APEC-VC - 1 Gasification and Power Generation of Woody Biomass

Features

１　Gasification and Power Generation of Woody Biomass

Kenichi Sasauchi

Biomass Application Plant Project
ChugaiRo Co.,LTD.

1. What is gasification and power generation of biomass?
When something burns it passes through a process called gasification. That is, the organic matter is gasified by heat decomposition from some kind of heat source, which reacts with the oxygen in the atmosphere for combustion. Furthermore, the heat gasifies the remaining organic matter. This process creates continuous combustion.
Since the energy density in woody biomass is low, a device is needed to obtain energy efficiently. For example, the biomass needs to be hardened into a small object like pellet fuel or carbonized to increase the energy density.
On the other hand, if the flammable gas is extracted just before being burnt, existing highly efficient equipment can be used to produce energy, such as a gas burner or a gas engine. Only a little heat can be obtained from the biomass using a small-scale direct fire steam generator, whereas using gasification with a small generator will require only 20% of the biomass and produce the same amount of electrical energy.
Woody biomass has comparatively little moisture, and the materials are suitable for energy conversion, such as combustion and heat decomposition.

2. How heat decomposed gasification works
Gasification is by no means new; there are over 200 methods throughout the world. Various combinations are devised using different furnaces, gasification agents, gasification temperatures, and gasification pressures. The classification for a typical gasification furnace is shown in Table 1.

Table 1　Domestic Woody Biogasification Furnace and Model Example

The gasification itself can be simply performed by drying the so-called biomass. The biggest problem is how to deal with the tar generated by the treating method. One method is to control tar generated at the gasification furnace itself and another is to remove tar during post-processing.

3. The model test facilities using woody biomass for gasification and power generation in Yamaguchi city

A gasification power generation plant for woody biomass was built in a sawmill in Yamaguchi city. It was the first practical use of woody biomass with equipment in Japan. The woody material came from Japanese cedar and cypress and chipped unused bamboo.In addition, the planar dust generated from the sawmill was also used. About 210kg of woody waste is usually produced in an hour (5t a day). Subsequently after refining the gas gasified by heat decomposition, about 180kWh of power was generated by sending this waste to a gas engine. A portion of this electric power was used to power the saws. Moreover, since the plant is a cogeneration facility, heat energy was also obtained and this heat, from steam, was used as a dryer for the wood produced by the sawmill. Fig.1 shows the sawmill. Moreover, the flow of energy is shown in Fig. 2.

Fig.1 Model test facilities for gasification and power generation of forest biomass at NEDO and ChugaiRo in Yamaguchi

Fig. 2 Flow of Demonstration Test Facilities

With a feeder, materials are cut in a fixed quantity automatically and are fed into a gasification furnace. Moreover, the air conveyer moves the planar dust from the sawmill. These materials can be adapted to various tests.
The gasification furnace called an indirect heated multireactor type rotary kiln, as shown in Fig.3, is composed of various reactors. Biomass material is collected in the hopper for raw materials at the entrance of the gasification furnace and then goes into the furnace by the rotating reactors, like a scoop. Since materials of any size can be extracted, it is possible to deal with woody biomass, such as fine sawdust, or long fibrous bamboo or bark. The materials in the reactor slowly move toward the exit, rolling inside the furnace in proportion to the number of rotations of the kiln. The reactor is heated to between 700 and 850 degrees C from the outside, and the biomass is gasified by this heat. By changing the number of rotations of the kiln, it is possible to control the detention time, i.e., the gasification time. The residue (carbide) from the gasification process is burned in a heat generator adjoining the gasification furnace. It is used as a heat source for gasification by heating the exterior of the reactor in the gasification furnace. The steam mentioned above is the waste heat.

Fig.3 Indirect heated multireactor type rotary kiln

Fig.3 Indirect heated multireactor type rotary kiln

Since the generated flammable gas contains tar and dust, the tar is removed by blowing oxygen in the high temperature gas cracker after the gasification process in the furnace, and then dust is removed with a high temperature filter. It is sent to the gas engine after being cooled by a Scrubber. Since the high temperature filter automatically drops off, the dust collected here was sent to the heat generator and is used as a heat source. As a result, tar and dust concentrations measured just before entering the gas-engine were extremely small, as shown in Table 2.

Table2 Operating Conditions and Results of Gasification Test Run

The gasification power generation was operated nonstop for 500 hours in February 2005. By continuing to extend operation time, we will be able to understand how the performance and maintenance changes, establish design criteria for a commercial machine, and implement many tests to improve efficiency.

Conclusion
A total scheme including not only energy plants but also supply and demand is important for the spread of biomass energy. Considering the material supply appropriate to specific areas is especially important because every area has its own problem to be solved. Moreover, energy plants also need to reduce pretreated materials as much as possible, and the biomass needs to be adapted to the area. We would like this design to serve as a model for a future total scheme plan by incorporating information from this study.

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Biogas and Biomass Energy