An introduction to point source emissions from waste incineration and combustion.
Pollution is the introduction of contaminants into the natural environment that can have adverse impact on human health and the environment. Pollutants can be Foreign Substances taking the form of chemical substances or Energy such as noise, heat or light. Pollution is often classed as point source or non-point source pollution.
In terms of waste management, one of the sources of air pollution is waste incineration. The U.K. Health Protection Agency concluded in 2009 that "Modern, well managed incinerators make only a small contribution to local concentrations of air pollutants. It is possible that such small additions could have an impact on health but such effects, if they exist, are likely to be very small and not detectable." According to Germany's Ministry of the Environment, waste incinerators reduce the amount of some atmospheric pollutants by substituting power produced by coal-fired plants with power from waste-fired plants. Nevertheless releases of pollutants into the environment need to be controlled to protect human health and the environment.
Pollution Abatement Systems
Waste management including waste incineration and combustion has its side effects mainly in the release of toxic compounds in the environment. The main pollutants released include acid gases (HCL, HF, SO2), Oxides of Nitrogen (Nox), heavy metals (Hg, Cd, Pb, etc) and particulates/dust. There are a number of pollution control technologies, commonly known as end of pipe treatment technologies which are currently being utilised. Depending on the pollutant(s) of concern, the abatement systems can either be used alone or in combination.
Acid Gas Removal
Scrubbers
Scrubbers are one of the primary devices that control gaseous emissions, especially acid gases, including Hydrochloric acid, Nitric acid, Hydrofluoric acid and other pollutants including, Mercury, Lead and other heavy metals. The efficiency of removal will depend on the specific equipment, the chemical composition of the waste, the design of the plant, the chemistry of reagents, and the ability of engineers to optimize these conditions, which may conflict for different pollutants. Basic scrubbers remove sulfur dioxide, forming gypsum by reaction with lime.
Dry Systems
Acid gas are controlled by reacting them with an alkali basically calcium hydroxide Ca(OH)2. This can be done wither by dry injection or dry scrubbing oprocess which invloves injecting the dry powdered lime into a reaction tower or directly into the flue gas downstream of the flue gas outlet.
Removal efficiency is temperature dependent and some processes reduce flue gas temperature to 140-180oC by injecting water into the flue gas. This allows the volatile heavy metals to condensate thereby enabling it to be easily removed, along with other pollutants either through a bag filter or an Electrastatic Precipitator (ESP). Temperature reduction also has the added advantage of reducing damage to abatement systems such as bag filters.
Removal efficiency for HCL is over 99% and SO2 removal is between 40-95% depending on temperature. Lowering the temperature to 40-60 degrees C further enhances removal to over 95%. Temperature is also a factor in the removal of heavy metals such as lead and cadmium with a removal efficiency over 99%. At 140oC and 110oC, mercury and dioxins respectively are efficiently removed. Mercury can be further removed by injecting activated carbon, fly ash or sodium sulphide with the lime (Ca(OH)2.
Dry systems require fewer number of plants and therefore lower capital cost. However operating cost can be high due to the use of large quantities of absorbents which may need to be disposed of. Relatively large quantities of residues are produced:
Ca(OH)2 + SO2 à CaSO3 + H2O,
Ca(OH)2 + 2HCL à CaCl2 + 2H20,
Ca(OH)2 + 2HF à CaF2 + 2H20
Typically 50 kg residue per tonne of MSW incinerated.
Semi-dry Systems
The semi-dry system involves reacting liquid alkali (CaO) with flue gases for the purpose of treatment to remove acid gases and other pollutants. Liquid is added to alkali in correct quantity so that complete evaporation occurs during reaction. For this reason, the process is known as semi-dry system.
The milk-like liquid alkali is atomized to enable as much contact and reaction as possible with the gasses. This results in mass transfer of the acid gases from the gas stream into the liquid droplets. The alkali neutralizes the acid while at the same time, evaporation of the water droplets occur. Depending on the required standard, the solid reagent can be recirculated (in part with fresh alkali) within the scrubbing process.
Most of the droplets would have evaporated leaving a dry powder consisting of calcium chloride, sulphate and sulphide, plus unreacted calcium hydroxide (approximately 20%). Most of this will be entrained in the flue gas and can be removed in a bag filter or an Electrostatic Precipitator. As usual, removal efficiency is temperature dependent with acid gases including HC, HF, SO2 reported to have been removed at a rate of 90%. Removal of heavy metals including mercury can be enhanced by introducing powdered activated carbon into the scrubbing tower. At temperatures ranging from 110% - 200oC, dioxins can be removed at an efficiency over 99%. However care should be taken to avoid condensation.
Wet Systems
This is often a two staged scrubbing process and involves the use of liquid absorbent to treat flue gases within a reaction tower. Typically HCL and SO2 are removed with high efficiency within the process. The gas stream is firstly passed through and Electrostatic Precipitator (ESP) to remove the dust/particulates and fly-ash. As mentioned in previous chapters, the removal efficiency is temperature dependent so the gas is cooled to 130oC-150oC in a heat exchanger before passing through the first scrubber. Here the gas flows in a counter current direction to the scrubbing liquid where some mass exchange occurs to remove HCL and the remaining dust and heavy metals.
As the flue gas enters the second scrubber, it is brought into contact with Calcium or Sodium Hydroxide solution which absorbs the remaining acid gases. At this stage, the flue gas would be saturated and is passed through a mist
eliminator before fed into a heat exchanger where it is heated to about 100oC before it is released via the stack.
Although less solid residue is produced, the wet system produces more liquid effluent (typically 450 kg per tonne MSW) which will require treatment before discharge.
NOx Removal
NOx is a collective term for Nitric Oxide (NO) and Nitrogen Dioxide (NO2) which are by-products of the combustion process. Nitorgen is present in atmospheric air as well as in fuel. Oxygen is needed for combustion, which is derived from air. The formation of thermal NOx is greatly dependent on the combustion temperature and the partial pressure of oxygen and the formation starts at temperature above 1200oC and increases rapidly with increasing temperature, O2 concentration and the residence time in the high temperature zone. Where NOx is generated from air, it is referred to as Thermal NOx. The formation of Fuel NOx is dependent on the the nitrogen content of the fuel and where NOx is generated from fuel, it is referred to as Fuel NOx.
Secondary method involves the use of end-of-pipe treatment to control NOx formed in the combustion zone. There are two main treatment technologies: Selective Non Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR). Both methods utilise Ammonia (NH3) or Urea to convert or reduce NOx to Nitrogen (N2) and water (H2O) but by different methods.
2NO2 + 4NH3 + O2 --> 3N2 + 6H2O
SNCR achieve NOx reduction by introducing ammonia or urea into the boiler whereas SCR achieve this by using a catalyst in a catalytic converter after the flue gas cleaning plant to achieve better results. Secondary methods can achieve NOx removal efficiency of between 50-90% (OU 2001).
Dioxin Removal
Dioxin is a generic term for Polychlorinated dibonzo-para-dioxin (PCDDs) and Polychlorinated dibonzofuan (PCDFs). Dioxins are a class of chemical contaminants
that are formed during combustion processes such as waste incineration. The most toxic chemical in the class is 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD). The highest environmental concentrations of dioxin are usually found in soil and sediment, with much lower levels found in air and water. Humans are primarily exposed to dioxins by eating food contaminated by these chemicals (National Institute of Environmental Health Sciences, 2011). Like Nox, Dioxins are controlled by both primary and secondary measures. Primary measures are mainly used to prevent or control the formation of dioxins and these include controlling feed rate, and temperature and mixing within the incinerator as well as, reducing combustion air (excess oxygen).
Good combustion minimises carry-over of organic material which are responsible for the formation of dioxins. Temperature control by avoiding critical temperature (450oC down to 200oC) following combustion is crucial. Frequently cleaning of boiler tubes also has its benefits. The use of activated carbon and catalytic oxidation are common examples of secondary control measures. Activated carbon is injected into the flue gas which causes dioxins to be adsorbed into the carbon. Catalytic oxidation destroys dioxins by concerting it into CO2, HCL and Water. This reaction is often achieved by over-sizing the SCR reaction used to reduce Nox emissions.
Management of Solid Residues (APCs)
APC residues, mainly fly-ash consist of a fine dusty material mainly in the size range 5-75 µg. They The elemental composition is shown here. Unlike bottom ash (IBA), most flyash (from incineration/coincineration) are classed as hazardous waste under the European waste catalogue (10 01 16). Due to their hazardous nature and leaching properties, APC residues would fail to meet the waste acceptance criteria leaching limits for chloride, lead and mercury. As a result, APC residues are pretreated before sent off to hazardous waste landfills for disposal.
Pretreatment Option for APC residues
Solidification and Stabilization (S/S): This is typically a process that involves the mixing of a waste with a binder to reduce the contaminants leachability by both physical and chemical means and convert the hazardous waste into an environmentally acceptable waste form for land disposal or reuse/recovery. It should be noted that chlorides are not entrapped in these process, so a preliminary washing stage may be required.
Thermal Treatment: This involves high temperature (1300oC – 1500oC) which results I the formation of an inert glassy material. In vitrification process, the residues are mixed with glass-forming material and heated. The final product is a single phase amorphous material. The process can also be carried out without glass materials which forms s a multi-phase material. Both materials can be sent to the landfill as back-fill material.