Performance of a Biomass-Gas Stove using Fuel of Rubber Wood Pellets

Transcription

1 Performance of a Biomass-Gas Stove using Fuel of Rubber Wood Pellets Dijan Supramono a dan Farah Inayati b ab Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Depok 16424, Indonesia Ph : (021) Facs : (021) a dsupramo@che.ui.ac.id ABSTRACT Conventional biomass stoves, which currently use direct combustion of biomass pellets or briquettes, have still a problem of emitting much higher CO gas emission compared to LPG stoves at the level of 100 s ppm,. These values are well above the minimum allowable CO gas emission of 25 ppm and therefore the emission is not safe for the stove users. In this research, a biomass-gas stove was designed using a method of top-lit updraft gasification where the combustion of pyrolysis gas evolving from the bed of rubber wood pellets by adding secondary air occurs at the top of the stove. The primary air is delivered upward the bed, while the secondary air flows through the annulus of the stove and bends horizontally towards a series of holes at the top of the stove. This research has an objective to obtain optimum value of flowrate ratio of the secondary air to the primary air (air flow ratio) where the stove produces minimum emission of CO and maximum thermal efficiency. The ratio was varied at 2.44, 6.29, 13.43, and This work concluded that the lowest CO emission and the highest average flame temperature occur at the air flow ratio of 6.29, while the highest thermal efficiency at Keywords Rubber wood pellets, biomass-gas stove, top-lit updraft gasification, CO emission, pyrolysis 1. INTRODUCTION Indonesia has abundant potential of biomass waste, which is currently not utilised massively. The production of the waste is estimated to reach million tons per year and is equivalent to 1, million GJ/year. The main resources are rice waste of 705 million GJ/year, rubber plantation waste of million GJ/year, bagasse waste of million GJ/year, palm plantation waste of million GJ/year, and coconut plantation waste of million GJ [1-2]. If the biomass waste is directly burnt as solid fuels for stoves in the forms of briquettes or pellets, CO emission resulting from the combustion is expected to be high, well above the minimum allowable CO emission of 25 ppm [3]. This occurs due to heat absorption by solid fuels through conduction during the combustion causing the surface temperature of briquettes or pellets to drop and consequently increasing CO emission. This condition encourages the research on the biomass waste fuel which utilises its gas phase to allow the combustion to produce low CO emission. This may be substantiated by pyrolysing the solid biomass waste to produce biomass gas and burning the gas. This is expected to produce much lower CO emission similar to that produced by LPG stoves. The combustion occurs due to oxidation reactions between gases (CO, H 2, CH 4 and hydrocarbons) produced by pyrolysis of the biomass material. This pyrolysis is carried out in limited air environment and combustion uses a separate flow of air. The air used for pyrolysis is usually termed as primary air, whereas that for combustion secondary air. This pyrolysis is usually called oxidative pyrolysis. Senneca et al [4] found that this type of pyrolysis results in much higher weight loss of solid fuels at certain range of temperature compared to the loss using inert-gas pyrolysis. The pyrolysis is initiated by heat radiation of the gas combustion at the top of the pellet bed. The gas produced by oxidative pyrolysis may subsequently be completely oxidised to produced CO 2 and H 2 O [5][6]. Stoves utilising gas produced by oxidative pyrolysis of biomass material as a fuel is usually called biomass-gas stoves. Figure 1 describes the schematic diagram of the stove. Fuel, in the current research of rubber wood pellets, is ignited at the top of the pellet bed. The ignition forms flaming pyrolysis front which moves downward. During its passage, the biomass pellets are pyrolysed (gasified) from the top to the bottom of the fixed bed [7]. Primary air is supplied from the bottom of the pellet bed at strictly limited amount to sustain flaming pyrolysis front. Biomass gas produced moves upward and after mixing with the secondary air supplied radially from the top side of the stove, it burns over the top of the fixed bed with high heat release. The gasification process is entirely autothermal and does not need any external heat. Part of the pellet bed left behind by flaming pyrolysis front forms biochar after releasing biomass gas. The char may be oxidised by Page 393

2 remaining oxygen from the flaming pyrolysis front to form CO 2 [8]. CO 2 and H 2 O produced in the flaming pyrolysis front may react with biochar to form gases CO and H 2 [7]. The pyrolysis-combustion of biomass material using this method was first introduced by using a principle of top-lit updraft (TLUD) gasification by Reed [9]. The present research is aimed to investigate the CO emission resulting from the combustion of the gas phase of the pyrolysis products of rubber wood pellets and the stove efficiency subject to the variation of ratio of the secondary to primary air flowrates. Although some researches relating to the CO emission has been conducted, but in general the emission figures were published in terms of grams as cumulative emissions [9]. None present in terms of the ppm during the combustion. Effect of CO emission to human being depends on the concentration of CO emission in ppm and the duration to which the human being is exposed [10]. Besides, the CO emission may also depend on the type of biomass used as fuel because different biomass types have different compositions of cellulose, hemicellulose and lignin part of which under pyrolysis environment will decompose to volatile matter and subsequently to smaller molecules [11]. The threshold CO emission stipulated by Ministry of Workforce Indonesia is 25 ppm. According to the result of proximite analysis by Energy Technology centre, BPPT, the content of volatile matter of the rubber wood sample was 68.32%. This gives indication that the rubber wood if used as fuel in TLUD stove will produce much volatile matter and therefore it is potential for biomass-gas stove. Figure 1a: Schematic description of TLUD stove [8]. Figure 1b: Mechanisms of biomass pellet pyrolysis and wood gas combustion [9]. 2. EXPERIMENT The experiment consists of biomass pellet preparation, and biomass pellet pyrolysis and combustion. Rubber wood pellets were prepared by first grinding the rubber wood branches to powder, drying the powder to reach moisture content of 10%. Subsequently, the dry powder was moulded to form pellets. During the pyrolysis-combustion, measurements of CO emission and flame temperature were conducted. Main measurement tools used were biomass-gas stove where the pyrolysis-combustion of pellets was undertaken, some thermocouples type K, a temperature data logger to log data of flame temperature, a gas analyzer to measure CO concentration in flue gas, and an anemometer to measure velocity of air to the primary and secondary air blowers, and a water pot of 17cm diameter. The water pot was used for determination of the stove thermal efficiency using water boiling test method. Pellets made of rubber wood were of cylindrical shape with 1.5cm diameter and 3cm length. The stove prepared has inside diameter of 15cm and height of 58cm (see Figure 2). These sizes were to accommodate the loading of rubber wood pellet of 1.4 kg. The stove consists of 2 concentric cylinders with an annulus in between the cylinders. The pellets were laid in the inner cylinder of the stove. Through the annulus, the secondary air is flowing to fulfil the need for the combustion of wood-gas produced by the pyrolysis. The construction material is made of mild steel. The outer side of the stove was lined with ceramic fibre and enclosed with an aluminium plate. A grate of stainless steel 314 was installed at the lower side of the stove to support the pellet bed to resist corrosion. Two centrifugal blowers have been used to deliver primary and secondary air respectively (see Figure 2). The first blower has diameter of 2 inches and input electricity ampere of 1A, while the second blower 2.5 inches and input ampere of 1.6A. The primary air was delivered to the base of the bed, while the secondary air flowed through the annulus of the stove and bent towards a series of holes at the circumference of the top of the stove. This design allows the crossflow between pyrolysis gas and combustion air, which intensifies mixing. For each run, the stove used 1.4 kg of the pellets and primary air flowrate set about m 3 /sec. The ratio of the secondary air to the primary air flowrates (air flow ratio) as the free variabel for this experiment was varied at 2.44, 6.29, and 20.6 and its effect on Page 394

3 flame temperature, CO emission and thermal efficiency were observed. The measurements of CO emission, flame temperature and water temperature in the water boiling test were carried out simultaneously. Figure 2: Fabricated biomass-gas stove for experiment with 2 blowers for primary air and secondary air respectively. 3. RESULTS AND DISCUSSION 3.1. Flame temperature and CO emission Figure 3 shows that at the air flow ratio equal to 2.44, the flame temperature is nearly constant during the course of the combustion. At this ratio, the use of the secondary air to oxidise the pyrolysis gas is limited and may be not sufficient (see Table 1). As a result, the concentration of CO as an indicator of the incomplete combustion is high as described in Figure 4. The heat radiated from the combustion flame to the flaming pyrolysis front to enhance the pyrolysis may be not high due to low temperature driving force between both flames. This weakens the rate of pyrolysis and consequently, the rate of pyrolysis gas production is low and the pyrolysis-combustion lasts longer to produce and oxidise all the pyrolysis gas compared to that at higher air flow ratios. This also affects the CO emission, where its level of emission is high at low flame temperature (see Figure 4) [10]. At higher air flow ratio of 6.29, Figure 3 shows that most of the time, the flame temperature is higher than that at air flow ratio of This may happen due to more complete oxidation as a result of sufficient supply of secondary air. In terms of the CO emission, this favours the low production of CO as described by Figure 4 and achieves the lowest average CO emission (see Table 1). Due to higher temperature driving force between combustion flame and flaming pyrolysis front at the air flow ratio of 6.29, the pyrolysis may occur at higher temperature and lasts shorter. Pyrolysis-combustion operation at air flow ratio of where the supply of the secondary air is higher than that at flow ratio of 6.29 exhibits opposite trend compared to the trend if the air flow ratio is increased from 2.44 to The flame temperature is lower (see Figure 3) and consequently the CO emission during the course of the pyrolysis-combustion is higher (see Figure 4) and its average value is also higher (see Table 1). This may happen due to excessive supply of the secondary air which cools the combustion flame. The temperature driving force between combustion flame and pyrolysis flame is lower at the air flow ratio of and thus extending the pyrolysis time as shown in Figure 3. At the air flow ratio of 20.6, at times after 9 minutes, the flame temperature is lower than that at air flow ratio of as expected due to excessive supply of the secondary air to the combustion flame. However, at times before 9 minutes, the flame temperature is slightly higher than that at air flow ratio of The temperature fluctuation at the air flow ratio of at initial stage of pyrolysis-combustion operation where the position of the thermocouples was kept at the combustion flame indicates that there may be non-uniformity of the secondary air supply because the secondary air inlet at the base of the stove is at one side of the stove. Consequently, this creates non-uniformity of the secondary air supply and forms lacking and excess air regions in the combustion flame front. The regions where the air supply is lacking form soot which intensifies radiation heat transfer [10]. At higher air flow ratio of 20.6, which means higher secondary air supply, turbulence inside the annulus becomes more rigorous and results in redistribution of inlet flow of the secondary air across the circumference of the top side of the stove [12]. As a result, the inlet flow is more uniform and flame temperature at this ratio is relatively unchanged from time to time. This also causes the portion of heat radiation is less than that at the air flow ratio of The less flame temperature at initial stage of the combustion using the air flow ratio of is due to non-uniformity preheating of the secondary air in the annulus. With the time proceeds, the wider region of the pellet bed with higher temperature allows expansion of the preheated Page 395

4 secondary air and intensified turbulence which gives effect similar to that which happens in the case of the air flow ratio of 20.4 where the flame temperature is less turbulent (see Figure 3). This comparison between flame temperature profile for the combustion using the air flow ratio of and that of 20.6 gives consequences on the CO emission profile of both air flow ratio. At initial stage of the combustion, the CO emission at the air flow ratio of is higher than that at the ratio of Contrary to that, at later stage of the combustion, the CO emission at air flow ratio of is less than that at the ratio of 20.6 (see Figure 4). Table 1. shows that the average CO concentration in the flue gas reaches the minimum value, i.e. 14 ppm, when the air flow ratio was set This implies that using controlled air flows both of primary air and secondary air, we can make the stove healthy to the users in domestic kitchens because the CO emission can be kept low. However, the stove fabricated for this experiment is considered too high to be practical, so the next research should be directed to fabricate a shorter stove with longer operation to suit the needs in the kitchens flame temperature ( o C) time (minute) air flow ratio 6.29 air flow ratio 20.6 air flow ratio air flow ratio 2.44 Figure 3: Flame temperature measured during the course of the combustion CO emission (ppm) time (minute) air flow ratio 6.29 air flow ratio 20.6 air flow ratio air flow ratio 2.44 Figure 4: CO emission during the course of the combustion Page 396

5 Table 1. Average CO emission, maximum flame temperature and maximum water temperature at different values of ratio of secondary air to primary air flowrates (air flow ratio) Primary air flow rate (m 3 /s) 3.2. Stove thermal efficiency Secondary air flow rate (m 3 /s) Air flow ratio Average CO emission (ppm) Maximum flame temperature ( O C) Maximum water temperature ( O C) Thermal efficiency is calculated using the following equation M c p1 T T M c T T b a 1 H W c p2 b a M H 2 L (1) where M is the initial mass of water in the water pot, M 1 is the mass of the water pot, C p1 is the heat capacity of water, C p2 is the heat capacity of the water pot, M 2 is the mass of evaporated water, H L is the latent heat of evaporation, H L is the high heating value of the rubber wood (3771 cal/gram), W is the mass of the rubber wood pellets in the stove, T a is the initial temperature of the water in the water pot and T b is the end temperature of the water in the water pot before the water is evaporated at constant temperature. The values of the thermal efficiency for all runs based on varied air flow ratio are shown in Table 2. The highest efficiency (58.05%) is achieved by the stove using air flow ratio of Even though in average the flame temperature at this ratio is low, insufficient supply of the secondary air forms soot particles in large number. These particles make the emissivity of the flame high which favours the heat radiation from the combustion flame to the base of water pot. In turns, this heat radiation intensifies the heat transfer to the water in the water pot and therefore the water temperature. Because the lack of secondary air supply uniformly occurs across the circumference of the top of the stove, the water temperature is higher than those measured at other air flow ratios (see Figure 5). In terms of the efficiency, this is desired, but as far as the CO emission is concerned, the stove at this ratio is not favourable because it produces high CO emission with the average value of 52 ppm (see Table 1). The highest thermal efficiency (56.98%) with appropriate CO emission of 32 ppm in average is achieved at air flow ratio of This is slightly higher than that achieved by the ratio of 6.29, i.e. 52.8% though the maximum flame temperature at the ratio of was lower, i.e o C. As discussed previously in Section 6.1 there may be non-uniformity of the secondary air supply on the circumference of top of the stove which exerts the formation of soot at circumferential positions where the air primary flow is low indicated visually by the yellow flame and intensifies heat radiation [10]. The high thermal efficiency was the result of the combined effect of heat convection and radiation, but at the ratio of 13.43, the heat radiation seems more predominant which intensifies the water temperature above the water temperature measured at the air flow ratio of 6.29 (see Figure 5). At the air flow ratio of 20.6, as discussed in Section 3.1, due to the uniformity of the secondary air supply across the circumference of the top of the stove, the soot formation can be lessened and the combustion flame is engulfed with excessive air. As a result, the thermal efficiency at this ratio is less than that at the ratio of (see Table 2). Table 2. Thermal efficiency at different values of ratio of secondary air to primary air flowrates (air flow ratio) Air flow ratio Thermal efficiency (%) Maximum flame temperature ( o C) Page 397

6 water temperature ( o C) time (minute) air flow ratio 6.29 air flow ratio 20.6 air flow ratio air flow ratio CONCLUSIONS Figure 5: Water temperature measured during the course of the combustion This experiment gives some conclusions as follows: 1. The stove operating with air flow ratio of 6.29 results in the lowest CO emission which reaches 14 ppm in average. 2. The stove operating with air flow ratio of 6.29 results in the highest average flame temperature which reaches the peak temperature of 739 o C. 3. The stove operating with air flow ratio of 2.44 results in the highest efficiency of 58.05%. 4. It seems that by exploiting a condition where the formation of soot is intensified, the thermal efficiency may be improved. REFERENCES [1] Milbrandt, A and Overend, R.P., Survey of biomass resource assessments and assessment capabilities in APEC economies, National Renewable Energy Laboratory (NREL), Colorado, [2] Zentrum fur Rationell Energieanwendung and Umwelt GmbH, Biomass in Indonesia-business guide, [3] Purwanto, W.W., Supramono, D., Nugroho, Y.S., and Rizqiardihatno, R.F., Designing biomass pellet stove of high efficiency and environmental friendly using heat recovery principle, International Seminar on Sustainable Biomass Production and Utilisation, University of Lampung, [4] Senneca, O., Chirone, R., and Salatino, P., Oxidative pyrolysis of solid fuels, J. Anal. Appl. Pyrolysis, 71, 2004, p [5] Belonio, Alexis T, Risk husk gas stove handbook, Department of Agricultural Engineering and Environmental Management, Central Philippine University, [6] Reed, T.B., Biomass gasification principles and technology, New Jersey : Noyes Data Corp, [7] Saravanakumar, A., Haridasan, T.M., Reed, T.B., and Bai, R.K., Operation and modelling of an updraft long-stick wood gasifier, Energy for Sustainable Development, Vol. IX, No. 4, p.25-39, December 2005 [8] Reed, T.B., Das, A., and Anderson, P.S., Gasification: A process common to all biomass stoves, ETHOS meeting, [9] Christa, Roth, Micro-gasification: cooking with gas from biomass, 1 st edition GIZ HERA Poverty-Oriented Basic Energy Service, [10] Turns, Stephen R., Introduction to combustion: concepts and applications, 2 nd edition, McGraw Hill, 2000 [11] Basu, Prabir, Biomass gasification and pyrolysis, practical design and theory, Elsevier, [12] McNair, J. N., Newbold, J. D., and Hart, D. D., Turbulent transport of suspended particles and dispersing benthic organisms: How long to hit bottom?., J. Theoretical Biology., 188, 1, pp.29 52, Page 398