40. Coking plants, coal-to-gas plants, gas production and distribution
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2. Environmental impacts and protective measures
2.1 Environmental impacts
2.2 Protective measures
3. Notes on the analysis and evaluation of environmental impacts
3.2 Summary of limit values and standards
3.3 Evaluation of environmental impacts
4. Interaction with other sectors
5. Summary assessment of environmental relevance
This environmental brief covers various coal upgrading technologies, incl. coking and low-temperature carbonization as processes yielding the target products coke and gas plus tar products and diverse raw chemicals.
The relevant facilities can be installed and operated either separately or in conjunction with neighbouring industrial technologies; cf. environmental briefs Planning of Locations for Trade and Industry, Spatial and Regional Planning.
Interconnected operations can be characterized by proximity to either collieries or iron works.
While in the former case the working product coal can be conveyed directly to the upgrading facility located only a short distance away, close proximity to a metallurgical plant avoids the necessity of transporting the product coke over long distances and enables direct supply of the gas to the consumer by way of a low-pressure network; blast furnace gas from the metallurgical plant can serve as low-sulfur fuel gas for, say, a coking plant.
If the coal upgrading facility is instead located separately and independently, substantial infrastructural measures will be required for conveying, loading, unloading and storing the working materials, process materials and products; cf. environmental briefs Transport and Traffic Planning, Railways and Railway Operation, Inland Ports.
In addition, the generated gas has to be compressed and purified to piped-gas quality before it can be supplied to the consumers.
The low-temperature-carbonization and coking processes, as applied to coal in the sense of this brief, are based on heating in exclusion of air in appropriate reactors.
Depending on the temperature at which the process takes place, distinction is made between:
- low-temperature carbonization (450 - 700°C)
- medium-temperature coking (700 - 900°C)
- high-temperature coking (>900°C).
While the above processes do not differ at all in principle, the different temperatures yield different products and process conditions and call for the use of different reactor systems.
a) Low-temperature carbonization processes
Low-temperature carbonization processes, as applied primarily to lignite (brown coal), take place in fixed-bed reactors, fluidized-bed reactors or entrained-bed reactors.
The heat supply derives from:
the use of hot coke as a heat transfer medium or
the direct supply of heat to the working material via hot cycle gas (circulation gas).
The gas produced by low-temperature carbonization is cooled (condensed), detarred, compressed and purified prior to delivery. Residual coke is cooled by means of either wet quenching or cold gas before being supplied to the users.
Low-temperature carbonization processes are used primarily for obtaining tar products, diverse raw chemicals and low-temperature-carbonization gas. The incidental coke is of no particularly high quality and therefore not used for metallurgical purposes. Consequently, other uses must be found for such coke, preferably uses that do not require high crush resistance.
b) Coking processes
Hard coal is coked in batteries of regenerative horizontal chamber ovens. Distinction is made between top charging and stamping operation, depending on the extent to which the coal is likely to produce high-quality, adequately stable blast-furnace coke.
Coke ovens are heated indirectly by fuel gas, the heat of which is transferred to the charge (feed coal) via heating walls. The fuel gas can consist of partially purified coke oven gas, blast furnace gas or a mixture of combustible gases. Even if the entire operation is heated exclusively with coke oven gas, the plant will still yield surplus quantities of gas with calorific values ranging from roughly 16 000 to 20 000 kJ/m³. Following appropriate purification, that gas can be supplied to diverse consumers.
Oven service equipment is needed to fill coal into the ovens (charging cars), transfer the coke to quenching stations (quenching cars) and convey the hot coke to the wet quenching or dry cooling plant.
Coke oven gas is produced above the charge at coking temperatures of 750 - 900°C. Through riser pipes, the gas passes into a so-called collecting main, where it is sprayed with recirculating water to induce cooling and partial condensation, thus precipitating most of its crude tar content.
The next stage of treatment consists of additional cooling to approximately 25°C, followed by final tar removal in electrostatic precipitators, and the primarily absorptive extraction of such constituents as H2S, NH3, HCN, CO2, benzene and naphthalene.
Those ingredients are further processed according to various techniques to obtain:
ammonium sulfate (after transformation of H2S into sulfuric acid)
Claus sulfur (with simultaneous cracking of ammonia)
crude benzene and
If the surplus coke oven gas cannot be injected into an LP (low-pressure) network, it is put through a gas compression stage that includes additional purification to remove H2S and raw benzene/naphthalene and to lower its dew point.
Wastewater deriving from condensation of the gas and from the H2S/NH3 scrubbing stage is treated in multiple stages including distillation in so-called strippers and dephenolating processes (extraction, biological elimination).
Modern coking plants can handle between 6 000 and 10 000 tons of feed coal a day for a coke output of 4 500 - 7 500 t/d.
The attendant gas production amounts to between 80 000 and 150 000 m³/h, and process wastewater accumulates at the rate of 80 - 150 m³/h.
c) Classification of process options
From the standpoint of environmental protection, carbonization and coking are roughly equivalent.
With regard to production capacities and technological applications, however, coking takes priority over carbonization, as evidenced by the fact that most laws, regulations and guidelines pertaining to the control of emissions refer mainly to coking processes. Nonetheless, carbonizing facilities should be dealt with under the same premises.
2. Environmental impacts and protective measures
2.1 Environmental impacts
The erection and operation of coking and/or coal carbonizing plants at locations previously not used for industrial purposes alters the landscape and consumes land to an extent that depends on the size of plant.
In addition to ascertaining the potential effects of emissive pollution, it must also be determined to which extent the requisite extraction of water from given resources would interfere with existing ecosystems, since make-up water is required at various points of both operations. The required volume of water can range from 200 to 500 m³/h; cf. environmental briefs Water Framework Planning, Water Supply.
Particularly in connection with coke oven batteries, emissions must be anticipated both from certain operating points (e.g., exhaust stacks) as well as from diffuse sources such as leaky shutoff valves and cracks in the masonry of coke ovens.
The following emissions are deemed particularly relevant:
a) Air pollutants
- suspended solids (coal and coke dust)
- gaseous and vaporous emissions, e.g.:
· sulfur dioxide (SO2)
· hydrogen sulfide (H2S)
· oxides of nitrogen (NOx)
· carbon monoxide (CO)
· benzene, toluene, xylene (BTX)
· polycyclic aromatic hydrocarbons (PAH)
· benzo(a)pyrene (BaP)
b) Wastewater pollutants
- various nitrogen compounds
- chemical and biological oxygen demand
- polycyclic aromatic hydrocarbons
- sum of all toxic substances classified as such on the basis of, say, toxicity toward fish (fish test).
c) Noise emissions
Coking plants have numerous noise emitters at all points of the operation. Each and every drive unit, for example, constitutes a source of noise.
The equipment used for mixing, crushing and screening coal and coke and for compressing gas is particularly noisy and therefore requires comprehensive noise-control measures. Otherwise, some emitters are liable to develop noise intensity levels significantly above 85 dB(A).
In order to preclude noise-induced detriment to human health, both the sound sources themselves and the general vicinity of such equipment are subject to certain emission and immission limits.
d) Soil and groundwater
The handling and storage of coking products, crude tar, crude benzene, sulfuric acid and various purchased chemical additives pose a hazard potential for soil and groundwater.
Environmental consequences result from emissions, the effects of which in the vicinity of the installations can be damaging both to human health and to nature and which are monitored by way of ground-level pollutant concentrations. Also, near-source pollution occurring directly at and around the workplace demands careful attention. In the interest of occupational safety, so-called maximum working-site concentrations (MAK-values), so-called occupational exposure limits, and technical concentration guidelines (TRK-values) have been specified.
Improper handling of substances constituting a hazard to water can cause contamination of the soil and groundwater; contaminated effluent can be toxic (toxicity as a common parameter), cause bad taste (phenols) and/or excessive fertilization and, hence, consumption of oxygen (nitrogen, phosphorus).
Consideration must also be given to the fact that the construction and operation of technical facilities affects the living conditions of sundry groups. The relevant socioeconomic and sociocultural aspects therefore have to be analyzed.
2.2 Protective measures
Environmental protection and occupational safety at coking plants are governed by statutory provisions; in Germany, these include the TA-Luft (Technical Instructions on Air Quality Control), the Gefahrstoffverordnung (Hazardous Substances Ordinance), and the Wasserhaushaltsgesetz (Federal Water Act).
In accommodation of amended laws with in part substantially more stringent provisions, technical advances have emerged which now enable comprehensive protection of the environment.
One such advance is the development of large coke ovens, so that the operation of new coking plants of equal capacity now requires less frequent opening (around 80%) and has a far smaller sealing area to be cleaned (approximately 65% reduction). At least the following basic emission control measures are now being implemented on new facilities:
a) Coal handling, including unloading, storage, conditioning (mixing, grinding) and hauling
- erection of stationary sprinkling systems over coal storage areas to keep the coal moist, with provision for climatic boundary conditions;
- minimized dumping heights for mobile discharge and transfer stations;
- use of enclosed conveyors;
- installation of dedusting facilities on grinding and mixing equipment, plus dust silos.
b) Coke oven batteries
- collection of charging gas and transfer by two separate routes to the crude gas, e.g., via so-called mini risers leading to the adjacent oven and then through the "real" riser into the collecting main;
- charging gas aspiration by means of stationary or mobile systems equipped for aftercombustion and dedusting;
- mechanical cleaning of charging hole lids and frames, plus sealing after each charge;
- mechanical cleaning (aspiration) of the oven roof;
- mechanical cleaning of the risers (closures equipped with water seals);
- installation of mechanical cleaning devices for the doors and chamber frames of the coke oven machines;
- collection and treatment of emissions emerging upon removal of the doors;
- use of special-purpose door maintenance cars;
- installation of tight door sealing systems with gas relief channels to avoid excessive gas pressure in the vicinity of the sealing elements;
- installation of aspiration hoods for the door and frame cleaning devices;
- leakage gas aspiration for emissions resulting from leakages around oven doors, with injection of exhausted air into the batteries' combustion air supply;
- use of combustion gases with sulfur contents safely below 0.8 g/m³ to limit SO2 emissions;
- gradual air feed and internal/external flue gas recirculation to reduce NOx emissions in connection with heating of the ovens;
- use of highly heat-conductive stone linings for the heating walls;
- collection and purification of emissions from coke pushing.
c) Coke cooling
- use of dry coke cooling technology, comprising:
· moistening of the dry cooled coke to suppress dust evolution at the transfer points
· dedusting of the delivered coke
· dedusting of surplus gas by means of bag filter
· intergas generation to replace cooling cycle gas based on the use of low-sulfur gas;
- emission control measures for wet quenching, e.g., provision of baffle plates for the wet quenching towers.
d) Coke treatment
- installation of enclosed coke conveying equipment;
- encapsulation of coke screening plant;
- collection and removal of particulate emissions, e.g., at the feed bunkers, sieving lines, crushers, belt feeders, etc.;
- installation of remoisteners for dry cooled coke to limit dust generation at the coke transfer points.
e) Gas treatment and coal-constituent recovery systems
- use of effective sealing systems/elements for pumps, valves and flanges;
- forced ventilation of tanks, water lutes, etc. and injection of the ventilation gases into the crude gas suction line;
- use of Claus systems with injection of tail gas into the crude gas (tail gas recirculation);
- provision of waste gas filtration and additional catalyst installation for the H2SO4 systems to extensively preclude SO2/SO3 emissions.
f) Wastewater treatment; cf. environmental briefs Wastewater Disposal and Mechanical Engineering, Workshops, Shipyards
- use of upstream strippers employing alkaline additives (e.g., caustic soda solution) to reduce the so-called fixed ammonia compound burden of the coking plant's process water;
- installation of multiple-stage biological wastewater treatment facilities, including a nitrification/denitrification stage to enable elimination of nitrogen compounds in the coking plant's process effluent.
g) Conservation of soil and water
- separate drainage systems for surface runoff and process wastewater (from gas treatment and coal-constituent recovery systems);
- placement of all tanks and apparatus used in the handling or treatment of substances constituting a hazard to water in collecting tanks; installation of intercepting sewers for wastewater, e.g., by way of biological water treatment;
- installation of monitorable tank bottoms (on strip footing); use of overfill protection devices;
- use of suitable materials and external anti-corrosion measures to substantially improve the availability of plant components.
h) Noise control
- noise control at the source, e.g., encapsulation of machines, pumps, etc.;
- noise control for structures, e.g., solid construction, sandwich construction, use of vibration dampers, partitions;
- erection of acoustical barriers;
- individual examination of noise sources with a view to satisfying equipment-noise and neighbor's-rights requirements.
The measures listed under a) through h) are technically tried and proven and routinely implemented for new facilities.
Stated in proportion to the total investment for a new coking plant, the cost of environmental protection measures accounts for 30 - 40 %.
The operational reliability and availability of environmental protection provisions - like that of the entire coking plant - is highly dependent on the qualifications of the operating personnel. Consequently, appropriate training is required to put the personnel in a position to operate and use the equipment in an expert, competent manner.
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