prop · GM03141 · 1999-01-01 00:00:00 — - Improved pretreatment of aluminium scrap shredders by using swim-sink separation techniques and grading through whirling stream deposition. a/ Remaining emission compared to unreduced mode. Measurements at sinter plants in the iron and steel industry have generally shown PCDD/F emissions in the range of 0.4 to 4 ng TE/m3. A single measurement at one plant without any control measures showed an emission concentration of 43 ng TE/m3 . Halogenated compounds may result in the formation of PCDD/F if they enter sinter plants in the feed materials (coke breeze, salt content in the ore) and in added recycled material (e.g. millscale, blast furnace top gas dust, filter dusts and sludges from waste water treatment). However, similarly to waste incineration, there is no clear link between the chlorine content of the feed materials and emissions of PCDD/F. An appropriate measure may be the avoidance of contaminated residual material and de-oiling or degreasing of millscale prior to its introduction into the sinter plant. The most effective PCDD/F emission reduction can be achieved using a combination of different secondary measures, as follows: (a) Recirculating waste gas significantly reduces PCDD/F emissions. Furthermore, the waste gas flow is reduced significantly, thereby reducing the cost of installing any additional end-of-pipe control systems; (b) Installing fabric filters (in combination with electrostatic precipitators in some cases) or electrostatic precipitators with the injection of activated carbon/open- hearth coal/limestone mixtures into the waste gas; (c) Scrubbing methods have been developed which include pre-quenching of the waste gas, leaching by high-performance scrubbing and separation by drip deposition. Emissions of 0.2 to 0.4 ng TE/m3 can be achieved. adsorption agents like lignite coal cokes/coal slack, an emission concentration of 0.1 ng TE/m3 can be reached. Primary and secondary production of copper Existing plants for the primary and secondary production of copper can achieve a PCDD/F emission level of a few picograms to 2 ng TE/m3 after flue-gas cleaning. A single copper shaft furnace emitted up to 29 ng TE/m3 PCDD/F before optimization of the Generally, there is a wide range of PCDD/F emission values from these plants because of the large differences in raw materials used in differing aggregates and processes. Generally, the following measures are suitable for reducing PCDD/F emissions: (a) Pre-sorting scrap; (b) Pretreating scrap, for example stripping of plastic or PVC coatings, pretreating cable scrap using only cold/mechanical methods; (c) Quenching hot waste gases (providing utilization of heat), to reduce time in the critical region of temperature in the waste gas system; (d) Using oxygen or oxygen-enriched air in firing, or oxygen injection in the shaft kiln (providing complete combustion and minimization of waste gas volume); (e) Adsorption in a fixed bed reactor or fluidized jet stream reactor with activated charcoal or open-hearth coal dust; and (f) Catalytic oxidation. PCDD/F emissions from converter steelworks for steel production and from hot blast cupola furnaces, electric furnaces and electric arc furnaces for the melting of cast iron are significantly lower than 0.1 ng TE/m3 . Cold-air furnaces and tube furnaces (melting of cast iron) have higher PCDD/F emissions. Electric arc furnaces used in secondary steel production can achieve an emission concentration value of 0.1 ng TE/m3 if the following measures are used: (a) Separate collection of emissions from loading and discharging; and (b) Use of a fabric filter or an electrostatic precipitator in combination with coke injection. The feedstock to electric arc furnaces often contains oils, emulsions or greases. General primary measures for PCDD/F reduction can be sorting, de-oiling and de-coating of scraps, which may contain plastics, rubber, paints, pigments and vulcanizing additives. Smelting plants in the secondary aluminium industry PCDD/F emissions from smelting plants in the secondary aluminium industry are in the range of approximately 0.1 to 14 ng TE/m3 . These levels depend on the type of smelting aggregates, materials used and waste gas purification techniques employed. In summary, single- and multi-stage fabric filters with the addition of limestone/activated carbon/open-hearth coal in front of the filter meet the emission concentration of 0.1 ng TE/m3 , with reduction efficiencies of . The following measures can also be considered: (a) Minimizing and separately removing and purifying differently contaminated waste gas flows; (b) Avoiding waste gas particle deposition; (c) Rapidly passing the critical temperature range; (d) Improving the pre-sorting of scrap aluminium from shredders by using swim- sink separation techniques and grading through whirling stream deposition; and (e) Improving the pre-cleaning of scrap aluminium by swarf decoating and swarf drying. Options (d) and (e) are important because it is unlikely that modern fluxless smelting techniques (which avoid halide salt fluxes) will be able to handle the low-grade scrap that can be used in kilns. Discussions are continuing under the Convention for the Protection of the of the North-east regarding the revision of an earlier recommendation to phase out the use of hexachloroethane in the aluminium industry. The melt can be treated using state-of-the-art technology, for example with nitrogen/chlorine mixtures in the ratio of between 9:1 and 8:2, gas injection equipment for fine dispersion and nitrogen pre- and post-flushing and vacuum degreasing. For nitrogen/chlorine mixtures, a PCDD/F emission concentration of about 0.03 ng TE/m3 was measured (as compared to values of > 1 ng TE/m3 for treatment with chlorine only). Chlorine is required for the removal of magnesium and other undesired components. C. Combustion of fossil fuels in utility and industrial boilers In the combustion of fossil fuels in utility and industrial boilers (>50 MW thermal capacity), improved energy efficiency and energy conservation will result in a decline in the emissions of all pollutants because of reduced fuel requirements. This will also result in a reduction in PCDD/F emissions. It would not be cost-effective to remove chlorine from coal or oil, but in any case the trend towards gas-fired stations will help to reduce PCDD/F emissions from this sector. It should be noted that PCDD/F emissions could increase significantly if waste material (sewage sludge, waste oil, rubber wastes, etc.) is added to the fuel. The combustion of wastes for energy supply should be undertaken only in installations using waste gas purification systems with highly efficient PCDD/F reduction (described in section A above). The application of techniques to reduce emissions of nitrogen oxides, sulphur dioxide and particulates from the flue gas can also remove PCDD/F emissions. When using these techniques, PCDD/F removal efficiencies will vary from plant to plant. Research is ongoing to develop PCDD/F removal techniques, but until such techniques are available on an industrial scale, no best available technique is identified for the specific purpose of PCDD/F removal. The contribution of residential combustion appliances to total emissions of PCDD/F is less significant when approved fuels are properly used. In addition, large regional differences in emissions can occur due to the type and quality of fuel, geographical appliance density and usage. Domestic fireplaces have a worse burn-out rate for hydrocarbons in fuels and waste gases than large combustion installations. This is especially true if they use solid fuels such as wood and coal, with PCDD/F emission concentrations in the range of 0.1 to 0.7 ng TE/m3. Burning packing material added to solid fuels increases PCDD/F emissions. Even though it is prohibited in some countries, the burning of rubbish and packing material may occur in private households. to increasing disposal charges, it must be recognized that household waste materials are being burned in domestic firing installations. The use of wood with the addition of waste packing material can lead to an increase in PCDD/F emissions from 0.06 ng TE/m3 (exclusively wood) to 8 ng TE/m3 (relative to O2 by volume). These results have been confirmed by investigations in several countries in which up to 114 ng TE/m3 (with respect to oxygen by volume) was measured in waste gases from residential combustion appliances burning waste materials. The emissions from residential combustion appliances can be reduced by restricting the input materials to good-quality fuel and avoiding the burning of waste, halogenated plastics and other materials. Public information programmes for the purchasers/operators of residential combustion appliances can be effective in achieving this goal. E. Firing installations for wood (<50 MW capacity) Measurement results for wood-firing installations indicate that PCDD/F emissions above 0.1 ng TE/m3 occur in waste gases especially during unfavourable burn-out conditions and/or when the substances burned have a higher content of chlorinated compounds than normal untreated wood. An indication of poor firing is the total carbon concentration in the waste gas. Correlations have been found between CO emissions, burn- out quality and PCDD/F emissions. Table 3 summarizes some emission concentrations and factors for wood-firing installations. Table 3: Quantity-related emission concetrations and factors for wood-firing installations Fuel Emission concentration (ng TE/m3) Emission factor (ng TE/kg) Emission factor (ng/GJ) Natural wood (beech tree) 0.02 - 0.10 0.23 - 1.3 12 - 70 Natural wood chips from 0.07 - 0.21 0.79 - 2.6 43 - 140 Chipboard 0.02 - 0.08 0.29 - 0.9 16 - 50 waste wood 2.7 - 14.4 26 - 173 1400 - 9400 Residential waste 114 3230 The combustion of urban waste wood (demolition wood) in moving grates leads to relatively high PCDD/F emissions, compared to non-waste wood sources. A primary measure for emission reduction is to avoid the use of treated waste wood in wood-firing installations. Combustion of treated wood should be undertaken only in installations with the appropriate flue-gas cleaning to minimize PCDD/F emissions. V. CONTROL TECHNIQUES FOR THE REDUCTION OF PAH EMISSIONS During coke production, PAHs are released into the ambient air mainly: (a) When the oven is charged through the charging holes; (b) By leakages from the oven door, the ascension pipes and the charging hole lids; and (c) During coke pushing and coke cooling. Benzo(a)pyrene (BaP) concentration varies substantially between the individual sources in a coke battery. The highest BaP concentrations are found on the top of the battery and in the immediate vicinity of the doors. PAH from coke production can be reduced by technically improving existing integrated iron and steel plants. This might entail the closure and replacement of old coke and the general reduction in coke production, for instance by injecting high-value coal in steel production. A PAH reduction strategy for coke should include the following technical measures: (a) Charging the coke ovens: - Particulate matter emission reduction when charging the coal from the bunker into the charging cars; - Closed systems for coal transfer when coal pre-heating is used; - Extraction of filling gases and subsequent treatment, either by passing the gases into the adjacent oven or by passing via a collecting main to an incinerator and a subsequent dedusting device. In some cases the extracted filling gases may be burned on the charging cars, but the environmental performance and safety of these charging-car-based systems is less satisfactory. Sufficient suction should be generated by steam or water injection in the ascension pipes; (b) Emissions at charging hole lids during coking operation should be avoided by: - Using charging hole lids with highly efficient sealing; - Luting the charging hole lids with clay (or equally effective material) after each charging operation; - Cleaning the charging hole lids and frames before closing the charging hole; - oven ceilings free from coal residuals; (c) Ascension pipe lids should be equipped with water seals to avoid gas and tar emissions, and the proper operation of the seals should be maintained by regular cleaning; (d) Coke oven machinery for operating the coke oven doors should be equipped with systems for cleaning the seals'''' surfaces on the oven door frames and oven doors; (e) Coke oven doors: - Highly effective seals should be used (e.g. spring-loaded membrane doors); - Seals on the oven doors and door frames should be cleaned thoroughly at every handling operation; - Doors should be designed in a that allows the installation of particulate matter extraction systems with connection to a dedusting device (via a collecting main) during pushing operations; (f) The coke transfer should be equipped with an integrated hood, stationary duct and stationary gas cleaning system (preferably a fabric filter); (g) Low-emission procedures should be applied for coke cooling, e.g. dry coke cooling. The replacement of a wet quenching process by dry coke cooling should be preferred, so long as the generation of waste water is avoided by using a closed circulation system. The dusts generated when dry quenched coke is handled should be reduced. A coke-making process referred to as "non-recovery coke-making" emits significantly less PAH than the more conventional by-product recovery process. This is because the ovens operate under negative pressure, thereby eliminating leaks to the atmosphere from the coke oven doors. During coking, the raw coke oven gas is removed from the ovens by a natural draught, which maintains a negative pressure in the ovens. These ovens are not designed to recover the chemical by-products from raw coke oven gas. Instead, the offgases from the coking process (including PAH) are burned efficiently at high temperatures and with long times. The waste heat from this incineration is used to provide the energy for coking, and excess heat may be used to generate steam. The economics of this type of coking operation may require a cogeneration unit to produce electricity from the excess steam. Currently there is only one non-recovery coke plant operating in the , and one is in operation in . The process is basically a horizontal sole-flue non-recovery coke oven with an incineration chamber adjoining two ovens. The process provides for alternate charging and coking schedules between the two ovens. Thus, one oven is always providing the incineration chamber with coke gases. The coke gas combustion in the incineration chamber provides the necessary heat source. The incineration chamber design provides the necessary dwell time (approximately 1 second) and high temperatures (minimum of 900 C). An effective monitoring programme for leakages from coke oven door seals, ascension pipes and charging hole lids should be operated. This implies the monitoring and recording of leakages and immediate repair or maintenance. A significant reduction of diffuse emissions can thus be achieved. Retrofitting existing coke to facilitate condensation of flue gases from all sources (with heat recovery) results in a PAH reduction of to more than in air (without regard to waste water treatment). Investment costs can be amortized in five years, taking into account recovered energy, heated water, gas for synthesis and saved cooling water. Increasing coke oven volumes results in a decrease in the total number of ovens, oven door openings (amount of pushed ovens per day), number of seals in a coke battery and consequently PAH emissions. Productivity increases in the same way by decreasing operating and personnel costs. Dry coke cooling systems require a higher investment cost than wet methods. Higher operating costs can be compensated for by heat recovery in a process of pre-heating the coke. The energy efficiency of a combined dry coke cooling/coal pre-heating system rises from 38 to . Coal pre-heating boosts productivity by . This can be raised to because the coking process is more homogeneous. All tanks and installations for the storage and treatment of coal tar and coal tar products must be equipped with an efficient vapour recovery return and/or vapour destruction system. The operating costs of vapour destruction systems can be reduced in an autothermal after-burning mode if the concentration of the carbon compounds in the waste is high enough. Table 4 summarizes PAH emission reduction measures in coke production plants. Table 4: PAH emission control for coke production Management options Emission level (%) a/ Estimated costs Management risks Retrofitting of old plants with condensation of emitted flue gases from all sources includes the following measures: Total < 10 (without waste water) High Emissions to waste water by wet quenching are very high. This method should be applied only if the waste is reused in a closed cycle. - Evacuation and after- burning of the filling gases during charging of ovens or passing the gases into the adjacent oven as far as possible; 5 (Amortization of investment costs, taking into account energy recovery, heated water, gas for synthesis and saved cooling water, may be 5 years.) - Emissions at charging hole lids should be avoided as far as possible, e.g. by special hole lid construction and highly effective sealing methods. Coke oven doors with highly effective sealings should be used. Cleaning of charging hole lids and frames before closing the charging hole; < 5 - gases from pushing operations should be collected and fed to a dedusting device; < 5 Higher investment costs than for wet cooling (but lower costs by preheating of coke and use of waste heat.) - Quenching during coke cooling by wet methods only if properly applied without waste water. Low emission procedures for coke cooling, e.g. dry coke cooling. No emissions into water Higher investment costs than for wet cooling (but lower costs by preheating of coke and use of waste heat.) Increasing the use of high-volume ovens to lower the humber of openings and the surface of sealing areas. Considerable Investment about higher than conventional plants In most cases total retrofitting or the installation of a new cokery is needed. a/ Remaining emission compared to unreduced mode. B. Anode production PAH emissions from anode production have to be dealt with in a similar fashion as those from coke production. The following secondary measures for emission reduction of PAH-contaminated dust are used: (a) Electrostatic tar precipitation; (b) Combination of a conventional electrostatic tar filter with a wet electrostatic filter as a more efficient technical measure; (c) Thermal after-burning of the waste gases; and (d) Dry scrubbing with limestone/coke or aluminum oxide (Al2O3). The operating costs in thermal after-burning can be reduced in an autothermal after- burning mode if the concentration of carbon compounds in the waste gas is high enough. Table 5 summarizes PAH emission control measures for anode production. Table 5: PAH emission control for anode production Management options Emission level (%) a/ Estimated costs Management risks Modernization of old plants by reducing diffuse emissions with the following measures: 3-10 High - Reduction of leakages; - Installation of flexible sealants at the oven doors; - Evacuation of filling gases and subsequent treatment, either by passing the gases into the adjacent oven or by passing the gases via a collecting main to an incinerator and a subsequent dedusting device on the ground; - Operating and coke oven cooling systems; and - Evacuation and purification of particulate emissions from coke. Established technologies for anode production in the : 45-50 Implemented in the in 1990. Scrubbing with limestone or cokes is effective for reducing PAH; with aluminium not know. - New kiln with dry scrubber (with limestone/cokes or with aluminium) - Effluent recycling in paste unit. - Electrostatic dust precipitation; and 2-5 Regular cleaning of tar is needed. - Thermal after-burning. 15 Lower operating costs in an autothermal mode. Operating in autothermal mode only if the concentration of PAH in the waste gas is high. a/ Remaining emission compared to unreduced mode. Aluminium is produced from aluminium oxide (Al2O3) by electrolysis in pots (cells) electrically connected in series. Pots are classified as prebake or Soederberg pots, according to the type of the anode. Prebake pots have anodes consisting of calcined (baked) carbon blocks, which are replaced after partial consumption. Soederberg anodes are baked in the cell, with a mixture of coke and coal tar pitch acting as a binder. high PAH emissions are released from the Soederberg process. Primary abatement measures include modernization of existing plants and optimization of the processes, which could reduce PAH emissions by . An emission level of B(a)P/tonne of Al could be reached. Replacing the existing Soederberg cells by prebaked ones would require major reconstruction of the existing process, but would nearly eliminate the PAH The capital costs of such replacements are very high. Table 6 summarizes PAH emission control measures for aluminium production. Table 6: PAH emission control for aluminium production using the Soederberg process Management options Emission level (%) a/ Estimated costs Management risks Replacement of Soederberg electrodes by: - Prebaked electrodes (avoidance of pitch binders); - Inert anodes. 3-30 Higher costs for electrodes about US$ 800 million Soederberg electrodes are cheaper than prebaked ones, because no anode baking plant is needed. Research is in progress, but expectations are low. Efficient operation and monitoring of emission are essential parts of emission control. Poor performance could cause significant diffuse emissions. Closed prebake systems with point feeding of alumina and efficient process control, hoods covering the entire pot and allowing efficient collection of air pollutants. Soederberg pot with vertical contact bolts and waste gas collection systems. > 10 Retrofit of Soederberg technology by encapsulation and modified feeding point: US$ 50,000 - 10,000 per furnace Diffuse emissions occur during feeding, crust breaking and lifting of iron contact bolts to a higher position Sumitomo technology (anode briquettes for VSS process). - Electrostatic tar filters; 2-5 Low High rate of sparking and electrical arcing; - Combination of conventional electrostatic tar filters with electrostatic wet gas cleaning; > 1 Medium gas-cleaning generates waste water. - Thermal after-burning. Pitch use with higher melting point (HSS + VSS) High Medium Low - medium Use of dry scrubbing in existing HSS + VSS plants. a/ Remaining emission compared to unreduced mode. PAH emissions from residential combustion can be detected from stoves or open fireplaces especially when wood or coal is used. Households could be a significant source of PAH emissions. This is the result of the use of fireplaces and small firing installations burning solid fuels in households. In some countries the usual fuel for stoves is coal. Coal- burning stoves emit less PAH than wood-burning ones, because of their higher combustion temperatures and more consistent fuel quality. Furthermore, combustion systems with optimized operation characteristics (e.g. burning rate) effectively control PAH emissions from residential combustion. Optimized combustion conditions include optimized combustion chamber design and optimized supply of air. There are several techniques which optimize combustion conditions and reduce emissions. There is a significant difference in emissions between different techniques. A modern wood-fired boiler with a water accumulation tank, representing BAT, reduces the emission by more than compared to an outdated boiler without a water accumulation tank.