Trends and technologies for hazardous waste combustion in Asia
By Michel Buron (MSc), June 2003
Some of the products used in our modern society are too toxic to be disposed of without particular treatment. Not only the industry generates such kind of poisonous and hazardous waste but also many household products fall under this category. If not disposed of correctly, some cleaners, solvents, pesticides, paints, etc. can contaminate a landfill, leak into the ground water or contaminate the ocean resulting in tremendous risks for the safety and health of human beings. For the respect of our environment, the basic 3R's rule (reduce, reuse, recycle) should prevail as a prevention instead of curing.
As good as it can be applied, this principle cannot totally avoid the generation of hazardous waste and long term solutions must be developed. Today treatment of hazardous waste includes biological treatment, chemical oxidation and reduction, neutralization, stabilization, incineration and energy recovery prior to landfill.
The role that combustion plays in hazardous waste management has changed dramatically over the past two decades. The recognition that land disposal of hazardous waste could present long-term pollution problems and that the development of low emission incinerators prompted combustion to become the preferred method of waste management.
We understand by thermal processing the combustion of waste in an enclosed area. Incinerators are used primarily for the destruction of waste with possible energy and material recovery; boilers and industrial furnaces burn waste not only for destruction but with main purpose the recovery of energy and material.
The increased use of incinerators to dispose of hazardous waste raised concerns about the proper role of combustion in waste management, as well as the safety of combustion. If conducted properly, the thermal processing of waste has several striking advantages. First, it is a process that substantially and permanently reduces the toxicity and volume of virtually all organic-bearing waste streams, by destroying organic compounds. Secondly, combustion devices can accommodate most types of waste, including liquids, solids, and sludge. Further, since combustion reduces a waste's toxicity and volume, residues from combustion are generally more amenable to land disposal than the original waste streams.
Despite these technical attributes, controversy surrounds the use of combustion since hazardous wastes burned in combustion units often contain toxic organic chemicals, heavy metals, and chlorine, trace amounts of which may be released into the atmosphere in the form of emissions.
To address these concerns and better ensure safe combustion of hazardous waste, various governments have focused on key issues involving the role of combustion and alternative technologies, emission and control standards, risk assessments, permitting priorities, enforcement and compliance assistance, and public involvement in the permitting process. The U.S. Environmental Protection Agency (EPA) and the EEC are the most advanced and comprehensive directives available today.
Incineration is the controlled burning of substances in an enclosed area. During the process, the waste is fed into the incinerator's combustion chamber and under the heat decomposes from solids and liquids into gases.
These gases pass through the flame and are heated further becoming so hot that the organic compounds in the gases are broken down into their constituent atoms. These atoms combine with oxygen forming stable gases that are further treated in the air pollution control devices before being released in the atmosphere.
Four main parameters influence the completeness of the combustion process and therefore the destruction of the waste: the temperature, the residence time in the combustion chamber, the turbulence (air/waste) and the size of the waste particles.
Efforts to achieve "low pollution" incinerators are mainly concentrating on the optimization of these parameters and the combination of air pollution control devices (APCD) to reduce emissions of particulate matter, metals and dioxins/furans, and acid gases. APCDs are grouped into three categories: wet, semi-wet and dry systems.
The gases produced by the combustion are primarily carbon dioxide and water vapors but small quantities of carbon monoxide, nitrogen oxides, HCl and other toxic gases may form due to the presence of pollutants in the waste. If the combustion is incomplete, compounds known as incomplete combustion (PIC) may be emitted as well.
Ash is another by-product generated by the combustion. It is an inert solid material composed mainly of carbon, salts and metals. Ash is usually collected at the bottom of the combustion chamber (bottom ash) and in the air pollution control device (fly ash) under the form of particulate matter (PM). Because of its characteristic or by the derived-from rule, ash is often a hazardous waste.
Typical hazardous waste incinerators include rotary kilns, cement kilns, liquid injectors, controlled air furnace and fluidized bed.
* Rotary kilns were introduced about 30 years ago and developed according to the "all burn" principle. It consists of a rotating drum, supported by two massive rings carried in cradles at an angle of 2 degrees to the horizontal and rotating slowly (0.2 to 0.5 rpm). Usually designed for capacities exceeding 2T/h, they can accept large lumps of solid waste, drums and containers, pasty, liquid and gaseous waste, which is fed at the front wall of the kiln via a chute. Kilns usually operate in the primary chamber at temperatures above 1000C in order to achieve an acceptable burn out of the waste (poor contact air/waste). Complete burn out of the gases is achieved in the second combustion chamber at a temperature above 850C (up to 1200C) and with a residence time of at least 2 second. High energy and maintenance (seals, refractory) costs, fused slag formation unfit for direct landfill, difficult operation and high investment cost make this technology only interesting for very large plants (>5 TPH).
Emissions from incinerators are under strict scrutiny from environmentalist which results in more stringent regulations. The uses of sophisticated and expensive flue gas treatment systems are now a requirement and makes small on site incineration plant less economical. Often supported by both government and private sector, central treatment plants are a solution in full development in the region and are taking advantage of the economy of scale by collecting wastes from throughout an area. It offers the industry a low cost means of meeting its environmental responsibilities. With the improvement of central treatment facilities and the new role of cement kilns in the treatment of hazardous wastes, the industry will largely benefit from lower disposal costs. Those who are operating incinerators on site will eventually close down their units and contract the disposal task to waste management firms. Except for difficult wastes, such as highly chlorinated streams, that will be expensive to send to the cement kiln or in the event that a company policy is to treat waste in situ (for confidentiality reasons, liabilities or else), on site hazardous waste incinerators are likely to disappear in the near future.
Singapore and Malaysia have both adopted the concept of central treatment plant operated mainly by the private sector. Thailand is another good example. Till 2001, only two facilities could properly treat hazardous waste. Both were constructed by the Government and partly operated and managed by the private sector. In 2001, the Government started the process of market liberalization by issuing additional licenses to the private sector. At the beginning of 2003, 11 companies had obtained a 101-type license (*) for the disposal and treatment of hazardous waste, 6 of them being cement companies offering new options to dispose toxic wastes by the alternate fuel program (AFP).
Today we estimate that cement kilns are the preferred disposal method for hazardous waste, followed by stabilization/secure landfill, on site incineration and recycling.
Driven by increasing environmental pressure, hazardous waste combustion is shifting from a backyard activity into a sophisticated business where only some waste management companies will have the capabilities, know-how and resources to compete in the future. Cement companies have entered the market seeing at first a way to reduce their operating costs but are now fully realizing the economical interest. To achieve their goal, they move towards an integrated approach by offering more and more waste management services to their customers by either developing their own expertise or by collaboration with well established waste management firms. It is most likely to see this step take place in South East Asia in the coming years. This evolution will drastically reduce illegal dumping of waste by offering competitive means for the industry to achieve their environmental responsibilities.