The high levels of CO2 in oxy-fuel combustion are expected to have important effects on the transformation of pyrite, a major contributor to ash deposition. As a successive work to the previous study that explored the chemical role of CO2 in pyrite decomposition, this paper is purposely designed to investigate the effects of CO2 on the oxidation of pyrrhotite generated from pyrite decomposition. Pyrrhotite oxidation in pure CO2 was respectively investigated at 900, 950 and 1000 °C on a well-designed thermo-gravimetric reactor (TGR). The time-resolved data of sample weight loss and gas evolution were collected online by a data collection module and a Horiba PG-350 gas analyzer, respectively. The solid products were characterized by X-ray diffraction (XRD). The results demonstrate that the influence of CO2 on the oxidation of pyrrhotite is chemical in nature. The transformation of pyrrhotite in CO2 consists of three stages, i.e., the fast weight loss stage, the slow weight loss stage and the slow weight gain stage. The mechanisms involving CO2 at these three distinct stages are quite different. At the first stage, ferrous sulfide is formed by the decomposition reaction of pyrrhotite with CO2. At the second stage, the oxidation reactions of ferrous sulfide with CO2 are responsible for the formation of magnetite or hematite. Hematite may also be formed through further oxidation of magnetite by CO2. The third stage is dominated by the oxidation of magnetite to form hematite with CO as the only gas product.

Coal combustion and mineral particle heating experiments were carried out in a drop-tube furnace at 1373 and 1573 K, respectively, to investigate the temperature effect on central-mode particulate matter (PM) formation during the combustion of coals with different mineral compositions. Two bituminous coals, coal A and coal B, with similar organic properties but different Ca/Fe mineral contents were tested. Typical minerals in the two coals, calcite and kaolinite, were used in the mineral particle heating experiments. An air atmosphere, a sample-feeding rate of 0.3 g/min, and a particle residence time of about 2 s were adopted in these experiments. The PM and bulk ash samples were collected by a low-pressure impactor and fiber filters, respectively, through a water-cooled N2-quenched probe. The elemental compositions, mass concentrations of PMs, mineral compositions, and morphologies of bulk ashes were characterized. The results show that the mass fraction size distribution of aluminum (Al) can be used to identify the different PM formation modes. When the temperature is increased from 1373 to 1573 K, the central-mode PM concentration for coal A increases by 61.8%, whereas that for coal B decreases by 13.2%. The remarkable difference is attributed to different fragmentation and coalescence behaviors resulting from different mineral compositions of the two coals. The criteria of optimal coal mineral composition for melting-phase generation and coalescence occurrence are developed. Interactions between calcite and kaolinite and their influence on central-mode particle formation with respect to the temperature are clarified by the mineral particle heating experiments.

The devolatilization process has important influence on the formation of PM10 (particulate matter with an aerodynamic diameter of ≤10.0 μm) in oxy-fuel combustion of pulverized coal but has been explored little. A bituminous coal was devolatilized in either CO2 or N2 at 1573 K on a drop-tube furnace (DTF) to produce CO2-char and N2-char. Coal and its char samples were burned at 1573 K and in 29 vol % O2/71 vol % CO2. PM10 was collected and segregated into 13 size fractions, which were subjected to subsequent analysis. The results show that the particle mass size distributions of PM10 from coal and chars have similar peak and trough sizes, suggesting that the devolatilization process has insignificant influence on the major pathways of PM10 formation. Three particle modes can be identified, i.e., ultrafine mode (<0.5 μm, PM0.5), central mode (0.5–2.5 μm, PM0.5–2.5), and coarse mode (2.5–10 μm, PM2.5–10). Coal combustion produces more PM0.5 and PM0.5–2.5 than char combustion, suggesting that the devolatilization process has important influence on the production of PM0.5 and PM0.5–2.5. In contrast, the PM2.5–10 yield is insignificantly affected by the devolatilization process under the investigated conditions. In addition, the combustion of CO2-char generates more PM0.5 and PM0.5–2.5 than that of N2-char, indicating that the devolatilization in CO2 favors the formation of PM0.5 and PM0.5–2.5.

Co-combustion is the most attractive option for extending the utilization of Zhundong coals from the newly discovered and the largest intact coalfield in China. However, operational practices have shown that power plants frequently encounter ash deposition problems during co-combustion with Zhundong coals. To address such an issue, in the present work, coal blends of a bituminous coal and a Zhundong sub-bituminous coal, with blending ratios of 90:10, 70:30, 50:50, 30:70, and 10:90 on a weight basis, were burned on a laboratory drop tube furnace at 1350 °C. For comparison, combustion experiments of the component coals were also carried out under the same conditions. The resulting ash samples were thoroughly characterized by using a Malvern particle size analyzer and a computer controlled scanning electron microscope. The obtained data were correlated to the ash deposition behavior in co-combustion with Zhundong coals in power plant boilers. The results show that particle mass size distributions of the ash samples from combustion of low-ZD-loaded fuels (with the proportion of the Zhundong coal investigated ≤50 wt %) are similar. The basic and acidic elements are partitioned similarly into ash particles. Ash deposition propensities, evaluated as the ratio of basic to acidic oxides (B/A) of the ash, are all low and show insignificant differences. These are consistent with the similarities in ash deposition behavior during co-combustion with Zhundong coals with low proportions in practical coal-fired boilers. In contrast, the ash properties are apparently different for the fuels with the proportion of the Zhundong coal higher than 50 wt % (denoted as high-ZD-loaded fuels). Small particles of <10 μm are more abundant in the ash samples and are more enriched in basic elements (especially Ca, Fe, and Mg) for the high-ZD-loaded fuels than for the low-ZD-loaded fuels. These data could well explain the more serious ash deposition problems arising during co-combustion with higher proportions of the Zhundong coals in practical boilers. It is also found that, for the ashes from combustion of the high-ZD-loaded fuels, both the amount of the small ash particles and the contents of basic elements in them increase with increasing the proportion of the Zhundong coal in the fuel. These data agree well with field observations of higher ash deposition propensities for coal blends with higher proportions of the Zhundong coals.

Coal fly ash is a potential candidate for CO2 mineral sequestration. If calcium is extracted selectively from coal fly ash prior to carbonation (namely indirect carbonation), a high-purity and marketable precipitated calcium carbonate (PCC) can be obtained. In the extraction process, recyclable ammonium salt (i.e., NH4Cl/NH4NO3/CH3COONH4) solution was used as a calcium extraction agent in this study. The influence of time, temperature, agent concentration, and solid-to-liquid ratio on calcium extraction efficiency was explored. NH4Cl/NH4NO3/CH3COONH4 are confirmed to be effective calcium extraction agents for the high-calcium coal fly ash investigated, and about 35–40% of the calcium is extracted into the solution within an hour. The calcium extraction performance is best for CH4COONH4, followed by NH4NO3 and NH4Cl. Increasing temperature from 25 to 90 °C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH4Cl and NH4NO3, while the positive effect of increasing temperature and agent concentration is more obvious for CH3COONH4. In the carbonation process, carbonation efficiency, namely conversion of Ca2+ into precipitated calcium carbonate(PCC), is only 41–47% when the leachate is carbonated by CO2. A newly proposed method of substituting CO2 with NH4HCO3 as the source of CO32– yields much higher carbonation efficiency (90–93%). Furthermore, the carbonation reaction rate is also largely improved when carbonating the leachate by NH4HCO3. In addition to these benefits, CO2 capture and storage can be simultaneously realized on-site if integrating the leachate carbonation process with an ammonia–water CO2 capture process using NH4HCO3 as a connector. In this way, the costs associated with CO2 compression and transportation can be eliminated. PCC with a purity up to 97–98% is obtained, which meets the purity requirement (≥97%) of industrially used PCC. It is estimated based on the experimental results that 0.17 tons of PCC can be produced from 1 ton of coal fly ash by this method, bounding 0.075 tons of CO2 at the same time, and 0.036 tons more CO2 can be avoided if the obtained PCC is substituted for the PCC manufactured by the conventional energy-intensive method.

The effect of low-NOx combustion technologies on particulate emissions from coal-fired boilers is much less known. It is investigated in this work by experiments on two 200 MW coal-fired boilers, one with low-NOx combustion and the other with conventional combustion. Collection of particulate matter with an aerodynamic diameter of up to 10 μm (PM10) was carried out at the inlet of the electrostatic precipitators of both boilers. The particulates were classified into 13 size fractions by a low-pressure impactor. Their emissions and size distributions were obtained. Particle chemical composition was characterized by an X-ray fluorescence analyzer. The data show that low-NOx combustion results in higher concentrations of total suspended particulate (TSP) and PM10 but lower concentrations of PM2.5 and PM1. The size distribution of PM10 from both boilers shows a trimodal feature that is not significantly affected by low-NOx combustion. However, it is found for the first time that the size distribution of PM10 from low-NOx combustion shifts to a larger size compared to that from conventional combustion. The size distributions of Al, Si, S, and Ca in the <10 μm size range suggest three particle modes that are formed by different mechanisms. The inconsistency in the literature as to whether there is a relationship between PM1 and NOx is thought to be due to the fact that PM1 is not totally formed by the vaporization and condensation mechanism. The preliminary data presented in this work suggest that, for the test cases, there seems to be a positive correlation between NOx and PM0.1 that is formed by solid–vapor–particle processes.

Little work has been performed on the transformation of iron and ash fusibility during oxy-coal combustion, which is of great significance to assessing ash deposition propensity. A high-iron bituminous coal was burnt at 1300 °C in a laboratory drop-tube furnace under four conditions: (1) 21 vol % O2/79 vol % N2 (air-firing), (2) 21 vol % O2/79 vol % CO2 (oxy-firing), (3) 27 vol % O2/73 vol % CO2 (oxy-firing), and (4) 32 vol % O2/68 vol % CO2 (oxy-firing). The bulk ash samples were subjected to X-ray fluorescence and Mössbauer spectroscopic analyses. The effects of changing from air combustion to O2/CO2 combustion and the effects of varying the O2 level in O2/CO2 combustion on iron transformation and ash fusibility were investigated. The results show that varying the combustion condition has insignificant effects on the elemental composition of coal ashes but has appreciable effects on the relative proportions of iron combustion products that were identified as hematite, magnetite, and Fe–glass phases. This indicates that speciation analysis is as important as bulk analysis for thorough ash characterization. Replacing N2 in air with CO2 results in a higher content of hematite but a lower content of magnetite. Increasing the O2 level in O2/CO2 combustion increases the formation of hematite but decreases the formation of magnetite. In contrast, the amount of Fe–glass phases remains almost unchanged. An appreciable fraction (about 8% Fe) of hematite/magnetite seems to crystallize out of the molten glass phases during combustion, and it is not significantly affected by changing combustion conditions. The fusion temperatures of the oxy-fired ashes are higher than those of the air-fired ash and increase with the inlet O2 level. A positive correlation between the hematite content and the ash fusion temperatures is observed.

Nanoparticles are thought to induce more severe health impacts than larger particles. The nanoparticles from coal-fired boilers are classified into three size fractions with a 13-stage low pressure impactor. Their physicochemical properties are characterized by the high-resolution field emission scanning electron microscope and X-ray fluorescence spectrometer (XRF). The results show that coal-derived nanoparticles mainly consist of individual primary particles of 20–150 nm and their aggregates. Inorganic nanoparticles primarily contain ash-forming elements and their aggregates have a dense structure. Organic nanoparticles are dominated by the element carbon and their aggregates have a loose structure. Nanoparticles from the same boiler have a similar composition and are primarily composed of sulfur, refractory elements and alkali/alkaline elements. Some transition and heavy metals are also detected. For different boilers, greater differences are observed in the production of the nanoparticles and their composition, possibly due to the use of low-NOx burners. Coal-derived nanoparticles have a small size, large specific surface area and complicated chemical composition, and thus are potentially more harmful to human health.

The formation of PM10 (particles less than or equal to 10 μm in aerodynamic diameter) during char combustion in both air-firing and oxy-firing was investigated. Three Chinese coals of different ranks (i.e., DT bituminous coal, CF lignite, and YQ anthracite) were devolatilized at 1300 °C in N2 and CO2 atmosphere, respectively, in a drop tube furnace (DTF). The resulting N2-chars and CO2-chars were burned at 1300 °C in both air-firing (O2/N2 = 21/79) and oxy-firing (O2/CO2 = 21/79). The effects of char properties and combustion conditions on PM10 formation during char combustion were studied. It was found that the formation modes and particle size distribution of PM10 from char combustion whether in air-firing or in oxy-firing were similar to those from pulverized coal combustion. The significant amounts of PM0.5 (particles less than or equal to 0.5 μm in aerodynamic diameter) generated from combustion of various chars suggested that the mineral matter left in the chars after coal devolatilization still had great contributions to the formation of ultrafine particles even during the char combustion stage. The concentration of PM10 from char combustion in oxy-firing was generally less than that in air-firing. The properties of the CO2-chars were different from those of the N2-chars, which was likely due to gasification reactions coal particles experienced during devolatilization in CO2 atmosphere. Regardless of the combustion modes, PM10 formation in combustion of N2-char and CO2-char from the same coal was found to be significantly dependent on char properties. The difference in the PM10 formation behavior between the N2-char and CO2-char was coal-type dependent.

•A novel compound demister with some advantages was proposed.•The performance was evaluated by experimental and numerical methods.•At low gas velocities or for small droplets, the compound demister is superior to the wave-plate demister.•At high gas velocities, the compound demister shows higher resistance to droplet re-entrainment.A novel compound demister that combines an upstream tube bank and downstream wave plates was proposed in this work for application in multistage flash (MSF) desalination process. Its performance was evaluated by experimental and numerical methods. Compared with the individual tube-bank and wave-plate demisters, the compound demister is found to have the highest separation efficiency (> 95%) with much less fluctuation for a wide range of gas velocities. At low gas velocities (≤ 4 m/s) or for removing small droplets (< 20 μm), the separation efficiency of the compound demister is much higher than that of the wave-plate demister mainly because of the large separation capability of the tube bank. At high gas velocities (> 4 m/s), the compound demister shows higher resistance to droplet re-entrainment that occurs at inlet gas velocity of approximately 7 m/s compared with the tube-bank demister. This is due to the compensation from the wave plates in the compound demister that separate secondary droplets generated by tubes. The compound demister possesses higher dry pressure drops than either the tube-bank or wave-plate demister, but is acceptable for industrial application. All these advantages make the compound demister a promising candidate for droplet removal in the desalination process.