Combustion Of Coal Blends Research Articles

Experimental investigation on combustion and NO formation characteristics of semi-coke and bituminous coal blends

Experimental studies were conducted by thermogravimetric analyzer and drop tube furnace to clarify the co-combustion and NO formation characteristics of semi-coke and bituminous coal. The same experiments were carried out on the blend of anthracite and bituminous coal for comparison. Results indicated that the ignition and burnout temperatures of the blends decreased as the blending ratio of bituminous coal increased, and there existed significant interaction between semi-coke (or anthracite) and bituminous coal. The ignition index C and comprehensive combustion index S presented negative correlations with fuel ratio, and the relationships of C = 6.31–0.35FR and S = 3.87–0.26FR can be used to estimate the co-combustion characteristics of bituminous coal and semi-coke (or anthracite). The addition of bituminous coal gave rise to two competitive effects on semi-coke burnout and NO formation. When the bituminous coal accounted for 80%, the promotion on burnout was superior and the inhibition on NO formation was significant. Fuel ratio, rather than nitrogen content played more important role in the NO formation, and relationship of XNO = 40.96–4.24 N + 1.60FR was established to predict the NO formation during co-combustion of binary coal-based fuels. Besides, semi-coke had better combustion behavior and lower NO conversion ratio compared to anthracite, which mainly resulted from the more porous structure in semi-coke.

Combustion interactions of blended coals in an O2/CO2 mixture in a drop-tube furnace: Experimental investigation and numerical simulation

Interactions, known as the ignition promotion and burnout inhibition, exist during the combustion of blended coals in the air. The oxy-fuel conditions influence these interactions due to the change of the gas physical properties and the gasification reaction. However, little research has focused on the oxy-fuel combustion interactions of blended coals. To investigate the combustion interactions of blended coals in an O2/CO2 atmosphere, experimental and numerical studies of the mixed combustion of less- and more-volatile coals are performed in a drop tube furnace. The results show that the more-volatile coal combustion brings the promotive effect on the devolatilization of the less-volatile coal and the inhibitive effect on the char combustion of the less-volatile coal. Compared with that in the air, the promotive effect is weaker, but the inhibitive effect is stronger in the 21%O2/79%CO2 environment due to the different physical properties of the gases. In the O2/CO2, increasing the oxygen concentration increases the promotive effect but weakens the inhibitive effect owing to the change of the physical properties of gases and the reactivity improvement of the less-volatile coal char. In addition, the promotive and inhibitive effects achieve a level similar to that in the air when the oxygen concentration is increased to approximately 30–35%. Furthermore, the char-CO2 gasification reaction in the O2/CO2 has little impact on the promotive effect but weakens the inhibitive effect. Increasing the reactor wall temperature weakens both the promotive effect and the inhibitive effect.

Effect of different conditions on the combustion interactions of blended coals in O2/CO2 mixtures

The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO2 emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O2/N2 mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O2/CO2 mixture. In the O2/CO2 mixture, both increasing the O2 concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O2/CO2 atmosphere.

Utilization of apricot seed in (co-)combustion of lignite coal blends: Numeric optimization, empirical modeling and uncertainty estimation

Utilization of apricot seed (AS) in lignite coal (LC)-based (co-)combustion process was aimed in the present study considering the apricot production capacity of Turkey. By this way, an alternative and also ecofriendly way was suggested for coal-based energy production plants located in Turkey. This purpose was tested by thermogravimetric analyses to demonstrate the advantageous sides of AS in reduction of ash amount and also environmental aspects based on harmful gases. The other important contributors of present study was the comparison of both statistical modeling and numeric optimization techniques for maximization of mass loss percentage (MLP, %) in response to (co-)combustion process. For this purpose, multiple non-linear regression (MNLR) and artificial neural network (ANN) models as data-driven modeling techniques, and response surface methodology (RSM) and particle swarm optimization (PSO) as numeric optimization approaches were utilized. Results demonstrated the accuracy of ANN and PSO in prediction of MLP (%) and optimization of operating conditions of (co-)combustion of AS and LC, respectively. Finally, Bayesian approach was applied to the best-fit MNLR model to identify the uncertainties in predictors of proposed model. Bayesian was found quite effective in identification of uncertainties that were not possible to be captured through deterministic ways.

Simulation of one-dimensional heat transfer system based on the blended coal combustion

In this paper, the supercritical boiler thermodynamic calculation model is studied. Three types of heat exchangers are proposed, namely furnace (total radiation type), semi-radiation and convection, and discussed. Two cases were simulated - mixing of two bituminous coals and mixing of a bituminous coal and lignite- order to analyze the performance on the flue gas side. The study shows that the influence of flue air leakage and gas distribution coefficient on the system.

Study on the coal mixing ratio optimization for a power plant

For coal-fired power plants, the application of blended coal combustion has been a great issue due to the shortage and rising prices of high-rank coal. This paper describes the optimization of blending methods between Xing'an lignite coal, Shaltala lignite coal, Ura lignite coal, and Inner Mongolia bituminous coal. The multi-objective decision-making method based on fuzzy mathematics was used to determine the optimal blending ratio to improve the power plant coal-fired economy.

Characteristics of fine particulate matter formation during combustion of lignite riched in AAEM (alkali and alkaline earth metals) and sulfur

The high contents of AAEM (alkali and alkaline earth metals) and sulfur in zhundong coal result in more fine particulate matter during coal combustion. In this paper, the emission of fine particulates from zhundong coal riched in AAEM-sulfur and zhunnan coal riched in aluminum-silicon was investigated in a drop tube furnace. Co-combustion of zhundong coal and zhunnan coal was further investigated to study the effect of interaction among different elements in coal on the PM formation. The size distribution, concentration and composition of the particulates were analyzed by low pressure impactor (DLPI) and scanning electron microscopy (SEM). The distribution of sodium, magnesium, calcium, iron and sulfur in fine particulates and their effects on the generation of fine particulates were obtained. The results show that the fine particulates from AAEM-sulfur rich coal combustion mainly consist of sulfates and oxides of AAEM. The amount of fine particulates from burning aluminum-silicon rich coal decreased significantly. The composition of PM0.4 and PM0.4+ are significantly different. Co-firing of AAEM-sulfur rich coal with aluminum-silicon rich coal has an obviously synergetic effect of reducing the fine particulates emission. The capture of AAEM and iron by aluminum-silicon compounds plays an important role in PM10 reduction during blended coal combustion. The change of sulfur content in PM0.4 is consistent with the change of AAEM content.

Study of flame combustion of off-design binary coal blends in steam boilers

Changes in the structure of the fuel consumption by the thermal power stations of Ukraine caused by failure in supplying anthracite from the Donets Basin are analyzed and the major tasks of maintaining the functioning of the coal industry are formulated. The possibility of using, in the near future, the flame combustion of off-design solid fuels in the power boilers of the thermal power plants and combined heat and power plants is studied. The article presents results of expert tests of the TPP-210A and TP-15 boilers under flame combustion of mixtures of anthracites, lean coal, and the coal from the RSA in various combinations. When combusting, such mixtures have higher values of the combustibles yield and the ash fusibility temperature. The existence of the synergetic effect in the flame combustion of binary coal blends with different degrees of metamorphism is discussed. A number of top-priority measures have been worked out that allow for switching over the boilers designed to be fired with anthracite to using blends of coals of different ranks. Zoned thermal analysis of the TP-15 boiler furnace was performed for numerical investigation of the temperature distribution between the furnace chamber zones and exploration of the possibility of the liquid slag disposal and the temperature conditions for realization of this process. A positive result was achieved by combusting anthracite culm (AC), the coal from the RSA, and their mixtures with lean coal within the entire range of the working loads of the boilers in question. The problems of normalization of the liquid slag flow were also successfully solved without closing the slag notch. The results obtained by balance experiments suggest that the characteristics of the flame combustion of a binary blend, i.e., the temperature conditions in the furnace, the support flame values, and the degree of the fuel burnout, are similar to the characteristics of the flame of the coal with a higher reactive capacity, which proves the existence of the synergetic effect in the processes of cocombustion of coals of various grades.

Effects of Coal Blending and Operating Conditions on Combustion and NOx Emission Characteristics in a Tangentially-fired Utility Boiler

The influences of operating conditions, such as excess air ratio and SOFA arrangement, on temperature distributions, species distributions, NOx emissions and burnout of a tangentially-fired utility boiler using bituminous and blended coal were investigated by numerical approach in the present study. The simulation results show that the fuel type exhibits certain effect on the temperature distributions and the NOx emissions. Moreover, higher excess air ratio could restrain the CO formation but promote NOx emission. The temperature and the species distributions are highly related to the SOFA arrangement. In addition, SOFA technology is still an effective approach to control the NOx emissions during blended coal combustion, while the disadvantageous effect on char burnout should be considered.

25 Simulation of PM_ emissions during combustion of coal blends : Development of a coalescence-fragmentation model for coal minerals

25 Simulation of PM_ emissions during combustion of coal blends : Development of a coalescence-fragmentation model for coal minerals

Co-firing coal and biomass blends and their influence on the post-combustion CO2capture installation

Co-firing of biomass with coal for energy production is a well-known technology and plays an important role in the electricity sector. The post-combustion capture integrated with biomass-fired power plants (Bio-CCS) seems to be a new alternative for reducing greenhouse gas emissions.This study refers to the best known and advanced technology for post-combustion CO 2 capture (PCC) based on a chemical absorption in monoethanolamine (MEA). The co-firing of hard coal with four types of biomass was investigated using a laboratory fixed bed reactor system. The comparison of gaseous products emitted from the combustion of coal and different biomass blends were determined using gas chromatography.Research proved that co-firing of biomass in fossil fuel power plants is beneficial for PCC process. It may also reduce the corrosion of CO 2 capture installation. The oxygen concentration in the flue gases from hard coal combustion was comparable with the respective value for a fuel blend of biomass content of 20% w/w. It was also noted that an increase in biomass content in a sample from 20 to 40 % w/w increased the concentration of oxygen in the flue gas streams. However, this concentration should not have a significant impact on the rate of amine oxidative degradation.

Effect of ash preparation method on the sintering characteristics of ashes from combustion of coal and biomass blends

Sintering behaviour of ashes from combustion of coal and biomass blends was experimentally investigated with a focus on the effect of ash preparation method. Blends of a Chinese anthracite, Jincheng coal, and two biomass materials, a wheat straw and a pine sawdust, respectively, were subjected to three different ash preparation procedures, namely, a low temperature oxygen plasma ashing device operating at ca 150°C, combustion in a Muffle furnace at 815°C and in a drop tube furnace at 1500°C. The resulting ash samples were then subjected to the sintering temperature measurement using a pressure-drop sintering device, morphological and mineralogical characterisation using scanning electron microscope (SEM) fitted with X-ray energy dispersive spectroscopy (EDS) and X-ray diffractometry analyser (XRD), respectively. For the same coal and biomass blends but different ash preparation methods, the sintering temperatures were always the lowest for the ash samples from the plasma ashing device and the highest from drop tube furnace. SEM imaging showed that the texture of ash samples from the plasma ashing device was irregular, loose and more fibrous, indicating little interactions among the different mineral and inorganic constituents in the individual fuels. The muffle furnace and drop tube furnace ashes were denser and more uniform in shape as the mineral matter and inorganics became fused together. In addition, the drop tube furnace ashes were mostly in spherical-shape, indicating ash melting had occurred during combustion. The XRD analysis revealed that different minerals were present in the ash samples due to different ash preparation temperatures, with high temperature minerals present in the drop tube furnace ash. Biomass addition in the blends lowered the sintering temperature of corresponding ash; the higher the proportion of biomass in the blend, the lower the ash sintering temperature regardless the ashing method. At the same biomass addition ratio, the ash sintering temperature of the blend with the wheat straw addition was lower than that with the pine sawdust addition, because the wheat straw had a higher K and Cl content but lower Ca than the pine sawdust. The EDS and XRD analyses confirmed that the ashes of coal/wheat straw blends contained more K-bearing minerals, while the ashes of the coal/pine sawdust contained more Ca and K-bearing minerals. Both Ca and K-bearing minerals reacted with other minerals in the ash to form low temperature eutectics, leading to the reduction of sintering temperatures of the coal and biomass blends. However, the melting points of the K-containing minerals are higher than the Ca-containing minerals, therefore, the sintering temperatures of ash with the wheat straw addition were lower than those with the pine sawdust addition.

Ash deposition behaviours upon the combustion of low-rank coal blends in a 3 MWth pilot-scale pulverised coal-fired furnace

This paper aims to clarify the characteristics of ash deposits formed upon the blending of two low-rank lignites that have distinctively different properties, i.e. one has a low ash melting point, rich in Na and S, while another has a higher ash melting point, rich in Ca and Cl. A 3 MWth pilot-scale furnace has been used for the combustion, with a variety of cooling oil-jacketed probes installed along the furnace for ash deposit sampling. As have been confirmed, there are strong synergetic interactions between the ash-forming elements of the two lignites. At the mass fractions of 50% and 70% for the high-Cl coal, the ash fusion temperatures are even higher than the two single coals. The synergetic interactions between two coals enhanced the formation of crystallised species such as hematite, sulfates and carbonates that are mixed with Ca-Si-O matrix to reduce its stickiness and slagging propensity in the burner vicinity. The blending of up to 50wt% high-Cl coal resulted in the enhanced emission of gaseous HCl and Cl2, whereas the high-Cl coal mass fraction above 50wt% enhanced the formation of NaCl in super-heater ash deposits, via the chlorination of sodium from low-Cl coal, which are detrimental to tube corrosion. The Cl chemistry is complicated for the lignite blending combustion. The emission of gaseous Cl2 is remarkable, accounting for 6–8% of the total chorine that is far higher than the predicted values based on the thermodynamic equilibrium. The Cl2 emitted at high temperatures is also stable, with almost all the consumption reactions being frozen upon the rapid quenching of flue gas. The blending of up to 50wt% high-Cl coal is viable in minimising ash slagging and fouling. However, special materials and/or coating technologies should be employed to minimise the Cl2-related corrosion for the water tubes.

The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace

Ash deposits formed on an air-cooled probe at different temperatures during combustion of coal blends of a Zhundong lignite (ZL) and an Australian bituminous (AB) were studied in the present work. The blends with various AB ratios were combusted in a drop tube furnace in air at 1400°C, and the ash deposits formed on the probe top surface maintained at 500°C, 600°C, 700°C, and 750°C, were collected and characterised using XRD and SEM-EDS for their mineralogy, morphology and elemental composition. The size distributions and sintering temperatures of the ash deposits were also determined. At the probe temperature of 500°C, the ZL ash deposit was dominated by fluxing minerals where significant amounts of fine particles (<10μm) and their aggregates existed, leading to low ash sintering temperatures. As the AB ratio increased to 20wt% and above, the amount of the fluxing minerals in the deposits decreased while the amount of refractory quartz and mullite minerals increased. Morphologically, the refractory ash particles from AB coal diluted the Na-bearing mineral phases and the contacts between fine ash particles, leading to the absence of nepheline and, correspondingly, an increase in the ash sintering temperature. In addition, as the probe temperature increased, mineral interactions and ash sintering were facilitated when the AB ratio was less than 20wt%, leading to the formation of new mineral phases and an increase in ash particle sizes. However, no significant mineralogical and morphological changes were observed when the AB ratio was higher than 40wt%, indicating that these ash deposits were more refractory and resistant to the temperature change.

Formation of Reductive and Corrosive Gases during Air-Staged Combustion of Blends of Anthracite/Sub-bituminous Coals

Air-staged combustion of blended coals is now a common choice for coal-fired boilers in China as a primary method to control NOx emission. Reductive gases, CO and H2, and corrosive gases, H2S and COS, are generated in the fuel-rich zone which leads to problems of high temperature corrosion, slagging and fouling under a reducing atmosphere. This study focuses on the generation properties of reductive and corrosive gases during blended coals air-staged combustion. Experiments of anthracite, sub-bituminous, and their blends with different mixing ratios were conducted in a drop tube furnace. Generation characteristics of reductive gases, CO and H2, and corrosive gases, H2S and COS, in the fuel-rich zone were carefully studied at 1573 and 1673 K with different stoichiometric ratios in primary combustion zone. Blending effects on char burnout and NOx emission were also reported. It was found concentrations of CO and H2 increase obviously in the fuel-rich zone by blending sub-bituminous into anthracite due to th...

Reduction of fine particulate matter by blending lignite with semi-char in a down-fired pulverized coal combustor

With the increasing use of low-rank coal, the upgraded semi-char produced through low-temperature pyrolysis of lignite is one of promising alternatives towards the clean and high efficient utilization. The co-firing of these pyrolytic semi-chars with parent coals has received more attention. In this paper, a kind of semi-char, produced from Hulunbuir (HB) lignite under a pyrolysis temperature of 504°C, was burned in a 25kW self-sustained pulverized coal combustor. Different mass blending ratios of semi-char (0%, 40%, 60% and 100%) were used. The results indicate that the PM0.1 yield from pure combustion of HB lignite is ∼1.6 times of that from semi-char. The PM0.1 yields of the coal/char blends are much less than the linear summation of yields of two blending fuels without considering their interactions. As for coarser PM mode, the reductions still exist for both PM0.1–1 and PM1–10 particles. The different PM yields of pure combustion cases are attributed to less volatile mineral species and also less fragmentation degree of semi-chars during combustion. A mechanism is postulated for PM reduction during the combustion of coal blends. PM0.1 amounts are decreased since the volatilized mineral species is considerably scavenged by the previously formed semi-char surfaces, as partly evidenced from a time scale analysis. The scavenged compounds then serve as “glue” to convert ash particles to larger agglomerates, which may result in the reduction of PM0.1–10 yields.

5-2-4 微粉炭ボイラにおける混炭燃焼時の灰付着特性評価に関する研究(Session 5-2 燃焼・熱利用II)

5-2-4 微粉炭ボイラにおける混炭燃焼時の灰付着特性評価に関する研究(Session 5-2 燃焼・熱利用II)

Examinations of the process of hard coal and biomass blend combustion in OEA (oxygen enriched atmosphere)

The benefits of oxygen enrichment have been demonstrated in a variety of industrial combustion applications, but to date no implementation of oxygen enrichment in boilers has been reported, primarily due to their already high thermal efficiencies and a very large scale of such systems, which require significant amounts of oxygen. But recently, oxygen combustion in boilers has become one of the CCS technologies which can be an effective tool for reducing greenhouse gases emissions, and oxygen enriched combustion is suitable for low-calorific fuels, including biomass.This paper analyses the use of oxygen enrichment in a furnace for co-firing of different kinds of biomass with hard coal in terms of emission and burnout impact (LOI). As a part of this research, the effect of injection oxygen mode and total oxygen concentration on the flue gas emission (SO2, NOx) and burnout from co-firing of straw and wooden biomass in different proportions (20% and 40%) with hard coal were studied. The co-firing tests were carried out in an isothermal flow reactor. One of the benefits from the OEA (oxygen enriched atmosphere) technology is more effective separation of CO2 owing to the higher CO2 concentration in the flue gas. The additional advantage of the OEA combustion technology in comparison with oxy-fuel combustion is that the OEA process needs lower O2 purities and therefore it is cost-effective.Experimental tests on co-firing of 20% straw-hard coal blend were conducted in oxygen enriched (up to 25 and 30%) atmospheres with three variants of O2 injection modes. NOx, SO2 emissions and burnout for the various atmospheres in the combustion chamber were studied. Moreover, co-firing tests were performed with 40% share of wooden biomass to examine the effect of the biomass share and a type on emission of NOx and SO2 in OEA. The two O2 injection modes were investigated.In each case, the emission of SO2 increases alongside an increase of oxygen concentration in atmosphere. However, the experimental results show that stable and relatively low SO2 emission and low NOx emission can be achieved for OEA when oxygen is supplied to the primary stream of air. Also, minimal LOI (loss on ignition) is obtained for OEA I+. The supply of oxygen to the secondary oxidizer stream into the burner results in highest NOx emission only in the case of OEA30. It can be therefore concluded that both the oxygen enriched atmosphere and the O2 injection mode affects NOx emission.

Numerical Modeling and Experimental Investigation on the Use of Brown Coal and Its Beneficiated Semicoke for Coal Blending Combustion in a 600 MWe Utility Furnace

In this paper, coal blending combustion performance in a 600 MWe supercritical boiler has been investigated through a series of on-site measurements and computational fluid dynamic modellings. A bituminous coal was used as reference, which was blended with brown coal or brown coal semicoke in an indirect way, i.e., different coals being fed separately into the boiler without prior mixing in the mill. The coal blending combustion behavior was assessed upon varying brown coal injection location, brown coal blending ratio, air staging ratio, and the use of brown coal pyrolysis-derived semicoke. ANSYS FLUENT 15.0 was employed for numerical simulations. Upon a successful validation, the model was further used to predict the influence of individual variables. The main conclusions achieved include (1) the bituminous coal-designed mill can be used safely for brown coal, at the expense of an increased mass ratio of primary air to coal, a decreased temperature for the outlet of the mill, and an incomplete drying of brown coal. (2) The allocation of brown coal in the middle burner zone is the best option that ensures a relatively long residence time of bituminous coal particles in the boiler, as well as provides sufficient heat to bituminous coal particles for a quicker ignition and burnout. (3) The 50 wt % blending ratio for brown coal is optimum, striking a good balance between the feeding amounts of combustible hydrocarbons and inherent moisture into the burner. (4) The separate over fire air of 30% ratio is beneficial in reducing total NOx emission through increasing the unburnt char quantity in the NOx reduction zone. (5) The high heating value of beneficiated Victorian brown coal and its semicokes ensures a lower feeding rate for each of them to be used as a substitute fuel. The high volatile-O2 and char-O2 oxidation reactivity of these samples also ensured a strong heat provision from their flame to the bituminous coal allocated in both the bottom and top of the burner zones. (6) The use of brown coal and semicoke is in favor of the reduction in NOx, particularly fuel-NOx, due to the generally low nitrogen content within these samples, relative to bituminous coal.

An online case-based reasoning system for coal blends combustion optimization of thermal power plant

Coal blending is becoming increasingly common as more and more off-specification coals are received in coal-fired power plants, given the present coal market in China. This situation requires optimization of the operating parameters for matching the varying coal properties. The motivation for such optimization includes confirming good performance of the units regarding the security, the economy and environmental protection. However, the current adjustments to operation of the plant rely mostly on human experience because of the imperfections of existing theoretical models for coal-blend combustion. In this paper, a Case-Based Reasoning (CBR) method providing online decision-making for optimization of coal-blend combustion was investigated using cases representing successful operation of the unit for specific coal blends and loads. A case base containing a wealth of knowledge about optimal operation modes was constructed from a large number of cases. The development process for the CBR system includes case design, case evaluation, case generation, case retrieval and case reasoning. Case evaluation focused mainly on heating surface security, output capability, slagging tendency, comprehensive fuel consumption and pollutant emissions. Five indexes were introduced to quantify the above characteristics based on actual combustion parameters. A case-generating algorithm employing an evolutionary strategy was proposed in which the case base evolves while retaining new cases. Two methods for measuring case similarity – termed entirely similarity and eigenvalue similarity – were used for case retrieval. Run-time optimization strategies were recommended by the case-reasoning model based on the current operating status. The CBR system using Browser/Server framework were successfully applied to a 600-MW power plant, which provided an opportunity for coal-blend combustion optimization.