Biomass is available from many sources or can be mass-produced. Moreover, biomass has a high energy-generation potential, produces less toxic emissions than some other fuels, is mostly carbon ...neutrality, and burns easily. Biomass has been widely utilized as a raw material in thermal chemical conversion, replacing coal and oil, including power generation. Biomass firing and co-firing in pulverized coal boilers, fluidized bed boilers, and grate furnaces or stokerfed boilers have been developed around the world because of the worsening environmental problems and developing energy crisis. However, many issues hinder the efficient and clean utilization of biomass in energy applications. They include preparation, firing and co-firing, and ash-related issues during and after combustion. In particular, ash-related issues, including alkali-induced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, and ash utilization, are among the most challenging problems. The current review provides a summary of knowledge and research developments concerning these ash-related issues. It also gives an in-depth analysis and discussion on the formation mechanisms, urgent requirements, and potential countermeasures including the use of additives, co-firing, leaching, and alloying.
Alkali species, particularly alkali chlorides and sulfates, cause alkali-induced slagging during biomass combustion. Thus, the mechanisms of generation, transformation, and sequestration of alkali species and the formation and growth of alkali-induced slagging, formed as an alternating overlapping multi-layered structure, are discussed in detail. For silicate melt-induced slagging (ash fusion), the evolutions of chemical composition of both the elements and minerals in the ash during combustion and existing problems in testing are overviewed. Pseudo-4D phase diagrams of (Ma2O)-MaeO-P2O5-Al2O3 and (Ma2O)-MaeO-SiO2-Al2O3 are proposed as effective tools to predict ash fusion characteristics and the properties of melt-induced slagging. Concerning agglomeration that typically occurs in fluidized bed furnaces, melt-induced and coating-induced agglomeration and coating-forming mechanisms are highlighted. Concerning corrosion, seven corrosion mechanisms associated with Cl2, gaseous, solid/deposited, and molten alkali chlorides, molten alkali sulfates and carbonates, and the sulfation/silication of alkali chlorides are comprehensively reviewed. The effects of alloying, salt state (solid, molten, or gaseous), combustion atmosphere, and temperature are also discussed systematically. For ash utilization, potential approaches to the use of fly ash, bottom ash, and biomass/coal co-fired ash as construction and agricultural materials are explored.
Several criteria or evaluation indexes are introduced for alkali-induced slagging and agglomeration, and chemical equilibrium calculation and multicomponent phase diagrams of silicate melt-induced slagging and agglomeration. Meanwhile, remedies, including the use of additives, co-firing, leaching, alloying, and the establishment of regulations, are discussed.
It is suggested that considerable attention should be focused on an understanding of the kinetics of alkali chemistry, which is essential for the transformation and sequestration of alkali species. A combination of heterogeneous chemical kinetics and multiphase equilibrium modeling is critical to estimating the speciation, saturation levels, and the presence of melt of the ash-forming matter. Further practical evaluation and improvement of the existing criterion numbers of alkali-induced slagging and agglomeration should be improved. The pseudo-4D phase diagrams of (Ma2O)-MaeO-P2O5-Al2O3 and (Ma2O)-MaeO-SiO2-Al2O3 should be constructed from the data derived from real biomass ashes rather than those of simulated ashes in order to provide the capability to predict the properties of silicate melt-induced slagging. Apart from Cr, research should be conducted to understand the effects of Si, Al, and Co, which exhibit high corrosion resistance, and heavy metals such as Zn and Pb, which may form low-melting chlorides that accelerate corrosion. Regulations, cooperation among biomass-fired power plants and other industries, potential technical research, and logistics should be strengthened to enable the extensive utilization of biomass ash. Finally, alkali-induced slagging, silicate melt-induced slagging, agglomeration, and corrosion occur concurrently, and thus, these issues should be investigated jointly rather than separately.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•The emission of CH4/NH3/air flames in a swirl combustor are investigated.•The emission of ammonia ratio from 0 to 1.0 are investigated experimentally.•Two chemical reactions are identified as the ...major reactions to produce NO and NO2.•The chemical reactor networks are performed to reveal the pathways of NOx production.
Ammonia is regarded as one of the potential hydrogen carriers and a kind of carbon-free renewable energy source. However, there are still some challenges on ammonia application in combustion devices, i.e., the high NOX emission. In this study, the combustion performances and emission characteristics of co-firing CH4/NH3/air flame with NH3 mole fraction (ηNH3) from 0 to 1.0 are investigated in a swirl combustor. The emissions in the exhaust gas are analyzed by the Gasmet DX4000 Fourier Transform Infrared (FTIR) gas analyzer. The simulations of the chemical reactor networks (CRN) with a detailed mechanism are employed to extend the understanding of experimental data. Results show that the combustion efficiency is very low at fuel lean conditions for high ηNH3 flames, and NOX emission significantly increases when blending a small amount of ammonia. The co-firing flames produce large amount of NOx at lean conditions (ϕ less than 1.0). There exits an optimized condition (ϕ≈1.1) where the NOx and NH3 emissions reach their lowest value simultaneously. The co-firing flame produces maximum NOx emission at ηNH3 = 0.5. Therefore, to control the NOX emission, the CH4/NH3/air co-firing flames should be operated far away from ηNH3 = 0.5 at rich conditions. NOX reaction pathway analysis shows the HNO pathway is dominant in fuel lean conditions. Thermal-NO pathway and NHi pathway are primary in stoichiometric ratio and fuel rich conditions, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The high sodium content in Xinjiang coal ash induces severe fouling and slagging in pulverized coal furnaces. To solve these problems, many people have reported to use silicon–aluminum additives. In ...this paper, the effects of silicon–aluminum additives (SiO2, kaolin, and fly ash) on ash fusion characteristic, mineral transformation, and sodium emission were investigated. The results showed that the ash fusion temperatures (AFTs) manifested a pattern of decrease first and increase later along with an increasing in silicon–aluminum additive ratios. To achieve higher AFTs than that of raw coal ash, the blending proportion of silicon–aluminum additives is recommended as high as 9%. The causes for these results were explored. When the proportion of the additives was 3%, Ertixiite (Na2Si2O5), nepheline (NaAlSi3O8), lazurite (Na7Al6Si6O24S3) and hauyne (Na6Ca2Al6Si6O24(SO4)2), were formed at 1150°C. These species with low melting temperatures are flux minerals and lower ash fusion temperature, resulting in the lowest AFTs. This is the main cause of lower ash fusion point in the case of lower blending ratios. Moreover, almost all of the tested additives show the capability of capturing sodium, in which, the capturing capability of SiO2 is the best. For sodium capture capacity of the three additives under most tested conditions, the optimum temperature was around 1000°C.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Ammonia, made up of 17.8% hydrogen, has attracted a lot of attention in combustion community due to its zero carbon emission as a fuel in gas turbines. However, ammonia combustion still faces some ...challenges including the weak combustion and sharp NOx emissions which discourage its application. It was demonstrated that the combustion intensity of ammonia/air flame can be enhanced through adding active fuels like methane and hydrogen, while the NOx emission issue will emerge in the meantime. This study investigates regulation effect of methane and hydrogen on the emission characteristics of ammonia/air flame in a gas turbine combustor. The instantaneous OH profile and global emissions at the combustion chamber outlet are measured with Planar Laser Induced Fluorescence (PLIF) technique and the Fourier Transform Infrared (FTIR), respectively. The flames are also simulated by large eddy simulation to further reveal physical and chemical processes of the emissions formation. Results show that for NH3/air flames, the emissions behavior of the gas turbine combustor is similar to the calculated one-dimensional flames. Moreover, the NOx emissions and the unburned NH3 can be simultaneously controlled to a proper value at the equivalence ratio (φ) of approximate 1.1. The variation of NO and NO2 with φ for NH3/H2/air flames and NH3/CH4/air flames at blending ratio (Zf) of 0.1 are similar to the NH3/air flames, with the peak moving towards rich condition. This indicates that the NH3/air flame can be regulated through adding a small amount of active fuels without increasing the NOx emission level. However, when Zf = 0.3, we observe a clear large NOx emission and CO for NH3/CH4/air flames, indicating H2 is a better choice on the emission control. The LES results show that NO and OH radicals exhibit a general positive correlation. And the temperature plays a secondary role in promoting NOx formation comparing with CH4/air flame.
•The regulation effects of methane and hydrogen on NOx emission of ammonia/air flames in a swirl combustor are investigated.•The emission characteristics are verified similar between 1D simulation and the experimental results.•The flame structure detected by OH-PLIF is used to reveal NO emission qualitatively.•The relation of NO and OH is revealed by LES for the NH3/CH4/air flame.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The catalytic effect of potassium doping on soot oxidation is studied in oxy-fuel combustion.•The catalysis extent is affected by potassium type, doping concentration, oxygen concentration, ...atmosphere.•The catalytic role of potassium in oxy-fuel combustion is lower than that in air combustion.•The promotion degree of potassium doping presents the minimum value around at 15% O2 concentration.•These observations strongly approve the hypothesis that potassium as the oxygen carrier.
In oxy-fuel combustion, switching from nitrogen to carbon dioxide atmosphere with higher oxygen concentration will affect the oxidation rate of soot, and it was observed in our recent study on soot emission from biomass pyrolysis that potassium (K) crystals are embedded in soot and its precursor clusters. In this study, the effects of K-doping (KCl and K2SO4) and O2 concentration on soot oxidation in O2/CO2 atmosphere are studied using thermogravimetric analysis (TGA), and the extent of catalysis is compared with that in O2/N2 atmosphere. The delays on start, peak and end temperatures of soot oxidation are observed in O2/CO2 atmosphere. However, increase in O2 concentration which promotes oxidation significantly reduces the delay. All the K-doping cases results in accelerated soot oxidation rate, but catalytic role of the K-doping in O2/CO2 is significantly lower than that in O2/N2 because the CO2-enriched environment inhibits the performance of potassium as oxygen carrier. The accelerating degree from K-doping is also affected by the potassium type, doping mass and oxygen concentration. KCl acts as a better, more efficient doping agent than K2SO4 with the increase in doping mass. The catalytic effect of K2SO4 will not change and even decrease at 375μmol(K)/g(soot) for K2SO4 while the catalytic role of KCl keeps increasing even at 600μmol(K)/g(soot) for KCl. In O2 concentration range of 5–30%, the accelerating degree from K-doping presents the minimum value around 15%. This phenomenon strongly approves the hypothesis that potassium as the oxygen carrier and accelerating the oxygen transportation, because in the cases of without K-doping and at a high O2 concentration there is no additional active site for more O2 adsorption thus inducing the slow accelerating degree. The kinetic analysis indicates the first order reaction for soot oxidation and also a good compensation relation between apparent activation energy E and logarithmic frequency factor A. E is generally reduced with the atmosphere changing from O2/N2 to O2/CO2, with K-doping, and with O2 concentration decreasing. This study is beneficial to demonstrate the mechanism of how potassium doping and oxygen concentration affect soot oxidation rate in oxy-fuel combustion environment.
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GEOZS, IJS, IMTLJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UM, UPCLJ, UPUK
•The synergetic effect of sewage sludge and wheat straw co-pyrolysis was studied.•The mass balance measurement of gas, tar, and char was performed.•The synergetic effect shows strongest under a ...certain biomass addition ratio around 60%.•The required heat of co-pyrolysis is significantly reduced.
Much attention has been given to the valuable products from the pyrolysis of sewage sludge. In this study, the pyrolysis of sewage sludge, biomass (wheat straw) and their mixtures in different proportions were carried out in a thermogravimetric analyzer (TGA) and fixed-bed reactor. The effects of pyrolysis temperature and percentage of wheat straw in wheat straw–sewage sludge mixtures on product distributions in terms of gas, liquid and char and the gas composition were investigated. Results indicate that there is a significantly synergetic effect during the co-pyrolysis processes of sewage sludge and wheat straw, accelerating the pyrolysis reactions. The synergetic effect resulted in an increase in gas and liquid yields but a decrease in char yield. The gas composition and the synergetic effect degree are strongly affected by the wheat straw proportions, and the strongest synergetic effect of sewage sludge and wheat straw co-pyrolysis appears at the biomass proportion of 60wt.%. With an increase of temperature, the gas yield from the pyrolysis of sewage sludge increased but the liquid and char yields decreased. Moreover, the required heat of co-pyrolysis is significantly reduced compared with the pyrolysis of sewage sludge and wheat straw pyrolysis alone, because of the exothermic reactions between the ash components in two fuel samples.
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GEOZS, IJS, IMTLJ, KISLJ, NUK, OILJ, SAZU, SBCE, UL, UM, UPCLJ, UPUK
Heat pretreatment is a promising method for biomass upgrading. However, PM formation from the combustion of such pretreated biomass has not been fully evaluated. In this work, the effect of ...pretreatment temperature on PM emission of the upgraded biomass and biochar combustion was studied in an entrained flow reactor. The physical and chemical properties of upgraded biomass, biochar and PMs at varied pretreatment temperatures were obtained to illustrate the PM formation mechanism. Results show that pretreatment temperature significantly affects the concentration and particle size distribution of PM emissions, through changing the char yield and K/Cl contents in char. With increase in pretreatment temperature, the PM1.0 emission of upgraded biomass and biochar combustion first increases, reaches maximum at 500°C, and then decreases. A linear relationship between the PM1.0 emission and Cl content in upgraded biomass and biochar was found. This result indicates that the combustion of upgraded biomass and biochar produced at moderate temperatures of 250–500°C result in aggravated fouling and PM emissions.
•Effect of pretreatment temperature on PM emissions of heating-upgraded biomass and biochar combustion is studied.•The maximum PM1.0 emission is observed from the combustion of biomass heating pretreated at 500°C.•A linear relationship between PM1.0 emission and Cl content in heating-upgraded biomass and biochar is observed.•Biomass upgrading at moderate temperatures significantly aggravates PM emissions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Deep eutectic solvents (DES) have attracted more attention due to the excellent performance and environmental protection. In the study and application of DES, the presence of water cannot be ignored. ...Thus, it is of importance to study the effect of water on DES. In this work, tetrabutylammonium chloride (TBAC) was chosen as the hydrogen bond acceptor (HBA). Decanoic acid (DEC) and octanoic acid (OC) were chosen as the hydrogen bond donor (HBD). Then, two kinds of DES were prepared with the mole ratio of 1:2 for HBA:HBD. The water was added into the sample to form the mixing system. The properties of these DES and their aqueous solutions were analyzed. The thermogravimetric (TG) experiment was conducted to obtain the TG curves for the thermal stability evaluation. The data of density and viscosity for (1 −
x
)DES–
x
H
2
O (
x
represents the molar fractions of H
2
O) were measured at the temperatures ranging from 303.15 K to 343.15 K. Furthermore, the hard sphere model was introduced for the predictions of viscosity with the average absolute relative deviation for the pure DES and their aqueous solution less than 2.71 % and 7.65 %, respectively. It indicates the fine prediction of these models. Moreover, the excess molar volume, the viscosity deviation and the excess molar Gibbs energy of activation were calculated for the further analysis of the interactions between DES and water.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
High temperature corrosion poses a great threat to boiler water wall safe operation. To investigate the corrosion root cause, a 300 MW level boiler water wall high temperature H2S corrosion case was ...reported. The typical hydrogen sulfide H2S corrosion feature was large amounts of sulfur which could be found in the cut down sample tube corrosion layer, with a thickness of 482 μm. In addition, huge amounts of lead (Pb) could be found in the corrosion layer, which resulted from the lead sulfide (PbS) deposition when the high temperature flue gas condensed at the water wall tubes. Meanwhile, the sulfur in the corrosion layer was closely related to the H2S concentration in the water wall ambience. The related ambience test showed that the H2S could achieve up to 1000 ppm when the boiler was in operation, far larger than the suggested reference value of 100 ppm. Hence, the overlarge H2S concentration was a vital factor for the tube corrosion. To further investigate the reason why the H2S was kept in such high concentration in the boiler long term operation, and the reasons for the over-high sulfur content in the coal and the over-large diameter of the imaginary circle of primary air (DICPA), two factors were obtained. The peak sulfur content reached 2.5% and the suggested sulfur content was below 1%. The DICPA was so large (1580 mm) that the pulverized coal easily scoured the water wall tubes, which would boost the thinning process of the tubes. To relieve the high temperature corrosion, improve the coal qualities, decrease the DICPA, adjust the operation diameter and adopt a coating technology four measures were suggested.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
To solve the intractable slagging problem in biomass utility boilers, the effects of Si, Al, K, Cl, S, and initial slagging layer on slagging were performed by comparing the distinct slagging ...characteristics of two cotton stalks. The criterion numbers of slagging, namely, Cl ratio (Cl+K2O+Na2O)/(SiO2+Al2O3) and S ratio (Svolatile+K2O+Na2O)/(SiO2+Al2O3), were proposed quantitatively. When Cl ratio and S ratio are higher than 2.4 and 1.9, respectively, the biomass is easy to slag; by contrast, the slagging is low when Cl ratio and S ratio are lower than 1.0 and 0.5, respectively. The slagging growth mechanisms were further improved by coupling the alternating layer structure of the whole slagging, which was formed by the re-enrichment of fine particles that primarily contained high concentrations of K, Na, Cl, and S in the form of KCl and K3Na(SO4)2 and by the re-capture of coarse large particles that primarily contained higher Si, Al and so on. K3Na(SO4)2, originated from the interaction between sulfates, promotes the growth of the slagging. Nonetheless, the influence of K3Na(SO4)2 on slagging is minimal in comparison with KCl. The disruption or inhibition of initial slagging layer significantly weakens slagging.
•Quantifying slagging criterion number Cl ratio (Cl+K2O+Na2O)/(SiO2+Al2O3)•Quantifying slagging criterion number S ratio (Svolatile+K2O+Na2O)/(SiO2+Al2O3)•Re-enrichment and recapture of fine particles result in alternate layered structure.•The influence of K3Na(SO4)2 on slagging is slighter than KCl.•The initial slagging layer has lethal effect on the slagging formation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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