Pulverized-coal Boiler Research Articles

Fly Ash Transformation and Fouling Tendency during Co-firing Biomass with Coal

The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coalfired power plants. This present study examined the effect of the co-firing of coal with biomass on the produced fly ash and the fouling tendency under condition representative of reheater region of pulverized-coal (PC) boiler. The fouling tests were conducted in a drop-tube furnace by inserting a water-air-cooled deposition probe to the point where the inner furnace temperature was 800-900 °C and probe’s metal temperature was kept at 500 °C. Bituminous coal was mixed with up to 50% (energy basis) of three types of biomass, namely; Palm Kernel Shell (PKS), Japanese cedar (without bark) and bark of cedar respectively. Fouling tendency was evaluated by determining the ash deposition ratio during co-firing tests. The properties of fly ashes and ash deposits for each sample were analyzed in detail by XRF and CCSEM analysis. Compared to coal firing, PKS or bark co-firing significantly increases the fouling tendency, whereas cedar co-firing does not affect the fouling tendency. The increase of minerals with low melting point, particularly Ca-Fe-Al-Si and K/Na-Al-Si, in fly ash accelerated the fouling tendency during co-firing.

Decreasing of the concentration quenching of fluorescent dye in a solution by introducing it to nanoparticles of silicon dioxide

The article presents the results of experimental studies of a double-chamber linear circuit DC electric arc plasmatron in experimental and industrial conditions. The electric arc plasmatron of the VORTEX-200 linear circuit with a rated power of 200 kW is designed for plasma ignition of a pulverized-coal torch, its stabilization and oil-free ignition of pulverized-coal boilers of thermal power plants.The linear circuit plasmatron contains two water-cooled casings isolated from each other, one of which has a hollow cylindrical electrode - a cathode having swirlers of the plasma-forming gas at both ends, and the other has an output electrode - an anode-diffuser having a channel with a cylindrical segment at one end and the exit nozzle in the form of a diffuser at the other end.The power of the plasmatron, depending on the process parameters, can vary from 80 to 230 kW. The plasmatron has a stable powerful air plasma torch. Tests of such plasmatrons showed their high reliability and possibility of their operation in difficult conditions of thermal power plants (high level of dust and humidity, high temperatures near the boilers). Measured volt-ampere characteristics of the plasmatron enabled us to determine the region of stable arc combustion from 350 to 475 A. Parameters of the plasmatron operation at its nominal power of 200 kW and the flow rate of the plasma-forming gas of 1000 l/min are attained at currents not exceeding 450 A. The use of mechanical ignition of the plasmatron with a needle from a refractory material, instead of breakdown the inter electrode gap by an oscillator, increased the reliability of its operation.The device for axial scanning of binding of the cathode spot of the arc, which allows us to increase the surface of cathode erosion many times,was developed. The use of azimuthal and axial scanning of the cathode spot of the arc of the plasma torch makes it possible to increase the service life of the cathode up to 200 hours with a service life of the massive anode exceeding 500 hours.

Combustion Process Analysis and Diagnostic Using Optical Flame Scanners in Front-Fired Pulverized Coal Boiler

The paper presents a novel concept and method of coal combustion process analysis using flame scanners supervision system. The combustion process analysis and diagnostic has a crucial influence on boiler effectiveness, especially in high variance of load demand, which is nowadays a top challenge for coal-fired power plants. The first indicator of combustion inefficiency is flame stability, which can be observed as variation of flame intensity. Nowadays, there are no validated measuring methods dedicated for industrial usage, which are able to give complete information about flame condition. For this reason, the research activity was launched and focused on usage of commercial flame scanners for fast combustion analysis based on on-line flame parameters measuring. The analysis of combustion process was performed for 650 t/h live steam power boiler, which is supplied by five coal mill units. Each coal mill supplies four pulverized coal burners pulverized fuel ((PF) burners). The boiler start-up installation consists of 12 heavy oil burners placed in PF burners equipped with individual supervisory system based on Paragon 105f-1 flame scanners, which gave the possibility to observe and analyze the PF burner flame and oil burner flame individually. The research included numerous tests in which the combustion conditions inside the combustion chamber were changed. During stable load of selected mills, the primary air flow, secondary air dampers, air–coal mixture temperature, and balance were changed. The results of the changes were observed by flame scanners and the available optical parameters of the flame were analyzed: power spectral density, average amplitude (AA) of flame fluctuation, and flame temperature.

Control of Thermal Power Plant Combustion Distribution Using Extremum Seeking

High demands for increasing robustness, safety, and efficiency in thermal power plants are the main motivation behind ongoing attempts to optimize combustion. This paper presents a study of modeling and control of the combustion process in a tangentially fired pulverized-coal boiler. It proposes an approach to flame geometry and position control by means of reallocation of firing. Such control ensures flame focus maintenance away from the walls of the boiler, and thus prevents many unwanted by-products of combustion. In addition, uniform heat dissipation over mills enhances the energy efficiency and reliability of the boiler. First, experimental data obtained from the 350-MW boiler of the Nikola Tesla power plant, Serbia, are analyzed in detail. This results in a model identification procedure using an adaptive parameter estimation method. Second, constrained multivariate extremum seeking (ES) is proposed in this paper, to optimally tune boiler operation in order to maintain the desired flame configuration in the furnace. Finally, the effectiveness of the ES adaptive controller in the presence of disturbances is demonstrated through simulations performed on the experimentally identified model of the boiler.

Co-firing high ratio of woody biomass with coal in a 150-MW class pulverized coal boiler: Properties of the initial deposits and their effect on tube corrosion

A demonstration test on the co-firing of a high-ratio of woody biomass (25% energy basis) with coal in a 150-MW class pulverized-coal boiler was conducted in Japan for the first time. To investigate the effect of the co-firing on the corrosion of the boiler heat-transfer tubes inside furnace wall and superheater, we collected the initial layer of the ash deposits (initial deposits) by inserting water-air-cooled probe simulating heat-transfer tubes during the test. Based on the results of the analysis of the collected initial deposits, we then prepared synthetic deposits and used them to conduct high-temperature laboratory-scale corrosion test for 500h. Carbon steel (STB410) and low-alloy steel (STBA22) were used to represent the furnace wall tubes, while two austenitic stainless steels (Ka-SUS304J1HTB and Ka-SUS310J1TB) were used to represent the superheater tubes. The co-firing was found to produce deposits in the furnace wall containing twice as much sulfates (mainly Na2SO4) compared to coal firing. A relatively high ratio of unburned carbon was also observed in the initial deposits of co-firing, indicating a strongly reducing atmosphere in the furnace wall. We found that the co-firing produced a higher corrosion rate in the furnace wall tubes at 500°C compared to coal firing. The corrosion risk, however, can be mitigated by decreasing the reduction capacity of the atmosphere. Further, the co-firing was found to produce two to three times as much alkali sulfate, but nearly the same low level of unburned carbon in the initial deposits obtained in superheater compared to coal firing. However, the increased sulfates concentration in the deposits had negligible effect on the corrosion of austenitic stainless steel tubes in the oxidizing atmosphere of the superheater at 650°C.

Numerical evaluation of heat transfer and NOx emissions under deep-air-staging conditions within a 600 MWe tangentially fired pulverized-coal boiler

Numerical evaluation of heat transfer and NOx emissions was conducted within a 600MWe tangentially fired pulverized-coal boiler configured with the deep-air-staging combustion technology, composing of concentric horizontal bias staged-air (CFS) burners and opposing tangential separated over-fire air (SOFA) burners. Refined char burning model considering gasification reactions and NOx model were implemented and validated with on-site measurement. Bell-shaped heat flux distributions with peaks severely deviating from the furnace center line appeared in the primary combustion zone, while heat flux distribution was uniform in the burnout zone. The results revealed that the larger the feed angle of CFS air, the more uniform the heat flux distribution with lower heat flux peaks. Concurrently, NOx emissions decreased by 12% from 250 to 220mg/m3 at 6% O2 and the optimum value occurred with CFS injection angle fixed at 15°. When the SOFA ratio was increased from 10% to 40%, distinct NOx reduction (levels from 358 to 174mg/m3 at 6% O2) associated with more uniform heat flux distributions was achieved. Owing to the intense turbulence mixing of the opposing tangential jets with the upward flow, saddle-shaped heat flux curves with slightly higher peaks appeared in the burnout zone as SOFA ratio was 40%.

Studies of ignition and combustion of coals subjected to electrochemical activation

Coal is one of the most important energy sources in the world. According to forecasts, by 2020, the share of coal in the global energy sector will reach 50%, primarily due to the very likely reduction in oil and gas consumption, as well as the revision of the policy on the development of nuclear energy. For thermal power plants with pulverized-coal boilers, a problematic economic and technological issue is ignition and lighting, carried out with high-reaction fuel - gas, fuel oil, and diesel fuel. The cost of this fuel is much higher than the cost of coal itself, which means that the introduction of new technologies that allow excluding petroleum products from the energy processes that take place at the CHP plant is topical [1]. In this paper, the technology of electrochemical ignition, which is an alternative to gas and fuel oil ignition of power boilers, is studied.

Industrial Trials of High-Temperature Selective Noncatalytic Reduction Injected in the Primary Combustion Zone in a 50 MWe Tangentially Firing Pulverized-Coal Boiler for Deeper NOx Reduction

This paper aims to demonstrate an industrial retrofit of NOx reduction through use of a new technology, high temperature SNCR (abbr. HTSNCR), in a 50MWe tangentially firing boiler of pulverized coal. Last several years, the characteristics of HTSNCR were investigated in the bench-scale experiments, as presenting the temperature window of SNCR can extend to higher temperature range in the absence of oxygen. Therefore, HTSNCR provides a promising option of greater NOx reduction via injecting urea solution or ammonia into primary combustion zone specialized with oxygen of nearly zero in furnace. These retrofit experiments in this paper successfully showed the ability of HTSNCR. And 68mg/Nm3 of NOx emission was finally achieved through use of the optimum hybrid application of HTSNCR, SNCR and OFA. The overall reduction efficiency is approximate 90%, in which 17% devoted by HTSNCR. The main factors in HTSNCR were studied extensively, including 1) primary stoichiometric ratio (SR1) of air staging, 2) normalized...

Mathematical modeling and experimental validation of ash deposition in a pulverized-coal boiler

A dynamic mathematical model of the fly ash transport and sticking in a pulverized-coal boiler was developed by a direct simulation Monte Carlo (DSMC) method. In this paper, the critical velocity model was used to calculate ash sticking rate during the initial deposition stage and the viscosity model was used to describe the outer layer deposition. The dynamic process of ash deposition was simulated and the distributions of adhesion particle size were obtained. The simulation results show that the ash deposit thickness increases by 47.52% with the load increases by 20% and the Alkaline Earth Metal (AAEM)-rich enhanced deposits of bulk ash particles are apparently stickier than the other ash particles. The experiments were carried out at two different loads and the morphology of ash deposit samples was analyzed by SEM/EDS. The propotion of particle size in 2μm were most in the deposition inner layer. The mean deposit thickness calculated by the DSMC method agreed with the measured value and the maximum relative error was 5.23%. A parametric study quantified the influence of changed operational conditions and ash material properties on the deposit formation process.

Modeling of ash deposition in a pulverized-coal boiler by direct simulation Monte Carlo method

In this paper, a mathematical model of the fly ash deposition in a 660MW supercritical once-through boiler was developed. The mechanism of particle-particle and particle-wall collisions has been considered. Particle collision is modeled by a direct simulation Monte Carlo (DSMC) method. Three submodels (impaction, thermophoresis, and sticking) were included in the mathematical model. In the ash deposition model, the critical velocity model was used to calculate the ash sticking rate during initial deposition, and the viscosity model was used for the outer layer deposition. An ash deposition probe was inserted into the peephole and ash deposits were collected. The morphology of these samples was analyzed using SEM. The deposit thickness and the distributions of adhesion particle size were obtained by mathematical model. The mean deposit thickness calculated by the DSMC method agreed with the value measured by the ash deposition probe with the relative error within 3.79%. The simulation results show that the deposition inner layer consists of fine particles less than 10μm in size and sticky fine particles were selectively captured during the initial deposition stage. The DSMC method may be a practical alternative for studies of ash deposit formation.

Effects of wall temperature on slagging and ash deposition of Zhundong coal during circulating fluidized bed gasification

Zhundong coal with a large reserve is faced severe ash-related problems including slagging and fouling during pulverized-coal boiler combustion. The gasification in circulating fluidized bed is a good way to use Zhundong coal due to the low reaction temperature, while it is unknown for the slagging and ash deposition mechanism of Zhundong coal during circulating fluidized bed gasification. In this paper, the gasification of Zhundong coal was tested in a 0.4t/d CFB test rig simulating the real industrial device. Ash deposition probes were installed along the gas flow direction to characterize its slagging and ash deposition at different wall temperatures. These probes were treated with three cooling methods (no cooling, air cooling or water cooling) to reach different wall temperatures (882–737, 665–429 and 81–45°C). The results reveal that deposition was affected greatly by wall temperature. Mineral elements including Na, Ca, Fe, Al and Si were likely to accumulate in deposits on high-temperature surfaces, and existed in the forms of silicates and sulfates at wall temperature above 882°C, resulting in slagging. Although the contents of most mineral elements in the deposits on cooled surfaces were always low, a large gas-wall temperature gradient triggered the condensation of gaseous species, such as Na, Cl and K. The CaSO4 and CaO in gases were condensed and restructured in an ordered crystalline array on the surfaces of ash particles, forming a coating layer that made the ash particles stickier. Chemical composition and crystallographic analyses show that the properties of deposits on the probes and gas-borne ashes were different. More mineral phases as well as sodium were present in deposits. Additionally, due to the Cl enrichment on air-cooled surfaces, the corrosion of the metal surfaces should be focused.

Numerical investigation of air-staged combustion emphasizing char gasification and gas temperature deviation in a large-scale, tangentially fired pulverized-coal boiler

A refined char gasification model, successfully validated in a pilot-scale 20kW down-fired furnace, is now applied to a numerical investigation of the characteristics of the flow, temperature, and species distribution under various air-staged levels of combustion in a 600MWe tangentially fired (T-fired) pulverized-coal (PC) boiler. The simulation results with char gasification show that the CO concentration profile in both the primary combustion zone and the reduction zone is much higher than the corresponding case without the gasification model for deep (burnout air rate, fS=0.42), middle (fS=0.30), and shallow (fS=0.17) air-staged cases. Moreover, this result is in accordance with the tests from an industrial pulverized-coal-fired furnace. It can be concluded that the char gasification mechanism should be considered in the numerical simulation of large-scale air-staged T-fired PC boilers. On the basis of a reasonable prediction of combustion characteristics, the gas temperature deviation in the crossover pass was also depicted under conditions of various air-staged levels. The result of the thermal load curve of the final super-heater panels clearly presents a saddle-type distribution for the existing two peak values. These inherent deviations originate from the residual swirling flow at the furnace exit. More specifically, parameters of swirling momentum intensity (δ) in the furnace and heat flow intensity (Ψ) at the entry of the final super-heater were employed to identify the temperature deviation in degrees.

Reduction of pulverized coal boiler's emissions through ReEngineered Feedstock™ co-combustion

This paper presents an experimental study of ReEF™ co-firing with coal in a pilot-scale PCB (pulverized coal boiler). ReEF™, or ReEngineered Feedstock™, is a novel fuel composed of both waste and alkaline sorbent, combining therefore the PCB emission control techniques of waste derived fuel co-firing and that of sorbent blending. The results show that ReEF™ co-firing resulted in a higher sorbent utilization and a higher sulfur emissions reduction than sorbent alone. Several combinations of calcium-based and sodium-based sorbents were tested at different stoichiometric ratios. Emissions of SO2, NOx and HCl at different feed compositions, furnace temperatures and air injection patterns were measured. SO2 emission reductions of up to 55% were observed during experiments at gas residence time 4 times lower than that of pulverized coal boiler. The slagging propensity of the ashes in the reactor was also studied. Finally, a model was developed that satisfactorily predicted the experimental SO2 emissions. It was further applied to conditions typical of existing PCB.

Ash transformation by co-firing of coal with high ratios of woody biomass and effect on slagging propensity

The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants, as well as for the utilization of forest resources. This present study examined the effect of the co-firing of coal with woody biomass on the produced combustion ash and slagging propensity under different conditions representative of the wall furnace region of pulverized-coal boilers. The slagging tests were conducted in a drop-tube furnace by inserting a water–air-cooled deposition probe to the point where the inner furnace temperature was 1300°C. Bituminous coal was mixed with up to 70% (energy basis) of four different Japanese woody biomasses, namely; sakura wood (Prunus spp.), sugi (Cryptomeria japonica), nara (Japanese oak), and bark of sugi respectively. For comparison purposes, pure coal firing was also performed. The collected combustion ashes and ash deposits were characterized by computer-controlled scanning electron microscopy analysis while the slagging propensity was evaluated by determining the ratio of the deposited ash to the fuel ash, the so-called ash deposition ratio. The results showed no increase in the ash deposition ratio or significant change in the properties of the ash for co-firing using up to 50% of the low-ash (0.4wt%) biomasses (sugi and sakura). In contrast, there was significant change in the properties of the ash and an increase in the ash deposition ratio with increasing biomass ratio for co-firing using the high-ash (>1.0wt%) biomasses (nara and bark of sugi). The significant transformation of ash with regard to its morphology and chemical composition, together with the formation of eutectic calcium mineral mixtures in the deposit layer enhanced the slagging propensity during the co-firing process, particularly when using the high-ash biomasses. The use of a low-ash biomass was found to be suitable for preventing severe slagging during the co-firing process.

Co-combustion of coal and waste in pulverized coal boiler

A novel reactor reproducing PCB (pulverized coal boiler) environment was used to evaluate the potential of sorbent co-firing and waste co-combustion as cheap retrofitting techniques to reduce SO2 emissions of existing boiler. Several co-feeding tests of coal and CaCO3 and coal and Ca(OH)2 have been performed for a gas residence time around 0.5 s (4 times shorter than PCB). Results show sulfur emission reduction up to 18% with Ca(OH)2 injection, and up to 20% with limestone for a Ca/S ratio of 2. Co-firing of a 20% thermal share of a waste derived fuel called ReEF™, led to SO2 emissions reduction of around 20%. Co-firing of this ReEF™ led also to HCl emissions of more than 20 ppm due to the PVC content of the ReEF™.In order to describe the gas and particles evolution along the reactor height, a model combining almost 30 kinetic reactions from the literature and without any fitting parameters was developed. The model shows that the ReEF™ particles had higher residence time in the furnace than coal and sorbent particles. For the temperature and air injection pattern tested, the model accurately predicted the emissions measured at the exit of the reactor for all the feed compositions.

Using a genetic algorithm and CFD to identify low NOx configurations in an industrial boiler

Abstract This paper focuses on a computational intelligence approach used for minimizing NOx emissions in a 600 MW tangentially-fired pulverized-coal boiler. Genetic Algorithms (GA) were used to correlate operating parameters to significant output data predicted by CFD simulations of the boiler. The operating parameters include the opening or closing of air dampers, changing the coal distribution through mill selection and feed rate and vertical tilting of the burners. A target function was introduced to estimate for each boiler settings defined by given operating parameters, the costs associated with corrosion on the water-wall tubes, heterogeneous heat flux distribution along the walls, unburned carbon in fly ash and NOx emissions. The GA was able to automatically generate innovative boiler configurations among thousands of CFD calculations performed. The target function allowed the search space to be explored to establish configurations offering a good compromise between NOx reduction and the cost associated with corrosion in particular. Moreover, the predicted NOx emissions from the GA model are consistent with the NOx levels measured during test campaigns.

An approach for estimation of water wall degradation within pulverized-coal boilers

The main aim of this paper is to estimate the lifetime of water walls of pulverized-coal boilers at nominal conditions as well as after degradation of water tubes. An approach for a pulverized-coal chamber degradation process has been formulated based on operational and experimental data. This model was formulated using on-line state monitoring of a pulverized coal burner with aim of preventing the fireplace screens from high degradation and reducing large amounts of coal in the ashes. The rate of metal loss in the pipe wall from the fireplace was defined by a closure comprising of two parts. The first part describes degradation induced by impact of un-burned carbon, and the second part describes degradation caused by stress corrosion. In this paper, high-temperature creep in the pipe of the evaporator working in the powder-fired boiler is modelled and calculated. It has been shown that the lifetime of the element under consideration is essentially shortened by the influence of oxide growth on the steam side, an increase in metal temperature and erosion–corrosion wear on the flue gas side.

ICOPE-15-C078 Effects of vent air angles on combustion and NO emission characteristics for a down-fired 600MW pulverized-coal utility boiler retrofitted with a novel combustion system

ICOPE-15-C078 Effects of vent air angles on combustion and NO emission characteristics for a down-fired 600MW pulverized-coal utility boiler retrofitted with a novel combustion system

Numerical investigation of low NOx combustion strategies in tangentially-fired coal boilers

A numerical model is developed to investigate the effects of horizontal bias combustion (HBC) and air staging combustion (over-fire air, OFA) technologies on the performance of a 200MWe tangentially-fired pulverized-coal boiler. The devolatilization rate and volatiles amount are determined by a kinetic devolatilization model, which predicts the coal devolatilization prior to the volatile matter and char combustion. The characteristics of the devolatilization, combustion, heat transfer and NOx emission are studied and compared to achieve a comprehensive understanding of the low NOx combustion. The prediction shows a good agreement with the on-site measurement results, which confirms that the model is capable of predicting the characteristics of the investigated boiler. The predicted results have shown that the OFA has a remarkable effect on the reduction of NOx emission. The HBC makes a significant NOx reduction in the primary combustion zone (PCZ) when there is no air staging. In terms of the NOx reduction, the air staging plays a dominant role in comparison with HBC burners. The application of OFA tends to lead to slagging in the PCZ, which can be avoided using HBC due to the higher stoichiometry close to the furnace wall. The details of this study improve the understanding of combustion and NOx emissions in tangentially-fired pulverized-coal boilers with low NOx combustion technologies, especially for boilers adopting HBC burners and OFA.

Fouling and Its Mitigation in PC Boilers Co-Firing Forestry and Agricultural Biomass

This paper presents a study of the fouling properties of solid fuels based on analyses of ashes from hard coal, forestry biomass, and agricultural biomass, ash deposits collected directly from the boiler surfaces, and ash deposits collected from a deposition probe. All samples were collected in the area of the steam superheater/reheater of a OP 380 pulverized-coal boiler co-firing forestry and agricultural biomass. In addition, the study concerns mitigation of unwanted fouling through application of a silica-based fireside additive. An analysis was carried out to obtain the following information: physical and chemical properties of ash produced during co-firing of a biomass mixture (50 wt.% forestry/50 wt.% agricultural biomass) and coal in order to determine its propensity for fouling and slagging without the use of a fireside additive; slagging/fouling indices during co-firing of a biomass mixture (50 wt.% forestry/50 wt.% agricultural biomass) and coal with the use of a fireside additive; and slagging/fouling indices during co-firing of pure agricultural biomass (sunflower husk) and coal with the use of a fireside additive. The mass flow of co-fired biomass fuel supplied to the boiler during reference measurements and mass flow when the additive was fed were the same and amounted to 33 wt.% of the total fuel mass flow. Finally, the effectiveness of fouling mitigation is determined by comparing the results of the deposition rate obtained with the use of a deposition probe for all three cases of co-firing.