional characteristics. Test Outlet water temperature (°C) Temperature difference (°C) Firewood 0 78,6 13,3 1 88,2 16,9 2 83,5 14,6 1a05 76,3 13,1 1a10 71,9 11,5 1b05 80,5 14,5 1b10 78,0 14,4 1b15 87,2 15,7 Coal 1bu10 78,7 12,4 Briquette 1bb05 75,3 12,2 1bb10 76,3 13,1 Comparative values of outlet water temperature and temperature difference for all three test fuels are given on Figure 5. 90 n 78,0 78,7 Temperature difference 80 70 60 50 40 30 20 10 0 76,3 14,4 12,4 13,1 Firewood Coal Fuel type Briquette Figure 5: Outlet water temperature and temperature difference for different fuel types (final model) Table 6: Classes of appliances based on CO emission Appliance class Maximum permitted CO content, calculated at 13 %v/v O2 content in flue gas (%v/v) Class 1 less than 0,3 Class 2 from 0,3 to 0,8 Class 3 from 0,8 to 1,0 Results of the CO emission measurements are given in Table 7, and comparative CO emissions (calculated on 13 %v/v O2 content in flue gas) for all test fuels are given on Figure 6. The adopted final version was the best, fired on coal. It is Class 2 appliance based on classification given in Table 6 but it was slightly above the limit for the Class 1. Firing this appliance on firewood and briquettes from wood didn’t reach even the limit of Class 3. This will be the principal target for further development of this appliance. Table 7: Results of CO emission calculated at 13 %v/v O2 content in flue gas Test CO emission (%v/v) Firewood 0 1,78 1 1,93 2 2,27 1a05 1,85 1a10 1,58 1b05 1,35 1b10 1,24 1b15 2,14 Coal 1bu10 0,31 Briquette 1bb05 1,71 1bb10 1,74 Environmental characteristics Under environmental characteristics only carbon monoxide (CO) emission level is defined. Based on the value of CO emission all appliances are, according to the EN 12815 classified in three classes given in Table 6. Investigations of combustion process in stove fired on biomass 429 Strojniški vestnik - Journal of Mechanical Engineering 51(2005) 7-8, 426-430 2,00 1,50 1,00 0,50 0,00 1 1 1,74 1,24 0,31 Firewood Coal Fuel type Briquette Figure 6: CO emission calculated at 13 %v/v O2 content in flue gas for different fuel types (final model) Conclusions Heating appliances fired on solid fuel could be a part of past but definitely a part of the future, especially having on mind that the biomass is renewable energy source with zero CO2 balance. The importance of this type of heating appliances is emphasized by the adoption of the latest European regulation in this field EN 12815. This regulation has brought clear test procedures and strict limits for heating appliances fired on solid fuel. Combined cooker-boiler MBS 90KV produced by Milan Blagojevic a.d. Smederevo is one of the newest products on the Serbian market. The results from the tests made completely according to the demands of EN 12815 during its development are presented in this paper. Based on these results it can be concluded that: - total efficiency required by Class 1 was achieved (more than 75 %) when fired on firewood and coal while slightly below this limit (Class 2) when fired on briquette made of wood residues from furniture production; - nominal heat output of 16 kW was reached firing on coal, and even 20 kW fired on firewood; - heat output to the water for central heating system was about 6 kW for all test fuels; - CO emission of a Class 2 appliance was reached firing on coal (0,31 %v/v); - CO emission was above the highest permitted limit when fired on biomass (firewood and briquette both); - further improvements in biomass combustion (reconstruction of firebox) are needed to reduce the CO emission. Acknowledgement The investigations described in this paper are part of a project financed by Ministry of science, technology and development of Republic of Serbia. The authors hereby express their gratitude for financial support. References [1] ***, EN 12815 Residential Cookers fired by solid fuel - Requirements and test methods, Österreihsches Normuginstitut, Wien, 2002. [2] D. Stojiljkovič, V. Jovanovič, M. Radovanovič, N. Manič, I. Radulovič, Tests of heating appliance MBS 90KV Model 0, Report No. 12-29-12.01/2002, Faculty of Mechanical engineering, Belgrade, 2002. [3] D. Stojiljkovič, V. Jovanovič, M. Radovanovič, N. Manič, I. Radulovič, Tests of heating appliance MBS 90KV Model 1 and Model 2, Report No. 12-28- 12.01/2002, Faculty of Mechanical engineering, Belgrade, 2002. [4] D. Stojiljkovič, V. Jovanovič, M. Radovanovič, N. Manič, I. Radulovič, Tests of heating appliance MBS 90KV Model 1bu, Report No. 12-54-12.01/2002, Faculty of Mechanical engineering, Belgrade, 2002. [5] D. Stojiljkovič, V. Jovanovič, M. Radovanovič, N. Manič, I. Radulovič, Tests of heating appliance MBS 90KV Model 1bb, Report No. 12-08-12.01/2003, Faculty of Mechanical engineering, Belgrade, 2003. [6] D. Stojiljkovič, V. Jovanovič, M. Radovanovič, N. Manič, I. Radulovič, Tests of heating appliance MBS 90KV Model 1b, Report No. 12-07-12.01/2003, Faculty of Mechanical engineering, Belgrade, 2003. 430 Stojiljkovič D.D. - Jovanovič V.V. - Radovanovič M.R. - Manič N.G. - Radulovič I.R. -Perišič