Incineration

Waste incineration

Incinerating plant in Malmö, Sweden.

The incineration is the complete combustion of organic matter until its conversion into ashes, used in some places for the treatment of waste or garbage: urban solid waste, dangerous industrial and hospital, agricultural (whose recovery alternative is crushing), among others. Both incineration and other high-temperature waste treatment processes are described as 'thermal treatment'.

Incineration is carried out in furnaces by chemical oxidation in excess oxygen. Some of the reasons for which this treatment is used may be the destruction of information (document incinerator) or the destruction of hazardous chemical products or compounds (organic solid waste incinerator). The products of combustion are ashes, gases, toxic particles and some with carcinogenic effects, as well as heat, which can be used to generate electricity.

Because of its harmful effects on health, its high economic price and its unsustainability, it is a method of waste disposal strongly criticized.

Advantages and disadvantages

This waste processing system presents a series of advantages over other treatment techniques, such as:

1. Possibility of energy recovery.
2. Possibility of treatment of many types of waste.
3. Possibility of implanting it near urban centers.
4. Little land area is needed.
5. Reduces the volume of solid waste by 80%-85 %.

It also presents a series of quite significant drawbacks, such as:

1. It does not completely remove the waste, so a special landfill is needed for the deposit of ashes from the incineration, part of which are very toxic.
2. They produce toxic gases that should not be generated or that should be treated, that carry greenhouse gases and air pollution, finding compounds called toxins, such as furans and dioxins, which are carcinogens, acid gases (such as sulfhydric acid and carbon dioxide) and heavy metals.
3. They need a contribution of external energy for its operation.
4. Low flexibility to adapt to seasonal changes in waste generation.
5. Economic investment and treatment costs are high (250 million euros for a treatment plant of about 450,000 t/year).
6. Possibility of breakdowns, so an alternative treatment system is needed.
7. It overrides the implementation of policies aimed at the valuation of waste reduction and reuse, because of the need to make the investment made or simply by the generation of externalities.

Parameters to check

First of all, the type of waste that we are going to incinerate must be controlled, we can have a mixture of waste that has not been previously selected (raw waste), in this case the combustion is more difficult to control since we have a mixture heterogeneous range of materials and part of these may be non-combustible. Another option is that we would have already treated the waste previously, to achieve a homogeneous mixture of combustible materials (fuel derived from waste), so that combustion control will be much better.

To achieve a correct incineration of waste and a minimization of polluting gases, the following parameters must be controlled, in addition to the type of waste:

1. The residence time of waste in contact with oxygen within the incineration chamber (retention time).
2. The relationship between the amounts of oxygen and waste that are mixed.
3. The temperature.

The control of these three parameters is essential for a correct incineration, and they are also related, so that if we vary one, we will have to vary the others in the right measure so as not to lose the effectiveness in the combustion.

Main components of waste

The main elements found in waste are carbon, hydrogen, oxygen, nitrogen and sulfur; Other elements such as metals, halogens, etc. are also present in small amounts. Let's see the products obtained from incineration based on each component:

Oxidation reactions

Under ideal conditions, using only the stoichiometric amount of O₂ and that this reacts only in its elemental state, the gaseous products derived from waste incineration would be made up of CO₂, H₂O, N₂ and SO₂ in fewer numbers. The combustion reactions (oxidation) that will take place between the carbon, hydrogen and sulfur contained in the waste and the oxygen in the air, would basically be the following:

• C (organic) + O2 → CO2 + heat
• 4H2 (organic) + O₂ → 2H2O + heat
• S + O₂ → SO2

But in practice, a stoichiometric amount of O₂ is not enough for the incineration process, and O₂ also participates in all its oxidation states (O₂.-RO.ROO., O., HO.). This, added to the fact that some oxidation reaction products can react with each other, takes us away from ideal conditions and creates many problems, in the form of pollutants, in incineration plants.

Pollutants from waste incineration

The use of incinerators as waste treatment produces a series of gaseous and particulate emissions, solid waste (ash) and liquid effluents that are not beneficial to the environment. Let's look at these contaminants:

• Nitrogen oxides (NO_X): The most important are NO and NO₂. Nitrogen oxides are precursors of ozone formation (O₃) and nitrates of peroxiacile (NPA), photochemical oxidants constituent of the smog (black mixed with smoke and particles

), and contribute to the formation of nitric aerosols that cause acid rain and fog.

• Sulphur dioxide (SO)₂): It is formed by the combustion of materials containing sulfur. The S0₂ is an irritating gas for the eyes, nose and throat, and in high concentrations it can cause disease or death in people affected by respiratory problems. The SO₂ is the main responsible for the production of acid rain.
• Carbon monoxide (CO): It is formed when the combustion of carbon materials is incomplete. It reacts with blood hemoglobin to form carboxyhemoglobin (HbCO), which replaces oxyhemoglobin (HbO)₂) that transfers oxygen to living tissues. The lack of oxygen can cause headaches, nausea and even death at high concentrations and for a high time.
• Particles: They are formed by incomplete combustion of fuel and by physical drag of non-fuel materials. Particle emissions cause reductions in the visibility and effects on health that depend on the size and composition of these.
• Metals: Some articles such as plastics, magazines, batteries, etc. contain metallic elements, these can remain in the ashes or be issued by the incinerators. Specifically, the presence of Cd, Zn, Sb, Ag, In and Sn in the output gases, as well as Hg in lower concentrations, has been observed. The possibility of a metallic compound becoming volatile or forming solid particles will depend on its nature

chemistry. In principle, three different groups of metals can be distinguished:

1. Group 1: Al, Ba, Be, Ca, Co, Fe, K, Mg, Mn, Si (semimetal), Mr and Ti. These elements have high boiling points and therefore are not volatilized in the incinerator combustion chamber. They are part of the same ash matrix.
2. Group 2: As, Cd, Cu, Pb, Zn, Sb and Se (the last two are semi-metallic), which are volatilized during combustion, but condense quickly when the exhaust gases cool, so they are usually on the surface of the ashes.
3. Group 3: It is formed by the Hg that is volatile and does not condense, so this element is more likely to escape into the atmosphere.

The location of the metals (in the matrix or surface of the ashes, or in the gaseous effluent), depends on their chemical nature and also on the constitution of the exhaust gases. The presence of sulfur and nitrogen oxides and/or hydrogen chloride can lead to the formation of volatile compounds (metal sulphates, nitrates or chlorides), which alter the volatility of metals. Due to the possible toxicity of the effluents discharged during incineration, the control that must be carried out must be exhaustive.

• Acid gases: Incineration of residues containing fluoride and chlorine generates acid gases, such as fluoride and hydrogen chloride. There are trace amounts of fluoride in many products, while chlorine is located in plastics, especially in vinyl polychloride, and in polystyrene and polyethylene, which usually carry additives containing chlorine.
• Dioxines and furans: The issue of organic compounds of the family of dioxins and furans (which can be gaseously and/or adsorbed on particles), dioxins are chlorinated organic compounds belonging to the family of polychlorodibenzodioxins (PCDDs). Its molecule is formed by a triple ring structure in which two benzene rings are joined by a couple of oxygen atoms. A furan is a member of the polychlorodibenzofuran family (PCDF), with a similar chemical structure, except that the two benzene rings are joined by a single oxygen atom. The importance of PCDD and PCDF families of organic compounds is that some of their isomers are among the most toxic substances that exist. PCDDs and PCDFs are issued at low concentrations from incineration systems that burn urban waste. There are some evidence that these substances are produced in all combustion processes. Three sources of dioxins and furans have been proposed in emissions from urban waste incineration:

1. Presence in waste.
2. Training during combustion due to chlorinated aromatic compounds acting from precursors.
3. Training during combustion by the presence of hydrocarbonated compounds and chlorine.

One of the most probable causes of the generation of dioxins and furans in incineration is the formation from their organic precursors in the coldest areas of post-combustion, due to the action of hydrogen chloride that is generated during the process. This favors the formation of a chlorinating agent that, in contact with the aromatic compounds present, gives rise to this type of compound. The temperature range in which dioxins are formed on the surface of ash particles is 250 to 400 °C, with a maximum at 300 °C. For this reason it is recommended that, in post-combustion areas, the temperature drop sharply, in order not to allow time for the formation of dioxins. To avoid the emission into the atmosphere of dioxins that may have formed during incineration, powdered activated carbon is usually injected, which is a good adsorbent for this type of compound.

• HAP: Polycyclic aromatic hydrocarbons are organic compounds analogous to the benzene that contain aromatic rings of six members connected between them by sharing a pair of adjacent C, which gives rise to merged rings. They are formed by partially burning carbon-containing materials, therefore they are products of a bad combustion. These compounds are common in the atmosphere of cities and their existence is worrying because many are carcinogens such as benzo[a]pireno or benzo[a]antraceno.

Components of a waste incineration plant

We are going to have different types of incineration plants depending on the type of waste that is going to be treated in them, whether it is solid urban, hospital or industrial waste. But the initial scheme is the same in all cases, what varies are the subsequent treatments of gaseous effluents, liquids and ashes to eliminate the pollutants we have discussed (which vary in each case). The basic scheme is as follows:

1. Deposit where the wastes to be treated are introduced.
2. From here they go to the combustion oven where the necessary amount of air is introduced.
3. The ashes and scum fall below a deposit.
4. The gases go to a post-combustion chamber where they go to air pollution control equipment.
5. From here the clean and low-temperature gases go out to the atmosphere through the fireplace and the solid ashes that have been formed are dragged by water to another reservoir for further treatment.

Urban waste incinerators can be designed to operate with two types of solid waste as fuel: raw waste or already processed waste.

Most current incinerators use a rotary kiln, to produce as homogeneous a mixture as possible, made of refractory material, in which the waste is burned at a temperature between 950 °C and 1,200 °C.

The residue that remains from the combustion is collected at the bottom of the furnace, while the gases generated are conducted to a secondary combustion chamber. This chamber ensures an efficient mixture of the combustion air with the extra fuel that is sometimes added and also provides the necessary residence time to homogenize the air flow. In the secondary chamber, which works at around 1,000 °C, the gases are finished burning. The residence time in this chamber is usually about 2 to 4 seconds. The exhaust gases, in addition to having a low temperature, must be free of contaminants. To reduce the generation of pollutants, it is important to control the gases from the upper part of the furnace, which is where CO, NOx and other compounds seen before are produced. NOx is formed where there is more excess oxygen and temperatures are higher. CO is generated in the coldest areas and where there is a lack of oxygen.

To control atmospheric pollution, the incineration plant can include, for example, the injection of ammonia in the combustion zone itself to control nitrogen oxides, a dry or wet treatment plant (for example, with milk of lime) to control sulfur oxides and a bag filter to separate particles. The clean gases are led to the chimney to go out into the atmosphere. The slag from the combustion falls from the furnace into a reject hopper located below, to be managed together with the ash formed in the post-combustion chamber and the fly ash from the bag filter.