Rational utilisation of energy is one of the major objectives of national economy and an element of sustainable development with specific ecological, social and economic effects. In order to meet the assumptions of sustainable development in the energy sector, Poland made a commitment to protect air and keep up with global environmental protection standards.
Data on emissions from 1989–2013 point to a reduction in emissions of dust in Poland by more than 80%, sulphur dioxide by about 70% and nitrogen oxides by nearly 40%. Initially, the reduction in air pollution was a consequence of the liquidation of obsolete, energy-consuming engineering processes and later of fuel utilisation effectiveness and replacing some conventional fuels with low-emission fuels that have smaller environmental impact.
The commitment to protect air in the energy sector is implemented mainly by reducing the emission of nitrogen oxides, dust, sulphur oxides and carbon dioxide. Therefore, new power units are equipped with comprehensive flue gas treatment systems, taking into account the links between the systems for removing nitrogen, dust and sulphur, and comply with BAT (Best Available Techniques).
A general principle of desulphurization is to convert SO2 so that it could be easily removed from gas and from the treatment system, therefore most often sorption processes combined with SO2 oxidation are applied. Desulphurization technology is selected depending on the resultant product – commodity or sulphur waste. In regenerative methods concentrated elementary sulphur or sulphuric acid is used but power industry prefers non-regenerative, that is, waste methods.
Combustion of different kinds of coal has an essential share in the emission of SO2 so most desulphurization methods are connected with coal treatment processes: preparation of coal for combustion, desulphurization during combustion and from exhaust gas.
The most effective desulphurization method would be desulphurization of coal itself but unfortunately – for economic reasons – it is used very rarely. The most effective desulphurization during combustion takes place in fluidized beds where the combustion temperature is limited to 850°C, and SO2 is bound by alkaline substances introduced into the bed. However, the most widely used method of desulphurization is desulphurization of exhaust gas, and in particular lime-based methods (ca. 90%). This is due to the fact that lime-based sorbents are inexpensive and readily available. Lime-based methods are split into dry, semi-dry and wet methods. Dry methods (sorption on solid sorbents, injection of SO2 sorbent into a gas stream, desulphurization in the process of spray drying) require the use of an expensive sorbent, whereas the consumption of energy and water is relatively lower (or water is not used at all). Semi-dry methods
(e.g. SDA or RP+FT – using a pneumatic reactor integrated with a fabric filter) are high-duty methods (e.g. in RP+FT the efficiency of removing SO2 is 90%÷95% (max. 98%), SO3 – above 95% (max. 99%) HCl – above 95% (max. 99%), and HF – above 90%). These methods can meet the still “exorbitant” limits of emission to the atmosphere. The same also applies to wet methods (e.g. lime and gypsum method). An advantage of such methods, following the application of a lime-based neutralizing reagent, is that it produces gypsum which can be sold later. Among non-lime methods, the most important is flushing of flue gas with seawater. Other methods (Wellman Lord process, including sulphur recovery, magnesite method, ammonia method, two-cycle alkaline method, radiation or SO2 absorption based on active coke) are used more rarely. Such preferences are due to economic reasons (high price of the sorbent and high cost of investment related to the necessity of building very complex process units).
The source of NOx is oxidation of N2 from air at a high combustion temperature and conversion of nitrogen chemically bound in fuel – to NOx. The emissions of NOx can be reduced directly at their source (primary methods) or by treatment of exhaust gas by means of reduction, oxidation in the gas or liquid phase with simultaneous sorption in sorbents or on solid sorbents (secondary methods).
As regards primary methods the combustion process can be modified through, e.g. modernisation of the furnace system such as nonstoichiometric combustion, recirculation of flue gas, supply of water or steam, feeding additional hydrocarbon fuel to the combustion chamber, modification of boiler design (taking into account, for example, the type of furnace, thermal load of the chamber, type and position of burners, angle of inclination, change in burner load, etc.), use of special design burners (low-emission burners, fuel staging burners, gas recirculation burners, etc.), introduction of boilers with a circulating fluidized bed or co-firing of biomass (the content of nitrogen in biomass is two times lower than in coal). Primary methods are most eagerly used due to the good flue gas denitrification to costs ratio.
When primary methods are not capable of ensuring the required level of NOx emissions, secondary denitrification methods are used, including: selective calatytic reduction, selective non-catalytic reduction, absorption methods combined with desulphurization [SHL (Saaberg-Holder-Lurgi), WSA-SNOX (WSA – Wet Sulphuric Acid), Bergau Forschung-Uhde], and the radiation method.
Combined methods, e.g. primary – two-zone fuel combustion – and secondary – adding a reagent (urea), are also used in the denitrification of flue gas. Combined methods ensure optimum operation of the boiler and lower operating costs.
Flue gas dedusting comprises the removal of aerosol particles from flue gas. The main types of dust collectors include settling and inertial chambers, cyclone separators, fabric and layer filters, electrostatic precipitators and scrubbers.
Electrostatic precipitators are commonly used as auxiliary equipment for boilers in commercial power engineering. Dust particles carried by flue gas are electrically neutral and must be electrified to enable the treatment process. Dust grains are electrically charged in the process of corona discharge, i.e. an electrical discharge taking place in a strong non-uniform electric field – using voltages of 40-80 kV. Dust grains receive an electric charge from gas particles ionized by the corona discharge. Electrically charged, they migrate to an electrode with a charge opposite to that on which they settle. On the electrode the particles are electrically discharged and then they are removed cyclically. The electrostatic force is determined by the charge of the dust grain, whereas the charge – which can be accumulated on the grain – depending, among other factors, on grain size. Electrostatic precipitators (ESP) are very effective (normally more than 99% for 1 µm particles) at catching dust grains. However, the increasingly stringent dust emission standards require measures improving the performance of ESP. ESP modernisations are design alterations involving: increased dimensions, expanded pitch, new supply system solutions, and even construction of a new, high-duty electrostatic precipitator. Another method is reducing the resistance of ash (ash from Polish power plants is highly resistant – its level of resistant is at least one or two times higher than the level deemed as optimum from the point of view of dedusting effectiveness).
Heating plants and combined heat and power plants also use cyclone separators based on inertia forces to remove dust from gas. They remove dust particles larger than 60 μm. Cyclone separators are more effective at higher inlet speed and smaller chamber radius.
There are different activities that can reduce CO2 emissions from power stations and combined heat and power plants: refinement of coal prior to burning, improving energy conversion efficiency in power stations (in particular building power units with supercritical and, ultimately, ultra-supercritical parameters), diversification of fuels (replacing coal with fuel oil or preferably natural gas), implementing coal gasification technology [gasification of coal in mines, IGCC (Integrated Gasification Combined Cycle), and carbon dioxide sequestration in every combustion process.
Soil pollution may occur within the power plant’s premises as a result of an industrial accident. Therefore, power plants have adequate procedures and instructions in case of an accident. They contain technical descriptions preventing accidents and procedures in case an accident occurs.
Despite consuming large quantities of water, electricity can be and normally is generated in a manner that is the least onerous for the aquatic environment. Water intake and discharge of effluents from power plants is continuously monitored by computer systems for controlling the plant’s operation including water intake and effluent discharge indicators. Power plants mostly have their own wastewater treatment plants and water treatment stations that also operate when water is purchased from water supply companies. In addition, a power plant can be equipped with a complex of protective facilities comprising, for example: a chemical plant for the treatment of flue gas desulphurization wastewater including a heavy metals precipitation module, a chemical plant for the neutralization of aggressive effluents from boiler etching or ion-exchange bed regeneration processes, a biological and mechanical plant for the treatment of sanitary wastewater, a mechanical and a chemical plant for the treatment of industrial effluents and rainwater, and a sludge composting plant.
In order to reduce the consumption of water, a closed cycle is used for engineering purposes, and water from respective stages of treatment is recycled for further use. Such wastewater can be used in production processes, for instance, for producing process water (so-called combined water cycles) or disposed of into a closed-cycle hydrotransport system where industrial effluents enable disposal of the ash and slag mixture to storage sites. Part of the treated industrial effluents is re-used in many technological processes for which water with lower quality parameters can be used (e.g. washing, supply of external fire protection systems).
Generation of wastes is a major environmental problem. Power plants and other plants burning fuels to generate energy produce combustion wastes (volatile ash and slag) – classified among industrial wastes generated in the largest amounts. Volatile ash is pozzolanic ash (fine-grained fraction) mainly composed of spherical vitrified grains containing mostly SiO2 and Al2O3. Volatile ash is a by-product of combusting fuel in boilers. Afterwards, it is carried away by flue gas and retained in electrostatic precipitators. Slag is a coarse-grained fraction and is carried away from underneath the boiler to storage containers or slurry pumps.
Pursuant to the Regulation of the Minister of Environment of 9 December 2014 (item 1923) concerning the catalogue of wastes, combustion wastes are classified in group 10 01 as wastes from power stations and other combustion plants. This group of wastes includes: slag, furnace ash and boiler dust, volatile ash from coal, volatile ash from peat and timber not treated by chemical methods, volatile ash and dust from liquid fuel-fired boilers, solid wastes from lime-based desulphurization of exhaust gas, products of lime-based desulphurization of exhaust gas disposed of as slurry, sulphuric acid, volatile ash from emulsified hydrocarbons used as fuel, furnace ash, slag and dust from boilers from co-combustion, volatile ash from co-combustion, wastes from treatment of exhaust gas, sludge from works wastewater treatment plants, hydrated slurry from boiler cleaning, sand from fluidized beds, wastes from storage and preparation of fuels for coal-fired power plants, wastes from treatment of cooling water, ash and slag mixtures from wet disposal of combustion wastes, microspheres from volatile ash, mixtures of volatile ash and solid wastes from lime-based desulphurization of exhaust gas (dry and semi-dry desulphurization of flue gas and fluidized bed combustion).
Plants that generate wastes are required to ensure that their production methods and forms facilitate keeping the quantity of generated wastes at the lowest level but the quantity of combustion wastes depends on the amount of generated electricity, quality of combusted coal and content of ash in coal. Good quality of ash, in terms of its possible applications, is determined by: low content of coal, high content of glass, low content of alkali, and fine grains. Apart from the composition of the coal gangue, the quality of ash is also affected by process factors. Ash contains more glass and has finer grains (better pozzolanic properties) when coal burnt in boilers is fine-grained.
At the end of the past century the amount of generated electricity decreased and the quality of coal improved. These changes resulted in a reduced amount of combustion wastes. This trend has been retained – according to information provided by ECOPA (European Coal Combustion Products Association) – in the European Union (EU 15) in 2010 about 48 million tonnes of combustion wastes were generated (to compare: in 1993 it was 57 million tonnes and in 1999 – 55 million tonnes), the largest part of which is volatile ash (ca. 66%). In Poland in 2014 the amount of wastes from electricity, gas, steam and hot water generation and supply plants totalled 21 942.3 thousand tonnes (in 2013 – 24 304.3 thousand tonnes), whereas volatile ash from coal constituted 3 835.8 thousand tonnes, and mixed ash and slag from wet disposal of combustion wastes totalled 11 950.9 thousand tonnes. From the total amount of generated volatile ash 128.8 thousand tonnes of ash were recovered, and 134.8 thousand tonnes were disposed, of which only 0.05% was deposited in landfills. 28.3 thousand tonnes of mixed ash and slag from wet disposal of combustion wastes was recovered and 10 395.2 thousand tonnes were subject disposed of (100% deposited in landfills). Third parties received 51.6 thousand tonnes of volatile ash and 201.9 thousand tonnes of mixed ash and slag. Temporarily stored ash accounted for 91.78% of all volatile ash generated (288.3 thousand tonnes), whereas mixed ash and slag only for 11% (1325.5 thousand tonnes). In total, at the end of 2014 as many as 26 861.4 thousand tonnes of volatile ash were deposited in landfills, whereas 285 883.6 thousand tonnes of mixed ash and slag from wet disposal of combustion wastes were stored. The decreasing amount of combustion wastes in landfills is evidence of the effective use of available technologies for the disposal of industrial wastes.
Combustion wastes are disposed of to a wide extent in the construction industry. In the European Union (according to ECOPA) about 43% of volatile ash, about 46% of bottom ash and 100% of slag is utilized. In most cases these wastes are used as substitutes of naturally occurring resources, which is connected with a positive environmental effect. The utilization of these wastes also contributes to a decreased requirement of energy and reduced atmospheric emissions (e.g. CO2), which is a result of the process of manufacturing products that would have been used instead of such wastes. Combustion wastes are used as an additive in the production of: concrete, mortar, slurry, structural concrete on the construction site, prefabricated concrete, cellular concrete, building ceramic ware, cement and pozzolanic binders, asphalt concrete, and polymer concrete.
In economic terms a significant role is ascribed to volatile ash, not only as a cheap substitute for a part of cement, but also as an important component having a positive influence on the properties of concrete. Spherical grains of volatile ash are at least 70% composed of silica, aluminium oxides and iron oxides. Pozzolanic activity of volatile ash is determined by the chemical composition and fineness of the Portland cement with which it will be used. Volatile ash, filling intergrain pores in cement, seals the structures of slurry and, thanks to its pozzolanic activity, improves concrete binding and strength parameters. The maximum dose of ash in the concrete should not exceed 20% of the cement weight, but it is determined individually for every type of ash. When added, ash delays the concrete binding time and slows down its setting. The mix with volatile ash has many positive properties: it is more compact, more resistant to water leaks, it can be pumped more easily and its surface can be finished better, but the strength of concrete with an admixture of ash increases more slowly than the strength of other types of concrete.
A characteristic method of ash disposal in Poland is using it in underground mining by means of suspension technology. It can also be used as mineral fillers and fertilizers. The chemical composition of combustion wastes from fluidized bed boilers is definitely different from that derived from pulverized-fuel boilers with dry desulphurization. Furnace products from fluidized bed boilers are characterized by good binding properties and bottom ash from fluidized bed boilers contains more silica and iron oxides and slightly less CaO than volatile ash does. Therefore, volatile ash from fluidized bed boilers is used in road construction where the use of silica ash is most recommended and in the production of silicate and aluminium binders from fluidized bed boilers. Their use considerably reduces the cost of the binder although the stabilization parameters are no worse than when lime or cement is used.
In road construction power industry wastes can be used as an alternative material, mainly for the road substructure or body. These materials can be loose or – after processing – they can have the form of pellets. Volatile ash from fluidized bed boilers is also used for the construction of settling ponds, for stabilization and reclamation of land, in mining and the construction industry where their binding properties are used. Due to its alkalinity, volatile ash from fluidized bed boilers can be used in agriculture (soil deacidification), environmental protection and municipal management (neutralization of wastewater).
One of the mass directions of utilizing energy wastes is macro-levelling and reclamation of land. In Poland, ash and slag from brown coal is commonly used to fill the brown coal, aggregate, clay and sulphur pits. The mixture of ash and slag from landfills is used in the construction of embankments of combustion and other waste landfills and for the construction of transport embankments. In connection with an increasing amount of flue gas desulphurization wastes their recovery must be taken into account. The best options for disposal are available for post-reaction gypsum as a material replacing natural gypsum.
The European Union aims to separate economic development and growth from the irrevocable consumption of natural resources, which provides options for wider utilization of by-products of combustion.
Poland – ratifying the Convention on Climate Change and the Kyoto Protocol – became included in the international scheme for preventing climate change. One of the main obligations is the reduction of greenhouse gas emissions by 6% in 2008-2012, in relation to the base year and by 20% in 2013-2020. So far, Poland has reduced the emissions of greenhouse gases by about 30% (at the required 6%), and despite this fact in 2013 it was ranked 5th in terms of greenhouse gas emissions (including carbon dioxide) in the European Union (according to Eurostat).
According to data gathered during the national stocktaking (KOBIZE) in 2013, the total emission of greenhouse gases in Poland was 394 891.52 kt of CO2 equivalent (without considering the sector Use of land, changes in the use of land and forestry), while the sector Energy emitted 323 470.71 kt of CO2 equivalent. The main source of CO2 emissions was the sector Fuel combustion and this category accounted for 92.4% of the total CO2 emissions in 2013, whereas 52.4% was the share of Energy industry. Taking into account changes in greenhouse gas emissions in 1988-2013, split into the main categories of sources, a decrease in emissions was noted in all categories, and the second sector with the largest reduction in emissions (by 33%), was Energy. Despite these positive changes, combustion of fossil fuels still causes atmospheric emissions of CO2 in amounts that cannot be absorbed by plants or dissolved in oceans, which results in the greenhouse effect causing global warming on the Earth. Therefore, the European Union has set an objective – a 20% reduction in emissions of CO2 by 2020 (in comparison to 1990) and the right solutions must be found to fulfil this condition. It is relatively easy and quick to implement the CCS (Carbon Capture and Storage) technology, that is, capturing, transporting and storing carbon dioxide, referred to as sequestration. CO2 sequestration makes it possible to utilize fossil fuels and at the same time reduce atmospheric emissions of CO2. Underground storage of CO2 is a phenomenon that is common in nature, and such natural tanks have existed for millions of years (e.g. a series of eight natural CO2 tanks in south eastern France discovered during oil extraction works in that area in the 1960s). It demonstrates that the right geological formations were selected for storage of CO2 as they are characterised by large capacity, safety and guaranteed long-term stability.
At present, it is expected that sequestration will be used for disposing of large amounts of CO2 (millions of tonnes), but for most combustion and industrial gases the concentration of CO2 is low and ranges from a few to several per cent, and a clean stream is emitted by some industrial processes only. When carbon dioxide is captured in industrial installations, it is compressed to form a thick liquid.
There are a number of technologies which make it possible to receive a concentrated stream of CO2, including: pre-combustion capture, post-combustion capture and oxy-fuel combustion of coal. Before it is injected under the ground the captured carbon dioxide must be separated from other combustion and industrial gases. Separation methods used include chemical absorption (the most common) and physical absorption, physical adsorption, cryogenic fractionation and membrane separation. Next, CO2 is transported, using vessels and pipelines, and pumped into tank formations.
For carbon dioxide sequestration in geological structures (possibly provided for in Polish conditions), injection of CO2 must be preceded by a thorough survey of the land – the presence of thick, considerably scattered sedimentary rocks with good collection properties is required. The rocks must form a so-called “trap” inside which the injected carbon dioxide is accumulated in the intergrain space, and in the cracks, pushing out and replacing the existing substances such as gas, water and oil. The “trap” must be covered by impermeable rocks (e.g. loam, mud rock, marl, salt rocks) that will prevent uncontrolled release of CO2 from the “trap”.
A very significant factor playing a key role in selecting the location for CO2 sequestration is the capacity of the geological structure and the lack of drinking water (CO2 cannot be injected into water which can be used or utilized by humans). In order to increase the amount of stored CO2 it is accumulated in the form of a compressed liquid, so the tank should be located more than 800 metres underground, where pressure and temperature are adequately high.
Having these factors in mind, CO2 sequestration can make use of:
For reasons of safety (primarily), including environmental, social and economic safety, CO2 sequestration must be continuously monitored in compliance with applicable legal regulations. The European Commission aims to construct demonstration facilities for carbon dioxide capturing and storage. Currently, this technology is at an early stage of development and the idea of capturing and storing carbon dioxide has not yet been implemented on a large scale (due to the lack of economic incentives as well as political and regulatory doubts).
Before 1988, the power engineering sector and industry had the largest influence on the deterioration of air quality in Poland. At present, reasons for the poor quality of air are sought for in the household and municipal sector. It is significant that since 1988 a downward trend has been observed in emissions of main industrial pollutants, which is connected with the transformation of the national economy, and with regard to environmental protection, with the start of implementation of best available techniques (BAT) and integrated permits in industrial plants. Since the Environmental Protection law was put into effect, power plants have been obliged to obtain integrated permits for all types of emissions. Failure to comply with the provisions of integrated permits results in severe criminal sanctions and further – leads to the discontinuation of production. Apart from complying with the provisions of integrated permits, most power plants have already adopted the Environmental Management System according to PN-EN ISO 14001, which guarantees improved environmental protection activity, reduced environmental threats, improve activities adapting to regulatory requirements and support and improvement of environmental measures (including environmental quality of goods and services).
Power plants pursuing sustainability policy also make attempts to obtain a conformity certificate according to EMAS (Eco Management and Audit Scheme) concerning voluntary participation in the eco-management and audit scheme. Some Polish power plants already have implemented EMAS certificate. These are: Opole, Łaziska, Jaworzno III, Siersza, and Łagisza.
EMAS is a Community instrument, a kind of trademark indicating that the organisation aims at excellence in environmental protection. EMAS is synchronized with ISO 14001 which forms a part thereof. EMAS requires more than ISO systems, at least to the extent of reporting of continuous sustainable development and full compliance with the European and national legislation on environmental protection.
A very significant element of the assumptions of EMAS is the requirement to include the team, both top management and employees, in activities for the protection of the natural environment. Activities undertaken for the protection of the natural environment are seen in the submitted rationalization solutions concerning: reduction of environmental load, correct supervision over subcontractors, adequate system documentation (procedures and manuals), correct management review and correct identification of environmental goals.
In order to comply with the requirements of EMAS, power plants must inform the public and interested parties on the environmental impact of the power plant, its products and services and on environmental measures they are going to undertake. The requirement for registration with EMAS is a multi-aspect environmental review of goods and services, including methods of their evaluation, regulatory requirements and existing environmental management practices and procedures. Also, an environmental declaration describing the management system and effects of environmental activities must be prepared and published. The EMAS regulation identifies six main indicators of environmental effectiveness to be included in the environmental declaration. They refer to: energy efficiency, rational use of materials, water, wastes, as well as biodiversity and emissions.
Sustainable power plants undertake investment and modernisation activities with reference to, for example: discontinuing the storage of ash and slag in heaps; building systems for dry collection of ash from electrostatic precipitators; building systems for semi-dry desulphurization of flue gas; building new power units with fluidized bed boilers and high-duty turbines; generating green energy from co-combustion of biomass; building soundproof screens or housings for power generation equipment; planting greenery to enhance the appearance and reduce noise emitted into the environment; reducing the content of suspension in effluents discharged into a receiving tank. Apart from environmental investments, the methods of business management also change. It must be emphasized that the EMAS certificate requires openness to environmental issues and transparency of the power plant's activities.