By Douglas Mulhall, Rachel Platin, Sonja Rickert-Kruglov, Alain Riviere, Tanja Scheelhaase, Christoph Semisch, & Christian Sinn.
This is a supplement to an earlier-published article in CATALYST magazine, February 2010, Transitioning From Waste Incineration To Beneficial Materials where most background is contained. This supplement focuses on potential transitions to beneficial incineration.
Incineration is a waste management technology involving a thermal oxidation process in which organic matter is destroyed as a carbon source in a process known as the Redox reaction. Burning carbon is an exothermic reaction that generates energy in the form of heat.
In a complete combustion process when every carbon atom is oxidised the end products are CO2 and mineralized materials. This complete combustion can be optimized for example by using small particle sizes and sufficient oxygen at a high temperature.
Incineration is a material destruction process where organic pollutants can be destroyed. For this reason incineration is sometimes referred to as a pollutant sink. However, as described in the referenced CATALYST article, conventional incineration has serious drawbacks. To solve these, modifications are required to the conventional process, and precise definitions are required for the types of materials that can best be used for incineration.
Criteria for Beneficially Optimizing Incineration
For the purposes of this discussion, incineration is considered to be distinct from other forms of rapid oxidation or reduction such as pyrolosis, thermolysis, or burning of gas generated from biodigestion. In many cases those types of rapid oxidation or reduction can be superior to incineration because they allow for a more organized recovery of high value nutrients. So, before considering incineration it is important to evaluate those other processes.
In that context, incineration can be considered from a Cradle to Cradle® (C2C) viewpoint if clean burning man-made materials or biomass are combusted after a defined use “cascade,” as defined here, with high value energy generation and production of beneficial ashes. For example;
In any case recovery of manufactured materials is preferable to destruction.
Example. Plastics which have a high calorific value and can be used as SRF (Solid Recovered Fuel). In C2C these materials are (a) first designed correctly and recovered in the technical cycle. (b) used in cascades, (c) after a defined period can be burned safely.
Special composites which can’t be separated like glass-fibre reinforced plastic are problematic and require special design considerations.
Combustion of biomass, especially wood, can be considered as using current solar income according to the second C2C principle, but only if the biomass is effectively renewed in balanced nutrient cycles, including carbon, nitrogen, phosphorous and other elements required for life processes.
Biomass incineration in the conventional way contributes to resource loss and to raw material shortages, especially wood shortages. To avoid this, byproducts from forestry and agriculture can be used in composting instead of incineration.
For C2C-combustion of biomass a “defined use cascade” before burning is key.
Cradle to Cradle® provides a concept for the use of materials, for example wood, in a defined cascade that includes materials and energy recovery as well as CO2 storage. In a cascade there are consecutive uses of lesser and lesser properties until at the end the process results in destruction or re-production of the material. At the end of that cascade the nutrients in biological materials are returned to the soil to support biodiversity and continued soil productivity. Throughout the cascade products are designed for the biological cycle and are free of harmful contaminants.
Ashes from plastic and glass fibre incineration can be used as building material if;
- The input is defined and no toxic matter is in the output stream.
- No dioxin or furan are generated during combustion.
- Heavy metal contents are limited to background levels.
- Input is positively defined and as a first step free of chlorine-, bromine, or fluorine compounds. Clean-burning of biomass can support plant growth by replenishing fertilizer.
Valuable nutrients may be recovered from the ash under defined conditions.
Example. Phosphate can be extracted from ash. The reutilization of biomass ashes in agriculture is important to create nutrient cycles and save fertilizer. Ashes from biomass combustion are valuable nutrient sources.
Example of Wood
The thermal use of wood has the highest CO2-savings among the different kinds of biomass as a renewable energy source. Wood, after use in a cascade, can be used for the production of heat and power. The main wastes are ashes, generated in an amount of 1-12% of initial mass, depending on incineration parameters (imperfect combustion) or type and shape of used combustibles e.g., high bark content will result in high ash generation.
In wood ashes all major macro- and micro nutrients except N are present. The plant uptake of those nutrients differs widely. Whereas K availability is close to 100%, only 50% and 10% of Mg and P, respectively, are bio-available.
Wood ashes are highly alkaline and may counteract soil acidification. With their high pH of 12-13.5 wood ashes can substitute lime to buffer forest soil acidity. Nevertheless, wood ashes can only be used in combination with lime at a rate <30%. Otherwise the high reactivity of ashes could impair soil quality. Besides nutrients, heavy metals are contained in wood ashes due to for example coatings. Very problematic is chromium, which accumulates and hinders the use of wood ashes as input material for composting plants. Today only ashes from untreated woods can be used safely in composting plants.
Separation for Recovery
Elements like phosphorous (also in phosphate) and metals can’t be burned and depending on their volatile properties can be found in the fly or bottom ash.
Resources such as metals should be recovered before burning, because in a high temperature incineration they get melted with the SiO2 fraction in glass as the inert fraction. Once glazing occurs, it is very energy-intensive to separate the elements again. Rare metals, which are not magnetic, like gold, silver, copper, platinum, palladium etc. can be recovered from the input stream. Iron and steel can be separated from bottom ash but require re-heating resulting in a negative energy balance.
Summary of Beneficial Criteria for Optimizing Incineration
Organizations that use incineration can take these measurable steps to improve their incineration processes.
- First consider other earlier-described rapid oxidation options where nutrient recovery can be more effective.
- Materials are designed for clean burning (i.e., without harmful emissions). Clean burning might still require filters because most incineration processes generate respirable ash. However, such filtration would be different than most filtration today because it would recover ash that can then be used beneficially.
- Materials destined for eventual incineration are first used in a cascade. Combustion is the last step of the cascade prior to reuse of ash.
- Separation of certain resources prior to incineration.
- Energy, steam and heat are produced through co-generation.
- Ashes are completely suitable to reuse.
- Flexible scalability to prevent unnecessary demand-side growth of incineration.
Near-Term Focus for Modified Conventional Incineration
Conventional incineration focuses on destruction of mixed and hazardous waste. This focus could be modified to emphasize hazardous waste, and “unmarketable” products where the component materials cannot be separated for recycling.
By destroying unmarketable products, incineration can contribute to a positive “suction effect” if at the same time specifications are established for new products that are recyclable or combustible in a defined-use cascade.
Incineration is at the moment the best conventional process to destroy toxic or unmarketable materials. Promising areas for modified conventional incineration as interim solutions are:
Destroying complex “unmarketable” materials which are badly designed and cannot be recycled effectively, and which would pollute recycling streams if introduced to them. Here toxic effect on the ash and fly should be taken into consideration. Sorting and separation of badly designed material from other materials may avoid contamination of ash and fly and could be an interim solution. Example; Incinerating materials like soft PVC which might result in HCl and dioxins emissions in uncontrolled combustion, and which are not recyclable at the same level of quality. PVC incineration is not a Cradle to Cradle® process, but rather a strategy for destroying unmarketable products.
Destroying highly toxic materials that cannot be degraded effectively. In that case the various material classes have to be put in a logical order and be separated from other incineration materials. Example; Industrial toxic waste and hazardous waste like contaminated products such as drain cleaners, oven and grill cleaners, turpentine, insecticides, paint removers, chemicals, fertilizers, photographic chemicals, wood preservatives, adhesives, coatings and paints, solvents, chrome cleaners, defroster, antifreeze, oil filters, anti-rust and others.