The pollutants are present as particles (dust) and gases such as Hydrochloric acid (HCl), Hydrogen fluoride (HF), and Sulfur dioxide (SO2).
• Particulate pollutants: Fly ash, including the heavy metals of antimony (Sb), arsenic (As), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni), thallium (Tl), and vanadium (V).
• Gaseous pollutants: HCl mainly from the combustion of PVC; SO2 from THE combustion of sulfurous compounds; HF from THE combustion of fluorine compounds; and nitrogen oxides (NOx) from part of the nitrogen in the waste and N2 in the air.
Some harmful compounds such as mercury, dioxins and NOx can be fully removed only through advanced and costly chemical treatment technologies. Primary and secondary measures can help reduce the emission of pollutants. Primary measures - which are initiatives that actually hinder the formation of pollutants, especially NOx and organic compositions such as dioxins - must be applied as much as possible.
Primary measures comprise an efficient combustion process (such as long flue gas retention time at high temperatures with an appropriate oxygen content, intensive mixing and recirculation of flue gases, etc), pre-precipitation of ashes in the boiler, and short flue gas retention time at intermediate temperatures. The content of carbon monoxide (CO) and total organic carbon excluding CO (TOC) in the raw flue gas before inlet into the cleaning system is a good indicator of the efficiency of the combustion process.
Secondary measures consist of the air pollution control (APC) system that precipitate, adsorb, absorb, or transform the pollutants.The following table gives an indication of the technologies used in the APC system.
|Parameter||ABATEMENT TECHNOLOGY USED|
|Electrostatic precipitator (wet - dry)|
|Acid gases||Dry sorption|
|Semi dry sorption|
|Nitrogen oxides||Selective non catalytic reduction|
|Selective catalytic reduction|
The selection of the APC system depends primarily on actual emission limits or standards, if any, and the desired emission level. In this context, the different APC systems can be grouped into basic, medium, or advanced emission control.
Dry and semi-dry scrubbing processes are simple and hence cheap concerning the investment required, and are thus used in many plants all over the world. In most cases the adsorbent is either injected directly into the gas duct or into a spray dryer downstream of the boiler in dry form (dry process), or as a slurry (semi-dry process). In most cases, the scrubbing products are removed from the flue gas by a fabric filter.
As flue gases enter the dry scrubber, milk of lime is sprayed to cool them down and react with acids like HCl and SO2, while partial mercury capturing occurs. Liquids evaporate in the vertical scrubber and thus the reaction products appear as a dry dust in the flue gases. Larger particles fall to the bottom of the scrubber and are then removed. It is recommended that lime milk be used as a reactant (suspension of fine Ca(OH)2 in water).
Semi-dry scrubbers offer several advantages, such as:
− At least 50% removal of mercury and cadmium when combined with other materials such as activated carbon
− No wastewater production
These advantages balance the disadvantage of slightly larger quantities of fly ash.
In wet scrubbers SO2 is reduced by means of a reaction with a NaOH solution or a CaCO3 suspension. Due to excess oxygen in the flue gas, the reaction products are a sodium sulfate (Na2SO4) solution and a gypsum (CaSO4,2H2O) suspension respectively. If NaOH is applied, the scrubber system must have an additional water treatment plant in which the sulfate ions of the Na2SO4 solution are precipitated as gypsum by Ca ions. If CaCO3 is used, the gypsum is formed directly and may be removed as sludge either by settling or into a hydro cyclone and then dewatered.
The gas from the SO2 scrubber is reheated in the gas or gas heat exchanger and guided into a bag house filter. Before this, activated carbon or a mixture of lime and activated carbon is injected into the duct. Thus, the bags are powdered and when the gas penetrates them, Hg and dioxins are removed to concentrations below the limit values of the advanced control level. In addition, dust, HCl, HF, SO2 and other heavy metals are further reduced. None of these processes, however, has any effect on NOx
Powdered Activated Carbon (PAC) is used to remove heavy metals and organic compounds. The system includes a PAC silo, a feeder, an injection blower and an in-pipe reactor with an injection nozzle and injection valve.
PAC is transferred pneumatically from the silo to the exit pipe of the scrubber and injected in the entrained reactor between the semi-dry scrubber and the bag filter.
The silo consists of a cylinder and two feeding funnels (con-shaped) made of special steel. In order to allow inspection, two sliding doors are situated at the lower part. The feeder should continuously supply PAC to the injection system. The amount of PAC is determined according to the flue gas flow after the bag filters (i.e. through a dosing screw)
For effective system function, at least three injection blowers are installed (one spare). At the exit of the blowers, pressure gauges should be installed for measuring pressure. Pressure transmitters should be located at the main injection lines for monitoring air input pressure.
Mercury, cadmium, thallium and partially arsenic are removed by the activated carbon, while molecules of these metals adhere to the small dust particles captured at the bag filters. Other heavy metals also cling onto dust particles and are removed.
Activated carbon also “captures” Volatile Organic Compounds (VOC’s), Dioxins/Furans and PolyAromatic Hydrocarbons (PAH), while the PAC dust is removed in the bag filters. Residue from the bag filters is stored in the fly ash silo and transported outside the plant for proper management.
An electrostatic precipitator (ESP) is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates. The particles are given an electrical charge by forcing them to pass through a corona, a region in which gaseous ions flow. The electrical field that forces the charged particles to the walls comes from electrodes maintained at high voltage in the centre of the flow lane.
Once the particles are collected on the plates, they should be removed from the plates without re-entraining into the gas stream. This is usually accomplished by knocking them loose from the plates, allowing the collected layer of particles to slide down into a hopper from where they are evacuated. Some precipitators remove the particles by intermittent or continuous washing with water.
Bag filters ensure very efficient collection of dust, while at the same time they further absorb the acidic residues. For the attainment of this further absorption, it is important that a layer of dust be maintained on the fabric, as it efficiently collects particles with a diameter smaller than micrometres (µm). In this way heavy metals and dioxins, usually the smallest particles, are removed efficiently.
The automatic system of filter control and cleaning (activated by the detected pressure difference of filters) ensures the presence of a continuous dust layer on the bags/filters. The cleaning of filters should take place when these are in operation so as not to affect the process of cleaning.
Gases flow through bag filters from the outside of the bag towards the inside and dust is collected on the outside. Gases reach the bag filters through a pipe and are distributed via openings to various filter sections. A special pipe ensures smooth flow of the gases and in this way, removal is optimized and the lifetime of the filters is extended.
Fly ash is captured on the dust layer formed on the bags, and the filter itself. Clean gases flow up the upper-part openings to the compartment outlet damper and through the outlet pipe, by means of the Induced Draft (I.D.) fan, they reach the chimney and finally the atmosphere.
The dust layer increases bag filter efficiency, while remaining quantities of lime react with acidic compounds. Dioxins and the rest of VOCs are absorbed by the activated carbon, and the PAC particles are captured by the dust layer. When the pressure of the filter is increased to a certain point, this means that the dust layer has become too thick and the cleaning process should be activated.
The fly ash that stays at the outer surface of the filter bags is periodically removed by an air pulse, blown into the bag from the inner side. This cleaning process releases the particles, which fall into the discharge hopper.
An air tank is positioned under each filter station, equipped with plunger valves. Compressed air is blown at the lower inner part of the bags in a very short pulse, no more than 0.1 s. The entire process, which takes place while the bag filter is in operation, should require minimum amounts of energy.
The production of NOx may be prevented with the following measures:
• Continuous mixing of waste in the bunker to ensure a better fuel mixture
• Good mixing of secondary air through ideal positioning of the secondary air nozzles so as to create turbulence in the combustion chamber, which subsequently causes good mixing of combustion gases and smooth flow
• Use of low NOx burners
• Use of natural gas
As an end-of-pipe measure for NOx removal, the Selective Non Catalytic Reaction is proposed. In SNCR, ammonia (NH3) or urea (CO(NH2)2) is injected into the furnace to reduce NOx emissions. The NH3 reacts most effectively with NOx between 850 and 950 °C, although temperatures of up to 1,050 °C are effective when urea is used. If the temperature is too high, a competing oxidation reaction generates unwanted NOX. If the temperature is too low, or the residence time for the reaction between NH3 and NOx is insufficient, the efficiency of NOx reduction decreases, and the emission of residual ammonia can increase. This is known as NH3 slip. Some ammonia slip will always occur because of reaction chemistry. Additional NH3 slip can be caused by excess or poorly optimized reagent injection.
Enough nozzles are placed in order to ensure ammonia spraying through the radiation zone, thus ensure good contact and less residual ammonia. The number and position of the operating nozzles should be controlled depending on the furnace temperature, which should be measured with advanced devices such as infra-red pyrometers or auditory systems.
The volume of injected ammonia solution is determined by NOx concentrations, measured at the chimney. The ammonia solution will be diluted with water coming from the boiler (blow down water) before it turns into droplets with the use of compressed air.
An induced draught fan is needed to overcome the pressure drop across the flue gas treatment system and maintain a certain underpressure in the furnace. This is normally placed at the rear of the flue gas treatment train and is furnished with a silencer. The flue gas passes into the stack. The stack height is decisive for the dilution of the flue gases in the environment, depending on the emission control level applied and other factors. A minimum height is required to prevent the plume from reaching the ground or entering tall buildings. This minimum height will depend on local atmospheric conditions, the topography (flat or hilly), and the height of the buildings within a radius of at least 1.0 km. The stack height should be decided on the basis of computer modeling while as a rule of thumb, stack height should be twice the height of the tallest building within 1.0 km, or at least 70 meters high.