Fundamental gasification processes are as follows:

(a) Evaporation of surface moisture

Surface moisture evaporates from the raw material at the water boiling point (depends on pressure). Inner moisture remains when the raw material is large.

(b) Evaporation of inherent moisture

Following surface moisture evaporation, inherent moisture evaporates at 110-120°C.

(c) Volatilization

Thermal decomposition of biomass begins at 200-300C, and CO, CO2, H2 and H2O are vaporized as gas. Thermal decomposition is a heat generating reaction which is a characteristic phenomenon of biomass CJdmOP).

(d) Volatilization and gasification reaction

The temperature is raised further during volatilization, and the volatile matter of the lightweight hydrocarbons (CxHy where x and y are integers of at least 1; a low value of x indicates lightness and a high value of xindicates heaviness) is transformed into heavy CxHy with a high boiling point. Subsequently, the CxHy reacts with the gasifying agent for conversion to lightweight molecule clean gas, although tar and soot can form when diffusion of the gasifying agent is slow and the CxHy condenses.

(e) Char gasification

Following volatilization of the volatile content in the raw material biomass, the fixed carbon and ash become char, and the char is heated to the surrounding temperature. The subsequent reaction with the gasifying agent transforms the carbon into CO and CO2. However, in cases where the gasifying agent contains excess steam and the surrounding temperature is over 750C, a wet gas reaction occurs (C+H2O ( CO+H2) producing gas composed mainly of CO, CO2 and H2.

(f) Char residue

The reaction rate of the wet gas reaction is slow, and char residue can easily form. The formation of tar, soot and char tends to reduce efficiency, as well causing equipment trouble.

4.2.3 Characteristics of gasification product gas

Gasification generally adopts the direct gasification method with partial combustion of raw material to raise the temperature. Raw materials are mainly wood chips and corn stalks. Most gasification furnaces use normal pressure and a direct gasification process. To keep the reaction temperature at 800°C and above for direct gasification, air, oxygen and steam (as appropriate) are required for the gasification agent. For this purpose, approximately 1/3 of the oxygen required for complete combustion (known as the oxygen ratio) is supplied, with partial combustion (partial oxidation) causing gasification. The calorific value of product gas depends on the percentage of inflammable gas (CO, H2, CdH) contained. Generally, gas can be divided into low calorie gas (4-12 MJ/m3), medium calorie gas (12-28 MJ/m3) and high calorie gas (above 28 MJ/m3). For the most part, direct gasification of biomass yields low calorie gas. Fig.

4.2.1 presents composition of the product gas from rice straw when steam and oxygen is employed as gasifying agent. The ratio between the calorific content of the biomass and that of the product gas (at room temperature) is called cold gas efficiency.

4.2.4 Gasification equipment and a practical example

Here is shown a fixed bed gasifier, based on the combustion or gasification of solid fuel, and featuring a comparatively simple structure and low equipment cost. Fig. 4.2.2 shows a concept diagram of the gasifier. Wood chips of about 2.5-5 cm are generally used as the raw material. They are supplied from the upper feed port, and layered in the furnace. The gasifying agent (air, oxygen, steam or a mixture thereof) is supplied from the bottom in a rising flow (some systems

use a descending flow). The gasification reaction proceeds from the bottom towards the top. From the bottom upward, individual layers are formed due to the changes accompanying gasification of the raw material, in the order of ash, char, volatilized and decomposed material, and product. The product gas is obtained at the top.

Further information

Kawamoto, H. in “Baiomasu, Enerugi, Kankyo”, Saka, S. Ed., IPC, 2001, pp.240-244 (in Japanese)

Sakai, M. in “Baiomasu, Enerugi, Kankyo”, Saka, S. Ed., IPC, 2001, pp.409-421 (in Japanese)

Takeno, K. in “Baiomasu Enerugi Riyono Saishin Gijutsu”, Yukawa, H. Ed. CMC, 2001, pp.59-78 (in Japanese)

Sakai, M. “Baiomasuga Hiraku 21 Seiki Enerugi”, Morikita Shuppan (1998) (in Japanese)

Yokoyama, S. “Baiomasu Enerugi Saizensen”, Morikita Shuppan, 2001, pp.87-95 (in Japanese)

4.2 Pyrolysis

4.3.1 What is pyrolysis?

Biomass is consisted mainly of carbon, hydrogen and oxygen. The photosynthesis and pyrolysis can be simply described as the following formulas,

Heat (500 ~ 600 °C)

Pyrolysis: "(CeH12O6)m ——- ► (H2+CO+CH4+.. .+C5H12)|+(H2O+.. .+CH3OH+CH3COOH+.. ,)+C

Biomass Gas Liquid Char

(4.3.1)

Light

Photosynthesis: m(6CO2+6№O)———— ► -(C6H12O6)m + 6mO2| (4.3.2)

Carbon dioxide & water Biomass Oxygen

The main chemical components of biomass are cellulose, hemicellulose and lignin. Fig. 4.3.1 shows the composition changing during pyrolysis.

The cellulose, hemicellulose and lignin are decomposed with temperature increase. Solid residue is char in the yield of 10 to 25%.

4.3.2 Characteristics of pyrolysis

During pyrolysis, moisture evaporates at first (- 110°C), and then hemicellulose is decomposed (200- 260°C), followed by cellulose (240-340 °C) and lignin (280-500 °C). When temperature reaches 500°C, the reactions of pyrolysis are almost finished. Therefore, at the heating rate of 10°C/s, the pyrolysis finishes in 1 min, while it finishes in 5 s at 100°C/s. Higher heating rate results in the more rapid generation of vapor product, increasing pressure, shorter residence time of vapor product in the reactor, and higher liquid yield; named fast pyrolysis or flash pyrolysis. Dynamotive (Canada) and BTG (Netherlands) have developed fast pyrolysis reactors, which show high liquid yield of 60 to 80%. Since heat conductance of wood is 0.12-0.42 W/(m K), which is around 1/1000 of copper, heat transfer is important for fast pyrolysis, and the milling to small particles is required.

4.3.3 Reactors at laboratory scale

Thermobalance is most commonly used at laboratory for the fundamental study. Very small amount of sample, around a few mg to 10s mg, is heated up from room temperature to the desired temperature at the desired heating rate to measure the weight changing. However, it is difficult to recover the products.

A few g to 10s g of sample is used at laboratory scale rector to recover the products. Sand bath or molten salt bath are used as the heater for the batch type reactor. Infrared rays heater is usually used for the continuous reactor. At these reactors, mass balance and product analysis are studied.