Incineration

Incineration is a technical term for controlled combustion. Often referred to as burning, combustion is a high-temperature exothermic chemical reaction between a fuel and an oxidant, in this case waste and atmospheric oxygen. Heat and light energy is released during this reaction and transferred to the surroundings, hence it is called an exothermic reaction.

Incineration is a thermal treatment method suitable for different types of waste. Municipal solid waste (MSW) and other non-hazardous waste can be readily incinerated. Medical waste is often required by law to be sterilised, for example in an autoclave, at the facility where it was produced before being transported to an incinerator. Hazardous waste requires a special combustion chamber design that is able to withstand corrosive and abrasive substances, and allows specific residual products to be collected for reuse. Rotary kilns are often used to process hazardous waste.

How it works

The main parts of an incinerator are a furnace, a boiler with heat exchangers, a water supply system, a turbine and a generator. The combustion of the waste happens in the furnace below the boiler. The heated gases from the furnace rise into a series of chambers in the boiler. This heat vaporises the water in the boiler’s heat exchangers, a series of pipes filled with water, and creates superheated high pressure steam. This steam forces the blades of the turbine to rotate, driving the generator and producing electricity. A tonne of MSW with suitable characteristics can result in 500-600 kilowatt hours (kWh). The water travels through the system in a closed loop cycle - after passing through the turbine as steam it cools down and condenses back to liquid, then it returns to the heat exchangers to be heated again.

In Japan, typical steam conditions are 400℃ temperature and 4 megapascal (MPa) pressure. These values are low in comparison with power plants fired by coal or natural gas, where steam conditions are 500℃ and 15-25 MPa. These conditions cannot be replicated in MSW incinerators due to corrosive gases like hydrogen chloride present in the flue gas. Higher efficiency could only be achieved at the expensive of fast pipe corrosion.

The waste is often mixed or shredded during pre-processing, since uniform quality of the waste is important for optimum incineration. Pre-processed waste is stored in bunkers before being transferred into the furnace. If the moisture content of the waste is high, it is desirable to keep it in a bunker for several days to allow the moisture to seep through. On the contrary, if the waste is very dry, it is desirable to process it as quickly as possible due to a high risk of accidental ignition. A mechanical grabber, capable of picking up 7 tonnes of waste at a time, transfers the waste into the chutes leading to moving grates in the furnace. Some incinerators feature three-stage combustion chamber - a “dry area” where the moisture evaporates from the waste, a “combustion area” supplied with most oxygen, and a “burn-out area” where the residue is retained until complete combustion occurs. Moving grates direct the waste through these areas. Complete combustion is achieved by allowing appropriate retention time in each area, which depends on the amount and properties of the waste. Even waste with high moisture content can be efficiently combusted given sufficient retention time in the “dry area”.

Fluidized beds have been developed as an alternative to moving grates. A layer of sand at the bottom of the combustion chamber and the air blown through it provide high temperature due to the sand’s high heating capacity. This instantly dries and combusts the waste, even if it has high moisture content. The advantages of fluidized bed technology include smaller amount of bottom ash produced and shorter starting time after a stop in operations. High combustion speed is an advantage when processing homogeneous materials, but can often lead to incomplete combustion and high levels of carbon monoxide when processing heterogeneous MSW. Due to this and the additional costs of fuel preparation and bed materials that need to be regularly replenished, this incinerator type is not widely used.

The combustion occurs at a temperature between 850℃ and 1200℃. Feeding uniform quality waste at the right speed avoids hot spots within the furnace and allows for a uniform temperature to be maintained. Some hydrocarbon gas components originating from the volatile organic compounds in the waste are dioxin precursors. Waste with higher content of chlorine and certain metals like copper produces more dioxins when incinerated. To form stable end products and break down the dioxins that form inside the combustion chamber it is essential to ensure complete combustion of both waste and flue gas. This is achieved by maintaining the required high temperature, regulating the oxygen content and the retention time of the waste and flue gas in the combustion chamber, and providing sufficient turbulence via the moving grates. Sustaining uniform high temperature allows to keep carbon monoxide levels low. Complete combustion is the main difference between incineration and open burning of waste which happens at much lower temperatures and hence leads to incomplete combustion and releases toxic pollutants.

The combustion process outputs thermal energy which has to be recovered on-site, since it is not readily transportable. The most efficient incinerators are Combined Heat and Power (CHP) plants that both generate electricity and deliver residual heat to the neighborhood. This is done by routing low pressure steam from the turbine through a second closed water loop leading out into the neighborhood. Locating an incinerator next to an industrial area that can use the heat is a way to maximise the use of the available energy. Alternatively, the heat can be incorporated into the local district heating system, used to heat up nearby swimming pools and tropical plant greenhouses or to melt snow on the roads. In the European Union, a tonne of MSW typically results in 500 kWh of electricity and 1000 kWh of heat. Further improving the efficiency of incinerators is an active area of research. For example, a recent study proposed using aluminium alloy-based Phase Change Materials in ceramic bricks in the combustion chamber to absorb temperature fluctuations and thus produce more heat.

Incineration by-products

Apart from the heat, the principal byproducts of combustion are bottom ash and flue gas. The bottom ash produced is normally 10-25% by weight of the MSW processed. The bottom ash is generally non hazardous and contains mostly incombustible waste, for example metals, glass and pebbles. The content of useful materials and contaminants in the bottom ash depends on the original composition of the waste processed. Iron and non-ferrous metals, for example aluminium and copper, are recovered from the bottom ash using magnetic separation, air table sorting or sieving. High value metals like gold and silver are usually present in particles of less than 2mm in diameter. Technologies that would allow the extraction and reuse of valuable materials in the bottom ash are an active area of research.

Once the metals have been extracted, the rest of the bottom ash is either sent to a landfill or cleaned and subsequently used by the construction industry. In Europe about 54% of bottom ash is used in construction. The bottom ash can be used for landscaping and in construction to produce cement, asphalt and glazed tiles.
Research conducted in Thailand showed that addition of bottom ash to Portland cement results in similar compressive strengths and decreased water requirement. Similarly, addition of bottom ash to calcium sulfo-aluminate cement, which is used in high early strength and rapid setting applications, results in similar compressive strengths and porosity. High-density glass-like aggregate material called “slag” can be produced by melting bottom ash at 1250℃ or more. Slag is a high quality construction material, but the construction and operation costs of a melting facility that produces it are high.

There is a concern that contaminants present in bottom ash, if not removed prior to use, might leach out if the asphalt or cement is exposed to particular conditions, for example constant running water, road salting during freezing temperatures, acid rain or simply time. A European literature review covers the pre-2019 research about leaching behaviour of cement produced with bottom ash. A 2020 Chinese study found that while alkaline elements (calcium, silicon and aluminium) accounted for over 70% of the total mass of the bottom ash sample examined, chlorine, sulfur, iron, magnesium, phosphorus, titanium, potassium, sodium and zinc accounted for another 25-30%, and copper, chromium, lead, strontium and barium accounted for up to 1.5%. The study concluded that bottom ash is an appropriate material for road construction due to its low metal leaching potential. Earlier studies in Denmark, China, Germany, Singapore and the US also showed that heavy metals (barium, cadmium, chromium, copper, lead, nickel and zinc) are often present in bottom ash. Their concentration is highest amongst particles smaller than 2 millimeters in diameter, hence sieving might be an option for separating the fraction of the bottom ash most suitable for use in the construction industry. Metals have different leaching profiles - for some the amount of leaching monotonically increases or decreases with time, for others it fluctuates based on several factors. A Singapore study showed that heavy metals can be washed out from the bottom ash with nitric acid. A recent European study provides an overview of different leaching tests. The US study showed that the amount of leaching from asphalt and concrete with added bottom ash meets the criteria of the US Secondary Drinking Water Standard. Ultimately the environmental and health impact of using bottom ash in construction depends on the original composition of the waste processed, methods used to clean the bottom ash before usage, and the type of material to which the bottom ash is added. It is important to consider that landfilling the bottom ash or the waste itself, without incinerating it first, does not necessarily reduce the leaching of the contaminants into the surrounding soil and groundwater.

Flue gas, the exhaust air generated during combustion, contains solid particles, acidic gases, dioxins and heavy metals. Some trace elements tend to evaporate at combustion temperatures and then recondense when the temperature falls in the heat transfer sections of a boiler. This recondensation results in both homogeneous submicron particles of trace elements and ash particles covered in a heterogeneous layer of trace elements. Flue gas must be treated before being released from the incinerator’s chimney stack; this is the longest part of the whole incineration process. Prior to treatment, flue gas is cooled to under 200℃ with water injections or more efficiently by indirectly heating the water in the heat exchanger pipes. This is followed by a series of absorption, scrubbing and filtering steps. A typical air pollution control system includes the following components: an acid gas removal system, dioxin and mercury removal system, a particulate removal system, a nitrogen oxide removal system, and a pollution control system. These may include wet scrubbers that spray water to remove large-scale particles, mechanical filters for capturing fine particles, oil traps, and chambers that house controlled chemical reactions for capturing harmful gaseous compounds. Compared to MSW, hazardous waste processing requires even more sophisticated flue gas treatment systems.

Series of electrostatic precipitators are often used to remove the particles from the flue gas. First the flue gas is funneled past electrodes that charge the particles with a negative voltage, then past electrodes that are charged with a positive voltage. The particles stick to the electrodes with opposite charge and are subsequently moved into a collector by an automated shaking or brushing mechanism. There are concerns that de novo synthesis of dioxins happens in electrostatic precipitators that operate at around 300℃. To mitigate this risk, sometimes bag filters are used (at a later stage) instead of electrostatic precipitators to remove the particles from the flue gas cooled to under 200℃, which is too cold for dioxin synthesis that happens only in 200-600℃ temperature range.

The next step of the treatment is injection of an alkali agent (lime powder, limestone slurry or sodium bicarbonate) and powdered activated carbon into the flue gas. Acidic gases such as hydrochloric acid (a solution of hydrogen chloride gas in water), hydrofluoric acid (a solution of hydrogen fluoride gas in water) and sulfur oxides react with alkali agent and are subsequently removed. Sulfur reacts with lime and produces gypsum. Powdered activated carbon removes dioxins and heavy metals. The next step is to remove the particles from the flue gas by passing it through the bag filter. Mercury, a volatile metal, is removed at this stage as it can be collected more efficiently at a lower temperature.

Nitrogen oxides, unlike other acidic gases, cannot be removed with bag filters and require a different system. A combination of three methods - combustion control, catalytic and non-catalytic denitrification - is used to reduce the amount of nitrogen oxides in the flue gas. Combustion control is a method for reducing emissions of nitrogen oxides by forming a low-oxygen atmosphere inside the combustion chamber, for example by recirculating some of the combusted flue gas to the furnace. Maintaining low-oxygen atmosphere results in an increase of carbon monoxide unless the temperature and the air ratio in the combustion chamber are finely controlled. A 2013 Swiss study provides an overview of this method. Non-catalytic denitrification method involves spraying an ammonia or urea solution into the combustion chamber to decompose nitrogen oxides. The amount of solution sprayed and the temperature of the flue gas (above 800℃) must be precisely controlled. Catalytic denitrification method involves facilitating the reaction of nitrogen oxides in the flue gas with ammonia and oxygen with the help of a catalyst, for example vanadium pentoxide/titanium dioxide. As a result nitrogen oxides decomposes into nitrogen and water and trace dioxins are decomposed as well. The efficiency of catalytic denitrification method is about 95%, higher than that of non-catalytic denitrification and combustion control. The catalyst requires that the flue gas is free from particles and at a temperature of 200℃ or higher, which means that the flue gas must be first filtered through bag filters and then reheated. The flue gas is reheated using steam which reduces the amount of power generated. If the emission standards can be met with a combination of combustion control and non-catalytic denitrification, catalytic denitrificationis is usually avoided due to its financial cost. New catalysts that are effective at temperatures below 200℃ are currently an active area of research.

Real-time emissions monitoring systems allow for the concentrations of pollutants in flue gas emissions to be measured continuously, in addition to periodical measurements conducted by external monitoring organisations. Until recently emissions of dioxins, furans and mercury could only be monitored by periodical measurements, new technologies like AMESA (Adsorption MEthod for SAmpling of dioxins and furans) are changing that.

It has been shown that dioxin emissions occur at high levels during startups, after startups and during shutdowns of an incinerator. A 2016 Taiwan study showed that dioxin emission levels characteristic of steady state operation are reached only 15 days after startup, for the first 9-12 of these the emissions exceed the amount allowed by the environmental regulations. For this reason, semi-continuous type (operated during the day and unused during the night) and batch-type incinerators that were common in Japan have been replaced with continuous incineration systems. While total dioxin emissions is what really matters, only emissions that occur during steady state operation are regulated by environmental laws. Installing backup air pollution control systems is essential for being able to operate an incinerator continuously. Most incinerators have a “filter bypass mode” which allows to bypass air pollution control systems and emit flue gas directly into the environment. Incinerators that are locked into contracts with electricity companies that require a certain amount of power to be provided per unit of time, sometimes have to use “filter bypass mode” during necessary repairs. Emissions can be tested indirectly, for example by monitoring concentration of dioxins and furans in chicken eggs in nearby farms. The visible white plumes coming out of incinerator smokestacks, that get people concerned, are water vapour (10-18% by volume), CO₂ (6-15% by volume), oxygen (7-14% by volume) and nitrogen. Unfortunately, pollutant emissions are invisible and hence cannot be monitored without specialised equipment. Sometimes smokestacks are build very high to disperse trace polutants across a larger area and hence minimize the concentrations. Apart from bottom ash and fly ash, by-products of incineration include used filters, oil from oil traps, contaminated water from flue gas treatment process and leachate generated from the waste while it is stored in the bunkers.

The stuff that collects in the bag filters (removed pollutants together with the injected alkali agent and activated carbon) is known as fly ash. The amount of fly ash generated is 3-5% of the waste input by weight. To prevent toxic pollutants from leaching, fly ash is either disposed of as hazardous waste or treated prior to landfilling or reuse. In Europe hazardous waste landfills are commonly used to dispose of fly ash, while in US fly ash is often mixed with bottom ash and disposed of in a sanitary landfill or used as landfill cover. Fly ash can be incorporated in concrete, which results in decrease of CO₂ emissions, environmental impact and costs associated with concrete and decrease of the heavy metal leaching potential of the fly ash. Trace heavy metals from the fly ash might be released from the concrete during exposure to CO₂-saturated water or acid rain. Fly ash can also be used as a filling material in asphalt. In Netherlands fly ash is processed to recover plaster and salt for de-icing. Sulfuric acid from the flue gas can be separated and recirculated into the furnace; as demonstrated in the incinerator in Gothenburg, Sweden. This reduces the chlorine content of bottom and fly ash, reduces dioxin formation in the flue gas by about 25%, and reduces corrosion by 60–90% thus allowing for higher steam temperature and hence more electricity production without increasing corrosion. Silica sulfuric acid catalyst can be synthesised from fly ash and used to simultaneously remove both sulfur oxides and nitrogen oxides from flue gas.

Researchers at Chalmers University of Technology in Sweden developed a method for extracting 70% of the zinc present in the fly ash. An acid wash is separated from the flue gas and mixed with the fly ash to release zinc and other metal ions. Hydroxide precipitation of the resulting slurry, followed by filtration of the formed crystals, produces a filter cake with about 80% zinc hydroxide by weight. This can be processed in existing production lines in the metal industry to generate high purity zinc metal. The residual ash is re-incinerated to break down the dioxins; 90% of it turns into non hazardous bottom ash that can be used by the construction industry. In late 2020, the ash washing facility with zinc recycling is under construction in Gothenburg, Sweden; there this method will be applied at scale for the first time.

What it looks like

Click on the image below to see the video of an incinerator operated by AEB Amsterdam.

While most incinerators look like any other industrial facility, some incinerators are architectural gems. Amager Bakke waste-to-energy (WtE) plant in Copenhagen, Denmark is also a ski slope and the world’s tallest man-made climbing wall (85 meters). A 450 meter artificial ski slope is covered in green synthetic bristles which provide friction similar to fresh show. The plant was designed by a Danish architect Bjarke Ingels. The plants capacity is 450,000 tonnes per year; it is predicted that by 2024 almost half of the material processed will be imported waste and biomass. The output is electricity for 30,000 homes and heating district heating for 72,000 homes. This is one of the cleanest WtE plants in the world, due to its air pollution control systems. The plant cost USD 600 million to built.

Maishima WtE plant in Osaka, Japan processes 328,500 tonnes of waste per year and attracts 12,000 tourists annually. It was designed by an Austrian artist Friedensreich Hundertwasser, who also designed Spittelau WtE plant in Vienna, Austria. The plant cost USD 600 million to built.

WtE plant in Roskilde, Denmark has won a number of architectural awards. A spire that wraps around the plant’s chimney, 97 metre tall, towers above the landscape, creating a contemporary counterpart to the steeples of the historic Roskilde Cathedral. The plant was designed by Dutch architect Erick van Egeraart. The plants capacity is 250,000 - 300,000 tonnes per year; it processes waste from nine surrounding municipalities and waste imported from abroad. The output is 19 megawatt of electricity and 52 megawatt of heat, that is electricity for 65,000 homes and district heating for 40,000 homes. The plant cost USD 215 million to built.

Richmond Hill WtE plant on Isle of Man processes all domestic and commercial waste produced on the island, as well as medical waste, old tyres and bio‑waste from sewage treatment. The plants capacity is 50,000 tonnes per year. It generates 25,000 megawatt hours per year, covering 10% of the island’s electricity needs.

Incineration can be financially sustainable

Incineration with energy recovery might be a good option for waste disposal if there are skilled staff available and a steady input of waste. Associated capital expenditure (CAPEX) and operating expenses (OPEX) are high, but incineration with energy recovery can be profitable as there are many possible revenue streams. Waste to Energy International estimated that CAPEX of an incineration plant grow almost linearly with capacity. According to the World Bank CAPEX is USD 190-1,000 per tonne of annual waste capacity, depending on the location and size of the plant. Since generated electricity is often the main source of revenue, CAPEX is sometimes expressed in terms of plant power output in kilowatts. Project Drawdown estimated CAPEX USD 7,474 per kilowatt of power output, which falls within the range of USD 7,000 - 10,000 per kilowatt of power output reported by the World Banks. As a concrete example, let’s take an incinerator in Phuket, Thailand built in 2009 and operated since 2012 by PJT Technology. The capacity of this incinerator is 700 tonnes per day and it is operated 7,000-8,000 hours per year, which means about 224,000 tonnes of annual waste capacity, assuming 320 days of operation a year. The CAPEX were USD 31.17 million, which is USD 139 per tonnes of annual waste capacity and USD 2,793 per kilowatt of power output.

Large scale incinerators are able to achieve low volumes of by-products, a high rate of efficiency, and financial profitability due to economies of scale. The efficiency of generating electricity generally increases with the scale of the plant, while CAPEX and OPEX per annual waste capacity decrease. Air pollution control systems are the most expensive part of the incinerator; and all stages of flue gas treatment are essential, even for the smallest incinerator. To ensure continuous operation, backup systems are required, which further add to the cost. air pollution control systems often account for half of the CAPEX of building an incinerator with energy recovery, and this proportion is even higher for small plants. In wealthy countries like Japan incinerators with capacities as low as 95 tonnes per day (30,000 - 35,000 tonnes per year) can be financially sustainable. Due to the high CAPEX, contracts for incineration plants are typically 25-30 years to allow the developer and their financial backers to recoup the investment made.

According to the World Bank, OPEX is about 1-3% of CAPEX, approximately USD 100-200 per tonne of waste. Project Drawdown estimated annual fixed operation and maintenance costs to be USD 335 per kilowatt of power output. The incinerator in Phuket reported USD 9.12 million in total expenses in 2017, which is USD 40.71 per tonnes of annual waste capacity and USD 817 per kilowatt of power output. In terms of treatment of bottom and fly ash, Denmark reported costs of USD 41.30 per tonne of bottom ash and USD 162.77 per tonne of fly ash. Some of the electricity produced is used to power the plant itself, which means that there is no electricity bill to pay. When the incinerator is no longer operational, there are no further OPEX, unlike landfills that require costly monitoring for several decades after closing.

The thermal energy produced during combustion, in the form of electricity and heat, is the major source of revenue for WtE plants. The incinerator in Phuket reported 382.6 kilowatt-hours per tonne of MWS incinerated in 2017, which is similar to Japan’s average of 400 kilowatt-hours per tonne of MWS. In China, Japan, Thailand and other countries feed-in tariff (FIT) schemes allow the WtE plants to sell the generated electricity to outside customers. In some countries governments hold the monopoly on electricity, which restricts or even prevents this. Selling recyclables separated from the waste before incineration and metals recovered from the bottom ash is another source of revenue. Bottom and fly ash can be sold as construction materials or fertilisers.

Often selling energy and materials is insufficient to cover the operating costs, and hence additional sources of revenue are required. Tipping fees (also known as gate fees) are a major source of revenue. These are paid by the local government or companies that deliver waste to the incinerator. In some cases, carbon credits under the Clean Development Mechanism (CDM) may be used to supplement the revenue of an incineration plant. Diverting waste from landfills, and hence preventing methane emissions, and producing energy that otherwise would be generated from fossil fuels qualifies WtE plants for carbon credits that they can sell to companies. Governments sometimes supplement the revue of WtE plants with tax refunds, local and national subsidies.

In 2017, the incinerator in Phuket reported revenue of USD 80.67 per tonne of waste processed - USD 17.24 in tipping fees and USD 63.43 from electricity sales. The high amount of revenue from electricity sales is due to Thailand’s national subsidy programme (Adder) and high FIT selling rates (about USD 0.17 per kilowatt hour). The OPEX were USD 40.71 per tonne, hence resulting in an annual profit of USD 8.95 million for processing 224,000 tonnes of waste. In the EU the total revenue, including government subsidies, is usually double that at about USD 100 per tonne of waste processed.

Waste incineration is used in many countries

How does incineration compare to other waste disposal methods?

Incineration destroying pathogens present in waste, thus preventing waste-associated disease outbreaks. Incineration produces less air pollution than open burning of waste, where combustion is incomplete due to low temperatures. Unlike landfills, this waste disposal method doesn’t require much land, and hence it doesn’t contribute to deforestation or competition for agricultural land. An incinerator can generate up to 10 times more electricity from the same amount of waste compared to a landfill with methane capture. Since incineration doesn’t involve storing waste for long time, environmental risks associated with storms, floods and unintended fires are significantly lower. Similarly to recycling and landfilling, incineration can be very environmentally damaging if done as cheaply as possible. Preventing environmental damage requires ongoing financial commitment. Environmental impact of incineration can be limited with regulations covering environmental assessments, bottom and fly ash disposal, emission standards for flue gas, and polluted water filtration requirements. In countries that lack the capacity to enforce environmental laws, these kind of regulations have minimal effect. Early incinerators that were inefficient and produced a lot of pollution resulted in negative public perception, that is now slowly changing. The dioxin emissions of all 53 waste-to-energy plants in US accounted for 0.09% of total dioxin emissions in 2012. A Greek study published in July 2020 examined different scenarios of pollutant emission rates and concluded that in the worst-case scenario operation of an MSW incinerator would add addition 0.03% to the existing environmental pressures (road traffic, industrial activity, etc.) in Thessaloniki area. Incinerators can act as sources of back-up power in the event of a power failure during a disaster; in Japan this is considered an important additional benefit. Unlike sunlight, wind or waves, waste can be stored and transformed into electricity when required. Incinerators that can produce varying amounts of electricity combined with machine learning algorithms powered by demand and meteorological data would accelerate the switch to renewable energy sources.

According to the research conducted by Project Drawdown wider adoption of incineration with energy recovery could reduce global CO₂ emissions. During incineration a lot of the carbon contained in the waste is released into the environment as CO₂. According to the Global CCS Institute, 1 tonne of MSW containes 190 kg of carbon which results in 698 kg of CO₂ emitted during incineration; other estimates range from 425 kg to 994 kg of CO₂ equivalent. Despite this in many cases incineration reduces greenhouse gas emissions associated with waste. The magnitude of this reduction varies substantially depending on the baseline used for comparison. The important factors are the efficiency of the incinerator (type and amount of additional fuel used, interaction with the electricity system), the emissions intensity of the energy sources being displaced (carbon-intensity of local electricity and/or heating generation), the type of the waste (caloric value of the waste, fraction of combustible materials in the waste, its methane generation potential) and likely alternative waste disposal methods. A 2014 Swiss study estimated that the reduction could be 67-752 kg of CO₂ equivalent per 1 tonne of MSW incinerated.

Combining incineration with carbon capture technology can result in a carbon negative waste disposal method.