Problems of coal utilization in Romanian thermopower plants

In this paper, we present the most important problems regarding coal utilization in Romanian thermopower plants (TPPs). Reflected are the economic and technical difficulties of coal utilization as fuel. Consideration of this fuel is from the perspective of Romania. Presented are the results of Romanian researches for resolving the necessary problems regarding ecological standards and obtaining good efficiency in TPPs functioning with coal. Proposals are made for application of such researches.     

Continuous lignite char combustion in a spouted bed

A simulation model of continuous lignite char combustion in a spouted bed has been developed to predict bed oxygen concentrations, bed particle size distribution, bed carbon loading, mean diameter of bed char, and the fractional combustion in spout, annulus, and fountain. The approach involves taking into account the spouted bed hydrodynamics, a burning law for individual particles, and the combines mass balances for bed char and oxidant in the three typical regions. The predicted results for various operating conditions are compared with some experimental data.     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

褐煤及其混煤燃烧特性试验研究

将小龙潭煤和先锋煤两种褐煤按不同比例混合进行热重分析研究.分析单一煤种及不同配比混煤的燃烧特性的差异,提出了反映煤燃烧着火及燃尽的燃烧特性综合判断指数.并根据化学动力学方法计算了各过程的化学动力学参数活化能和频率因子.     

Studies of transformations of inorganic constituents in a Texas lignite during combustion

The mechanisms of coal ash formation were studied under closely controlled combustion conditions. Monticello lignite, from Titus County, Texas was combusted in a drop-tube furnace at 1500°C and fly ash was collected and aerodynamically size segregated into six stages. Short residence time chars were also produced in the drop-tube furnace. The coal was analyzed to determine both mineralogical and organically bound components using computer controlled scanning electron microscopy (CCSEM) and chemical fractionation techniques, respectively. Scanning electron microscopy/microprobe techniques were used to classify and determine the distribution of various ash particle types in each size fraction. Over 80% of the Monticello mineral content consisted of quartz and clay minerals and relatively large quantities of Ca, Mg, and possibly Fe were organically bound. Extensive reaction between the quartz and clay minerals and organically bound Ca resulted in amorphous and crystalline Ca-silicate and Ca-aluminosilicate phases in the fly ash. Finely subdivided discrete minerals or organically bound cations of Mg and Fe were concentrated in the finer fraction of the fly ash.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Laboratory studies on combustion cavity growth in lignite coal blocks in the context of underground coal gasification

Cavity formation is an important phenomenon in the underground coal gasification (UCG) process. In the early stages of cavity formation, only the combustion reaction is performed in order to stabilize the temperature field. In the present work, we study the formation of the combustion cavity and the effect of various design and operating parameters such as the distance between the wells, feed flow rate and operation time, on its evolution. This paper presents details of laboratory-scale experiments that demonstrate the shape and size of the combustion cavity, and their dependence on various parameters. Empirical correlations for the cavity volume and dimensions in various directions are developed, which indicate the strong effects of mass transport. Results from computational fluid dynamics (CFD), which map the velocity profiles in the cavity, support the experimental observations.     

Ecological efficiency in thermoelectric power plants

This work evaluates the environmental impact resulting from the natural gas and diesel combustion in thermoelectric power plants that utilize the combined cycle technology (CC), as regarding to Brazilian conditions according to Thermopower Priority Plan (TPP). In the regions where there are not natural gas the option has been the utilization of diesel and consequentily there are more emission of pollutants. The ecological efficiency concept, which evaluates by and large the environmental impact, caused by CO₂, SO₂, NO_x and particulate matter (PM) emissions. The combustion gases of the thermoelectric power plants working with natural gas (less pollutant) and diesel (more pollutant) cause problems to the environment, for their components harm the human being life, animals and directly the plants. The resulting pollution from natural gas and diesel combustion is analyzed, considering separately the CO₂, SO₂, NO_x and particulate matter gas emission and comparing them with the in use international standards regarding the air quality. It can be concluded that it is possible to calculate thermoelectric power plant quantitative and qualitative environment factor, and on the ecological standpoint, for plant with total power of 41 441 kW, being 27 170 kW for the gas turbine and 14271 kW for the steam turbine. The natural gas used as fuel is better than the diesel, presenting ecological efficiency of 0.944 versus 0.914 for the latter, considering a thermal efficiency of 54% for the combined cycle.     

Safety aspects of hydrogen combustion in hydrogen energy systems

Safety aspects will become essential for the introduction and acceptance of gaseous and liquid hydrogen as an energy carrier and fuel in energy supply systems. Prevention and control of accidental formation and ignition of large volumes of fuel-air mixtures are of primary importance when safety aspects of released gaseous hydrogen are discussed. Detailed knowledge of the overpressure in an accidental situation is essential for the protection of the public as well as for the corresponding plants and safety installations. Considerable progress has been made in the last few years concerning the understanding of the complex phenomena involved in combustion processes of gaseous mixtures. This holds in particular for flame acceleration and maximum turbulent flame speeds in unconfined and confined geometries. Fast turbulent deflagrations often transit spontaneously to detonations if flame speeds are high enough, depending on the combustible and boundary conditions. This paper discusses the potential hazards of hydrogen in the energy market as compared with other and already familiar energy carriers like natural gas and propane.     

Unburnt carbon and ashing behavior for slow burning of lignite under oxygen-enriched combustion conditions

Oxygen-enriched combustion of coal has so far been investigated using either entrained flow reactors or fluidized-bed combustors where burning takes place rapidly due to high heating rates. On the contrary, this paper presents the results of slow burning of coal under oxygen-enriched combustion conditions. Ashing behavior of mineral matter-rich low-rank coal from Turkish Tekirdag-Malkara lignite under oxygen-enriched conditions was investigated to determine the effect of this combustion technique on unburnt carbon, mineralogical characteristics and the burning performance. These experiments showed that the influence of O₂ concentration on ashing is much more evident than the temperature during oxygen-enriched combustion.     

Migration and transformation of sodium and chlorine in high-sodium high-chlorine Xinjiang lignite during circulating fluidized bed combustion

The Xinjiang lignite mined from Shaerhu coalfield (SEHc) easily causes severe fouling and corrosion because of its high sodium and chlorine contents. Therefore, it is necessary to study the migration and transformation behavior of sodium and chlorine during combustion in order to reveal the mechanisms of fouling and corrosion, and propose the effective solutions of above problems. In this study, based on the 0.4 T/D circulating fluidized bed (CFB) test system, the migration and transformation behavior of sodium and chlorine in SEHc during combustion at 950 °C was explored. The migration and transformation paths of sodium and chlorine were proposed through the chemical characterization of ash samples along the flue gas flow direction, as well as the thermodynamic equilibrium calculation by the software of Factsage 6.1. The experimental studies show the sodium and chlorine mainly in the form of NaCl crystal in raw coal underwent a series of physical and chemical changes during combustion, and subsequently distributed in bottom ash/circulating ash, fly ash and gas phase in various forms including sodium aluminosilicates, chlorides and sodium oxides. Sodium was more inclined to be resided in ash in the form of aluminosilicates through the reactions with other minerals (SiO₂ and Al₂O₃), while chlorine was easily released into the flue gas in forms of HCl, Cl₂, NaCl, etc. The Cl-based species might result in the corrosion of metal heating surfaces because of the presence of corrosion products (metal chlorides) in fly ash. As temperature decreased, the sodium or chlorine vapors would successively deposit in fly ash via physical condensation or chemical reaction. At 840～570 °C, the sodium-based species (Na₂O and NaCl) would first deposit in fly ash, then gaseous chlorine species (NaCl, FeCl₃ and so on) primarily deposited at 570～180 °C.     

Some aspects of fluidized bed combustion of paddy husk

Some agricultural wastes—for example, bagasse, paddy husk, etc.—are frequently used as fuels, paddy husk appearing, in particular, to be quite suitable for fluidized bed combustion. The conventional method of combustion of paddy husk in grate-type furnaces is slow and inefficient.

This paper reports certain aspects of the fluidized bed combustion of paddy husk. Fluidized bed combustion was carried out by feeding husk in a bed of sand particles. The unexpanded bed height was 10 cm and the size of the sand particles, 351–420 μm. The superficial velocity of the ambient fluidization air through the bed ranged from 11·1 to 22·2 m/min.

A combustion intensity of about 530 kg/h/m² of distributor area could be achieved. This is about 7·5 times higher than the maximum combustion intensity possible in a grate-type furnace per unit grate area. The efficiency of combustion, which ranged from 81 to 98 per cent was found to increase with the air flow rate. There was significant carry-over of inert sand particles from the bed under conditions of high air flow rate.

Combustion intensity increased as the bed height rose from 10 cm to 15 cm, but increased sand entrainment also occurred.     

Technoeconomic aspects of central photovoltaic power plants

The economic feasibility of central photovoltaic power plants is investigated from the perspective of an electric power utility company. The maximum acceptable price of the system is established, as a function of conventional fuel costs. Factors which would enhance the economic attractiveness of the system are analyzed. These factors include; increases in conventional fuel costs, decreases in photovoltaic system costs and subsidies to attract utility companies.

Abstract

On étudie ici les possibilités économiques d'installations d'énergie par centrales photovoltaïques du point de vue d'une Société de services d'énergie. Le prix maximum acceptable pour le système est établi en fonction des coûts habituels du carburant. Les facteurs qui pourraient mettre en valeur l'intérêt économique du système sont analysés. Ces facteurs comprenment: les hausses des coûts des carburants habituels, les baisses des coûts du système photovoltaïque et les subventions accordées pour intéresser les Sociétés de service.     

Local community opinions regarding the socio-environmental aspects of lignite surface mining: Experiences from central Poland

Surface lignite mining covers large areas and usually generates social conflicts which pose one of several energy security threats to certain states. Therefore, defining the social conditions determines the success of a mining project. Two communes were chosen for a public opinion study: Kleszczów, where the Bełchatów mine is located, and Złoczew, where a lignite deposit will soon be developed. The analysis shows, as opposed to other areas in Poland that have been projected for development, that both local communities are characterised by a high level of acceptance for lignite mining. In both cases, awareness about the profits was stronger than anxiety about the investment's negative effects. However, most inhabitants could not assess the mining company's diligence concerning its responsibility for mining damages as well as the diligence of external experts assessing the environmental impacts of excavation. Most respondents also could not assess if the legal regulations of public participation in the decision process were sufficient, but the negative opinions outweighed the positive ones. From the perspective of the energy policy, dialogue-type social communication is needed for every case of a new energy-mining project. Research on local public opinion should be the first step to opening up a social debate.     

Technical-economic aspects of small geothermal power plants

This paper describes the criteria followed in designing small geothermal steam turbines with particular reference to 3–5 MW portable units. Attention is drawn in particular to certain features of the machinery directed at a more rational exploitation of the geothermal resource.     

Particulate matter and gaseous emission rate from combustion of Thai lignite and agricultural residues in a fixed-bed combustor

This research has been conducted in order to obtain a database of emission rate of particulate matter and gases (CO, NO, and SO₂) from combustion of lignite and agricultural residues, such as rice husk. The experimental investigation was performed in a fixed-bed combustor. Thirteen stages–electrical low-pressure impactor was used to collect particles ranging in sizes from 40 nm to 10 μm. The results show that emission rate of total mass of particulate matter from combustion of rice husk is lower than that of lignite combustion but the total number of particles emitted is higher. This implies lower particle density from agricultural residue combustion. For co-firing lignite and rice husk, particulate matter emission is found to be higher than combustion of either lignite or rice husk and an increase in rice husk mass fraction in fuel mixture leads to an increase in particulate matter emission. From these quantitative data, it could be mentioned that the fuel characteristics influenced directly on particulate emission. For gaseous emission factors, CO and NO_x concentration decrease as SA/TA ratio increases. Meanwhile, SO₂ emission tends to increase. Both NO_x and SO₂ emissions are reduced as increased rice husk mass fraction in fuel mixture.     

Comparison of bituminous coal and lignite during combustion: Combustion performance,coking and slagging characteristics

The combustion characteristics such as combustion performance, coking, and slagging—at high temperatures (700–1300 °C) of bituminous coal and lignite were investigated and compared. The results show that the ignition temperature and the activation energy of lignite are lower than those of bituminous coal, and the combustion index and the burnout index are less than those of bituminous coal. Lignite has almost no coking while bituminous coal tends to coke at high temperatures. The larger the content and reflectivity of the vitrinite, the more severe the degree of coking. In the range of 700–1300 °C, the increase of temperature has little influence on the coking characteristics of lignite and bituminous coal. The low-rank lignite has larger amounts of mineral content which tend to form low-fusion-temperature eutectics. Furthermore, there is a connection between the combustion performance, coking and slagging characteristics through the maceral compositions: the coal which is hard to ignite but easy to burn out is more likely to have strong coking ability. Meanwhile, coking tends to keep alkaline oxides stay in the char and reinforce slagging at high temperatures.     

Mineralogy,geochemistry and leachability of ashes produced after lignite combustion in Amyntaio Power Station,northern Greece

The Kozani–Ptolemais–Amyntaio basin constitutes the principal coal field of Greece. Approximately 50% of the total power of Greece is generated by five power stations operating in the area. Pulverized lignite, a bottom ash, and a fly ash sample from the power plant of Amyntaio were subjected to a leaching procedure. The relative mass leached (RML) of three potentially toxic trace elements (Cu, Cr, Ni) was determined by use of AAS. Results indicate that the determined elements are more readily available in fly ash than in bottom ash or pulverized lignite. Mineralogical examination of the samples has revealed that calcite, gypsum, quartz, and feldspar are the constituents of lignite; quartz and calcite are found in fly ash particles; while gypsum, quartz, and calcite are found in bottom ash.     

Monitoring the availability of electrostatic precipitators (ESP) in automated biomass combustion plants

To achieve emission limit values on particulate matter emissions of less than 20 mg·m⁻³ at 11% O₂ volume fraction for 1 MW–5 MW, automated biomass combustion plants are often equipped with electrostatic precipitators (ESPs). To ensure low emissions, a high availability of the precipitators, i.e. a high ratio between the uptime of the ESP and the uptime of the boiler, has to be guaranteed including operation at part load and during start-up. In the present work, an investigation on seven heating plants with tube-type and plate-type ESP in the size range of 450 kW–3.5 MW was conducted. The signals on load, temperatures, fans, lambda sensors, voltage and currency were measured during two years. From these data, the availability is determined based on specific definitions for the operation modes of the ESP and the boiler. To evaluate threshold values for the ESP operation and to validate the method, gravimetric measurements on the particulate matter emissions in different operation modes were performed.The investigation reveals that ESPs in today's biomass plants can achieve availabilities of greater than 90%. Further, malfunctions and maintenance issues that are not immediately repaired are identified as main source for low availabilities, while availabilities of greater than 95% are considered achievable for new plants. To ensure optimum ESP operation with low particle emissions under real-life conditions, a reliable monitoring of the availability based on the presented method is recommended for automated biomass combustion plants.     

Employment benefits of electricity generation: A comparative assessment of lignite and natural gas power plants in Greece

This paper aims at developing an integrated approach for estimating the employment benefits associated with power-generation technologies. The proposed approach exploits the input–output methodology for estimating the direct, indirect and induced employment effects associated with the energy project in question, as well as two different valuation techniques, namely the “opportunity cost of labour” approach and the “public expenditures” approach, for expressing these effects in monetary terms. This framework has been implemented to estimate the employment benefits resulting from the development of a lignite-fired and a natural gas-fired power plant in Greece, taking into account all the stages of the corresponding fuel cycles that are undertaken domestically. The results of the analysis clearly show that lignite-fired electricity generation results in significant employment benefits amounting to 2.9–3.5 €/MWh in the basic scenario. On the other hand, the employment benefits associated with the examined natural gas unit were estimated at 0.4–0.6 €/MWh in the basic scenario. It is also worth mentioning that the significant environmental externalities of the lignite-fired electricity in Greece that have been presented in a number of studies can only be partially compensated by the estimated employment benefits.