from junk to juice

In a bid to help reduce the environmental burden of waste disposal, municipalities across the nation are beginning to harness the energy that escapes into the atmosphere as heat during trash incineration. Inefficiency has long plagued this type of power generation, but new technologies are making the option attractive to the many plants that are now adopting them.
Plenty of Power from Garbage
Not surprisingly, waste incineration, like other forms of energy generation, involves using heat obtained by burning fuel to boil water; steam-driven turbines then generate electricity. What is surprising is the long history behind waste incineration. An Osaka incinerator was outfitted to produce electricity in 1965, and the late 1970s saw a similar operation by a Tokyo factory looking to sell power to electric utility companies. The practice has been growing steadily since then. Of the some 2,000 incinerators in the nation, 130 were producing a total of 640 megawatts of power, or an oil equivalent of 232,000 kiloliters, as of the end of March 1996. This is far more than the amount currently produced by solar power (400 kl) or wind power (1,000 kl).
Waste-incineration is a stable source of energy, being unaffected by changes in the weather as are solar and wind power. Moreover, other thermal, hydroelectric, and nuclear plants are generally built in remote places; as a result, the cost of getting the energy to the consumer is higher. Waste incinerators, however, have access to more fuel in highly populated areas, and large generators are easy to install. And the need to burn the trash at a constant temperature to produce a steady flow of power affords the additional benefit of reducing the amounts of dioxin and other dangerous substances produced.
There is a downside to this method of generating power, though. In general, the efficiency of thermal electricity production goes up along with the temperature of the steam. But garbage burned at high temperatures gives off chlorine and other corrosive gases that can damage the steam pipes. Steam produced by waste incineration is therefore kept between 250 and 300 degrees centigrade, giving the method an efficiency of only 15% to 16%, compared with close to 40% efficiency for other types of thermal power generation where the steam is heated to between 500 and 600 degrees. But recent technological developments are now making it possible to cope with this problem.
Toward More Efficient Generation
Two methods for boosting the efficiency of waste-incineration power have been developed. The first involves using more heat-resistant materials for the steam pipes in the plants. One incinerator in Saitama Prefecture that has been generating electricity since 1995 has been able to hike its efficiency to 21% by replacing its pipes with chlorine-resistant stainless steel conduits and boosting steam temperature to 380 degrees. The plant is now able to generate 720 kilowatts per ton of trash, as opposed to the 200 to 300 kilowatts that a normal incinerator is able to obtain.
The second technological development is a dual generating system involving both gas turbines and trash incineration. Turbines are powered by natural gas to produce electricity; the exhaust from the turbines, which reaches temperatures from 500 to 600 degrees, is then used to further heat the steam produced by the trash incinerator to about 400 degrees. This type of power plant, called a "super waste incinerator," has been generating electricity on a trial basis since the end of last year in both Gunma Prefecture and in the city of Sakai, Osaka Prefecture. Another "super" plant should go on line in Kitakyushu, Fukuoka Prefecture, in summer 1998. The Gunma dual-system plant is capable of producing 25 megawatts of electricity and has achieved an efficiency rate of over 30%.
These highly efficient waste-incineration power plants consume about 20% to 30% of their generated electricity themselves; the remainder is sold back to utility companies. One plant in Saitama Prefecture uses one-third of the 24 megawatts it generates and earns 1.3 billion yen (11.3 million dollars at 115 yen to the dollar) annually by selling the remaining two-thirds of the electricity. This is quite a difference from before the switch to high-efficiency equipment, when the plant paid 180 million yen (1.6 million dollars) each year for the power it needed.
Next on the drawing board are ways to reduce the cost of plant construction and make it easier to store electricity produced overnight until it can be used. As these new technologies are developed, the number of waste incinerators producing electricity is sure to leap.