Experimental Study on Combustion of Biomass in a Boiler with Gasification

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1 Experimental Study on Combustion of Biomass in a Boiler with Gasification TĂNASE PANAIT, GHEORGHE CIOCEA, ION ION Thermal Systems and Environmental Engineering Department Dunarea de Jos University of Galati 47 Domneasca St., 88, Galati ROMANIA tpanait@ugal.ro, termaexim@yahoo.com, ion.ion@ugal.ro, Abstract: - This paper summarizes the results of an experimental study on combustion of briquettes made from different agricultural residues in a heating boiler with gasification of 42 kw. The results are similar to those obtained for fuels recommended by the boiler manufacturer. The maximum combustion efficiencies were obtained for briquettes made from pure reed, mixture of sawdust 5% +corn stalk 5%, sawdust 5%+wheat straw 5%, respectively. Key-Words: - biomass using, down-draught boiler, combustion efficiency, pollutant emissions. 1 Introduction Biomass is the third largest primary energy resource in the world, after coal and oil [4]. The use of biomass as partial substitute of fossil fuels is also important in reaching higher energetic independence and Kyoto Protocol targets since biomass combustion is CO 2 neutral. Biomass materials with high energy potential include agricultural residues and residues from forest-related activities. Residues from forest-related activities (excluding wood fuel) account for 65% of the biomass energy potential whereas 33% comes from residues of agricultural crops [4]. Corn, wheat, sunflower and soy are Romania s major crops. Romania has one of the largest expanse of reed in the world (about 2 million ha). In choosing the process to convert the biomass into useful form of energy a great influence is exerted by the available type and quantity of biomass, the desired form of the energy, environmental standards and the economic conditions. The gasification of biomass is the most efficient conversion technology, but due to very high costs of plant construction, the combustion remains the most attractive technology. The use of small-scale boilers in rural areas, could offer the highest advantages for both the biomass producer and the user. The interest in using more efficient boilers, user-friendly and environmental friendly is increasing. In a boiler with reverse combustion known also as down-draught boiler or boiler with gasification the combustion process occurs in two steps. Through separation of devolatilization and char combustion phases, the mixing of fuel gas and secondary combustion air is improved leading to higher combustion temperatures and reduced emissions from incomplete combustion. 2 Purpose of the work The goal of this study is to analyse the use of different agriculture residues in a down-draught boiler with a power of 42 kw for domestic heating (Figure 1). The boiler is provided with a refractory plate. The plate has three slots and each slot is provided with secondary air holes. While the flue gases are guided down they transfer their heat to the water space and then flow from the bottom to the chimney. The fuels selected for tests were one briquette produced from pure reed, four briquettes produced from mixture of sawdust with ground wheat straw and with ground corn stalk and one briquette produced from pure sawdust and wood log (acacia) as fuels recommended by the boiler manufacturer. Only the results for pure reed and mixtures with equal parts are discussed since the results obtained for the other two mixtures are similar. The briquettes have a diameter of 7 cm and different lengths. The draught had almost the same value ( -.3 hpa) during all tests and the load was the same for all fuel samples (6 kg). 3 Method of investigation The fuels were characterised by means of the higher heating value (HHV), elemental analyses and energy density (Table 1). The higher heating value, in ISSN: ISBN:

2 [MJ/kg], was determinated according to the European standard CEN/TS 14918:25 in oxygen Bomb Calorimeter and predicted by the following equation [2]: HHV.3491C.134O.151N H.211A.15S (1) where C, H, O, N, S and A represents carbon, hydrogen, oxygen, nitrogen, sulphur and ash contents expressed in mass percentages on dry basis. This correlation offers predictions with average absolute error of 1.32%. The temperature, CO, CO 2, NO x and O 2 concentrations in flue gas, the combustion efficiency and the excess air ratio (λ) were analysed using the Eurotron Ecoline 4 gas analyser. The test rig includes also thermocouples to measure the temperature in the combustion chambers and the Riken analyser for CO content in fuel gas (Figure 1). Briquettes ignition is achieved by means of hot embers of wood chips. Hot water Flue gas evacuation when opening the door Primary air CO measurement Thermocouples Thermal shield Ember Air fan Fuelling Secondary air Ash pan Primary combustion Flue gas analyser Secondary combustion Fig. 1. Scheme of the FI-GS boiler. Table 1. Fuel properties. Fuel sample Ultimate analysis (wt% of wet fuel with ash) C H N S O Moisture Ash Higher heating value, HHV [kj/kg] Energy density [GJ/Nm 3 ] 1. Acacia wood log Reed %+ corn stalk 5% 5. 5% + wheat straw 5% ISSN: ISBN:

3 The time dependence of combustion efficiency, NO x and CO emissions and excess air ratio for three briquettes are shown in Figure 2. The results are compared with those obtained for acacia wood logs and sawdust briquette. The combustion efficiency decreases in time for all fuel samples due to increase of excess air ratio. This happens because the fuel composition varies continuously as a function of burnout degree. The lower reactivity of char as compared to the volatile fraction and the constant air flow rate lead to a lower fuel consumption rate and a larger amount of excess air. The boiler should be equipped with a system to optimize the amount and distribution of air between the combustion chambers. It can be seen the influence of the HHV on combustion efficiency. The highest carbon content in fuel leads to the highest combustion efficiency. The CO emission is high for all fuels despite the high excess air ratio. This could be explained by lower level of combustion temperature. The highest CO emission is obtained for sawdust 5% +corn stalk 5% and the minimum CO emission for reed. The highest NO x emission corresponds to the fuel with the highest content of N (sawdust). The trend of NO x emission is to decrease in time as the N from fuel is consumed and the combustion temperature decreases. 95. Combustion efficiency vs time 1.4 CO vs combustion time Combustion efficiency [ % ] Time [min] Reed 5%+corn stalk 5% 5%+straw 5% C O in d ry flu e g as [ % ] Time [min] Reed 5% + corn stalk 5% 5% + wheat straw 5% NOx [p p m ] NOx vs combustion time Time [min] vs combustion time Time [min] Reed 5%+corn stalk 5% 5%+straw 5% Reed 5%+corn stalk 5% 5%+straw5% Fig. 2. Time dependence of combustion process. ISSN: ISBN:

4 Com bustion tem perature [ C] Combustion temperature vs time Primary combustion temperature Secondary combustion temperature Time [min] C O in flam e [p p m ] Volatile combustion CO in flame vs time 5% + corn stalk 5% Char combustion Time [min] Fig. 3. Time dependence of combustion temperature and CO in flame. NOx in dry flue gas [ppm] NOx vs excess air ratio 5% + corn stalk 5% CO in dry flu e g ases [% ] CO in flue gas vs excess air ratio 5% + corn stalk 5% Fig. 4. Dependence of NO x and CO emissions on excess air ratio. N O x in d ry flu e g as [p p m ] NOx vs first combustion temperature 5% + corn stalk 5% Temperature [ C] Fig. 5. Combustion temperature dependence of NO x emission. Figures 3-5 shows for the mixture sawdust 5%+ corn stalk 5% variation in time of combustion temperature and CO in gas fuel, variation of NO x and CO emissions with excess air ratio, the influence of O 2 in flue gas on combustion efficiency Com bustion efficiency [% ] Combustion efficiency vs O2 in flue gas 5%+corn stalk 5% O2 in dry flue gas [%] Fig. 6. Combustion efficiency variation with O 2 concentration in flame. and the influence of combustion temperature on NO x emission. The temperature varies a little in primary combustion chamber while in the secondary combustion chamber it decreases as volatiles are ISSN: ISBN:

5 burnt. Variation of CO in gas fuel confirms the separation of volatile and char combustion phases. The decrease of NO x emission with excess air ratio shows that the main NO x formation mechanism is fuel-no X and not thermal-no x. The NO x is produced mainly in the volatile combustion phase when the combustion temperature is higher. That is how the increase of NO x with temperature can be explained (Figure 5). The CO emission also decreases with excess air ratio due to a better mixing of fuel with air. The combustion efficiency decreases with O 2 content in flue gas, which is a normal trend (Figure 6). Higher O 2 content means either an insufficient mixing of air with fuel or/and higher excess air. 4 Conclusion The behaviour of the briquettes made from agricultural residues in the 42kW with reverse combustion is similar to that of the wood logs and sawdust briquette recommended by the boiler manufacturer. The maximum combustion efficiencies were obtained for reed, sawdust 5% +corn stalk 5%, sawdust 5%+wheat straw 5%, respectively. The results could be improved by a better air distribution in combustion chambers and by partially thermal insulating of primary combustion chamber. In this way, the overall and local excess air will be reduced and the combustion temperature will be increased. References: [1] Lundgren J., Hermansson R., Dahl J., Experimental studies of a biomass boiler suitable for small district heating systems, Biomass and Bioenergy, Vol.26, 24, pp [2] Van Loo S., Koppejan J., The Hanook of Biomass Combustion and Co-firing, Earthscan, 28. [3] Vasen N.N., Grassi G., Dahl J., Pigaht M., Janssen R. Agripellets, an opportunity to convert waste into a modern fuel. The 14th European Biomass Conference, October 25, Paris, pp [4] Werther J., Saenger M., Hartge E.-U., Ogada T., Siagi Z. Combustion of agricultural residues. Progress in Energy and Combustion Science, Vol. 26, 2, pp [5] Zabaniotou A.A., Skoulou V.K. Application of pilot technologies for energy utilization of agricultural residues in Northern Greece. Thermal Science Vol. 11, 27, 3, - pp Acknowledgments: This paper has financial support grants PN II nr /27 and 51-63/27, Romania. ISSN: ISBN: