Gasification of Rice Hulls

Man discovered fire and with it the combustion process between 0.5 and 1.4 million years ago. It took him much longer, until 1669, to develop the art of generating a combustible gas from a carbonaceous fuel (Wyer, 1907). It may be appropriate to ask why it took so long. It is even more appropriate to explore why the history of gasification was so chaotic and why the heyday of gasification was so short. Over 12,000 large gasifiers were installed in North America within a period of 30 years, providing electricity and street lights to many large cities. Moreover, in the 1930’s and 1940’s over 1 million automotive gas producers were operated worldwide. The period of commercial use of gas producers was quite short, considering the almost total disappearance of the technology after 1950.

The present and future high energy prices which have affected Developing Countries most have made the possibility of gasification of waste products such as rice hulls and other crop residues a popular choice in Third World Countries and elsewhere. The alleged advantages of gasification seem to be intriguing for several reasons:

Gasification research and commercial production came to an almost complete standstill after World War II. The current revitalization of the technology is due to the growing belief that fossil fuel supplies will be exhausted within 50–100 years and that transitional energy technologies will be necessary until direct utilization of the sun’s energy can be instituted on a large scale.

Gasification of coal and biomass had its heyday at a time when very little was known about the process except that incomplete combustion generated a gas which in turn could be further oxidized (burned).

As outlined previously, gasification is a complicated interaction of a gas-solid phase system. Based on a literature study and a survey of existing units, it is concluded that the physical properties of a fuel are usually underestimated while the influence of the heating values and ash content are overestimated with regard to the performance of a gas producer.

During my first trial runs using rice hulls in standard wood gasifiers of the updraft and downdraft type, it became apparent that the quickly changing flow properties and the unique thermal decomposition characteristics of rice hulls cause severe operational problems in small gasifiers of 15 cm to 30 cm hearth diameter. For instance, common slot grates used in coal and wood gasification can neither remove the rice hull ash continuously nor prevent the rice hulls from falling through. Another extreme example of the variable nature of rice hull flow properties is shown in Figure 7-1. An experimental hopper with 70° and 45° walls was used to feed the rice hulls into a downdraft gas producer. The rice hulls were loosely poured into the hopper, but because of gravity feeding the rice hulls became compacted and formed a vertical wall during the trial run. Additional difficulties were encountered in updraft gasification. Tar vapors condensed in the cooler upper layers and formed a sticky coating of charred rice hulls and tarry liquids, as shown in Figure 7-2. The frequent flow distortion and channel generation within the fuel bed could not he attributed to one particular cause. The next step was to examine closely the physical appearance of rice hulls and rice hull pellets before and after gasification to see if any insights into fuel bed disturbance could be found.

Combustion of a carbonaceous fuel includes a pyrolytic component. Pyrolysis takes place when the fuel is heated to a temperature between 250–500°C. At these temperatures, the carbonaceous fuel will begin releasing its so-called volatile components. The pyrolysis process can be characterized as the first stage of combustion. This stage takes place at relatively low temperatures of 250–500°C. Pyrolysis is not limited to solid fuels. A diffusion flame of producer gas from rice hull gasification and a schematic sketch of the flame is shown in Figures 8-1 and 8-2.

The renewed interest in small gas producer-engine systems (5–100 hp) has mostly involved downdraft (co-current) gas producers because of their ability to convert condensable pyrolytic products into noncondensable combustible gases. The evolution of gas producers for use with internal combustion engines has taken place as shown in Figures 9-1 through 9-3. The trend toward downdraft designs is due to the difficulties that occur in cleaning the highly contaminated (tar and water) gas generated in an updraft gas producer. However, the myth that downdraft gas producers generate a “tar-free gas” has never been proven and the fact is that all known operational downdraft and updraft gas producer-engine systems are equipped with extensive gas cleaning trains for tarry liquids and solid contaminants. The notion of a tar-free gas has been persistently associated with downdraft gas producers in scientific literature. It is therefore of interest to:

It was mentioned in Chapter 4 that small rice hull gas producers have never been used commercially. In this context small refers to a unit with a furnace diameter smaller than 50 cm. Commercial installations like the Italian Balestra unit or the Chinese unit (which is attached to the rice mill in Suzhou in Jiangsu Province) have furnace diameters of 1.5 – 2.5 m and mechanical power outputs of 60 – 186 hp.