| Boiling Point No. 21 - April 1990 |
By Professor Yu-shi Mao, Chinese Academy of Social Sciences, Beijing
Coal is a cheap fuel because of its abundant reserves, but it emits harmful gases and particulates, leaves ash when it burns, and contaminates the environment because of transportation. It is also hard to ignite as compared to gas or petrol fuels. To change coal into a clean and convenient fuel has been people's long time expectations. Briquetting is one of the approaches; the other approach is gasification that requires large scale factory, big investment and may produce secondary pollution.
Briquetting had developed into a mature technology, especially in some resources restricted but technically advanced countries such as Japan, before these countries shifted to high quality energy (oil, gas and electric). In Britain coal was widely used before smokeless zones became widespread. Now these countries no longer use it but briquetting is receiving high attention in developing countries where oil and gas are scarce and woodfuel supply diminishes. The output of briquettes has increased very fast in India, Pakistan, South Korea and particularly in China where the output has almost tripled in the nineteen eighties. The technology of coal briquetting has been developed for almost a 100 years. There are numerous useful results, but a lot of work remains to be done. Research on briquetting technology has been focused on the following: Easy ignition. The common briquette has to be ignited from the bottom by burning wood. During ignition the upper part of briquette upon heating, emits volatile
matter which is a mixture of carbon and hydrogen compounds and is a strong pollutant. The newly developed briquettes can be ignited from the top just by striking a match and the volatile matter will be burnt away when it has passed the upper flame. But the technology of making such new briquette is much more complicated. Since it should be coated with a thin layer of combustibles and oxidizing agent the briquette should be kept in a dry place lest moisture retard ignition.
Smokeless combustion. If a high temperature zone can be established over the burning briquette, the volatile gas will be burnt and little smoke will come out. But this condition is difficult to achieve especially while starting the fire. The design of stove is an important factor for smokeless combustion. Here the key point is adequate air supply that should result a coefficient of surplus air around 1.1 to 1.3. Some advanced stove designs provide secondary air, ensuring complete combustion with a smaller coefficient of surplus air, so the combustion temperature can be very high. It is natural, that the less volatile matter in the coal, the less the smoke produced while burning. This is the reason why the briquette is best made of anthracite but is much less abundant than bituminous or lignite in reserves.
Engineers have tried hard to use bituminous coal to make smokeless briquette, but not satisfactorily succeeded. Carbonization is an important way to change the properties of bituminous coal to fit the requirement in making briquettes. The technology of carbonization varies with production scale: In case of small scale it creates heavy pollution to the environment; in case of large scale, the pollutants turn to be by-products but large investment is needed.
Simplification technology. The process of making briquettes usually consists of crushing, screening, mixing (with binder or adhesive) and pressing. The details of each process vary widely. For example, the pressure employed in pressing can be from 70 to 5600 kg/cm and the temperature can be from room temperature to around 500° C depending on the property of raw coal and the usage of briquettes.
Reduction of cost. By changing the proportion of raw materials and simplifying technology, the cost of production can be reduced. The management of briquetting factories has also been greatly improved including appropriate location of factory, convenient way of distributing to the households and more utilization of indigenous raw materials.
Expanding market. Briquette is essentially for household use, but there are possibilities to be used in other occasions, such as institutions, commercial restaurants, industrial boilers and steam locomotives.
Research on stoves. Briquette stoves vary in design and capacity, and so do their efficiencies. A good briquette stove should ensure smokeless combustion, flexible control, and high heat efficiency over a wide range of combustion rate. Light in weight and easy to move sometimes is important for household use. High combustion temperature is required for restaurant use. Convenience in ash removal is also a consideration in design. The method of feeding new briquettes to the stove often contradicts the requirement of smokeless combustion. Since if the new briquette is to he put on top of the burning briquette, smoke is likely to come out because volatile matter in coal leaves under high temperature; if the new briquette is fed under the burning briquette, smoke may burn away while coming out but it creates a lot of problems in designing the feeding mechanism. A new way to solve the problem is so called reverse combustion, where the new briquette is fed on the top but the combustion proceeds downward by a forced draft (see article on page 25 of this issue).
Stoves for household use can be classified as for space heating or for cooking. The former requires stable low temperature combustion; the latter requires quick start and high temperature combustion. Multipurpose stoves have been developed.
Although briquettes are a relatively convenient, safe and cheap fuel, they are not ideal in many aspects such as clean operation and stock, flexible fire control, compact space occupation. Therefore, briquettes are an appropriate fuel only for medium income households in developing countries. When income rises, some of the households may turn to high quality fuels, i.e. piped or bottled gas and electricity. In an economy under quick economic development, the time period when briquettes serves as a major fuel may be short. In South Korea, briquette production has had very fast development in the past decade, but it peaked in mid-eighties, and the market is diminishing now.
Impact of carbon dioxide. Global warming due to high concentration of CO: in atmosphere is a worldwide concern. Burning of coal produces carbon dioxide, but if the coal is used to replace fuelwood, the consumption of coal can save fuelwood from trees which will absorb carbon dioxide in the atmosphere. Therefore the net result is not as clear as it first appears.
Calculation show that the longer the remaining life and larger the growth rate of fuelwoods, the better it is to burn coal to replace fuelwood.
In general tests, the sulphur content of fuelwood is much less than that in coal, thus more sulphur dioxide will be produced when briquettes replace fuelwood. Mixing a small amount of limestone in briquette raw material composition before pressing can reduce sulphur dioxide up to 50%.
Coal briquettes can be a very dangerous fuel if they produce carbon monoxide while burning in a closed room but in normal combustion these conditions will not appear. If a chimney is provided to extract the smoke, briquettes are 100% safe even if the stove is installed in a bedroom.
If the coal used for briquette is rich in volatile matter, as in the case of Botswana coal, the coal has to be carbonized first. Carbonized briquettes are superior to uncarbonized briquettes for the same reasons that charcoal is preferred to fuelwood. The sulphur content is also reduced during carbonization as the sulphur is converted to hydrogen sulphide gas.* Coal carbonization can be carried out by a wide range of technologies and coal types, from one-person "village coal piles" to advanced technology plants which recover a series of by-products with few pollutants discharged but rather high investment. The scale of production for such an advanced plant varies from hundreds of tons to thousands of tons per day. The primitive technology plants practice limited or no by-product recovery. Where no by-product recovery is pursued, these plants are heavy polluters, but with merit of saving investment. To build an advanced technology plant may be too expensive for developing countries, especially when the briquette market is limited. So intermediate technology may be appropriate, where major pollutants can be recovered and the environmental damage is tolerable.
Impact on employment. Fuel shift from fuelwood to briquettes will promote a serious economic adjustment. The people felling trees, making charcoal and transporting fuelwood will lose their jobs; while coal mining, transportation and briquetting will increase employment. The difference of job loss and increased employment is the net aggregate effect on the economy. But the person who loses a job is not necessarily the person who will get employed, because the employment adjustments happen in different places. Retraining and population move may be necessary. Employment changes happen not only with the direct production process of fuelwood and briquetting and tools for felling trees. The briquetting machine can be manufactured by local machine building industry if an appropriate manufacturing technology has been developed. This is a favorable chance to promote domestic industry.
Briquette fuel is adopted mostly because of the depletion of fuelwood resources in surrounding areas and high labour cost to get it. Therefore, shift to briquette fuel probably will reduce aggregate employment. If released labour can find other jobs in local areas such as vending, transportation, intensive farming, all the people involved will be better off and the GNP of the economy will increase. But if there is no outlet for the released labour, fuel shift to briquette may be questionable.
Costs incurred for households in burning briquettes include the installation of stoves and buying the briquettes. If it can not compete with the cost of burning fuelwood, briquettes cannot be accepted. The competitiveness of briquettes is sensitive to many factors such as the price of coal, transportation distance, the opportunity cost of labour, the production scale; among them the most important is the difference between the prices of coal and fuelwood. Fuelwood market is a competitive market; its price reflects its relative scarcity. But the future price of fuelwood that is to be compared with the price of briquettes is different from the current one and has to be predicted. Future price of fuelwood is a crucial element in the feasibility study of briquette development.
It can be a wise policy to lease the coal resources to some small independent miners, so that the coal market turns to be a competitive one. Coal production cost is not sensitive to scale, so competition between large mines and small mines is possible. According to China's experience, small coal mines can absorb a great number of surplus labour, can flexibly adjust their output to meet market fluctuation, can get quick development with very limited investment and can keep the coal price low by the force of market competition. Another difficulty in economic assessment of briquettes versus fuelwood is that the market price of fuelwood is usually lower than its real social cost that has to include the impact of environment, i.e. the damage due to deforestation. In some ecologically fragile conditions, the social cost of deforestation can be very high, and there is no way to determine it solely by economics. The difference of market price and social price of fuelwood imposes an unfavorable competition for briquette. In this case, the government subsidy for briquette productions is justifiable, but the amount of subsidy is still hard to identify.