Ash Chemistry Research Articles

Zinc speciation in fly ash from MSWI using XAS - novel insights and implications

The chemical forms of zinc in fly ash from municipal solid waste incineration (MSWI) crucially affect ash management, influencing both material recovery options and the risk of unwanted leaching into ecosystems. The zinc speciation was investigated in fly ash samples sourced from full-scale MSWI plants, including four grate fired boilers (GB) and one fluidized bed boiler (FB). We applied X-ray Absorption Spectroscopy (XAS), and the spectra were analyzed against a unique library of over 30 relevant compounds, tailored to the nuances of zinc chemistry of fly ash. Nano-XANES and sequential leaching were employed as complementary analytical methods. Multiple chemical forms of zinc were found in the ash, whereof potassium zinc chloride salts (K2ZnCl4) emerged as the predominant form in GB fly ash representing 41–64 % of the zinc content, while less for FB fly ash (19 %). The mere exposure to humidity in the air during storage resulted in hydroxylation of the alkali zinc chlorides into Zn5(OH)8Cl2·H2O. Other forms of zinc in the ash were Zn4Si2O7(OH)2·H2O, ZnFe2O4, ZnAl2O4, surface adsorbed zinc, and Zn5(CO3)2(OH)6. Notably, the proportion of zinc in spinel forms (ZnFe2O4 and ZnAl2O4) increased threefold in FB ash compared to GB ash, representing ∼60 % and ∼10–20 % of the zinc, respectively.

Model-Based XPS Technique for Characterization of Surface Composition on Nano-Scale SiCOH Sidewalls

The objective of this study is to develop an in-line metrology technique that employs X-ray Photoelectron Spectroscopy to measure photoelectron intensity from patterned trench surfaces of low-k dielectric film. A mathematical model uses linear regression method to separate the intensities into individual elemental compositions of the constituent surfaces. The initial segment of the model uses measured data from CD-SEM and spectroscopic Ellipsometry to determine the respective surface areas visible to the XPS electron analyzer. The second segment derives the surface composition of the line top, trench bottom, and vertical sidewalls based on a linear relationship between photoelectron intensity, emitting area, and characteristic elemental composition of each surface. The study has verified the predicted compositions from the model through physical measurements, demonstrating excellent agreement and concurrence with the physical mechanisms expected from the applied etch and ash chemistry in a commercial CCP RIE etch chamber.

Effects of Proximate Analysis on Coal Ash Fusion Temperatures: An Application of Artificial Neural Network.

The temperature at which coal ash melts has a significant impact on the operation of a coal-fired boiler. The coal ash fusion temperature (AFT) is determined by its chemical composition, although the relationship between the two varies. Therefore, it is important to have mathematical models that can reliably predict the coal AFTs when designing coal-based processes based on their coal ash chemistry and proximate analysis. A computational intelligence model based on the interrelationships between coal properties and AFTs was used to predict the AFTs of the coal investigated. A model that integrates the ash, volatile matter, fixed carbon contents, and ash chemistry as input and the AFT [softening temperature, deformation temperature, hemispherical temperature, and flow temperature] as an output provided the best indicators to predict AFTs. The findings from the models indicate (a) a method for determining the AFTs from the coal properties; (b) a reliable technique to calculate the AFTs by varying the proximate analysis; and (c) a better understanding of the impact, significance, and interactions of coal properties regarding the thermal properties of coal ash. This study creates a predictive model that is easy to use, computer-efficient, and highly accurate in predicting coal AFTs based on their ash chemistry and proximate analysis data.

Agglomeration and transformation of different types of inorganic potassium in biomass during co-gasification with coal

Co-gasification of biomass and coal has many advantages, which also mitigates some of the ash-related problems induced by potassium (K) in biomass. However, this effect varies with the occurrence mode of inorganic K in biomass, which varies with the species of biomass. Therefore, the present work investigated the effect of a bituminous coal on the transformation of K and the ash chemistry of bed agglomeration (without considering the effect of fluidization) during co-gasification (steam and CO2 atmospheres) for different types of inorganic K (K2CO3, K2SO4 and KCl) in biomass. The results show that for both K2CO3 and K2SO4 in biomass, K is mainly retained as KAlSiO4 and K2SO4 in the gasification ash with KAlSiO4 being the dominant phase. In addition, for K2CO3 in biomass, more KAlSiO4 and less K2SO4 are formed under the steam atmosphere than under the CO2 atmosphere. However, the K retention of the ash or the agglomerate for both K2CO3 and K2SO4 in biomass is similar and is much higher than that for KCl in biomass, which tends to release and to induce other ash-related problems. In addition, both the organic compounds of coal and the gasification atmosphere significantly alter the reaction between coal with KCl in biomass: The majority of KCl reacts with coal to form KAlSiO4, the formation of which is quite limited for the coal ash; More K is retained in the ash and more KAlSiO4 is formed under the steam atmosphere than under the CO2 atmosphere. Lastly, although co-gasification with coal (silica sand as the bed material) cannot stop the formation of agglomerates for K2CO3, K2SO4 and KCl in biomass, these agglomerates are more easily to be broken than those produced from the gasification of biomass alone, thus mitigating bed agglomeration in fluidized-bed reactors.

Biomass ash chemistry in oxygen carrier aided combustion: Interaction between potassium and red mud

Oxygen Carrier Aided Combustion (OCAC) is a process that uses oxygen carrier particles to improve oxygen distribution in circulating fluidized bed reactors. Its application using biomass as fuel can increase fuel conversion, thus enhancing combustion efficiency. However, ash generated during combustion is a major challenge, with potassium-based ash being particularly problematic. Red mud is considered a potential low-cost oxygen carrier. In this study, the interaction mechanism of organic potassium (CH3COOK), as well as inorganic potassium (K2CO3, KCl, K2SO4, K3PO4) and red mud oxygen carriers, was investigated under an oxidizing atmosphere at 950 °C. Experiments were carried out on a TGA test bench and a fixed bed reactor. XRD and SEM-EDS were used to characterize the reaction products of the mixtures. Thermodynamic equilibrium calculations to support the experimental data. The results show that K enters the red mud oxygen carrier and forms mainly KAlSiO4 and KAlSi3O8. Some of the potassium salts lead to the formation of K-Fe-O material. The agglomeration of the bed material depends on the proportion of elements in the oxygen carrier and ash mixture. CH3COOK and K2CO3 are more likely to cause the reacting particles to stick together. KCl, K2SO4, and K3PO4 react with the red mud particles but have no obvious effect on the particles’ agglomeration.

The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

Abstract. We employ the ECHAM5/MESSy2 atmospheric chemistry general circulation model (EMAC) that incorporates calculations of gas-phase and heterogeneous chemistry coupled with the ozone cycle and aerosol formation, transport, and microphysics to calculate the 1991 Pinatubo volcanic cloud. We considered simultaneous injections of SO2, volcanic ash, and water vapor. We conducted multiple ensemble simulations with different injection configurations to test the evolution of SO2, SO42-, ash masses, stratospheric aerosol optical depth, surface area density (SAD), and the stratospheric temperature response against available observations. We found that the volcanic cloud evolution is sensitive to the altitude where volcanic debris is initially injected and the initial concentrations of the eruption products that affect radiative heating and lofting of the volcanic cloud. The numerical experiments with the injection of 12 Mt SO2, 75 Mt of volcanic ash, and 150 Mt of water vapor at 20 km show the best agreement with the observation aerosol optical depth and stratospheric temperature response. Volcanic water injected by eruptive jet and/or intruding through the tropopause accelerates SO2 oxidation. But the mass of volcanic water retained in the stratosphere is controlled by the stratospheric temperature at the injection level. For example, if volcanic materials are released in the cold point above the tropical tropopause, most of the injected water freezes and sediments as ice crystals. The water vapor directly injected into the volcanic cloud increases the SO42- mass and stratospheric aerosol optical depth by about 5 %. The coarse ash comprises 98 % of the ash injected mass. It sediments within a few days, but aged submicron ash could stay in the stratosphere for a few months providing SAD for heterogeneous chemistry. The presence of ash accelerates the SO2 oxidation by 10 %–20 % due to heterogeneous chemistry, radiative heating, lofting, and faster dispersion of volcanic debris. Ash aging affects its lifetime and optical properties, almost doubling the ash radiative heating. The 2.5-year simulations show that the stratospheric temperature anomalies forced by radiative heating of volcanic debris in our experiments with the 20 km injection height agree well with observations and reanalysis data. This indicates that the model captures the long-term evolution and climate effect of the Pinatubo volcanic cloud. The volcanic cloud's initial lofting, facilitated by ash particles' radiative heating, controls the oxidation rate of SO2. Ash accelerates the formation of the sulfate layer in the first 2 months after the eruption. We also found that the interactive calculations of OH and heterogeneous chemistry increase the volcanic cloud sensitivity to water vapor and ash injections. All those factors must be accounted for in modeling the impact of large-scale volcanic injections on climate and stratospheric chemistry.

Fouling and Slagging Investigation on Ash Derived from Sasol Coal Using ICP and XRF Analytical Techniques

During coal combustion in boilers, light fly ash particles are carried away along with the hot flue gases and the heavier bottom ash particles fall to the bottom of the boiler. The fly ash particles stick on the convective heat transfer surfaces and the furnace wall, causing fouling and slagging deposition problems during the boiler operation. The fouling and slagging effect reduces the boiler’s operational efficiency. This study was motivated by the decline in the operational efficiency of the installed boilers at Sasol synfuel operations in Secunda, Mpumalanga province in the Republic of South Africa. It was assumed that the drop in the boiler efficiency was caused by the coal ash deposition during the boiler operations. The rate of ash deposition and accumulation in the convective heat transfer tubes and furnace water walls during the boiler operation depends on the chemical composition of the coal ash produced during combustion. Coal fly and bottom ash samples were collected from the operational site for laboratory analysis to determine their chemical composition using induced coupled plasma optical emission spectroscopy, induced coupled plasma mass spectroscopy (ICP-OES, ICP-MS) and X-ray fluorescence (XRF) analytical methods. The major, minor and trace elements by mass (%) in the ash samples were obtained from the ICP-OES and ICP-MS, whereas the elemental composition in an oxidised atmosphere was obtained from the XRF analysis. The amount of unburnt coal particles within the ash samples was determined from the loss on ignition (LOI) test. The fouling and slagging prediction during Sasol boiler operation was evaluated using previously developed fouling and slagging indices as a guide using the analysed ash chemical composition results obtained in this study. It was concluded from the analysed results using the guided evaluated indices from the analysed coal ash chemistry that during the operations of Sasol boiler(s) there is a low to medium fouling prediction occurrence on the convective heat transfer tubes and a low slagging in the boiler furnace walls.

Biomass ash chemistry in chemical looping: Interaction between organic-K and Fe2O3/Al2O3 oxygen carrier using cellulose-CH3COOK as model compound

Biomass fueled-Chemical Looping Combustion (CLC) provides a way for negative CO2 emission due to its property of carbon neutrality. Current knowledge is still lacking for organic K interaction behavior with oxygen carrier in CLC. Biomass ash chemistry mainly focusing on the interaction between organic-K and a Fe2O3/Al2O3 oxygen carrier was investigated using cellulose-potassium acetate (CH3COOK) as model compound. The decomposition characteristics of cellulose and CH3COOK in the presence of oxygen carrier and interaction mechanism between K and Fe2O3/Al2O3 were studied in a fixed bed at a reaction temperature of 900 °C. The influences of reducing atmosphere and addition of steam on the interaction behavior and mechanism were investigated. ICP-OES, XRD and cross section EDS/mapping analyses were employed to characterize the reacted oxygen carrier samples. Further, thermodynamic calculations were performed based on Gibbs energy minimization principle, to detect phase transition of biomass ash chemistry. The K accumulation in the oxygen carriers were 83.18% in dry condition, higher than that of 75.32% in wet condition when pure CH3COOK was used. K2CO3 formation may be directly derived from the decomposition of CH3COOK or combination of K(g) with CO2 in the presence of lattice oxygen provided by oxygen carriers. Most of K turned into K2CO3 and the melting of K2CO3 caused the particles to stick together, creating agglomerations. Introduction of steam could suppress the agglomeration affinity. CH3COOK was more converted into KOH in the case of water vapor. The steam atmosphere enhanced K penetration into the particles in comparison to dry atmosphere.

Mitigation of particulate matter emissions from co-combustion of rice husk with cotton stalk or cornstalk

PM emission is one of key issues in the biomass combustion of heat and power plants. In this paper, rice husk (RH) was co-combusted with cotton stalk (CSK) or cornstalk (CS) to study the PM emission behaviors. The experimental results show that the addition decreases PM1 yields by 20.13–54.65% for CSK and 45.99–76.70% for CS in comparison to the CSK or CS combustion alone. A strong synergistic effect exists during the co-combustion process, which can appreciably inhibit the generation of fine particulate matter. The synergistic effect is caused by the physical dilution effect, and mainly by the reaction between alkali metals species in cornstalk/cotton stalk ash and Si-containing species in rice husk ash to inhibit the volatilization of alkali metals. However, the PM reduction degree is also affected by the ash chemistry, especially the Si/(Ca + Mg) ratio, as confirmed by the higher synergistic effect of rice husk/cornstalk compared to rice husk/cotton stalk. The results suggest that co-combustion of biomass with high Si-containing rice husk is a promising approach to reduce PM1 emissions during biomass co-combustion.

Co-Processing Lignocellulosic Biomass and Sewage Digestate by Hydrothermal Carbonisation: Influence of Blending on Product Quality

Hydrothermal carbonisation (HTC) can be integrated with anaerobic digestion (AD) for the treatment of digestate, resulting in a solid hydrochar or bio-coal and a process water, which can be recirculated back into AD to produce biogas. The properties of digestate-derived hydrochars do not lend themselves to producing high quality bio-coal and blending with lignocellulosic feedstocks can improve its properties. This study investigates the co-processing of sewage sludge (SS) digestate with three lignocellulosic biomass (grass, privet hedge, and woodchip). The calorific value of the resulting bio-coal is increased following co-processing, although feedstock interactions result in non-additive behaviour. The largest increase in calorific value was observed for co-processing with woodchip. There is evidence for non-additive partitioning of metals during co-processing resulting in only moderate improvements in ash chemistry during combustion. Co-processing also effects the composition of process waters, influencing the potential for biogas production. Experimental biomethane potential (BMP) tests indicate that grass clippings are the most suitable co-feedstock for maintaining both calorific value and biogas production. However, above 200 °C, BMP yields appear to decrease, suggesting the process water may become more inhibitory. Co-processing with wood waste and privet hedge produce the higher CV bio-coal but significantly reduced BMP.

Fate of trace elements in Oxygen Carrier Aided Combustion (OCAC) of municipal solid waste

Oxygen Carrier Aided Combustion is a novel fluidized bed concept for burning waste. This study analyzed solid samples from an industrial OCAC application using municipal solid waste and the oxygen carrier ilmenite. The presence of oxygen carriers impacts the ash chemistry, which can influence corrosion and ash characteristics. By investigating samples obtained from industrial applications, unique and highly relevant information on the solid-state chemistry and the fate of important elements can be obtained. In total, 20 bottom ashes and 17 fly ashes were sampled over a period of 38 days. In a preceding study, the surface interaction between ilmenite and Zn, Cu and Pb was investigated. In this paper, the distribution of these elements throughout the particle cross-section and the influence of residence time has been studied using XRD, SEM-EDX and XPS. The results show that Zn is incorporated in the Fe-rich ash layer over time in the form of Zn ferrites, while Cu accumulates inside the ilmenite particles with time, and Cr is enriched in the magnetically separated bottom ash. Low concentrations of Pb were detected in the bottom ashes, suggesting that a significant part is released in the gas phase. The influence of temperature, bed material and reduction potential were evaluated using multicomponent, multiphase equilibrium calculations. It is shown that an ilmenite bed is less prone to form melts in comparison to a bed of silica sand and that the addition of sulfur could decrease the volatilization of Pb.

Comparing fuel additives for fireside corrosion inhibition in pulverised fuel boilers using thermodynamic modelling

This study presents a method for comparing corrosion inhibiting fuel additives using the thermodynamic modelling software FactSageTM. Two biomass ashes were investigated while using a range of loadings of three additives: Fe-based additive and two coal ashes, one with an 80% Si and Al contents and one with less than 50% of Si and Al contents but having a significant Ca content. The metrics used for analysis were the formation of various corrosive compounds and by-products. The Fe-based additive could inhibit the formation of corrosive species but not as well as either of the coal ashes, as it was key to increase the Si and Al content of the deposits. The coal ash with the greatest Si and Al content proved most capable of inhibiting fireside corrosion, while the contaminants present in the other coal ash proved detrimental to reducing the production of certain corrosive species. This method can be used as a qualitative predictive tool by research and development teams to quickly and economically understand the consequences of utilising fuel additives on ash chemistry or of changes in proportion/sources of fuel in a power plant.

Role of Curing Conditions and Precursor on the Microstructure and Phase Chemistry of Alkali-Activated Fly Ash and Slag Pastes.

Understanding the role of curing conditions on the microstructure and phase chemistry of alkali-activated materials (AAMs) is essential for the evaluation of the long-term performance as well as the optimization of the processing methods for achieving more durable AAMs-based concretes. However, this information cannot be obtained with the common material characterization techniques as they often deliver limited information on the chemical domains and proportions of reaction products. This paper presents the use of PhAse Recognition and Characterization (PARC) software to overcome this obstacle for the first time. A single precursor (ground granulated blast-furnace slag (GBFS)) and a binary precursor (50% GBFS–50% fly ash) alkali-activated paste are investigated. The pastes are prepared and then cured in sealed and unsealed conditions for up to one year. The development of the microstructure and phase chemistry are investigated with PARC, and the obtained results are compared with independent bulk analytical techniques X-ray Powder Fluorescence and X-ray Powder Diffraction. PARC allowed the determination of the type of reaction products and GBFS and FA’s spatial distribution and degree of reaction at different curing ages and conditions. The results showed that the pastes react at different rates with the dominant reaction products of Mg-rich gel around GBFS particles, i.e., Ca-Mg-Na-Al-Si, and with Ca-Na-Al-Si gel, in the bulk paste. The microstructure evolution was significantly affected in the unsealed curing conditions due to the Na+ loss. The effect of the curing conditions was more pronounced in the binary system.

Inorganic PM and K emissions during ashing of solid biofuels and Kaolinite – Data measurement in laboratory scale

During combustion of solid biofuels, a broad variety of particulate matter (PM) is released into the atmosphere. Combustion systems are constantly being further improved according to the 3‑T rule (time, temperature and turbulence), thus further reducing the proportion of organic compounds in the PM. The inorganic share in PM is affected by this optimization in a rather small amount and therefore gains relevance. Woody and herbaceous biofuels are characterized by a varying elemental composition responsible for a different ash chemistry and thus for various amounts of emitted particles. To estimate the potential PM emissions as well as the corresponding K content related to the respective elements in the solid biofuels during combustion a new inorganic PM sampling method in the laboratory scale has been developed and tested. This method can forecast the amount of released inorganic PM and K during ashing of different biomasses. Here the inorganic PM of five different biomasses (i.e., spruce wood, wheat straw, spruce bark, corn cob and miscanthus) is measured. In addition, the influence of the additive Kaolinite (1 mas-%) is tested regarding the inorganic PM and K emissions during ashing of the biomasses spruce wood and miscanthus. Based on the obtained results from the ash analyses, conclusions concerning the applicability of the presented inorganic PM sampling method are derived. It can be shown, that the addition of 1 mas-% Kaolinite reduces the amount of K in the inorganic PM in the spruce wood and miscanthus sample by 55% and by 78%, respectively.

Superheater deposits and corrosion in temperature gradient – Laboratory studies into effects of flue gas composition, initial deposit structure, and exposure time

The heterogeneous nature of the ash chemistry of biomass fuels gives rise to challenges in predicting the deposit melting, sintering, and enrichment of corrosive ash species. An experimental method has been developed to study the evolution of ash deposit chemistry and morphology in temperature gradients simulating the conditions of real superheater deposits. The method is based on applying synthetic ash mixtures on an air-cooled corrosion probe, which is inserted into a tube furnace. The focus has been on how the melting behavior of alkali salt-rich deposits, i.e., KCl–K2SO4–NaCl–Na2SO4 mixtures, affects the chemistry and morphology. Intradeposit vaporization-condensation of alkali chlorides has been of interest. The interaction of reactive gas components (H2O + SO2), with the deposits, was also studied. The vaporization-condensation mechanism leads to enrichment of alkali chlorides in crevices and voids within deposits, leading also to build-up of chlorides on the steel surface, which causes accelerated corrosion, due to the formation of low-melting FeCl2 mixtures. Liquid phase sintering and temperature gradient zone melting (TGZM) were the main mechanisms for the supersolidus sintering of the deposits. Iron and nickel oxides were found within the deposits and at the outer edge of deposits, due to the TGZM mechanism.

Distribution of rare earth elements in the pilot-scale processing of fly ashes derived from eastern Kentucky coals: Comparisons of the feed and processed ashes

Beneficiated fly ash from the combustion of Central Appalachian high volatile bituminous coals was extracted with HNO3 in a pilot-scale processing plant. Several major oxides (notably CaO and SO3, but also including Fe2O3, MgO, K2O, and P2O5) and minor elements (Mn, As, Sr, Ba, and Pb) are depleted in the post-HNO3-extraction spent ash. The total lanthanides, Y, and Sc concentration is reduced by about 20% in the spent ash, with Gd showing the greatest decrease. Along with Gd, Nd and Dy are also well differentiated between the feed and spent ashes, with La and Sm showing minimal partitioning. The Gd decrease is correlated with the depletion of Fe2O3. The heavy rare earth elements (REE heavier than Eu) and Y are disproportionately concentrated in the HNO3-leachate compared to the light REE. For the ashes studied, Sc did not partition between the feed and spent ashes. Pozzolanicity tests show that the compressive strength and strength activity indices of the spent ash + ordinary Portland cement (OPC) mixes are comparable to 100% OPC, indicating that the spent ashes produced in the pilot-scale runs have the potential to be sold as a Class F fly ash. Ultimately, the beneficiated ash chemistry influences the chemistry of the post-HNO3-extraction spent ash and the HNO3-leachate. A 500-ppm-REE fly ash will presumably be a more economically favorable feedstock than an ash with a significantly lesser concentration.

Rheological and chemical interaction between volcanic ash and thermal barrier coatings

Combustion temperatures (>1200 °C) in gas turbines cause softening and eventually melting of ingested volcanic ash particles. In the downstream section, lower temperatures lead to droplet deposition onto the thermal barrier coatings (TBCs) of the turbine blades. The intensity of interaction (wetting and chemical interaction) depends on the ash and TBC chemistry as well as the TBC structure. We have determined the spreading behavior of five different volcanic ash melts on four types of TBCs. The TBCs differed in their composition - yttria-stabilized zirconia (YSZ) and gadolinium zirconate (GZO) – and in their fabrication - air plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD). The spreading properties have been parameterized on the basis of four parameters, 1) the CaO-SiO 2 -ratio, 2) optical basicity, 3) the R b/a ratio and 4) the viscosity of the volcanic ash melts. Infiltration efficiency of the melts into the TBCs and corrosion characteristics have been determined via electron microprobe analysis of cross sections. In this study, the highest damage potential was found following interaction with basaltic melt, as its low viscosity is favorable to extensive spreading and high chemical reactivity. • Basaltic (SiO 2 -lean) silicate melts show the most extensive spreading on TBC surfaces and the most intense chemical reaction with the TBC materials • Spreading of silicate melts on APS TBCs is enhanced over EB-PVD TBCs and spreading on GZO TBCs is enhanced over YSZ TBCs • Spreading ability of silicate melts can be estimated based on a parametrization deduced from our spreading experiments

Integrated modeling methodology for ash agglomeration in poly-disperse fluidized beds using particle population framework

This paper discusses a unique agglomeration modeling methodology developed based on binary collisions to combine the effects of heterogeneity in ash chemistry and granular physics. A simple population balance is defined to find changes in the particle size distribution (PSD) of a fluidized bed. Thermodynamic equilibrium calculations and a computational fluid dynamics (CFD) code, are used to obtain hydrodynamic parameters. A method to calculate and use a distribution of collision frequencies in fluidized beds, corresponding to a poly-disperse particle size distribution, was developed in order to incorporate the particle level heterogeneities. A distribution of collision frequencies obtained for poly-dispersed particles showed a three orders of magnitude higher collision frequency amongst the smaller particles than the coarser ones, at the initiation of agglomeration. Ash agglomeration occurred when the slag amount (binder) was less than 10 wt% at temperatures <850 °C for Pittsburgh No. 8 coal.

Slagging Characteristics and Optimization of Operating Temperature on High-Alkali Coal Gasification

In order to mitigate the slagging risks of high-alkali coal (HAC) gasification while optimizing the operating temperatures for practical application, the experimental and calculational works were performed on the horizontal tubular furnace system and the FactSage 7.2 software, respectively. The slagging tendencies of three HACs were anticipated by applying different indexes of ash chemistry characteristics prior to experiments, but the determined results were found inconsistent. The relationship between Na retained ratio (RNa) and the ash fusion temperatures (AFTs) of gasification residues demonstrated that the AFTs of coal with high RNa are low, while AFTs of coal with low RNa are high. Nevertheless, when the residues contained a large number of calcium-containing silicates and aluminosilicates, the ATFs were reduced significantly even if its RNa stayed at a low level. Furthermore, the operating temperatures of the three coals (in order of Mulei coal, Shaerhu coal, and Tietou coal) were suggested to be controlled below 1150°C, 1100°C, and 950°C, respectively.

Influence of feedwater pH on the CO2 reactivity of hydrochars. Co-carbonisation with a bituminous coal

Hydrothermal carbonisation (HTC) is recognised as an environmentally advantageous method to pre-treat biomass feedstocks. This study examined the hydrothermal carbonisation of two lignocellulosic biomasses at different pH levels. Lower pH promoted hydrolysis, dehydration and decarboxylation reactions, which led to more condensed and aromatic structures of hydrochars. These observations were confirmed by means of elemental analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The initial pH also had a strong influence on the ash chemistry of the hydrochars. Lower pH promoted the removal of alkali and alkaline earth metals from the biomass. This was reflected in a decrease in the reactivity of bio-cokes produced from hydrochars compared with those obtained from untreated biomass. Based on these findings, the possibility of adding 20 wt% of hydrochar samples to a low volatile bituminous coal was studied. A significant decrease in the reactivity of the resulting bio-cokes compared to the values obtained for the same blends with raw biomass was observed (37% and 20% for PS and RH hydrochars, respectively).