Co-firing Of Coal Research Articles

The potential oxidation characteristics of CaCr2O4 during coal combustion with solid waste in a fluidized bed boiler: A thermogravimetric analysis

CaCr2O4 (Cr (III)), mainly generated through the decomposition of CaCrO4 (Cr (VI)), is a significant intermediate for toxic Cr (VI) formation during solid fuel combustion. In this study, the formation, oxidation and sulfation kinetics of CaCr2O4 were analyzed to forecast the potential of CaCr2O4 oxidation during co-firing of coal and solid waste in a circulating fluidized bed boiler. The results indicated that the formation and oxidation of CaCr2O4 were fitted to a single step nucleation and growth model while CaCr2O4 sulfation was fitted to a shrinking core model. CaCr2O4 formation through CaCrO4 decomposition was favored in oxygen-lean atmosphere and considerably suppressed in the presence of oxygen. In contrast, CaCr2O4 oxidation was mainly determined by the contacts between CaCr2O4 and CaSO4/CaO, which influenced not only oxidation rates but also the product species. Moreover, the oxidation reactivity of CaCr2O4 was higher in the presence of CaO than that of CaSO4. On the other hand, CaCr2O4 sulfation can occur more easily than CaCr2O4 oxidation, the reaction rate of which was deeply affected by sulfate product layer. Findings in this study suggested that spraying calcium in furnace for desulphurization may increase the risk of CaCr2O4 oxidation. Measures including the adjustment of Ca/S ratio in blended fuel (with added limestone) and operating conditions (such as temperature and local atmosphere) in co-firing system could be taken to control CaCr2O4 formation and oxidation.

Torrefied biomass fuels as a renewable alternative to coal in co-firing for power generation

This study aims to assess the torrefaction of biomass as alternative renewable energy fuel to coal during co-firing. It was evaluated that torrefaction improves biomass grindability to such an extent that it can be used in coal mills with coal in co-firing without capital intensive modification. Torrefaction of beech wood was performed on a batch scale reactor at three different temperatures (200, 250 and 300 °C) with 30 min of residence time. The chemical structural changes in torrefied biomass were investigated with binding energies and FTIR (Fourier transform infrared) analysis. Monocombustion and co-combustion tests were performed to examine the combustion behaviour regarding flue gas emissions (CO, NOx and SO2) at 0.5, 1.5 and 2.5 m distance from the burner opening along with fly ash analysis. The FTIR and binding energies showed that lignin hardly affected during light torrefaction while hemicellulosic material was significantly depleted. The Hardgrove grindability index (HGI) was calculated with three methods (DIN51742, IFK and ISO). The medium temperature torrefied biomass (MTTB) yields HGI value in the range of 32–37 that was comparable with HGI of El Cerrejon coal (36–41). A slight change in temperature enabled the torrefied beech wood to be co-milled with coal without capital intensive modification and improved grindability. Comparing the combustion behaviour of single fuels, low temperature torrefied biomass (LTTB) produces less amount of NOx (426 mg/m3), CO (0.002 mg/m3) and SO2 (2 mg/m3) as compared MTTB and raw beech wood. In the case of co-combustion, it was found that blending of coal with raw biomass does not show a stable behaviour. However, premixing of 50% of coal with 50% of torrefied biomasses (MTTB and LTTB) gives most stable behaviour and reduces NOx almost 30% and SOx up to almost 50% compared to coal. The fly ash contents analysis proved that K2O contents much decreased during co-firing of coal and torrefied fuels that could cause ash related issues during combustion of raw biomass.

Three-dimensional simulation of the co-firing of coal and biomass in an oxy-fuel fluidized bed

The co-firing of coal and biomass in fluidized beds under oxy-fuel conditions is an important approach with significant industrial prospects, for both carbon dioxide capture and disposal of biomass waste produced by agriculture and forestry. Numerical simulation is essential in the design of structural and operational parameters for oxy-fuel fluidized beds, as is experimentation. Following our previous research on the oxy-fuel combustion of coal in a fluidized bed (Powder Technol. 2019, 349, 40–51), this paper develops 3D Eulerian-Lagrangian simulations for the co-firing of coal and biomass in an oxy-fuel fluidized bed based on the multiphase particle-in-cell (MP-PIC) scheme. The particle field is described by the discrete particle method (DPM) and the gas field is calculated by large eddy simulation (LES). Different models are used for the reaction of coal and biomass, including devolatilization, combustion of char and volatiles, and the production of pollutants. The numerical simulation was verified by combustion experiments carried out in a micro-fluidized bed reactor with an online mass spectrometer. Based on the simulations, the effects of biomass mass ratio, combustion atmosphere, oxygen concentration, and combustion temperature on the co-firing of coal and biomass in an oxy-fuel fluidized bed are discussed.

The ash fusion characteristics of bamboo and coal co-firing process

The ash fusion characteristics of bamboo and coal co-firing process were determined by a thermogravimetric analyzer, X-ray fluorescence and X-ray diffraction. The results showed that the ash of bamboo and coal co-firing process had similar chemical compositions with coal ash. Ash components occurred to synergistic interaction during co-firing process of bamboo and coal. Bamboo increased ash fusion temperatures of blends. The ash fusion process included moisture evaporation, oxidation of organic matters, removal and reaction of inorganic matters. The transformation of chemical compositions resulted in mass loss of ash. Fusion mechanism included G–B model, Mampel power model, second-order reaction and the Avrami–Erofeev model. The optimum blend ratio of bamboo and coal was suggested as 30%:70%. The results will be helpful to provide a scientific guidance to use bamboo residues for energy products.

Feasibility Study of Co-Firing of Torrefied Empty Fruit Bunch and Coal through Boiler Simulation

Torrefied empty fruit bunch (EFB) co-firing is a promising technology to reduce emissions from coal-fired power plants. However, co-firing can influence the combustion and heat transfer characteristics in a coal boiler. In order to study the feasibility of co-firing application of torrefied EFB (T-EFB) in boilers, the combustion characteristics, gas emissions and heat flux distribution were analyzed, respectively. First, the kinetic parameters of T-EFB devolatilization and char oxidation were obtained by experimental analysis. Second, the computational fluid dynamics (CFD) analysis was applied to the actual 500 MWe boiler simulation to further evaluate the differences in the co-firing performance parameters (combustion characteristics and emissions) of the T-EFB and the heat transfer characteristics within the boiler. Numerical results show that T-EFB co-firing can improve the ignition characteristics of pulverized coal, reduce the formation of unburned particles. When the blending ratio was increased from 10% to 50%, significantly NOx (oxides of nitrogen) reduction (levels from 170 to 98 ppm at 6% O2) was achieved. At a blending ratio above 40%, boiler combustion efficiency decreases as the total heat flux of the boiler decreases due to an increase in the amount of unburned carbon. In addition, T-EFB co-firing can affect the heat transfer characteristics of the boiler.

Experimental Studies on the Emission of Gaseous Pollutants in an Oxy-Fuel-Fluidized Bed with the Cofiring of Coal and Biomass Waste Fuels

The co-firing of coal and biomass waste is an important technological direction in oxy-fuel combustion for both CO2 capture and waste disposal. The emission of pollutants during co-firing in an oxy...

Generalized Energy and Ecological Characteristics of the Process of Co-Firing Coal with Biomass in a Steam Boiler

One of the ways used to reduce the emission of carbon dioxide and other harmful substances is the implementation of biomass co-firing processes with coals. Such processes have been implemented for many years throughout many countries of the world, and have included using existing high-power coal boilers. Despite numerous experiments, there are still no analyses in the literature allowing for their generalization. The purpose of this paper is to determine the generalized energy and ecological characteristics of dust steam boilers co-firing hard coal with biomass. The energy characteristics determined in the paper are the dependence of the gross energy efficiency of boilers on such decision parameters as their efficiency and the share of biomass chemical energy in fuel. However, the ecological characteristics are the dependence of emission streams: CO, NOx, SO2, and dust on the same decision parameters. From a mathematical point of view, the characteristics are approximation functions between the efficiency values obtained from the measurements and the emission streams of the analysed harmful substances and the corresponding values of the decision parameters. Second-degree polynomials are assumed in this paper as approximation functions. Therefore, determining the characteristics came down to determining the constant coefficients occurring in these polynomials, the so-called structural parameters. The fit of the determined characteristics was assessed based on the coefficients of random variation and the test of estimated significance of structural parameters. Boiler characteristics can be used when forecasting the impact of changes in operating conditions on the effects achieved in existing, modernized, and designed boilers. The generalization of the characteristics was obtained from the measurement results presented in 10 independent sources used to determine them.

Cofiring low-rank coal and biomass in a bubbling fluidized bed with varying excess air ratio and fluidization velocity

Cofiring coal and biomass is one of the alternatives to reduce emissions from thermoelectric power plants. The influence of biomass fraction in the mixture is an important research topic to increase coal substitution and achieve lower GHG emissions. A combustion system based on laboratory-scale bubbling fluidized bed technology was used to analyze the burning of different ratios (5, 10 and 15%) of mixed biomass and low-rank mineral coal. Two operational parameters were varied, fluidization velocity (3umf and 6umf) and excess air ratio (50 and 70%). Previous studies focused mainly on gasification processes, where excess air is a negative parameter. For combustion processes, high values of excess air are mandatory. Experimental results showed that fuel mixtures containing up to 15% of biomass gives good results in terms of emissions and temperature stability control. Emissions were slightly increased as well as the operating temperature with the presence of biomass. The operating regimes with higher fluidization velocity and excess air ratio presented better performance, however, biomass volatiles release at the freeboard should be the main concern for this type of application. Therefore, it was concluded that the combined burning of coal with biomass is a feasible alternative to reduce emissions without impairing efficiency.

Effect of biomass ratio on co-firing of biomass with coal on pozzolanic properties

Recently, fly ash from the coal combustion process in power generation has been used in the cement industry as an additive to cement or as partial replacing binder in cement-based structural mortars. The use of fly ash can reduce costs in cement production, including a reduction in the environmental impact. Co-firing biomass with coal is one of the existing technologies for utilizing biomass energy in the power generation system. However, this technology also influences the quantity and quality of the ash produced. The present work, therefore, aims to investigate the pozzolanic properties of ashes obtained from the co-firing ofcoal with corncob. Pozzolanic and physical properties of such ashes were investigated by varying the percentages of corncob at 20, 30 and 40 during combustion. The Pozzolanic compounds (SiO2+ Al2O3+ Fe2O3) according to ASTM C618 were determined by using X-ray fluorescence spectroscopy (XRF). Scanning electron microscopy (SEM) was used to investigate the structure of ash samples. The results show that content of pozzolanic compounds in ashes produced from cofiring corncob with coal decreased when increasing ratio of corncob and became stable when ratio of corncob was higher than 30%. It is observed that the morphology of such ashes illustrated in the form of small spherically shaped particles which can improve rheological behavior, viscosity and flow performance. Considering the requirements for pozzolanic-cementitious material, it is found that the ashes produced in this study has lower content of pozzoolanic compounds than the standard specification for pozzolan class C.

Characteristics of Corrosion Related to Ash Deposition on Boiler Heating Surface during Cofiring of Coal and Biomass

In order to investigate the regularity and mechanism of corrosion related to ash deposition on the boiler heating surface during cofiring of coal and biomass, the influence of fuel property, type of metal tubes (heating surface), proportion of blended biomass, and atmosphere in the furnace was studied by using the static corrosion mass gain method with the high-temperature tube furnace system. The results indicated that the effect of biomass property on ash corrosion is greater than that of coal, which was mainly due to high content of alkali metals and chlorine in biomass fuels. The corrosion resistance of metal pipes is T91 > 12CrMoVG > 20G. T91 is the most appropriate one, and it can effectively inhibit chlorine corrosion and can be used as the ideal material for the biomass-fired boiler and the biomass and coal cofired boiler. In addition, ash deposition can significantly aggravate the corrosion of metal tubes, and the degree of corrosion tends to become significant with increasing proportion of blended biomass fuels. HCl can aggravate metal corrosion, which can be inhibited by SO2.

The Generation and Suppression of NOx and N2O Emissions in the Oxy-Fuel Combustion Process with Recycled CO2 (an Overview)

It is shown that combustion of solid fuels in an oxygen atmosphere with recycled flue gases is an efficient method for capture of CO2. The use of the circulating fluidized bed (CFB) technology in the oxy-fuel combustion process reduces the CO2 capture costs and nitrogen oxide emissions compared with flare combustion. The mechanisms of generation and suppression of nitrogen and nitrous oxides during CFB combustion of solid fuels are considered. The basic reactions that occur during the exit and burning of volatiles and combustion of char resulting in generation of NO, N2O, and N2 are indicated. The effect of the secondary air delivery on nitrogen oxide emissions is studied. It is shown that the degree of converting fuel nitrogen into N2O is reduced when the degree of converting nitrogen into NOx is increased. Comparative analysis that the effect of flue gas recycling and other process factors has on generation of NO and N2O during combustion in air, oxygen, and CO2 has been made. It is shown that, owing to recycling, the fractions of NOx and N2O generated from fuel nitrogen during oxy-fuel combustion are significantly lower than those during combustion in air. Mathematical simulation of various complicated processes related to generation and suppression of hazardous pollutant emissions has recently become a generally accepted method that allows the prediction at relatively low costs of such processes during the combustion of various fuels. However, as applied to new technologies for combustion of various biomass types and cofiring of coal and biomass, these methods have not yet been properly developed. The most preferable mathematical models for calculating nitrogen oxide emissions are provided, and the predicted results are compared with experimental data. The lines of further research, predominantly in the simulation of cofiring of coal and biomass, are outlined. It is shown that comprehensive investigations of generation and suppression of hazardous pollutant emissions during cofiring of coals and biomass of various kinds, in particular, during oxy-fuel combustion with recycled CO2 are of topical interest.

System Performance and Pollution Emission of Biomass Gas Co-Firing in a Coal-Fired Boiler

To reduce greenhouse gases emission and increase the renewable energy utilization portion in the world, the biomass gasification coupled with a coal-fired boiler power generation system is studied. It is a challenge to achieve optimum performance for the coupled system. The models of biomass gasification coupled with co-firing of coal in a boiler have been established. A comparative study of three kinds of biomass (Food Rubbish, Straw and Wood Pellets) has been done. The syngas produced in a 10 t/h gasifier is fed to a 330 MWe coal-fired boiler for co-combustion, and the co-firing performances have been compared with pure coal combustion case under the conditions of constant boiler load. Results show that co-firing decreases the furnace combustion temperature and raises the flue gas temperature for Food Rubbish and Straw, while, flue gases temperature decrease in case of Wood Pellets. At the same time NOx and SOx emissions have reduced. The system efficiencies at constant load for Food Rubbish, Straw and Wood Pellets are 83.25%, 83.88% and 82.56% when the optimum conditions of gasification and co-firing process are guaranteed.

Best Available Technologies for Waste Co-Firing Applications

Abstract Thermal transformation of waste is one way in which the negative impact of human activities on the environment can be reduced. Thermal waste treatment is generally negatively associated by the public as hazardous and harmful to the environment. In this study the authors present the possibility of waste management in the process of co-firing with coal using BAT (Best Available Techniques) for both the combustion process and the exhaust gas purification process. Co-firing of coal with RDF (Refuse Derived Fuel) while ensuring stable production of electricity and heat supports the process of waste management which, if not burned, will end up in landfills, which, according to the authors, is the worst possible way to handle waste both from the point of view of environmental protection itself and from the point of view of energy efficiency.

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

This research focuses on the co-firing of low-quality coal with refuse derived fuel (RDF) as a means to reduce the volume of waste dumped in landfill sites. The co-combustion behaviour and kinetics of various RDF/coal blends at different weight ratios, along with their physicochemical characteristics were investigated. The physicochemical analysis revealed that the run-of-mine and discard coal have relatively low calorific values of 21.7 MJ/kg and 16.7 MJ/kg, respectively. The RDF samples, plastic blend (31.2 MJ/kg) and paper blend (22.4 MJ/kg), were found to have higher energy contents. The thermogravimetric analysis was performed in an atmosphere of air, over a temperature range of 25–850 °C, and the results showed that the RDF samples had lower ignition, devolatilisation, and burnout temperatures compared to the coals. The ignition temperatures for the blended fuel occurs in the lower temperature region when RDF is added to the blend, likewise the peak temperatures and burnout temperature shifted to a lower temperature zone. The activation energies (Ea) were determined using the Coats-Redfern method. The Ea for the run-of-mine (ROM) coal of 104.4 kJ/mol, was found to reduce to 31.4 kJ/mol for the 75% PB + 25% ROM coal blend and 35 kJ/mol for the 75% PL + 25% ROM coal blend, respectively. The discard coal which had an Ea of 109.9 kJ/mol was reduced to 30.9 kJ/mol for the 75% PB + 25% discard blend, and 33.5 kJ/mol for the 75% PL + 25% discard coal blend. It was determined that the most favourable blend for co-combustion was 70% discard coal + 30% PL RDF due to the similarity of the combustion profile to that of 100% coal and the simultaneous reduction in apparent activation energy.

The effect of co-firing coal and woody biomass upon the slagging/deposition tendency in iron-ore pelletizing grate-kiln plants

Woody biomass is being considered a potential co-firing fuel to reduce coal consumption in iron-ore pelletizing rotary kilns. An important consideration is the slagging inside the kiln caused by ash deposition that can lead to process disturbances or shutdowns. In terms of ash chemistry, co-firing woody biomass implies the addition of mainly Ca and K to the Si- and Al-dominated coal-ash (characteristic of high-rank coals) and Fe from the iron-ore that are both inherent to the process. An alkali-laden gaseous atmosphere is also present due to the accumulation of alkali via the recirculation of flue gas in the system. The slagging propensity of blending woody biomass with coal in the grate-kiln process was studied based on the viscosity of the molten phases predicted by global thermochemical equilibrium modeling. This was carried out for variations in temperature, gaseous KOH atmosphere, and fuel blending levels. Results were evaluated and compared using a qualitative slagging indicator previously proposed by the authors where an inverse relationship between deposition tendency and the viscosity of the molten fraction of the ash was established. The results were also compared with a set of co-firing experiments performed in a pilot-scale (0.4 MW) experimental combustion furnace. In general, the co-firing of woody biomass would likely increase the slagging tendency via the increased formation of low-viscosity melts. The fluxing behavior of biomass-ash potentially reduces the viscosity of the Fe-rich aluminosilicate melt and intensifies deposition. However, the results also revealed that there are certain conditions where deposition tendency may decrease via the formation of high-melting-point alkali-containing solid phases (e.g., leucite).

ANALISIS SWOT UNTUK DIRECT CO-FIRING BATUBARA DENGAN PELLET SAMPAH PADA BOILER TIPE CFBC

Indonesia to be the world’s second-largest contributor of plastic waste in the oceans. Every resident in Indonesia can produce 0,52 kg/person/day of waste. Efforts to overcome the problem of waste by processing it into Pellets, which contain calories from 2,800 to 3,300 kcal/kg using the peuyeumisasi method, so that they can use for co-firing in PLTU. Trial for direct co-firing coal and RDF Pellet from Klungkung Bali, will be carry out on Circulating Fluidized Bed Combustion (CFBC) boiler. The goal of study to analyze the feasibility for co-firing with SWOT analysis. The SWOT analysis questionnaire was compiled based on internal and external factors, which was carried out through the study of literature and document. After the questionnaire was compiled, the questionnaire was given to respondents who collected 10 people, selected based on factors of relevance and understanding of the issues discussed. The results of the questionnaire show the main indicators, which are divided into 2 parts, internal factors consisting of strengths, weaknesses and external factors consisting of opportunities and threats. Then implemented in the SWOT matrix, to get the best strategy and whether the decision is feasible or not. The results is feasible to be implemented in the PLTU MSW, with internal factor value of 3,03 or rounded to 3 (strong) and an external factor value of 2,6 or rounded to 3 (strong). Several strategies are designed to use the strength to take advantage of opportunities, reduce weaknesses and threats.

Ash deposition of Zhundong coal in a 350 MW pulverized coal furnace: Influence of sulfation

The huge reserve of Zhundong (ZD) coal makes it important for energy utilization of China. However, big concerns on fouling and slagging in pulverized coal-firing boilers were risen due to its high alkali content. It’s important to investigate deposition in different heating surfaces of a boiler burning ZD coal. In this paper, XRF, XRD, ICP-OES were conducted to analyze the properties of ash deposits collected from different heating surfaces in a 350 MW boiler burning 30% Wudong (WD) and 70% Wucaiwan (WCW) coals. Thermodynamics calculations were applied to study the mineral composition of coal ash in the range of 200–1600 °C under different coal co-firing ratio. The analytical results of the ash deposits showed that in medium-temperature (800–1100 °C) flue gas zone (FR and FS), ash deposits have higher sulfate content. While in high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al content and lower S content. The Na/S contents in FR (~1000 °C) are much higher than in other deposits. The thermal equilibrium calculation results showed that a large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition.

Thermochemical upgrading of coconut husk and rubber seed to coal co-firing feedstock via torrefaction

This research seeks to investigate thermochemical upgrading of Malaysian coconut husk and rubber seed to coal co-firing feedstock via torrefaction. Torrefaction experiments within a range of temperatures (200-300°C) and holding times (30 and 60 min) were carried out using a tubular furnace. Mass yield, higher heating value (HHV), energy densification ratio (EDR) and energy yield as well as proximate analysis were determined for the torrefied biomass sample. Both torrefaction temperature and holding time decreased the mass yield and the energy yield of the torrefied biomass. The HHV and EDR of coconut husk and rubber seed increased upon torrefaction. The VM decreased while the FC increased after torrefaction. Rubber seed torrefied at 300°C and 60 min had a HHV of 29.22 MJ/kg which was comparable to that of bituminuous coal. Linear correlations (R2=0.7764-0.965) were developed between mass yield and EDR, as well as mass yield and energy yield. Up to 70 wt% and 60 wt% of coal with a fuel ratio of 1 could be replaced by coconut husk and rubber seed torrefied at 300 °C and 60 min, respectively. Overall, the study indicated the feasibility of torrefied rubber seed (300°C and 60 min) as a coal co-firing feedstock in comparison to torrefied coconut husk due to its higher HHV cum lower ash content as well as its high co-firing proportion with coals having fuel ratio of at least 1.

Systematic study on ash transformation behaviour and thermal kinetic characteristics during co-firing of biomass with high ratios of bituminous coal

Co-firing of biomass and coal represents a sustainable and renewable energy option that ensures reduction of air pollutants and disposal of biomass residues. However, it adds much complexity to ash transformation. Here, the ash transformation mechanism and kinetic characteristics of Neimongol coal (NM), soybean straw (SS), and their blends were examined from the aspects of ash composition, morphology, mineralogy and thermodynamics. The results indicated that for a given temperature, the content of basic oxides in SS ash (SSA) was larger than that in NM ash (NMA), indicating a higher tendency of the former to slag. As temperature increased, molten floccules were found in SSA, and the surfaces consisted mainly of sylvine. For the ash from blends, the molten and solidified zones on agglomerate surface featured high contents of K, Si and O, indicating a layer of K-bearing silicates. With increasing SS content, the total weight loss and maximum weight loss rates decreased while their exothermic peaks shifted to a higher zone, and the ash agglomeration degree was enhanced. The K2CO3 addition significantly aggravated slagging. For all ashes, the peak temperatures corresponding to weight loss rates matched those of exothermic segments. The calculated kinetic results accurately predicted the ash transformation process.

Experiment and Kinetics Studies on Ash Fusion Characteristics of Biomass/Coal Mixtures during Combustion

Cofiring of biomass and coal significantly reduces carbon emissions and alleviates ash-related problems of biomass. This study investigates the effect of the biomass/coal blending ratio on ash fusi...