Hirotatsu Watanabe 1, Yasuyuki Kawakami 1, Yoshio Morozumi 1, Hideyuki Aoki 1, Shoji Tanno 1, Takatoshi Miura 1 and Yoshirou Honma 2

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1 ICLASS-26 Aug.27-Sept., 26, Kyoto, Japan Paper ID ICLASS6-93 Measurement of diameter distribution of dispersed droplets in W/O emulsified fuel and the effect of emulsified fuels on boiler performance Hirotatsu Watanabe, Yasuyuki Kawakami, Yoshio Morozumi, Hideyuki Aoki, Shoji Tanno, Takatoshi Miura and Yoshirou Honma 2 Department of Chemical Engineering, Tohoku University, hiro@tranpo.che.tohoku.ac.jp 2 Network Future Group Corporation ABSTRACT In this study, we measure diameter distribution of dispersed water droplets in the W/O emulsified fuels with and 2vol.% of water. Then, the emulsified fuel was applied to 233kW oil-fired water heater. The exhaust gas concentration of NO x, CO and SO x, and the thermal efficiency were measured under high and low combustion load. With an increases in the water content, both exhaust NO x and CO concentration decreases under both high and low combustion load. In high combustion load, the thermal efficiency increases as the water content increases. However, the thermal efficiency decreases under low combustion load because of latent heat of vaporization of water contained in the emulsified fuel. Especially, thermal efficiency increases by 2% when 2 vol.% W/O emulsified fuel was used in high combustion load. Keywords: : W/O emulsified fuels, Combustion characteristics, Emissions. Introduction Environmental destruction such as acid rain derived from NO x and SO x, are now global problems. One of the most efficient methods to reduce NO x and SO x emissions is to use alternative clean fuels. Emulsified fuel is regarded as one of the possible alternative fuels for reducing the emission of pollutants from practical combustion systems. A number of diesel engine tests using emulsified fuels have been conducted[-3], and indicated that NO x, CO and soot emissions decreased using emulsified fuels. The occurrence of micro-explosion in emulsified fuels is widely accepted to explain the decrease in CO and soot emissions. Micro-explosion is the secondary atomization caused by the rapid evaporation of dispersed water in emulsified fuel droplets. Although there are different opinions about micro-explosion in spray flame, Mizutani et al.[4] have observed the micro-explosion and puffing in spray flame. Puffing is that small droplets spout from surface of emulsified fuel droplets[5]. Micro-explosion in spray flame is related with the diameter of dispersed water droplets in emulsified fuel because the micro-explosion is caused by the rapid evaporation of the dispersed water. The size of water droplets affects the rapid evaporation. Therefore, it is important to measure diameter distribution of the dispersed water droplets and to investigate the effect of the diameter distribution on combustion characteristics. While the use of emulsified fuel in diesel engine and boiler has been active area of recent research, some of these studies have only measured external effects on exhaust emissions or engine performance[]. There has been few systematic investigation of the effect of emulsified fuel on the performance and exhaust emissions in practical heaters or boilers. In this study, we measure diameter distribution of dispersed water droplets in the W/O emulsified fuels with and 2 vol.% of water. Then, the emulsified fuel is applied to an oil-fired water heater to investigate the exhaust gas concentration of NO x, CO and SO x and thermal efficiency. 2. Experiment The fuel properties of heavy oil used in this study are shown in Table (JIS K 225 No. ). The commercialized surfactant, Concol-S(NEO CS, Co., Ltd.), is used as an emulsifying agent to prepare the W/O emulsified fuel. The volume flow rate of emulsifying agent is /35 by that of heavy-oil. The water content in the emulsified fuel is ranged from to 2 vol.%. Fig. shows the schematic diagram of the mixing device used in this study. The emulsifying agent and water are Table Characteristics of the heavy oil used in this study[6] Ultimate analysis [wt%] C 85.8 H 3.2 N. S.9 Net calorific value [MJkg - ] 42.2 Density [kgm -3 ] 856 Emulsifying agent Water Heavy oil Flow meter Static mixer Controller Valve Pressure gauge Fig. Schematic diagram of mixing device Flow meter Emulsified fuel Valve

2 Emulsifying agent Water Heavy oil Outflow water Inflow water Exhasut gas Sampling probe Gas analyzer Filter injected into the oil stream before reaching the static mixer. After mixing by the static mixer, the emulsified fuel discharged from the pump is back to upstream of the pump for the purpose of further mixing. In the measurement of the size distribution of dispersed droplets in W/O emulsified fuel, the flow rate of the emulsified fuel is set to.26 lmin -. Water droplet sizes in the emulsified fuel with, 2 vol.% of water are measured by photomicrography. Fig. 2 shows the schematic diagram of experimental apparatus for the measurement of combustion characteristics of emulsified fuel. A hot-water heater (KFL-2A, TAKUMA Co., Ltd.) is used. The emulsified fuel is produced by mixing system shown in Fig.. Pure heavy oil and emulsified fuels with % and 2% of water are applied to the heater. The heater has two pressure nozzles. These injection pressures are.6 MPa. In low combustion load, the heater runs with one pressure nozzle. In high combustion load, the heater runs with two pressure nozzle. In this study, the fuel flow rates are set to.26 lmin - and.46 lmin - for low and high combustion load, respectively. The combustion chamber is cooled by a coolant that exhanges heat with the inflow water. The flow rate of the inflow water is 55 lmin -. The temperature of inflow water, outflow water and exhaust gas are measured with K-type thermocouples. The combustion behavior is considered to be reached at steady state when the exhaust temperature is almost constant. O 2, NO x, CO and SO x concentration in exhaust gas are measured with a gas analyzer(testo-35xl, Testo Co., Ltd.). The air flow rate is adjusted to obtain concentration of oxygen in the exhaust gas of about 6. vol%(dry volume.) Thermal efficiency is defined as ( T η = ρ T ρ out w,out in w,in ) H W ρ L Mixing device f f W C w p,w In this study, the difference between T out and T in is about 5K. Therefore, the temperature dependence of specific heat is neglected because specific heat of water is almost constant in this temperature range. Each experiment is repeated three times to obtain the mean value of experimental data. Injector Boiler Flame Coolant Fig. 2 Schematic diagram of experimental apparatus µm (a) vol. % of water 3. Results and Discussion water 3. The size distribution of dispersed water droplets Fig. 3 shows the microphotographs of emulsified fuel with vol.% and 2 vol.% of water. The size of dispersed droplets in 2 vol.% emulsified fuel is larger than that in vol.% emulsified fuel. Fig. 4 shows the size distribution of water droplets. The dispersed droplets smaller than µm are not measured because of experimental uncertainties. Number of sampling droplets for vol.% and 2 vol.% emulsified fuel are 262 and 2542, respectively. The number of dispersed droplets in the size range from to 2µm in vol.% emulsified fuel is larger than that in 2 vol.% emulsified fuel. However, no significant changes is observed for the number of dispersed droplets larger than 3µm. The linear diameter of water oil µm (b) 2 vol. % of water Fig. 3 Photomicrograph of emulsified fuel

3 Frequency [-] Frequency [-] Water droplet size [µm] (a) vol. % W/O emulsified fuel Water droplet size [µm] (b) 2 vol. % W/O emulsified fuel Fig. 4 Size distribution of dispersed droplet droplets for vol.% and 2 vol.% emulsified fuel are 5.6 and 5.73 µm, respectively. 3.2 The effect of emulsified fuel on boiler performance Fig. 5 shows the exhaust O 2 concentration. Exhaust O 2 concentration is maintained at about 6 vol% in all case. Fig. 6 shows the effect of the water content in emulsified fuel on the exhaust NO x concentration. As the water content in the emulsified fuel increases, the exhaust NO x concentration decreases. In this study, since the volume flow rate of emulsified fuel are kept constant, the amount of nitrogen compounds contained in the emulsified fuel decreases as the water content in emulsified fuel increases. Table 2 shows the exhaust NO x concentration and the amount of decrease in the exhaust NO x concentration in the emulsified fuel compared to the concentration in the heavy oil assuming that fuel nitrogen is perfectly converted NO x. The conversion rate of fuel-bound nitrogen to NO x is about 5-3%[7]. However, the decrease in exhaust NO x concentration in the emulsified fuel is smaller than 8%. Three different possible explanations have been found in the literature that emulsified fuels decrease NO x emission. The formation of thermal NO is governed by highly temperature-dependent chemical reactions called the extended Zel dovich mechanism[8]. Some researchers[-2] attributed the decrease in NO x emission to the decrease in flame temperature. Some researchers said the increase in the OH radical due to the presence of the additional water can reduce the formation of NO x by lowering the concentration of O atoms[9-]. Ballester et al.[] shows O2 concentration [dry volume %] NO concentration NO concentration Small fuel flow Large fuel flow rate combustion rate combustion Fig. 5 Exhaust O 2 concentration Fig. 6 Exhasut NO concentration the visible flame length decreases to about.6m by using W/O emulsified fuel while that is about m by using heavy oil. Therefore, the residence time in high temperature region by using emulsified fuel is shorter than that by using heavy oil. The decrease of residence time in the high temperature region has the significant effect on the reducing exhaust NO x concentration[]. Although, spatial distribution of combustion characteristics can help in assessing the importance effects of emulsified fuels, these are not measured in this study. More detailed work on emulsion flame is needed. Fig. 7 shows the effect of water content on the exhaust CO concentration. As the water content in the emulsified fuel increases, the exhaust CO concentration decreases in low and high combustion load. This improvement of the combustion behavior is attributed to the secondary 2 2

4 Table 2 Exhaust NO x concentration and NO x concentration from nitrogen compounds in decreased heavy oil Water [vol.%] 2 Exhaust NO x concentration Decreased heavy oil flow rate [lmin - ] NO x concentration from nitrogen compounds in decreased heavy oil Water [vol.%] 2 Exhaust NO x concentration Decreased heavy oil flow rate [lmin - ] NO x concentration from nitrogen compounds in decreased heavy oil CO concentration SOx concentration CO concentration Fig. 7 Exhasut CO concentration atomization induced by the micro-explosions of the emulsified fuel. Fig. 8 shows the effect of the water content on the exhaust SO x concentration. With an increase in water content, the exhaust SO x concentration shows little change in high combustion load, however, exhaust SO x concentration decreases in low combustion load. Fig. 9 shows the exhaust gas temperature for low and high combustion load. As the ratio of water content increases, the exhaust gas temperature decreases. The exhaust gas temperature under high combustion load is higher than that under low combustion load by 5-6 degree Celsius. When SOx concentration Fig. 8 Exhasut SO x concentration the exhaust gas temperature is lower than acid dew point, low-temperature corrosion takes place because H 2 SO 4 is formed. Thus, low-temperature corrosion may occurs in low combustion load. Fig. shows the effect of water content on the thermal efficiency. Table 3 shows the effect of latent heat of water on thermal efficiency. As the water in the emulsified fuel increases, the thermal efficiency decreases in low combustion load primarily because latent heat of vaporization of water lower the flame temperature. However, in high combustion load, as the ratio of water content in the emulsified fuel increases, the thermal

5 Exhasut gas temperature [ ] Exhasut gas temperature [ ] (a) Low combustion load (b) High combustion load Fig. 9 Exhasut gas temperature efficiency increases. This is because that the effect of the improvement of the combustion due to micro-explosion on increasing in thermal efficiency is higher than the effect of latent heat of vaporization of water. Moreover, radiative heat transfer to the wall may be improved. The radiative heat transfer to the wall is related to the temperature of coolant that exchanges heat with the inflow water. Temperature distribution of coolant may not be constant. When radiative heat transfer to the wall increases upstream of the chamber, thermal efficiency increases because the heat exchange between inflow water and coolant is improved. Ballester et al.[] showed that radiative heat transfer to the wall in flame zone is increased owing to a higher particle concentration caused by a larger number of drops after the secondary atomization. In fact, Mattielo et al.[2] reported a significant increase in particle number density in the first part of flame. 2 2 Thermal efficiency [-] Thermal efficiency [-] (a) Low combustion load 2 (b) High combustion load Fig. Thermal efficiency oil-fired water heater to investigate the effect of emulsified fuels on the exhaust gas and thermal efficiency. The more increases the ratio of water content, the more decreases in both exhaust NO x and CO concentration. In high combustion load, the thermal efficiency increases as the ratio of water content increases, although the thermal efficiency decreases in low combustion load due to latent heat of vaporization of water. Exhaust SO x concentration is almost constant in high combustion load. However, in low combustion load, exhaust SO x concentration decreases with increasing the ratio of water content because low exhaust temperature causes SO x to change H 2 SO 4. The results indicate that low-temperature corrosion may occurs when W/O emulsified fuel is used. It is necessary to raise the exhaust gas temperature to over acid dew point. 6. CONCLUSION In this study, we measure the size distribution of dispersed droplets in the W/O emulsified fuels with and 2vol. % of water. The emulsified fuels are applied to an Low combustion load High combustion load Table 3 Decrease in latent heat of vaporization of water water [vol%] Increase and decrease in the thermal efficiency [%] Decrease in the thermal efficiency from latent heat of vaporization of water [%]

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