Incineration (Large-scale)

Integrated Waste Management (IWM) refers to the complementary use of a variety of practices to safely and effectively handle municipal solid waste. The strategy used to develop an integrated waste management system is to identify the level or levels at which the highest values of individual and collective materials can be recovered. The most favourable is reduction, which suggests using less to begin with and reusing more (e.g. separation of plastics, bottles, etc.; composting or anaerobic digestion and production of biogas from organic waste), thereby saving material production, resource cost, and energy. The least desirable is landfilling (AFRICAN DEVELOPMENT BANK 2002).

The primary benefit of municipal solid waste (MSW) incineration (see also small scale incineration) is a substantial reduction of the weight (up to 75%) and volume (up to 90%) of solid waste, which can be valuable if landfill space is scarce. The generation of revenues from energy production, known as waste-to-energy incineration, can also partially offset the cost of incineration, although there are typically less expensive forms of energy production available. Moreover, incineration breaks down some hazardous, non-metallic organic wastes and destroys bacteria and viruses, which is the main benefit of incineration of medical wastes (read more about incineration of medical waste in UNEP (1996, Chapter 1.7.2). Moreover, some precious metals may be recovered from the residual ashes. However, the design of large-scale incinerators is high-tech but crucial for an environmentally sound and healthy operation: in particular, the correct treatment of the toxic flue gases and the safe disposal of residual ashes (preventing soil and groundwater pollution) have to be considered.

In considering the MSW incineration option, decision makers must weigh the benefits of incineration against the significant capital and operating costs, potential environmental impacts, and technical difficulties of operating an incinerator (adapted from UNEP 1996). Open burning of MSW must be avoided. Read more about it here.

MSW incineration can constitute a sound practice only in situations where most or all of the following conditions hold (adapted from UNEP 1996):

• Suitable landfill space is scarce, making incineration a cost-effective alternative.
• The necessary environmental controls are properly installed and maintained.
• The facility is properly sized and sited to fit well with other components of the MSW management (MSWM) system.
• The material to be burned is combustible and has sufficient energy content.
• There are nearby energy markets.

(Adapted from UNEP 1996)

Mass-burn Incinerator

Mass-burn systems are the predominant form of MSW incineration. Mass-burn systems generally consist of either two or three incineration units ranging in capacity from 50 to 1,000 tons per day; thus, facility capacity ranges from about 100 to 3,000 tons per day. These facilities can accept refuse that has undergone little pre-processing other than the removal of oversized items, such as refrigerators and sofas. Although this versatility makes mass-burn facilities convenient and flexible, local programs to separate household hazardous wastes (e.g., cleaners and pesticides) and recover certain materials (e.g., iron scrap) are necessary to help ensure environmentally responsible incineration and resource conservation.

Fluidised-bed Incinerator

In a fluidised-bed incinerator, the stoker grate is replaced by a bed of limestone or sand that can withstand high temperatures, fed by an air distribution system. The heating of the bed and the increasing of the air velocities cause the bed to bubble, which gives rise to the term ‘fluidised’. There are two types of fluidised-bed technologies, a bubbling bed and a circulating bed. The differences are reflected in the relationship between air flow and bed material, and have implications for the type of wastes that can be burned, as well as the heat transfer to the energy recovery system. They are widely used in Japan and have a capacity between 50 and 150 tons per day.

Modular Incinerator

Modular incinerator units are usually prefabricated units with small to medium capacities of between 5 and 120 tons of solid waste per day. Typical facilities have between one and four units for a total plant capacity of about 15 to 400 tons per day. The majority of modular units produce steam as the sole energy product. Due to their small capacity, modular incinerators are generally used in smaller communities or for commercial and industrial operations. Their prefabricated design gives modular facilities the advantage of shorter construction times. On average, capital costs per ton of capacity are lower for modular units than for other MSW incineration options.

Modular incinerators employ a somewhat different process than mass-burn incinerators, typically involving two combustion chambers. Gases generated in the primary chamber flow to an afterburner, which ensures more complete combustion and often serves as the primary means of pollution control. In addition, smaller-scale plants (i.e., less than 50 tons per day) sometimes operate using a batch process, rather than continuously, operating only 8 to 16 hours per day. The modular incineration option has become less common, however, in part due to concerns over the consistency and adequacy of air pollution controls.

Sludge Incineration

Incineration of dewatered sludge (check out some techniques to dewater sewerage sludge: drying beds; settling - thickening ponds; mechanical dewatering) from wastewater treatment plants reduces the volume of the dry sludge and produces a sterile non-harmful residue that is free from toxic organic chemicals and pathogens. It provides a safe alternative solution when faced with land scarcity for waste disposal and if anaerobic digestion or composting is not an option. Moreover, it also helps to recover some of the energy used in the combustion process especially in large treatment plants whereby there is a huge quantity of sludge generation (WASTEWATER SYSTEM n.y). The disposal of sewage sludge along with the incineration of household rubbish can be integrated both in newly planned plants as well as retrofitted in older plants (PUTZMEISTER 2000).

Furthermore, with the implementation of new technologies (e.g. Ash Dec), multi-nutrient fertiliser can be produced out of sewage sludge. The thermo-chemical process in Ash Dec removes toxic heavy metals and makes it harmless for the environment. This allows closing the nutrient cycle.

MSW incineration is typically only cost-effective in regions where land suitable for landfilling is scarce. Such landfill scarcity can arise due to geographical constraints, as with a highly urbanised region or island, or environmental conditions, as in regions where the water table is high. Jurisdictional and political boundaries can also constrain the size and number of sites available for landfilling, thereby increasing the attractiveness of incineration (adapted from UNEP 1996).

In addition to heat, the products of incineration include the normal primary products of combustion—carbon dioxide and water—as well as oxides of sulphur and nitrogen and other gaseous pollutants; nongaseous products include fly ash and unburned solid residue. Emissions of fly ash and other particles are often controlled by wet scrubbers, electrostatic precipitators, and bag filters (ABUBAKAR 2006). Read more about incinerators and their impacts on human health and the environment in GAIA (2008).

Incineration systems must have complex air pollution control (APC) systems in order to meet the required limits for protecting the quality of the ambient air and human health. Therefore, the institutional framework and policies are crucial in order to enforce that environmental and health standards are met. The complexity is a result of the fact that modern APC systems include provisions for controlling a number of pollutants to very low concentrations (e.g., parts per million or per billion). The provisions include control and manipulation of the combustion process itself within the combustion chamber and the use of post-combustion techniques, including the use of chemical reagents and of special mechanical and electrical systems to process the combustion gases (UNEP 2005).

Highly specialised personnel must service APC equipment regularly. Monitoring equipment is costly and requires aggressive maintenance and servicing by trained technicians. In summary, when incineration is done in a manner that has low adverse health and environmental impacts, it is expensive. When it is done poorly (with low financial costs) it can be expensive in terms of human health and environmental impacts.