DEVELOPMENT OF UTILIZATION TECHNOLOGIES OF BIOMASS ENERGY

Transcription

1 J. Environ. Eng. anage., 19(2), (29) 67 DEVELOPENT OF UTILIZATION TECHNOLOGIES OF BIOASS ENERGY Shigekatsu ori * Department of Chemical Engineering Nagoya University Nagoya 464-1, Japan Key Words: Biomass, wood, pulverization, vibration mill, fine powder, combustion, gasification, hot water treatment, fuel cell, Stirling engine ABSTRACT A new pulverization process, which produces fine wood powder economically, was introduced. Then two types of pulverized combustion processes using wood powder produced by this pulverizer were developed. A new gasification process of the wood powder was also introduced. This gasification process was applied to produce fuel gas for a new power generation system of molten carbonate fuel cell and this process was demonstrated during Aichi Expo held in 25. A new hot water treatment process for fine wood powder was also introduced. INTRODUCTION Since biomass is carbon-neutral and renewable resource, the utilization of biomass can contribute to the reduction of CO 2 emission. ain resources of biomass in Japan are woody biomass, agriculture wastes, farming waste, wastes from food industry, garbage and leftover foods. The largest amount of generation in this biomass is woody biomass including thinning timber, forest-residual, pruned-residual, saw residual, woodwork residual and construction wastes. Following utilization technologies have been developed to utilize wood biomass as the CO 2 free energy resources. (1) Gas fuel produced by methane fermentation. (2) Solid fuel for combustion, such as, for thermal utilization by steam or hot water. (3) Gasification for fuel gas. (4) Gasification for power generation processes by gas engine, gas turbine and fuel cell. (5) Production processes of liquid fuel by ethanol fermentation, Fischer-Tropsch Synthesis, methanol synthesis and dimethylether synthesis. Although the performance of these processes could be able to be increased drastically by using pulverized fine wood biomass as the raw materials, conventional pulverizations of wood biomass usually consume too much energy to be adopted for the pretreatment for these processes. Here, at first, a new pulverization process, which produces fine wood powder economically, is introduced. Then two types of combustion processes using wood powder produced by this pulverizer were introduced. A gasification process of the wood powder is also introduced. This process was applied to produce fuel gas for a new power generation system of CFC (molten carbonate fuel cell) which was demonstrated in Aichi Expo held in 25. A new hot water treatment process for wood powder is also introduced. 1. Production of fine powder by vibration-mill For effective wood biomass utilization, pulverization was very important. ass production of wood powder at low cost was the key technology to create new biomass utilization processes and also to develop new wood powder markets. There were several conventional pulverization techniques such as the cutting, mechanical shocking and grinding methods. However, wood biomass, which consists of cellulose, hemicelluloses and lignin, is too strong to pulverize into micron-size particles with low costs by those conventional pulverization techniques. New pulverization techniques such as freezing pulverization or burst pulverization were developed in recent years, but economical and practical issues still remained for their commercialization. Here, the pulverization of wood biomass has been conducted by using a vibration mill equipped with rods or balls [1,2]. Figure 1 shows the schematic diagram of continues type vibration mill for the pulverization of wood biomass. Its type has two pulver- *Corresponding author shigekatsu-m@mf.ccnw.ne.jp

2 68 J. Environ. Eng. anage., 19(2), (29) (a) (b) 1µm 1µm (c) Fig. 1. Continues type vibration mill. (%) Cumulative particle size distribution Vibration mill Cutter mill Particle diameter (μm) Fig. 2. Size distribution of wood powders produced by the vibration mill and the cutter mill. izetion cylinders and fed wood chips flow in these vessels in series. Figure 2 shows typical cumulative distribution of particle diameter of pulverized woody biomass by the vibration mill and 5% diameter of this particle was about 2 μm that was compared with about 2 μm of the particles pulverized by the cutter mill. The SE pictures of typical pulverized particle by the vibration mill are compared with particles produced by cutter and hummer mill in Fig. 3. The shape of the pulverized particle by vibration mill was almost round and its surface was smooth. The biomass fiber was totally broken by the strong mechanical impact of rods and the edge of the particle was rounded. The distributions of aspect ratio (ratio of long and short lengths of particles) for pulverized particles are shown in Fig. 4. The aspect ratio of 8% of the pulverized 3µm Fig. 3. Shape of the fine powder produced by (a) the vibration mill, (b) the hummer mill and (c) the cutter mill. Cumulative distribution (%) Vibration mill Saw dust Cutter mill Hammer mill Grinder Aspect ratio (-) Fig. 4. Comparison of the aspect ratio for fine powders produced by various process. particles by the vibration mill was less than 2.. Typical analysis of produced wood powder is shown in Table 1 and it is found that the properties of the power were almost same as the raw wood except moisture content. It is found that high volatile matter as 83%, very low ash as.6, low fixed carbon and high oxygen contents as 44%. In addition, it contained no sulfur, very low chlorine, and low nitrogen. Heating value was about 18 J kg Combustion process of pulverized wood biomass Since the pulverization technology of wood

3 ori: Technologies for Biomass Energy Generation 69 Table 1. Typical analysis of wood powder Proximate analysis (wt%) oisture 4.6 Volatile matter 82.7 Fixed carbon 16.6 Ash.6 Ultimate analysis (wt%) Carbon 48.7 Hydrogen 6.3 Nitrogen.1 Oxygen 44.1 Heating value (J kg -1 ) 18.2 Wood powder Screw feeder City gas Storage tank Ejector ain Burner Torch buener furnace stuck Fig. 5. The test unit of burner combustion for pulverized wood biomass. Combustion efficiency ηc (%, LHV base) Burner (A),- 5 µm Burner (B), - 5 µm Air ratio (-) Fig. 6. Results of the burner combustion test of wood powder. biomass by the vibration mill was able produce fine wood powder, the pulverized combustion of wood biomass by using the burner shown in Fig. 5 was adopted as the combustion process. Wood powder fuel was transported by from the fuel hopper and the fuel feed rate was controlled by the rotating rate of the feeder. Combustion was supplied by a forced draft fan and it was distributed into three nozzles. The primary was fed for the fuel transportation and used also for the combustion, secondary was the main combustion and tertiary was fed as the over firing. The internal diameter of the combustion chamber of the test unit [3] was 1 m and the length of the chamber was easily changed from 3 to 6 m. aximum feed rate of wood powder was.3 kg s -1 and the designed burner thermal capacity was 4 kw. As shown in Fig. 6, the combustion efficiency Table 2. Burner combustion results of wood powder Wood powder with 2 mm under was burned at higher temperature than 12 ºC and over 99% combustion efficiency was established. Sufficiently lower concentration of SOx and NOx in flue gas. If wood powder involved no PVC and with low chlorine atom concentration, very low emission of dioxin was achieved. Potassium components in wood were volatilized during combustion and it was deposited at boiler surface. Stock silo of wood powder and pneumatic transfer line to the burner are necessary in this plant. Table 3. Design of the small scale industrial boiler Steam generation:.5 to 1 t h -1 Combustion capacity of wood:.1 to 2 t h -1 Burner: 1 to 3 Boiler type: cylindrical furnace with flue pipes Table 4. Example of specifications for 1 ton h -1 industrial boiler Furnace diameter.47 m Diameter of primary flue tubes.31 m Diameter of secondary and tertiary flue tubes.48 and.42 m, respectively Total surface area of heat transfer 15.2 m 2 Boiler pressure 2 kg cm -2 Combustion rate of wood 1 kg h -1 Combustion heat of wood powder 16.7 J kg -1 Flue gas rate 612 Nm 3 h -1 Steam evaporation rate 626 kg h -1 Boiler efficiency 84.4% was considerably high even under such low ratio as 1.1. Results of the combustion test are summarized in Table 2. As shown in this table, the proposed pulverized combustion of wood powder by using burner nozzle showed considerably high efficiency with quite low emission of pollutant components. However, one remaining problem, which should be paid attention, was the deposition of potassium components volatilized from the wood during combustion. Small-scale industrial boiler shown in Table 3 was designed to confirm the feasibility of the industrial application. The calculated specifications for 1 ton h -1 industrial boiler are shown in Table 4 and this boiler system is schematically shown in Fig. 7. It is found in this table and the figure that considerably high boiler efficiency was established and estimated boiler size was quite compact. However, a handling system including storage tank, powder feeder, and pneumatic conveying line was required. A new Stirling engine power generation system with high temperature pulverized wood combustion was developed [3]. However, potassium components volatilized during burner combustion was deposited at the heat-exchange surface of Stirling engine since

4 7 J. Environ. Eng. anage., 19(2), (29) Wood powder delivery Wood powder storage tank Air City gas Blower Screw feeder Ejector Boiler Torch burner Wood powder burner Water Filter Ash Fig. 7. Typical design of the industrial boiler with burner combustion. Feeder Air fun Air fun Wood powder LPG Hopper De-potassium - Cyclone Air-heater Bagfilter Air fun Stirling engine Fun Stack Fig. 8. Power generation system of Starling engine with wood powder gasification. their fin interval was quite narrow. Therefore, the combustor was operated at reducing condition (gasifier) and a cyclone was installed between the gasifier and the combustor of Stirling engine to remove the potassium components and ash as shown in Fig. 8. As a result, the trouble encountered by potassium deposition was successfully cleared and a long operational period could be achieved. 3. High temperature gasification of wood powder In this paper, a new entrainment type of a high temperature gasifier for wood powder [4] is introduced. Figure 9 shows the outline of the experimental down flow type gasifier. Both wood powder and wasted plastic powder were able to be gasified simultaneously. Fed particles were pneumatically transported with nitrogen to the main burner and injected into the gasifier with oxygen gas. Two methane burners for heating up and ignition were also assembled on the top of the furnace. Inner diameter of the reaction zone of the furnace was 255 mm with 2 m height. The feeding of wood powder was started after the gasifier was heated up to 13 K by methane combustion. Oxygen or was used for high temperature gasification by the partial combustion. However, since this gasifier was small, the heat loss from the gasifier could not be ignored and hence methane was supplied rotary valve Heating burner ignition burner nitrogen knockout cyclone hopper cooling water powder burner heating fan cooling water adsorber incinerator control valve city gas Fig. 9. Test unit for gasification of wood and wasted plastics powders simultaneously. Cold gas efficiency and carbon to gas conversion (%) Carbon to gas conversion Cold gas efficiency 14 Temperature O/ C (-) Fig. 1. Carbon conversion and gasification efficiency. Gas composition (%) Hydrogen Carbon dioxide 1 Carbon monoxide 14 ethane Temperature O/ C (-) Fig. 11. Produced gas composition. to keep a higher temperature in the furnace. The feeding rate of wood powder was controlled from 15 to 25 kg h -1. Typical examples of the carbon conversion and the cold gas efficiency are shown in Fig. 1 as the function of O/C ratio. Produced gas composition is shown in Fig. 11. It is found that considerably high efficiency was achieved in this small test unit since the particle size of fed wood powder was small enough and gasification temperature was high enough. Almost tar free gas components and only small amounts of hydrocarbon were produced. A small scale gasification process of the wasted plastics and wood powder Temperature (K) Temperature (K)

5 ori: Technologies for Biomass Energy Generation 71 Powder supply icrowave reactor hopper Chlorine and surfer removal Wood powder-water slurry mixer Wood powder Pre-heater Tubular reactor P CW Cooler Bag filterー cyclone adsorber Gas cooler Pressurize produced gas compressor CF C water AC H H High-pressure pump Slurry pomp ake-up N2 AC AC Tank reactor Fare stack Gas separator Storage Letdown valves FL gasifier Gas cooler Removal of tar and carbon Drain tank pressure tank Pressure tank Fig. 12. Gasification plant demonstrated in Aichi EXPO. Gas GT Cathode blower Fuel cell module cathode anode Pre-heater mixer reform Heat up combustor Heat recovery boiler compressor Heat exchanger freezer City gas Exhaust, water Water treatment P blow Cat. combustor neutralization Air heater T combustor icro gas turbine Wasted Nitrogengas C Cat. combustor GT freezer Fig. 13. CFC Power generation plant with gasifier of wood and plastic powder demonstrated in Aichi EXPO. with a similar down flow gasifier (Fig. 12) was demonstrated to produce fuel gas for the CFC power generation process (Fig. 13) in the previous Aichi Exposition in Hot water treatment of wood powder In this paper, a new hot water treatment process of wood powder [5] is introduced. Figure 14 shows a flow diagram of the continuous process. Water slurry of fine wood powder was prepared and fed by a highpressure pump into the reactor tank. The temperature in the system was controlled to be about 2 to 25 ºC and the pressure was maintained at the saturated condition. This continuous test unit was operated with 1.2 kg h -1 of wood slurry and wood particles were decomposed in the reactor. Approximately 4-6% of the products was liquid products and residues was solid charcoal. Liquid products were composed with phenol group and saccharides including glucose and oligosaccharide. The origin of phenol group was lignin, glucose was produced from cellulose and oligosaccharide was produced from hemi-cellulose. The maximum yield of glucose about 1% was obtained at 25 Fig. 14. Test unit of the hot water treatment for wood powder. Table 5. Solid products by the hot water treatment Properties of solid product (573 K, 4 Pa) lndustrial analysis (% dry) Volatile matter = 72.5, fixed carbon = 27.3, ash =.25 Ultimate analysis (% dry) C = 58., H = 5.6, N =.3, O = 36.1, S =. Heating value 23.3 J kg -1 High quality Solid fuel High heating value equivalent to lignire Very low ash content No surfer and quite low nitrogen Little content of heavy metals Combustible with high volatiles Biomass origin with no CO 2 emission load Table 6. Utilization methods of produced materials from the hot water treatment Liquid materials: Cellulose Saccharides Lignin Ash: Solid materials: Plastic cast, board Oligosaccharide, glucose, organic acid ethanol production Glue, plastic for cast, board wood cast Soil conditioner Reformed solid fuel ºC and the maximum yield of oligosaccharide about 15% was obtained at 2 ºC. The fuel properties of solid product are shown in Table 5. It was shown in the table that the produced solid residues were quite higher quality as the solid fuel rather than ordinary coal. Various utilization methods of produced materials from these hot water treatment processes can be developed. Typical promising utilization methods are summarized in Table 6. CONCLUSIONS The vibration mill developed to produce fine wood powder and the property of the produced powder were briefly introduced. Then two types of pulver-

6 72 J. Environ. Eng. anage., 19(2), (29) ized combustion processes using wood powder produced by this pulverizer were presented. The gasification process of the wood powder was also introduced. Finally, the hot water treatment process of the wood powder was presented. REFERENCES 1. Kobayashi, N., T. Satou, N. Okada, J. Kobayashi, S. Hatano, Y. Itaya and S. ori, Evaluation of wood powder property pulverized by a vibration mill. J. Jpn. Inst. Energy, 86(9), (27). 2. Kobayashi, N., Y. Ueda, Y. Ohshika, K. izuno, S. Tukada, H. Kiyokawa, J. Kobayashi, S. Hatano, Y. Itaya and S. ori, Pulverization of woody biomass by a continuous vibration mill. Kagakukogaku Ronbunsyu, 34(1), (28). 3. Satou, K., N. Ooiwa, A. Ishikawa, A. Nishiyama, S. ori and H. oritomi, Development of smallscale biomass power plant with 55 kw Stirling engine. Energ. Resources (Japan), 29(5), 31 (28). 4. Kobayashi, N.,. Tanaka, G. Piao, X. Wu, J. Kobayashi, S. Hatano, Y. Itaya and S. ori, A study on high temperature -blown gasification process of woody biomass in an entrained downflow gasifier. J. Jpn. Inst. Energ., 86(11), (27). 5. Kobayashi, N., T. Sato, N. Okada, S. Hatano, Y. Itaya, J. Kobayashi and S. ori, The liquefaction technique of pulverized super fine ligneous biomass powder by hot compressed water treatment. The 7th World Congress Chem. Eng., Glasgow, Scotland (25). Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. anuscript Received: September 2, 28 Revision Received: January 15, 29 and Accepted: January 21, 29