Development of Biomass Charcoal Combustion Heater for Household

Development of Biomass Charcoal Combustion Heater for Household Utilization. Masayuki ... Publication Date (Web): August 27, 2008 ... The distinct adv...
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Ind. Eng. Chem. Res. 2009, 48, 361–372

361

Development of Biomass Charcoal Combustion Heater for Household Utilization Masayuki Horio, Amit Suri,* Junji Asahara, Shinichi Sagawa, and Chizuko Aida Department of Chemical Engineering, BASE, Tokyo UniVersity of Agriculture & Technology, 2-24-16 Naka-cho, Koganei, Tokyo 1848588, Japan

In the present work a prototype powdered biomass charcoal fired heater with a heat output of 6 kW is designed and developed so that a new powdered charcoal market can be initiated to enhance greenhouse gas (GHG) reduction through massive biomass utilization. The combustion heater was designed based on the concept of charcoal combustion in a thin bed cross-flow (TBCF) mode, where a very thin uniform bed of charcoal is fixed by air flow on the wall of a cylindrical chamber with an air-penetrable wall. The distinct advantage of using such a thin bed cross-flow is low fuel inventory and good air-fuel contact, resulting in fast startup/ shutdown and low CO emissions in the exhaust gases. Fundamental data for realizing such a combustion heater are presented, and the performance characterization of the thus manufactured heater is investigated. The combustion heater was characterized for charcoal prepared from Japanese oak (Quercus serrata) and from several waste biomass sources, such as a pruned apple branch and charcoal formed from spent coffee waste and soybean fiber. For wood charcoal the heater’s thermal efficiency was about 65-86%, and for waste biomass charcoal species it was found to be in the range of 60-81%. When the combustion heater was operated at the stable combustion mode, the CO concentration in the exhaust after the flue gas passed through catalyst was less than 5 ppm. 1. Introduction The Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) has been successful in attracting the world’s attention to global warming and greenhouse gas (GHG) emissions, bringing into focus the importance of the Kyoto Protocol, which addresses key issues in reduction of GHG emissions, and the post Kyoto Protocol framework design. To suppress the global temperature rise within 2 °C to avoid extreme climate disasters, it is said to be necessary to reduce the GHG emission to half of its year 1990 value.1 With a simple calculation (cf. Appendix A) as shown in Table 1, for the case where equal right to spend fossil fuels but equal duty to utilize low GHG emission technologies, as reasonable as and as low as Japan’s year 2000 conditions’ per capita basis, are assumed as developing countries request, we obtain the result that the global GHG emission reaches 2.66 times as big as those of the 1990s for the year 2000. If global GHG emissions are to be reduced based on this Japan’s per capita equivalent CO2 emissions for year 2000, to obtain half of the world’s 1990 CO2 emissions to avoid extreme climate disaster (rise of 2 °C) a substantial reduction is necessary for almost all countries. The required reduction for the year 2000 was in the range of 73-91% for industrialized nations (cf. Table 1, right-most column). However, developing countries, such as China, have less duties, 23% (cf. Table 1), but substantial reduction is still requested based on year 2000 emission results. Since the reduction duty of GHG emissions is quite high, the industrialized nations (participating Annex I countries in the Kyoto Protocol) have to take the initiative to introduce a fair rule to change the modern petroleum-dependent lifestyle by putting effort into the development of renewable energy sources not just on an industrial scale, but also on a small/household scale. However, to use renewable energy sources, the energy source should be abundantly available, well distributed, and in a * To whom correspondence should be addressed. Tel.: +81-42-3887067. Fax: +81-42-386-3303. E-mail: [email protected], amsuri@ gmail.com.

perennial supply. In the case of Japan, about 67% of Japan’s land area is covered with forests2 and biomass is the single most abundantly available renewable energy source that can be used on demand. Wood biomass combustion/gasification can thus play a significant role in the domestic electricity supply by following load changes. Moreover, small-scale to mid-scale biomass thermal power stations can be installed which can help reduce distribution losses and also serve as district heating and for regional hot water supply lines, thus reducing double conversion as in the case of using electricity for heating. In Japan, about 28% of the total energy consumed is utilized for domestic usage, of which about 50% is used in hot water supply and space heating.3 By replacing only 5% of this heating energy, we can obtain a CO2 reduction effect of about 4.46 million tons of CO2/ year. Direct biomass combustion boilers and heaters for individual buildings were used in Japan only until the 1960s, and reintroducing firewood and charcoal distribution system should be still not too much unrealistic, if biomass utilization style can be made a little more convenient. “Improved biomass stoves” used in Turkey and U.S. residential space heating offer the advantage of biomass utilization, but are not preferred due to their low thermal efficiency, in the range of 46-54%.4,5 This is due to the smoke emission and the inefficient nature of conventional chimneys, which may not be easily improved due to tar and soot deposition problems. Previous researchers5 have characterized these “improved biomass stoves” for flue gas emissions and thermal efficiencies for combustion of different biomass species. Biomass in its raw form with its low calorific value (15 MJ/kg) is very ineffective in its storage, transportation, and combustion and cannot compete with kerosene or gas combustion. More recently, becoming popular are wood pellet combustion heaters, having higher thermal efficiency and automatic feed control.6 Even airconditioning systems fueled by wood pellets have been designed and operated in Kagoshima prefecture in Japan.7 The point of it is a higher heating value (18 MJ/kg) and the convenience of easy handling and automatic control. Nevertheless, the improve-

10.1021/ie8006243 CCC: $40.75  2009 American Chemical Society Published on Web 08/27/2008

362 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 Table 1. Energy Consumption and CO2 Emissions for Different Countries energy consumption

CO2 emissions

oil equivalent energy consumption [MtOe]

carbon equivalent emissions [Mt(C)]

achievement year countries

1990

Japan USA Canada U.K. Germany France Italy China India Kenya OECD

466 2133 277 234 374 230 171 680 199 3 4517

561 2494 326 245 359 274 192 945 339 4 5316

world

8755

10 033

a

2000

achievement year Japanese per capita equivalent,a Y2000

1990

2000

561 1251 139 264 364 271 256 5621 4448 135 4661

272 1350 127 162 265 99 112 605 155 2 3073

321 1572 151 150 229 108 120 786 268 2 3463

26 881

5777

6413

Japanese per capita equivalent,b Y2000

if CO2 emissions is half of Y1990 and standard of consumption and emission is based on Japanese per capita equivalent for Y2000 oil equivalent energy consumption [MtOe]

CO2emissions [Mt(C)]

reduction for Y2000 [%]

321 716 79 151 208 155 146 3217 2546 77 2891

105 235 26 49 68 51 48 1055 835 25 875

60 134 15 28 39 29 27 604 478 15 543

81 91 90 81 83 73 77 23 -78 -522 84

15 387

5046

2888

55

Oil equivalent energy consumption. b Carbon equivalent emissions.

Figure 1. Types of air-fuel contact in a bed of solid fuel particles illustrated for constant-pressure cases.

ment by making wood chips or sawdust into pellets is small due to additional energy requirements for drying, milling, and extrusion and due to the basic degrading nature of pellets in the long term. Contrary to the good intention of pellet stove supporters to reintroduce wood biomass into domestic heating demand, the pellet stove system has not yet solved the issue of biomass utilization successfully. However, once carbonized, wood biomass becomes a stable, clean, and high-caloric fuel, i.e., charcoal (25-30 MJ/kg). Although charcoal produced in traditional ways is inefficient in terms of yield and high GHG emissions,8 a near theoretical charcoal yield can be achieved under elevated pressure.9-13 These high-yield processes can be integrated with district heating or other heat utilization facilities for coproduction of charcoal and thermal energy by directly utilizing the volatiles produced during carbonization for combustion. Furthermore, if charcoal is crushed, the mechanical strength of charcoal is very much less and it can be easily crushed to powder.14 For the case of coffee bean waste after dripping, the crushing energy of its char prepared at different temperatures (maximum 800 °C) was measured experimentally and was found to be less than 48% of that of raw coffee bean waste.15 If powdered charcoal is packaged in cartridge containers, the fuel quality of charcoal

can be preserved for long-term storage with the convenience of easy transportation.16 These charcoal cartridges can then be used in conjunction with some combustion equipment that has a special flow-controlling device and a completely automatic combustion control system; this can provide a promising viable alternative for heat energy requirements for small-scale utilization. 2. Design Concept for a Solid Fuel Combustion Heater The combustion system aimed at in the present work for biomass charcoal particle combustion is the one that can be operated in continuous mode, with low fuel inventory, and with good contact between fuel and air for fast startup and extinction and complete oxidation for low CO in flue gas. The low fuel inventory is essential because, as shown in Figure 1, if the particle bed is thick, or if the bed support wall is impenetrable to air, the combustion of particles will only take place at the top surface of the bed. However, if the bed is made very thin and if the air penetrates into the bed and then out from the bed support wall, the contact between fuel and air is made good, resulting in a quick ignition/quick extinction. We name this the “thin bed cross-flow” (TBCF) mode of solids combustion.

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The TBCF mode of combustion has advantages in terms of good air-fuel contact and low bed inventory. Comparing TBCF combustion with fluidized bed combustors, both offer good air-fuel contact, resulting in almost complete oxidation of CO. However, in the case of fluidized bed combustors, the bed cannot consist of solid fuel only. If a bed contains only solid fuel, oxygen is consumed only in the grid zone, lowering the contact efficiency and forming local hot spots and agglomerates of ash. Hence, a fluidized bed combustor needs to be diluted by incombustible bed material and the solid fuel particles are floated in the bed. This requires a large solid inventory and as a result a very long time for startup and complete extinction. If TBCF combustion is compared with a packed bed combustor and a rotary kiln, although they all offer an advantage of operation with no bed diluents material, the air-fuel contact in packed beds and rotary kilns is nonuniform, and combustion takes place only in a zone or on the bed surface. Hence, if one wants to take advantage of low bed inventory and good air-fuel contact, for quick ignition/extinction, TBCF combustion should be preferred. However, to form a uniform thin bed of charcoal particles on the wall of the combustion chamber for TBCF combustion, particles are needed to be uniformly fixed on the wall. Fixing a thin fuel layer all over the circular chamber wall can be done either by rotating the chamber or by attaching a rotating feeder or a solid flow conditioner, so that the particle flow is directed radially to the wall of the chamber. However, in the present design, the chamber was supposed to rotate since by this rotation the fuel distribution over the wall can be done more easily. The front end of the rotating chamber was covered by a nonrotating high-temperature glass plate. If the combustion chamber rotates and is sealed by a nonrotating glass plate, the combustion chamber needs to be housed in a stationary cylinder of little larger dimensions. Hence, an air seal for the gap between the combustion chamber and its surrounding needs a design delicate, but robust, enough for possible combustion chamber deformation during high-temperature operation. An induced negative draft was applied to keep the temperature of the glass plate relatively low and to avoid any spillover of burning char in case of glass breakage. To fix fuel particles on the wall, even on its top wall, it is necessary to counter the gravitational force for the particles to form an upside-down fixed bed on the penetrable wall of the combustion chamber.17 In the following section the design procedure is discussed based on design principles and data are determined experimentally for the particle fixing and ignition condition with an air heater. 3. Design and Development Procedure 3.1. Design Basis. A heat output of 6 kW was adopted as a design basis, which is of the same capacity as the commercially sold heaters for household utilization in Japan. Charcoal prepared from Japanese oak (Quercus serrata) at 700 °C, still popular in Japan as one of the major classical fuels at home, was crushed and sieved into 150-250 µm size range. Table 2 summarizes the physical and chemical properties of the char particles used in the design of a charcoal combustion heater. The stoichiometric air ratio was assumed to be 2. The combustion heater was designed based on the steadystate heat balance at 850 °C. The cross section of the front glass window was determined so that about 45% of the thermal heat capacity is transferred for space heating through radiation from the front window as shown in the heat balance in Figure 2. The

Table 2. Physical and Chemical Properties of Wood Charcoal (Japanese Oak) property

wood charcoal

size range (µm) density (kg/m3) proximate analysis fixed carbon (wt %, dry) volatiles (wt %, dry) ash (wt %, dry) moisture (wt %) ultimate analysis carbon (wt %, dry ash free) hydrogen (wt %, dry ash free) nitrogen (wt %, dry ash free)

150-250 1200 93.50 5.68 0.81 5.42 93.02 1.32 0.43

combustion air flow rate can be calculated from stoichiometry (including excess air). The length of the combustion chamber was determined from the air flow rate and the air velocity necessary to fix particles on the circular side wall as discussed in the next section. The heat exchangers in the downstream section of the combustion chamber were designed to recover as much as possible of the remaining 55% of the thermal heat capacity of the heater. 3.2. TBCF Combustion Chamber Design. The penetrable wall of the combustion chamber was developed by covering the inner circular wall of the combustion chamber by a ceramic fiber mesh (Rubylon CS-40, Nichias Corp., Japan) on a punched SUS304 cylindrical plate. To achieve a TBCF mode of charcoal particle combustion, the length of the combustion chamber should satisfy hydrodynamic and reaction kinetics conditions, i.e., to fix the charcoal particles’ bed on the wall of the combustion chamber vertically upward and to check if the bed thus formed on the wall of the combustion chamber is thin, respectively. 3.2.1. Hydrodynamic Condition. To fix charcoal particles on the penetrable wall of the combustion chamber, the drag force exerted by the combustion air should be higher than the gravitational force acting on the particles. The minimum velocity, Uudp, to fix the fuel particles in an inverted packed bed mode, i.e., vertically upward onto the wall of the chamber, is determined experimentally. The length of the combustion chamber is then calculated from the lateral surface area obtained by dividing the air flow rate by Uudp. 3.2.2. Experimental determination of Uudp. The experimental apparatus consisted of a distributor, prepared by the same ceramic fiber mesh as used in the combustion chamber and connecting it to a cylindrical tube of 215 mm length and 26 mm i.d.; the base of the tube was fitted with a plug and connected to Teflon tubing through a flow meter to a vacuum pump. The top of this tube was connected coaxially to another tube of 315 mm length and 32 mm i.d. to form the arrangement shown in Figure 3a. A known weight of charcoal particles (150-250 µm) was placed on the distributor and the vacuum pump was started. The apparatus was then inverted slowly until it was completely upside down, and the mass of charcoal that did not form the inverted packed bed was measured. The experiment was repeated for varying air velocities, to form an inverted packed bed with 100% of charcoal weight placed on the ceramic fiber mesh. Figure 3b shows the percentage weight of charcoal sticking to the mesh on varying air velocities. The air velocity in the combustion chamber should be greater than Uudp to form TBCF in the chamber. 3.2.3. Reaction Kinetics Consideration. The condition of the thin bed of charcoal particles is then checked by calculating the thickness of the bed for the obtained length and diameter of the combustion chamber. The thickness of the charcoal bed is calculated from the inventory of charcoal particles in the combustion chamber,

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Figure 2. Heat balance for charcoal combustion heater: Qout ) ∑(Qrad + Qconv + Qloss) + Qflue.

which is obtained by determining the time for TBCF combustion with multilayers based on the shrinking core model without ash layer diffusion (cf. Appendix B). If the thickness of the bed calculated by this procedure is too large, the burnout time of charcoal particles and also the pressure drop across the bed increase. In such a case, the length of the chamber has to be recalculated by readjusting the air ratio and the heat balance. Based on this design procedure, the dimensions of the TBCF combustion chamber were determined to be of 200 mm i.d. and 100 mm length. 3.3. Ring Duct Air Distributor Design. To prevent charcoal particles from colliding with the front glass, the combustion air was injected into the combustion chamber from the front side (around the front glass) through a ring duct air distributor. The ring duct air distributor acts as a flange mounted on the stationary part of the combustion chamber to seal the chamber from the front. Since the ring duct was fixed to the stationary part, there existed a gap between the rotating combustion chamber and the stationary cylinder of 0.6 mm (constrained by mechanical design). Thus, the ring duct has a role to provide seal and lubrication air to this gap, reducing the friction between

the flanges of the fixed and rotating chambers and preventing the leakage of both flue gas and charcoal particles from the gap. Another role of this front side aeration is to keep the temperature of the front side including both the ring duct and the front glass sufficiently low to minimize thermal expansion to reduce the chances of glass breakage to as rare as possible. However, even in the case of glass breakage, due to induced draft and a negative pressure in the combustion chamber, the burning charcoal particles would not flow out of the combustion chamber. For a smooth flow of combustion air from the front glass to the wall of the combustion chamber, the ring duct air distributor was designed by calculating the permissible pressure drop in the ring duct air distributor and by assuming the total air flow rate as well as the number of orifices and orifice diameter. The ring duct air distributor was designed so that the air velocity through each orifice was same for all orifices. To prevent the combustion air from completely bypassing the rotating combustion chamber to the annulus between the rotating and the stationary part of the combustion chamber, the rotating combustion chamber had a circular flange of length 15 mm on the front side facing the ring duct air distributor with 5

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Figure 4. Optical photograph of circular combustion air inlet distributor.

Figure 3. (a) Experimental setup to measure Uudp. (b) Measurement of Uudp for charcoal particle size range 150-250 µm.

mm inside the rotating part and 10 mm outside in the annulus region. The flange on the rotating combustion chamber acted as a barrier and a jet of sealing air was introduced through the orifices of smaller diameter to the outer flange of the rotating combustion chamber. The sealing air flows through the 0.6 mm gap partly inward into the combustion chamber and partly outward through the annulus region between the flange of the rotating combustion chamber and the stationary part of the outer chamber, thus preventing the combustion air from bypassing the rotating combustion chamber. The sealing air orifice density per distributor area was adjusted so that the air velocity from the gap mentioned above was greater than the terminal velocity of charcoal particles only for the orifices in the bottom half of the combustion chamber to avoid the charcoal particles falling down/out of the combustion chamber. The ring duct air distributor thus designed had 100 orifices of 1.6 mm diameter for sealing air and 16, 7.4 mm × 5 mm slits for combustion air injection. To prevent vortex formation due to the sudden change in air velocity from the orifices as air entered the combustion chamber, a part of the combustion air was injected through 36, 4.7 mm diameter orifices, placed at the bottom and inclined at a 45° angle as shown in Figure 4. 3.4. Automatic Charcoal Ignition System Design. The distance between the outlet nozzle of electrically heated air and the bed surface should be close enough to ignite the charcoal particles effectively and at the same time should be far enough not to blow off the charcoal bed. The temperature and velocity of the heated air from the nozzle were about 470 °C and 0.66 m/s, respectively. The minimum distance to not blow off the

Figure 5. (a) Experimental for ignition of charcoal particles. (b) Ignition time for varying ignition air velocity and distance between the ignition air heater nozzle and charcoal bed.

charcoal particles for this condition was experimentally obtained as 20 mm for fast ignition of charcoal particles. For test runs of ignition system, 6 g of charcoal was fed into the combustion chamber and then ignited by preheated air flowing through a 350 W air heater as shown in Figure 5a.The ignition time of charcoal particles in the bed was determined by placing a K-type thermocouple in the charcoal bed and measuring the temperature at a frequency of 1 Hz. A steep rise in dT/dt of the charcoal bed temperature indicated

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Figure 6. Process flow schematic of charcoal combustion heater.

the time of ignition. Figure 5b shows the ignition times of charcoal in the bed for varying ignition air flow and the distance between the air heater nozzle and the bed. From the plot it can be seen that, as the ignition air velocity was increased, the time for ignition of charcoal reduced. However, the rate of increase of air velocity was possible only to a limiting value, beyond which the charcoal bed dispersed, which inhibited fast charcoal ignition. From these tests an optimum value of ignition air flow rate and the distance between the ignition air heater nozzle and the charcoal bed were determined. 4. Prototype Charcoal Combustion Heater Testing 4.1. Experimental Apparatus. A prototype 6 kW charcoal combustion heater of 560 mm width, 360 mm breadth, and 800 mm height was manufactured (Figure 6) based on the design concepts and parameters established above. The charcoal combustion heater consisted of a TBCF combustion chamber, which was fabricated from a punched 1 mm thick SUS304 plate to form a horizontal cylinder of 200 mm i.d. and 100 mm length and had an outer circular flange on the front side of 15 mm length. The combustion chamber was coupled to a gear assembly and rotated by a 15 W single phase induction motor. The inner curved surface of the TBCF combustion chamber was covered with the ceramic fiber mesh to form the penetrable wall. The TBCF combustion chamber was housed in a stationary cylinder fabricated from 1 mm thick SUS304 plate of 270 mm

i.d. and 110 mm length. The front portion of this stationary cylinder was sealed by a Tempax glass plate of 196 mm diameter and 3 mm thickness. The glass plate was housed in the ring duct air distributor acting as a flange for connecting the glass plate to the stationary cylinder. The combustion air was injected in the combustion chamber through the ring duct air distributor by an induced draft with the aid of a 50 W radial fan type air blower of 250 L/min design capacity. The gases from the combustion chamber flowed through the annular gap between the rotating and the stationary parts of the cylindrical combustion chamber to a 40 mm i.d. outlet pipe which was connected radially to the stationary cylinder. From the outlet pipe the exhaust gases passed through a plate type heat exchanger placed on top of the combustion heater, which could be used as a hot plate for heating a water kettle or a small pan. The exhaust gases after leaving the plate type heat exchanger flowed through an alumina-based CO oxidation catalyst (Almite). The catalyst was prepared by spray coating of the catalyst material on a ribbon heating element (35 W) for quick response to the demand. The exhaust gases then flowed through another plate type heat exchanger for space heating and then into the second-stage CO oxidation catalyst (0 W) to oxidize any residual CO in the combustion heater exhaust. A double pipe heat exchanger was connected in the combustion heater exhaust to preheat the inlet combustion air to recover flue gas losses.

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At the center of the rotating chamber were placed an “Lshaped” ignition air pipe of 16 mm i.d. connected to a 350 W electrical air heater and a charcoal powder feeder nozzle connected to the cartridge type powder fuel storage. The powder fuel cartridge feeder developed by the author’s group18 was equipped with a set of couplers for easy mounting and removal and utilized subliminal fluidization of powder fuel using two 4.5 W aquarium air pumps for constant metric feeding of charcoal particles. For an automated operation of the charcoal combustion heater, the charcoal feeding, ignition air heater, catalyst electrification, and other sequences of operations were controlled by a microprocessor. 4.2. Heater Performance Monitoring. The feeding rate of charcoal particles was measured by replacing the fuel cartridge at different time intervals during the test run and measuring the change in weight. The combustion air flow rate was determined by direct measurement of the air volume for a time duration.19 The temperatures at various locations in the combustion heater were measured using K-type thermocouples as shown in Figure 6. The exhaust gas compositions were measured at two locations: (1) before the first-stage CO oxidation catalyst with a flue gas analyzer (Testo 350-S); (2) at the combustion heater exhaust outlet with a portable gas analyzer (Horiba PG-250). The temperature and emission data were recorded in a computer at a frequency of 1 Hz with the aid of a data logger. 4.3. Experimental Procedure. All the test runs of the combustion heater were carried out by using the same experimental procedure, as described below: 1. Charcoal particles were fed into the combustion chamber after switching on (to), for 30 s until 6 g of charcoal particles was built up in the combustion chamber. 2. The charcoal particles thus fed were then ignited in about 300 s by flowing 8 L/min ignition air through the ignition air heater. 3. The combustion air blower was started at 300 s (t1) to attain a TBCF mode of combustion after igniting the charcoal particles. 4. To cover the penetrable wall of the TBCF combustion chamber with charcoal particles, a pulse sequence was applied; i.e., the charcoal was fed into the combustion chamber for 5 s/min and, during the feeding, the combustion chamber was rotated by an angle of 22.5°. 5. The combustion chamber completed one complete rotation in 905 s (t2, 15.08 min) to cover its whole curved surface with charcoal particles. 6. The charcoal feeding rate and the combustion chamber rotation (0.75 rpm) were changed from pulse to constant, to allow the combustion and the inventory in the combustion chamber to stabilize. 7. The ignition air heater was switched off at 1500 s (t3, 25 min). 8. After 3600 s (t4) from switching on, the combustion heater was switched off and the charcoal feeding was stopped. The charcoal inventory in the combustion chamber was then burnt out with the aid of combustion air. 9. The combustion air blower and the combustion chamber rotation motor were switched off at 2400 s after switching off of the combustion heater (t5). 5. Results and Discussion 5.1. Characterization of Charcoal Combustion Heater for Wood Charcoal Combustion. The powdered charcoal combustion heater manufactured in the present work was

operated trouble-free for a continuous period of 4 h during demonstrations. However, for characterization tests of the charcoal combustion heater, the heater was tested experimentally by applying the procedure described in the previous section. The characterization of the combustion heater was done by identifying the different phases of operation, viz., the ignition phase, the transition phase, the steady-state phase, and the extinction phase from times tA-tG, from the sequential actions made at times to-t5 for the characteristic temperature of the combustion chamber TCC shown in Figure 7. In Figure 7 other thermocouple responses as well as gas concentration responses are also presented. All raw data sampled at 1 Hz were averaged for every 80 s time period because they oscillated with a period of 80 s due to the rotation of the combustion chamber. 5.1.1. Transient Temperature Profile of the Combustion Heater. The ignition phase begins with the combustion heater switch-on at time to when the charcoal feeding and the blowing of high-temperature ignition air on the charcoal bed are started. However, the combustion chamber rotation is not yet started at time to. The ignition of charcoal particles was achieved at time tA, which was identified by the change in rate of increase of TCC. To ensure ignition of charcoal particles, the ignition air was continued until time t1, when the combustion air was introduced into the combustion chamber to satisfy the hydrodynamic condition for the TBCF mode of operation. The transition phase begins at time t1 with the “turn-on” of combustion air and pulse rotation of the combustion chamber and pulse feeding of charcoal particles. As can be seen in Figure 7 at time t1, TCC initially dropped due to the introduction of cool combustion air. The penetrable wall of the combustion chamber was eventually covered by charcoal particles through the pulse sequence until time t2, when the first full revolution of the chamber was completed. From time t2 onward, the charcoal particle feeding and the combustion chamber rotation were changed from pulse to continuous. The changeover of combustion mode from partial to full area in the chamber can be identified at time tB from the change in rate of increase of TCC. The balance between the rates of charcoal feeding and combustion was achieved at time tC. Point C was identified by the intersection of lines drawn for constant rate of increase of TCC from time tB with the line of a steady TCC. Slight changes in the temperature of TCC after tC in the steadystate phase should be due to some fluctuations in the charcoal feeding rate and the change in permeability of penetrable ceramic fiber mesh on the wall of the combustion chamber. Figure 8 shows the mean steady-state temperature of TCC averaged over tC to tD against the charcoal feeding rate, which indicates a possibility of automatic temperature control by varying the charcoal feed rate. At time t4 the feeding of charcoal was stopped. The effect of this change was reflected from time tD, when TCC started reducing rapidly, until time tE, when the rate of decrease of TCC ceased for a while. From video observation it was clear that, at around time tE, the fraction of charcoal inventory which was not fixed well on the penetrable wall of the combustion chamber and rotated freely in the combustion chamber was burnt out. Then depending on the thickness of the thin bed the charcoal particles burnt out in the TBCF mode of combustion, resulting in a pseudosteady-state period. Obviously, when the feed rate was higher, the pseudosteady state after time tE was longer. At time tF the inventory of charcoal particles in the combustion chamber burnt out, as can be seen by the rapid decrease of TCC.

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Figure 7. Transient temperature and concentration response on varying wood charcoal feeding rate.

With no more combustion, cooling off takes place rapidly by the air blowing until ambient temperature was reached at time

tG. The phase of operation from time tD to time tG was identified as the extinction phase of the combustion heater.

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(i.e., the ignition and the catalyst heaters, the blower, the feed pump, and the rotation motor), which amounts to roughly 4% of the heat input for the design condition, to the heat generated by combustion of charcoal during the time of operation. The heat of combustion of charcoal was calculated from the elemental composition of charcoal, i.e., mass % C, H, and N (cf. Tables 2 and 3) corresponding to mass percent of carbon, hydrogen and nitrogen, respectively, by substituting these values in the ordinary-least-squares (OLS) equation of Frieldl et al.20 given by

Figure 8. Averaged steady-state temperature in TBCF combustion chamber vs wood charcoal feeding rate.

5.1.2. Transient CO Emissions from the Charcoal Combustion Heater. The changes in the combustion rate during the different phases of operation can be analyzed by monitoring the CO and CO2 concentrations in the exhaust. With the catalyst presence, the heater exhaust CO concentrations do not reflect the actual changes in the rate of combustion of charcoal particles. However, by following the transient CO concentration response in the combustion chamber exhaust gas (cf. COCE in Figure 7), the changes in combustion rate can be clearly identified for different phases of operation. From the point of ignition of charcoal particles at time tA, the CO concentration in the combustion chamber exhaust started to increase. However, at around tA the combustion rate was not high due to the low temperature in the combustion chamber and the small surface area of the charcoal bed for the combustion to take place. In the transition phase, COCE increased with the increase in total combustion rate. At around time tC, COCE reached a maximum, even higher than the steady-state value because of the combustion chamber overheating with ignition air which was also turned off at around tC. The constant charcoal inventory in the combustion chamber caused a steady-state TBCF mode of combustion of charcoal particles from time tC to tD. The CO concentration of the combustion chamber exhaust gas during the steady-state phase of operation was less than 1 mol %, indicating CO2 as the primary product of combustion. In the beginning COExhaust followed COCE, and at around time tC it also reached a maximum and then quickly reduced to almost zero until complete extinction. The first increase of COExhaust was due to the insufficient catalyst preheating; this increase can be avoided by electrically heating the catalyst to a much higher temperature during the ignition phase. However, installing a higher heat capacity catalyst would increase the electrical load of the combustion heater and correspondingly reduce its energy efficiency. 5.1.3. Efficiency of Charcoal Combustion Heater. 5.1.3.1. Thermal Efficiency. The thermal efficiency of the charcoal combustion heater was obtained by calculating the total heat out from the charcoal combustion heater by adding the heat transferred by radiation through the front glass of the combustion chamber and the heat recovered from the exhaust gas by the heat exchangers provided in the downstream of the TBCF combustion chamber. The total energy input was calculated by adding the individual electrical component’s power

HHV(OLS) ) 1.87C2 - 144C - 2820H + 63.8CH + 129N + 20147(1) Figure 9 shows the thermal efficiency of the charcoal combustion heater for different feeding rates of charcoal for the overall operation time. From the plot it can be seen that the thermal efficiency is highest, 86%, when the charcoal combustion heater is operated near the design conditions. In these test runs, the air flow rate was not changed while changing the charcoal feeding rate. Hence, for a low feeding rate of charcoal the thermal efficiency was low, because of the high air-fuel ratio in the combustion chamber. The excess air reduced the temperature in the TBCF combustion chamber, thus reducing the radiation heat and the efficiency of the combustion heater. However, in the case of a charcoal feeding rate greater than the design value, the thermal efficiency reduced because of a thicker charcoal bed in the combustion chamber, reducing the permeability of the penetrable wall, thus reducing the air flow. The lower air flow caused poor air-fuel contact with a higher amount of unburnt charcoal, thus reducing the thermal efficiency. The ash deposited on the ceramic fiber mesh is collected at the bottom of the outer chamber after passing through the ceramic fiber mesh. However, for a long operating stability, an ash cleaning device should be introduced inside the combustion chamber so that the permeability of the ceramic fiber mesh can be maintained high for a long period, avoiding possible reaction with ash at high temperature. 5.1.3.2. Carbon Combustion Efficiency. The combustion quality in the TBCF combustion chamber is compared with the combustion efficiency of the combustion chamber at different charcoal feeding rates. Combustion efficiency was calculated by integrating the CO and CO2 emission data obtained at the TBCF combustion chamber outlet for the duration of operation. Figure 10 shows the carbon combustion efficiency of the TBCF combustion chamber on changing the feed rate of charcoal, which remained constant at about 88% for all the feed rates of charcoal. 5.2. Characterization of Charcoal Combustion Heater with Waste Biomass Charcoal. To test the performance of this prototype charcoal combustion heater with different kinds of fuels or fuel blends, charcoal prepared from different biomass wastes, coffee residue, soybean fiber, and pruned apple branches were tested to obtain the overall operable efficiency range of the prototype charcoal combustion heater. Charcoal prepared from the present biomass wastes had low fixed carbon and high volatile content (cf. Table 3), making the charcoal easier to ignite, but caused flaming during the combustion operation. The different phases of operation were identified from the transient temperature response of the temperature in the combustion chamber for the charcoal from biomass wastes in a way similar to what was done for wood charcoal. Figure 11 shows the overall thermal efficiencies of different charcoal species at varying feed rate of charcoal. The thermal efficiency of the combustion heater for waste biomass charcoal combustion

370 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 Table 3. Properties of Waste Biomass Charcoal proximate analysis

ultimate analysis (wt %, dry ash free)

charcoal sample

particle density (kg/m3)

ash (wt %, dry)

volatiles (wt %, dry)

fixed carbon (wt %, dry)

moisture (wt %)

H

C

N

spent coffee and soybean char (CSC) spent coffee char (SCC) apple branch char (ABC)

650 672 1200

15.60 3.57 12.99

9.57 18.94 14.88

74.83 77.48 72.13

6.97 3.95 4.11

1.32 2.20 1.79

74.14 79.19 74.72

4.42 2.26 1.13

was found to be in the range of 60-81%, which was slightly lower than that for the wood charcoal due to high ash content. The thermal efficiency of spent coffee and a blend of spent coffee char and soybean fiber char was lower than that of the pruned apple branch char mainly because of the low particle density of spent coffee char (671.9 kg/m3) causing fluctuations in the feeding rate of charcoal, which changed the inventory of charcoal in the combustion chamber, thus reducing the efficiency of the combustion heater. Figure 12 shows the carbon combustion efficiency of the present biomass waste charcoal which was almost constant (88%) for the different charcoal species at varying feed rates. 6. Conclusions To develop a solid fuel combustion heater using a sustainable source of energy, a novel charcoal combustion heater was

designed and developed. The combustion of charcoal was conducted by feeding powdered charcoal automatically with a cartridge type powder feeder into a specially designed TBCF (thin bed cross-flow) combustion chamber forming a thin uniform charcoal bed to achieve good air and charcoal contact. Charcoal ignition in less than 5 min and the shift to a steady state in about 20-25 min were achieved by the low charcoal inventory in the combustion chamber. The TBCF mode of combustion for Japanese oak (Quercus serrata) charcoal, with high fixed carbon and low volatiles, and the charcoal derived from different waste biomass sources, i.e., pruned apple tree branches, spent coffee, and soybean fiber, with low fixed carbon content and high volatiles, resulted in carbon combustion efficiency as high as 88% in the combustion chamber. The high radiation heat transfer using a front glass

Figure 11. Thermal efficiency for ABC, CSC, and SCC chars (cf. Table 3) with different charcoal feed rates. Figure 9. Thermal efficiency of charcoal combustion heater vs wood charcoal feed rate.

Figure 10. Carbon combustion efficiency of TBCF combustion chamber vs wood charcoal feed rate.

Figure 12. Carbon combustion efficiency of TBCF combustion chamber for ABC, CSC, and SCC chars (cf. Table 3) with different charcoal feed rates.

Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 371

resulted in a thermal efficiency in the range of 65-86% for a wood charcoal feed rate of 7-14.4 g/min. The charcoals derived from biomass wastes were tested for the charcoal feed rate of 8.5-12.8 g/min, resulting in a slightly lower thermal efficiency in the range of 60-81% because of high ash content. The charcoal combustion heater developed in the present work, with its fast startup, high efficiency, and possible automated control, would open a new era of massive but smallscale biomass utilization for a sustainable society. Acknowledgment This work was supported by the Ministry of Environment Japan (Research Project: Development of Biomass Charcoal Network for Household and Small Scale Applications.) The authors are grateful to Mr. Y. Kawajiri of Hinomaru Ltd., Hiroshima, for supplying charcoal and Mr. N. Watanabe and Mr. M. Suzuki of Koganei Tech., Tokyo, for fabrication of the prototype charcoal combustion heater. Appendix A Assuming an equal opportunity both in consuming energy and in emitting CO2 for all countries per capita basis but forcing them to do it with reasonably high efficiencies and to collaborate in reducing the global CO2 emission to half of the 1990 value, calculations are done for the year 2000 population. To provide equal opportunities to a country X for energy consumption and at the same time to allow it to emit CO2 using high-efficiency processes, the per capita energy consumption * and per capita CO2 emission cR,2000 for a reference country of reasonably high efficiency are multiplied by the population of country X for year 2000 to obtain equal opportunity based energy consumption and corresponding CO2 emission in year * 2000, CX,2000 . ** CX,2000, the reduced CO2 emission of country X to achieve 50% reduction of the global CO2 emission of the year 1990 value (let us call this “equal right/equal obligation CO2 emission”), can then be obtained as ** ) CX,2000

1 CGlobal,1990 * CX,2000 2 C*

(A1)

Global,2000

where CGlobal,1990 is the global CO2 emission of the year 1990 * and CGlobal,2000 is the equal right based global CO2 emission for the year 2000. Then, the required percent reduction for country X for the year 2000 is given by reductionX,2000 % )

(

)

** CX,2000 - CX,2000 100 CX,2000

Figure B1. (a) Multilayer combustion of charcoal particles in a thin uniform bed. (b) Calculated burnout time of charcoal particles of each layer in multilayer TBCF combustion.

The number of charcoal particles fed into the combustion chamber is given by Nch-f )

Appendix B: Charcoal Particle Combustion in a TBCF Mode of Combustion Assumptions: 1. Charcoal particles are spherical and of median particle diameter, which is assumed to be representative for all the particles in a particular size range. 2. Combustion of charcoal particles takes place on the surface, without ash layer diffusion (shrinking-core model without ash layer diffusion).

πdch3Fch

(B1)

where Wch is the design feed rate of charcoal particles in the combustion chamber. The combustion of a single particle of charcoal is given by dWch j AC ) -K O2 dt

(B2)

j is the overall rate constant, A is the surface area of where K reacting char, and CO2 is the effective concentration of O2 near the charcoal particle surface.

(A2)

In the present calculations for Table 1, we chose Japan as the reference country. It should be noted here that the above calculations are based on the year 2000 population and do not take into account issues related to population growth/control.

6Wch

j) K

1 1 1 + kf ks

(B3)

where kf )

ShDO2 dch

(B4)

where Sh is the Sherwood number and DO2 is the diffusivity of oxygen in air, kf is the mass transfer rate constant, and ks is the reaction rate constant21 given by ks ) 71.7RTe-17970/T

(B5)

Substituting eq B1 for a single particle in eq B2 and integrating from the outer surface of the particle, i.e., from rch

372 Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009

to the center of the particle, i.e., rch ) 0, which in terms of diameter gives the limits as dch to dch/2, we obtain an expression for burnout time of a single charcoal particle, tc. Hence, the number of charcoal particles accumulated in the combustion chamber is given by Nch-t ) Nch-ftc

(B6)

The number of charcoal particles in a single layer on the wall of the combustion chamber is given by Nch-w ) 4

Acomb πdch2

(B7)

Hence, the number of layers of charcoal particles in the combustion chamber is nlayers )

Nch-t Nch-w

(B8)

nlayers was found to be 4 based on the length of the combustion chamber obtained by the hydrodynamic condition. Based on nlayers, the effective burnout time of charcoal in the bed was determined using the concept shown in Figure B1a. By solving for oxygen concentration in series for charcoal particles in each layer, the burnout times for different layers for a median charcoal particle diameter, 180 µm, are plotted in Figure B1b. Since the effective burnout time of particles increases with the number of layers, it is desirable to keep the bed thin for lower burnout times of charcoal particles. Nomenclature A ) surface area of reacting particle [m2] Acomb ) area of combustion chamber [m2] CO2 ) concentration of O2 near particle surface c*R,2000 ) per capita CO2 emission of reference country in year 2000 [million tons/person] CGlobal,1990 ) global CO2 emission in year 1990 [million tons] C*Global,2000 ) global CO2 emission based on reference country’s per capita in year 2000 [million tons] * CX,2000 ) CO2 emission of country X based on reference country’s per capita in year 2000 [million tons] C** X,2000 ) equal right/equal obligation to reduce global CO2 emission to half of 1990 in year 2000 for country X [million tons] dch ) diameter of charcoal particle [m] DO2 ) diffusivity of O2 in N2 HHV ) higher heating value [kJ/kg] j ) overall rate constant K kf ) mass transfer rate constant ks ) reaction rate constant Nch-f ) number of charcoal particles in feed Nch-t ) number of charcoal particles total Nch-w ) number of charcoal particles in single layer nlayers ) number of layers of charcoal particles rch ) radius of charcoal particle [m] R ) gas constant Re ) Reynolds number reductionX,2000 ) percentage reduction required to reduce the global CO2 emission to half of year 1990 in year 2000 of a country X, based on per capita emission of a reference country [%] Sh ) Sherwood number T ) charcoal particle temperature [K] t ) time [s] to-t5 ) time for sequential action in combustion heater [s]

tA-tG ) time for combustion characterization of combustion heater [s] tc ) burnout time for a single particle [s] Uudp ) velocity of air to form an upside-down bed of charcoal particles [m/s] Wch ) feed rate of charcoal particles [kg/s] Fch ) density of charcoal particles [kg/m3]

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ReceiVed for reView April 18, 2008 ReVised manuscript receiVed July 1, 2008 Accepted July 2, 2008 IE8006243