41. Thermal power stations
Contents - Previous - Next
2. Environmental impacts and protective measures
2.1.1 Dust control
2.1.3 De NOx
2.1.4 Greenhouse effect
2.1.5 Diffuse emissions
2.3 Soil and groundwater
2.4 Human health
3. Notes on the analysis and evaluation of environmental impacts
3.1 Immission limits for air
3.2 Emission limits for air
3.3 Monitoring of pollution levels
3.4 Emission limits for wastewater/effluent
4. Interaction with other sectors
5. Summary assessment of environmental relevance
Appendix A-1 - Flow diagram of energetically and environmentally relevant materials in a thermal power plant
Appendix A-2 - Schematic diagram of a thermal power plant equipped with various flue-gas cleaning systems
Appendix A-3 - Details of various desulfurization processes
Appendix A-4 - Immission limits as per the German TA-Luft
Appendix A-5 - German laws and regulations governing the limitation of emissions from thermal power plants
Appendix A-6 - Emission limits for air pollutants from large firing
Appendix A-6 - SO2 and nox Emissions
Appendix A-6 - Emission limits for new, large-scale, coal-fired power plants in various countries, plus pertinent EC and World Bank standards
Appendix A-7 - Minimum requirements as per German Federal water act (WHG*), section 7a
Thermal power stations are facilities in which the energy content of an energy carrier, i.e., a fuel, can be converted into either electricity or electricity and heat. The type of power plant employed depends on the source of energy and the type of energy being produced.
Possible energy sources include:
fossil fuels such as coal, petroleum products and natural gas
residual and waste materials such as domestic and industrial refuse and fuel made from recovered oil
Thermal power plants can be designed for different fuel sectors in the interest of greater fueling flexibility and/or higher efficiency - one example being a combination power plant with a gas turbine running on natural gas and an oil- or coal-fired steam generator feeding a steam turbine.
Renewable sources of energy such as wood and other forms of biomass are not dealt with here, as they are the subject of a separate environmental brief. Nuclear thermal power plants have also been omitted from this catalogue. The frame of reference concentrates extensively on fossil-fueled power plants, in particular types using coal and petroleum products, the present and near-future use of which is of eminent importance in most developing countries. With regard to hydropower, the reader is referred to the environmental brief Large-scale Hydraulic Engineering.
As far as the form of energy being generated is concerned, there are three main types of thermal power stations:
condensing power plants used exclusively for generating electricity
steam- or hot-water producing heating stations for domestic or industrial purposes
district heating power stations, or cogenerating plants, for the simultaneous generation of electricity and available heat.
It is important to note that, for economic reasons, process heat and heat for heating purposes should only be generated in close proximity to the users. For thermal outputs ranging from 50 to 100 MW, the distance between the power plant and the user should not exceed 2 to 5 km. Conversely, electricity can be economically transmitted over very substantial distances; cf. environmental brief Power Transmission and Distribution.
The unit power ratings of fossil-fueled thermal power plants range from a few hundred kW (diesel stations) to more than 1000 MW (oil- and coal-fired stations). In many countries, preference is given to unit ratings of 200 - 300 MWel with deference to power system stability. The better the boundary conditions, the higher the achievable capacities.
2. Environmental impacts and protective measures
The environmental consequences of any given plant are both plant- and site-dependent. Thermal power stations can impact the environment in different ways and at different locations. A typical thermal power plant is likely to comprise the following principal components:
facilities for preparing and storing working materials
facilities for burning fuel and generating steam
facilities for generating electricity and available heat
facilities for treating exhaust gases and solid and liquid residues
In Appendix A-1, a thermal power plant is reduced to a block diagram showing the most likely material inputs, outputs and environmentally relevant flows of material.
Table 1 surveys the potential emissions occurring at different stages of the generating process:
Table 1 - Potential emissions from thermal power plants
|Step of process|
|Type of emission||Fuel storage and
|Combustion and steam generation||Flue-gas cleaning||Power
|Cooling systems||Treatment of residue|
As the table indicates, thermal power stations can affect the media air, water and soil, as well as human beings, plants, animals and the landscape.
The disposal of residues, e.g., those associated with oil- and coal-fired facilities, is dealt with in section 2.3.
The main environmentally relevant effects of a thermal power plant derive from the combustion process and its particulate and gaseous emissions. As a rule of thumb, the environmental impacts of thermal power plants, i.e., pollution, spatial requirements and residues, tend to increase in severity for gas, light fuel oil, heavy fuel oil and coal, in that order.
Prior to examining the environmental consequences and possible protective measures for the various domains, let us begin with a few basic introductory remarks. The running text contains information essential to the subject environmental consequences and protective measures, while the relevant technical measures are detailed in the appendix.
In addressing the environmental consequences of a thermal power plant, distinction is drawn between emissions, i.e., the release of pollutants from various parts of the plant, the smokestack in particular, and immissions, i.e., the actual environmental effects of the pollutants, normally referred to ground level - as indicated by the terms "ground level concentration/pollution" and "ambient air quality concentration/standards". Emissions and immissions are interlinked by a number of factors, e.g., plant-specific technical parameters (emission volumes, outlet velocity, temperature), meteorological factors (weather category, wind speed) and range-specific data (distance between the emitter and the ground-level pollution point). Parameters belonging to the first and last categories, e.g., height of stack and distance from residential areas, are more or less freely selectable for new power plants, while the actuating variables for existing plants all belong to the first category. According to the laws of physics (conservation of matter), practically all noxious emissions with the notable exception of CO2, for example, eventually fall to earth. The height of the smokestack, the outlet velocity of the exhaust, and the prevailing wind velocity determine the size of area that will be affected. From a technical standpoint, it is relatively easy to reduce ground-level concentrations for a given area by increasing the height of the smokestack. Since, however, the specific emission volume does not change, but is simply distributed over a wider area, the extent to which such a measure would aggravate the environmental impact outside of the subject area would have to be clarified.
Measures aimed at reducing the environmental consequences of thermal power plants can be categorized as follows:
- alteration of boundary conditions
· incentives for the efficient utilization and conservation of energy, e.g., cost-covering power rates and taxes
· appropriate siting
- nontechnical protective measures
· regulations dictating the mandatory use of piped energy (district heating) in congested urban areas
· compensation models for the replacement of major emitters
- technical protective measures
· reduction of ground-level concentration, e.g., by extending the height of the smokestack
· emission-reducing measures
* pollution-control measures to prevent or reduce pollution by combustion modification, e.g., choice of an appropriate low-impact fuel such as natural gas (in place of coal), homogenization of fuel to avoid peak emissions, efficiency-enhancing measures, and NOx limitation by combustion engineering measures
* post-combustion measures, i.e., flue gas clean-up.
The order in which the protective measures are taken is subject to the principle of attaching priority to avoidance or reduction over subsequent rectification. First of all, pollution-control measures must be taken to preclude, or at least minimize, the occurrence of pollutants, before any further-reaching post-combustion technical remedial processes are initiated.
One very important relevant measure is to achieve higher efficiency, e.g., by erecting combination power plants or opting for the combined generation of heat and power (cogeneration) in efficient heating power stations with an accordingly low specific pollution level. High efficiency is also the most effective way to reduce CO2 emissions and, hence, their greenhouse effect. For additional means of reducing CO2 emissions, e.g., through the use of renewable sources of energy for power generation, cf. the brief Renewable Sources of Energy.
With regard to environmental consequences, distinction is made between direct consequences, e.g., the emission of pollutants, and indirect consequences such as the transfer of pollutants from the atmosphere to water via scrubbing processes (assuming that the liquid effluent is not subsequently processed) or the environmental impact of limestone mining and attendant road traffic, e.g., the transfer of limestone by truck from the mine to the power plant. Moreover, consequential problems can arise, e.g., the need to properly dispose of the gypsum resulting from flue-gas desulfurization processes (FGD).
The environmental consequences and potential protective measures applicable to the aforementioned areas are explained below.
The particulate and noxious gas emissions from thermal power plants primarily and directly pollute the air.
Eventually, the particulate emissions and, for the most part, the noxious gases and any atmospheric transformation products that may have formed (e.g., NO2 and nitrate from NO) fall to earth either by way of precipitation or dry deposition, thereby imposing a burden on the water and/or soil, with resultant potential damage to flora and fauna.
Depending on the fuel employed (type, composition, calorific value) and the type of combustion (e.g., dry or slag-tap firing), given amounts of pollutants (particulates, heavy metals, SOx, NOx, CO, CO2, HCl, HF, organic compounds) become entrained in the exhaust gases. Table 2 shows the potential concentration ranges of different emissions for various fuels in facilities devoid of flue-gas emission control measures.
Table 2 - Potential ranges of pollutant concentration levels in untreated gas Type of fuel
|Type of emission
||Natural gas||Light fuel oil||Heavy fuel oil||Hard coal||Lignite
|Sulfur oxides (SOx)
20 - 50
300 - 2000
1000 - 10000
500 - 800
500 - 18000
|Oxides of nitrogen
100 - 1000
200 - 1000
400 - 1200
600 - 2000
300 - 800
0 - 30
30 - 100
50 - 1000
3000 - 40000
3000 - 50000
Table 2 lists the noxious emissions in mg/m³STP, as prescribed by the applicable German rules and regulations [TA-Luft (Technical Instructions on Air Quality Control), and Großfeuerungsanlagenverordnung (Ordinance on Large-scale Firing Installations)]. SOx and NOx are postulated as SO2 and NO2. Some emissions are limited in terms of mass flow, e.g., in kg/h, or of minimum separation efficiency (cf. Appendix A-6). With a view to enabling conversion of the stated concentrations to other units such as ppm, g/GJ or lb. of pollutant per 106 BTU energy input, as commonly employed in the U.S.A., Appendix A-6 includes an appropriate conversion table.
The ranges quoted in table 2 for oxides of sulfur relate to differences in fuel-specific sulfur content, whereas many countries use large quantities of indigenous fuels like lignite with comparatively low calorific values and high sulfur contents. Such a combination naturally produces relatively high SOx concentrations in the (untreated) flue gas.
The lesser part of the NOx concentrations derives from the nitrogen content of the fuel (fuel NOx). The major share results from the oxidation of atmospheric nitrogen at combustion temperatures exceeding 1200°C (thermal NOx). Consequently, high combustion temperatures go hand in hand with relatively high NOx emission levels. Appropriate combustion engineering measures that are
relatively inexpensive for new plants can keep the emissions at the lower end of the respective range. However, care must be taken to ensure that a high quality of combustion is maintained. Otherwise, excessive combustion engineering measures aimed at reducing NOx emissions could result in a disproportionate increase in other emissions, e.g., carbon monoxide and combustible (unburned) hydrocarbons.
In general, CO2 emissions are mainly limited by controlling the burnout process such as to minimize the discharge of CO and the escape of combustible hydrocarbons. Unlike particulates, SO2, NOx and halogen compounds, CO and combustible hydrocarbons effectively defy retentive measures. Combustible hydrocarbons in particular include numerous chemical substances that can cause toxicological problems, e.g. benzpyrene.
Plants fueled with coal or heavy fuel oil also emit small amounts of hydrogen chloride and hydrofluoric acid (HCl and HF) ranging from 50 to 300 mg/m³STP. As a rule, the concentrations stay well below the SO2 levels and respond favorably to desulfurization processes, by which they are reduced even more than S2.
There are many combustion-stage and post-combustion alternatives for use in reducing air pollution from thermal power plants. Appendix A-2, for example, sketches out an integral set of DeNOx, particulate-control and desulfurization measures for the flue gas of a steam generating facility. The various measures are individually described in the following subsections.
2.1.1 Dust control
Dust control for power plants can be based on ordinary and multiple cyclone separators and electrostatic precipitators or fabric filters - with the order of mention corresponding to their respective separation efficiencies: from 60 % - 70 % for cyclone separators to >99 % for electrostatic precipitators and fabric filters. To be sure, the cost of the various options rises disproportionately for increasing separation efficiency. The separation efficiency of electrostatic filters depends on the number of consecutive fields. Like fabric filters, they can achieve extremely low residual emission levels, i.e., about 50 and 30 mg/m³STP, respectively. The drawback of cyclone separators is that they tend to eliminate coarse particles much more efficiently than respirable - and, hence, toxicologically critical - microparticles. Fabric filters are very good at separating out fine dust and its accumulated heavy metals. The capital outlay for flue-gas dust control depends
on such parameters as the type of fuel, the required separation efficiency and the technique employed. As a rule, the initial cost ranges from 20 to 70 DM/kWel, while the operating expenses amount to 0.1 - 0.6 DM/MWh. The high-ash fuels used in some countries makes flue-gas dust control a difficult problem - including
the proper disposal of the dust yield, either by recycling it in, say, building materials, or by depositing it in a landfill. Certain characteristics of the fly ash may require the use of additives to obtain a solidified product that is less susceptible to leaching, this with a view to preventing groundwater contamination.
SOx from combustion plants can be reduced either by combustion modification measures (use of low-sulfur fuel, direct desulfurization in the furnace, dry-additive method) or by post-combustion clean-up measures such as extracting the SOx from the flue gas.
The use of low-sulfur fuels is frequently precluded by economic considerations. In each case, the lowest total-cost concept must be ascertained. For example, while the use of a low-sulfur fuel may increase the cost of operation, it could save the cost of installing and operating desulfurization equipment, thus yielding lower overall costs for the power station. Of course, such considerations must also account for other criteria, e.g., using indigenous fuels in order to assure their safe supply.
Like solid fuels, sulfurous petroleum products are also amenable to pre- and post-combustion measures. The pollution-control measure of choice is to hydrate the sulfur by adding hydrogen in order to extract the product from the oil, e.g., as vacuum gas oil or a remanent of atmospheric or vacuum distillation. Such processes are only cost-efficient for large capacities and therefore only feasible for oil refinery applications. In a thermal power station, the appropriate measures for reducing SOx emissions are restricted to the use of a low-sulfur petroleum product, mixing different fuels and, primarily, flue-gas desulfurization according to the same principle as that employed in solid-fueled facilities (described below and in Appendix A-3).
For coal-fired power plants, particularly in response to the pronounced compositional variance observed in the indigenous coals of many countries, appropriate mixing and homogenization can have the positive effect of lowering the peak-value extremes that have to be accounted for in the design of desulfurization systems. Consequently, major importance must be attached to a conscientious analysis of the calorific values and the water, ash and sulfur contents
of fuel deriving from, say, different sections of a coal mine. It is also important to ascertain the possible extent of spontaneous desulfurization attributable to the presence of calcium compounds in the coal.
Coal can be desulfurized directly at the mine or pit as part of a process in which sulfur and various inert constituents are extracted primarily by wet methods. Depending on the type of coal and on the kind of sulfur linkage, the sulfur content of the coal (glance coal in particular) can be reduced by 5 % to 80 %. No such conditioning measures, however, can reduce the organosulfur content. Sulfite in the form of pyrite (FeS2) can be separated out if it is freely present in the raw coal or is so coarse-grained in intergrowths that it becomes amenable to removal following crushing.
Direct in-boiler desulfurization is applied to solid fuels in fluidized-bed combustion systems. Separation efficiencies of 80 % to 90 % are achievable. Dry additive techniques remove between 60 % and 80 % of the sulfur from coal (cf. Appendix A-3 for details).
Flue-gas desulfurization techniques enable SO2 separation efficiency levels of 90 - 95 %. Since flue-gas desulfurization equipment is expensive to install and operate, it is more judicious in some cases to install a component-flow desulfurization system in which only part of the flue gases are desulfurized, while the remaining, undesulfurized flue gases can be used for heating the treated gases.
Of all the described alternatives, flue-gas desulfurization is the most expensive and elaborate. In each case, particularly for retrofitting projects, the spatial integration options must be carefully investigated in advance.
A comparison of the aforementioned pre- and post-combustion desulfurization measures shows that the former offer the lower separation efficiencies, but are also less expensive and more conducive to retrofitting. Fluidized-bed combustion, however, is an exception to the rule, as it can only be implemented in new facilities (maximum capacity of commercial-scale systems to date: 150 MWel).
All methods of desulfurization and dust control involve the consequential problem of properly recycling or disposing of the residues and, possibly, of wastewater resulting from operation of the equipment (cf. section 2.3).
Depending on the size of the plant, the process employed, the separation efficiency achieved, and other factors, the investment cost of a desulfurization system can amount to anywhere from roughly 30 to 550 DM/kWel. Also the increase of auxiliary power consumption to run the system is unavoidable. Dry-additive methods are the least expensive, while regenerative techniques producing compounds of sulfur as their end product are the most costly.
The various desulfurization processes also and incidentally precipitate halogen compounds such as HCl and HF even more efficiently than sulfur.
2.1.3 De NOx
The available means of nitrogen removal also comprise pre- and post-combustion alternatives. With regard to sulfur content, a careful choice of fuel can do much to limit NOx emissions. On the other hand, the NOx formation process is more complicated than the conversion of fuel sulfur into SO2, as described in section 2.1. The combustion modification measures aim to reduce the rate of NOx formation during the combustion process, essentially by lowering the maximum temperature of combustion. This can be achieved by design measures, e.g., the combustion-chamber geometry, burner design and configuration, staged air supply, reduced excess air, and such operational measures as reduced combustion-air-preheating temperature or the use of low-nitrogen fuel.
The post-combustion DeNOx measures are concerned with reducing the exhaust-side NOx emissions by various means designed to remove the NOx either alone or together with SOx.
The only process to have gained commercial-scale acceptance to date is the selective catalytic reduction of NOx (SCR method). In this process, ammonia (NH3) reacts with NOx in a catalytic converter to form water and nitrogen. The process therefore produces no residues (like those from dust-control and desulfurization processes) that would require subsequent disposal. The SCR process takes place at temperatures of 300 - 400°C and can be integrated either on the raw-gas end, e.g., upstream of the air preheater (SCR (r) economizer) or on the clean-gas end behind a desulfurizing system (SCR (r) FGD).
SCR-base processes achieve NOx separation efficiencies of approximately 80 - 90%.
Another approach that is particularly well-suited for relatively low separation efficiencies of about 60 % or less is the SNCR process (selective non-catalytic reduction), in which NOx reduction is achieved by spraying ammonia into the boiler at a temperature of some 1000°C.
The initial cost of flue-gas DeNOx equipment depends on the size of the plant, the required separation efficiency, configuration, etc. and ranges from roughly 120 to 250 DM/kWel.
2.1.4 Greenhouse effect
The greenhouse effect, i.e., the long-term warming of the earth's atmosphere due to the presence of anthropogenic trace gases, is chiefly attributable to the accumulation of gases such as carbon dioxide (CO2), methane (CH4), chlorinated fluorocarbons (CFCs), tropospheric ozone (O3) and nitrous oxide (N2O) - with the order of mention corresponding to the relevant significance of the gases. Their specific contributions to the greenhouse effect are widely variant. Methane, for example, has roughly 21 times the effect of CO2, but occurs globally in much smaller mass volumes than does CO2 as the end product of any combustion process involving carbonaceous (organic) fuel.
The principal protective measure to counter CO2 emissions is to ensure high combustion efficiency, e.g., by way of a combination or cogeneration process.
Other measures like the use of renewable sources of energy - hydropower in particular - for generating electricity, in addition to measures aimed at steering the demand for electricity, are very important, but would never suffice to render superfluous the generation of electric power in fossil-fueled thermal power plants.
2.1.5 Diffuse emissions
In addition to the aforementioned types of emissions, most of which emanate from the smokestack, thermal power plants can also emit pollutants from other areas (cf. table 1). Particulate emissions, for example, can occur in connection with fuel storage, handling and processing. Such emissions can be extensively reduced by suitable measures such as moistening with water or enclosing/encapsulating critical areas. The same applies in effect to the storage and handling of petroleum products, i.e., via suitable contrivances on the tanks and pumping facilities, either to minimize evaporation or to return the condensate to the system. Such measures can be of major importance in countries with a warmer climate than that encountered in Central Europe.
Most water in thermal power stations is used for cooling. After absorbing enough heat to raise its temperature by 4 - 8°C, the water normally is returned to the extraction point. Power plants designed for non-circulating water cooling require about 160 - 220 m³/hMWel (with cooling water losses usually staying below 2 %).
In pure power generation, the cooling water absorbs approximately 60 % to 80 % of the fuel's energy content as waste heat. Less energy is wasted by plants with inherently higher efficiency, e.g., cogenerating facilities. Depending on local conditions, the waste heat can impose a thermal burden on surface water, e.g., cause an increase in the temperature of a river, with the volumetric flow and/or water regimen as an actuating variable. Particularly in developing countries, water bodies are subject to pronounced seasonal variation. Oxygen depletion therefore has two main causes: accelerated consumption due to rapid metabolism, and the lower solubility of oxygen in warm water. Oxygen deficiency can be seriously detrimental to aquatic life.
The in/out temperature gradient of cooling water can be limited by putting it through a cooling tower (once-through or circulation cooling) before it is returned to the river. Depending on the prevailing climatic conditions, however, such cooling systems involve major evaporative water losses and, hence, locally elevated atmospheric dampness. Such problems can be avoided or minimized by the use of closed-loop cooling systems in combination with dry or hybrid cooling towers. Natural-draft cooling towers are relatively expensive to build but comparatively inexpensive to operate, while induced-draft cooling towers have the disadvantage of operating on electricity, the generation of which increases the overall ecological burden.
Apart from their cooling-water consumption, power plants have very modest water requirements (0.1 - 0.3 m³/hMWel) for topping up the steam cycle, cooling the ashes and operating certain types of flue-gas purification equipment (spray absorption, wet processes).
Water effluent from thermal power stations, particularly from coal-fired plants, can pollute surface waters.
The following types of wastewater can occur in power plants:
regenerate from the conditioning of makeup water and desalination of condensate
water used for washing condensate filters
effluent from coal handling and coal storage
sensitive wastewater, e.g., from pickling and conservation
ash-laden water (deslagging water) from liquid ash removal
water from the boilers, turbines and transformers
cooling-tower discharge and makeup-water conditioning
wastewater from flue-gas purification.
The quantities of such wastewater depend on the type of fuel and on various plant-specific boundary conditions and can be expected to range between 10 and 100 l/h for each MWel power output. Such effluent can be polluted by entrained suspended solids, salts, heavy metals, acids, alkalies, ammonia and oil.
Wastewater treatment can be based on physical, chemical and thermal methods. For some forms of wastewater, e.g., filter backwashing water and coal-storage effluent, physical treatment in the form of filtration, sedimentation and/or ventilation will usually suffice. Other forms of wastewater such as regenerate from makeup-water and condensate polishing, flue-gas cleaning water or other wastewater streams require chemical treatment - flocculation, precipitation, neutralization - or even thermal treatment - evaporation, drying - before they can be discharged; cf. environmental briefs Wastewater Disposal and Mechanical Engineering Workshops, Shipyards.
As mentioned in section 2, wastewater occurring as a consequence of certain flue-gas desulfurization processes can contain various pollutants deriving from the flue gas. The composition of such wastewater depends on a number of parameters, e.g., the type of fuel, the process water and the quality of the additives.
As a rule, wastewater from flue-gas cleaning requires physicochemical conditioning in the form of neutralization, flocculation, sedimentation and filtration to remove heavy metals and suspended solids (gypsum, etc.).
The amount of wastewater occurring in connection with wet desulfurization methods having gypsum as a by-product depends mainly on the chloride content of the coal and on the permissible concentration of chloride in the washings. In a typical hard-coal power plant, flue-gas desulfurization processes can yield wastewater in quantities between 20 and 50 l/h per MW of power output.
The high water solubility of calcium chloride (CaCl2) entrained in the wastewater makes it an unprecipitable saline emission.
If no salt is allowed to be discharged into the receiving water, the FGD wastewater can be evaporated to yield dry, water-soluble salts requiring controlled disposal, e.g., in an underground sensitive-waste depot. Since the evaporation process requires high energy inputs, it should be ascertained for such cases whether or not an effluentless method (dry process, spray absorption) would be suitable.
Apart from the aforementioned direct effects, power plants can also have indirect effects on water. Consider, for example, the "acid rain" phenomenon involving the washout of airborne pollutants (SOx, HCl, NOx) from power plants in connection with natural precipitation.
2.3 Soil and groundwater
Thermal power plants can have multifarious impacts on soil and groundwater. The soil quality, for example, can be adversely affected by dust sediment, particularly in the near vicinity of the plant. The seriousness of ground-level pollution depends on the heavy-metal content of the dust. The chemism of the soil can be altered by acidic precipitation (acid rain) characterized mainly by the acid formers SO2 and NOx. Under unfavorable conditions, acidification can pass from the soil to both the groundwater and surface waters. The extent of soil and groundwater pollution does not depend on how much particulate matter and acid formers are contained in the exhaust, but rather on the absolute quantities emitted in the course of a year (total annual emissions) and on the conditions of distribution. Thus, it is important to limit such emissions by separation capacities commensurate with the size of the power plant.
The ground and, even more so, the groundwater in the immediate vicinity of the power plant are threatened by the escape of water-polluting substances, the main sources of which are various weak points in the collection and purification of wastewater, the leakage of oil and oil-containing liquids, and storage areas for oil, coal and residues.
The deposition of residues also has consequences for the soil and, even more so, for the groundwater. Power-generating residues consist primarily of slag, fly ash, remanents from flue-gas desulfurization, and sludge from the treatment of raw water and effluent. The residual quantities depend in part on the processes employed; in general, however, it may be said that, the lower the quality of the coal, the higher the quantity of residues.
Slag and fly ash can be put to various uses (for roadbuilding or as cement aggregate), depending on their composition. To the extent that they cannot be recycled, such substances must be disposed of at suitable dumps (e.g., above groundwater level). In Germany, these matters are regulated by TA-Abfall (Technical Instructions on Waste Management).
Part 1 in Appendix C of the catalogue of particularly sensitive wastes specifies aboveground deposition in the form of a mono-type hazardous waste dump for solid reaction products resulting from the purification of combustion-plant exhaust gases, excluding gypsum; cf. briefs Solid Waste Disposal, Disposal of Hazardous Wastes.
The nature of FGD residues depends on the method employed (cf. Appendix A-3) and may occur in recyclable form, e.g., gypsum. The quantities depend on the sulfur content and the calorific value of the fuel, the degree of desulfurization, and the additives involved. Prior to choosing a particular desulfurization process, it should be ascertained whether or not the respective remanent substances occurring as by-products of the different processes could be marketed in the respective country. This would require a detailed local market analysis, appropriately involving local contractors/consultants. Potential uses for the residues (as building materials) must be investigated; in their absence, it must be clarified whether or not and under which conditions the substances can be safely disposed of.
The following table compares the quantified residues of flue-gas desulfurization in facilities fired with heavy fuel oil and two different types of coal:
|Hard coal||Lignite||Heavy fuel oil|
|Calorific value [kJ/kg]||28 000||10 000||40 000|
|Sulfur content [weight %]||2.0||2.0||2.0|
|Degree of desulfurization [%]||85||85||85|
|SOx in raw gas [kg/MWelh]
|SOx in treated gas
|Hard coal||Lignite||Heavy fuel oil|
When both fly ash and desulfurization products (gypsum or a sulfite/sulfate mixture) require disposal, it is advisable to mix the products first. A blend of fly ash and desulfurization products can be hardened to stabilize the water-soluble constituents (stabilizate) and reduce their leachability.
Desulfurization processes with useful end products require appropriate treatment of the wastewater. The resultant sludge contains large amounts of heavy metals and therefore should be treated as sensitive waste.
2.4 Human health
Adverse effects of thermal power plants on human health can derive from the direct impact of noxious gases on the organism and/or their indirect impact via the food chain and changes in the environment. Especially in connection with high levels of fine particulates, noxious gases like S2 and NOx can lead to respiratory diseases. SO2 and NOx can have health-impairing effects at concentrations below those cited in the German smog ordinance. The duration of exposure is decisive. Injurious heavy metals (e.g., lead, mercury and cadmium) can enter the food chain and, hence, the human organism by way of drinking water and vegetable and animal products. Climatic changes such as warming and acidification of surface waters, Waldsterben (forest death) caused by acid rain and/or the greenhouse effect of CO2 and other trace gases can have long-term detrimental effects on human health. Similarly important are the effects of climatic changes on agriculture and forestry (and thus on people's standard of living), e.g., large-scale shifts of cultivation to other regions and/or deterioration of crop yields. Hence, the construction and operation of thermal power plants can have both socioeconomic and sociocultural consequences; appropriate preparatory studies, gender-specific and otherwise, are therefore required, and the state of medical services within the project area must be clarified in advance. Early, comprehensive involvement of the concerned sections of the population in the planning and decision-making process can help reduce and avoid points of conflict.
Noise, as an item of emission from thermal power plants, has direct effects on humans and animals. The main sources of noise in a power plant are: the mouth of the smokestack, belt conveyors, fans, motors/engines, transformers, flues, piping and turbines.
At least some of the personnel working in power plants are exposed to a more or less substantial noise nuisance.
Diverse noise-control measures can be introduced to reduce immissions to a tolerable level, whereas the primary goal must be to protect the power plant staff. To the extent possible, power plants should be located an acceptable distance from residential areas, and all appropriate noise-control measures must be applied to the respective sound sources at the planning and construction stages.
Two particularly effective measures are the use of sound absorbers to reduce flow noises and the encapsulation of machines and respective devices to reduce airborne and structure-borne sound levels. Appropriate enclosures constitute an additional means of simultaneously reducing both the emission and immission of noise. Incidentally, enclosures also provide weather protection and are therefore used widely in power plant engineering.
Power plants have substantial spatial requirements. The extent of land consumption is generally higher for coal-fired facilities than for gas- or oil-fueled plants. With regard to siting, cf. environmental briefs on Spatial and Regional Planning, Planning of Locations for Trade and Industry.
The landscape is also affected by construction of the roads needed for delivering operating media and disposing of residues; cf. environmental briefs on railways, roads and waterways. The associated mining activities to obtain coal and, say, limestone (for desulfurization purposes) and for disposing of residues not to be recycled also tend to alter the landscape. In connection with the disposal of residues, priority should be given to landfilling schemes (e.g., in worked-out strip mines) or land reclamation in coastal areas. Both alternatives avoid the need for separate dumping facilities and put the residues to an advantageous use. The residues, of course, should be environmentally benign, either by nature or by reason of appropriate treatment to impart, for example, a low level of leachability. Additionally, it must be ascertained whether or not and which measures (sealing, controlled drainage, conditioning of percolating water) will be required to keep soluble heavy metals and other substances contained in the residues from passing into the groundwater or coastal water (cf. sections 2.1.1 and 2.3).
Moreover, attendant pollution can cause damage to forests, lakes and rivers, resulting in serious permanent changes in the landscape.
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