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 cocombustion 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 CO 2 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., NH 4 Cl/NH 4 NO 3 /CH 3 COONH 4 ) 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. NH 4 Cl/NH 4 NO 3 /CH 3 COONH 4 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 CH 4 COONH 4 , followed by NH 4 NO 3 and NH 4 Cl. Increasing temperature from 25 to 90°C and agent concentration from 0.5 to 3 mol/L only subtly increases calcium extraction efficiency for NH 4 Cl and NH 4 NO 3 , while the positive effect of increasing temperature and agent concentration is more obvious for CH 3 COONH 4 . In the carbonation process, carbonation efficiency, namely conversion of Ca 2+ into precipitated calcium carbonate(PCC), is only 41− 47% when the leachate is carbonated by CO 2 . A newly proposed method of substituting CO 2 with NH 4 HCO 3 as the source of CO 3 2− yields much higher carbonation efficiency (90−93%). Furthermore, the carbonation reaction rate is also largely improved when carbonating the leachate by NH 4 HCO 3 . In addition to these benefits, CO 2 capture and storage can be simultaneously realized on-site if integrating the leachate carbonation process with an ammonia−water CO 2 capture process using NH 4 HCO 3 as a connector. In this way, the costs associated with CO 2 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 CO 2 at the same time, and 0.036 tons more CO 2 can be avoided if the obtained PCC is substituted for the PCC manufactured by the conventional energy-intensive method.

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The pulverized coal combustion in both air and oxy-fuel conditions generally experiences successive coal devolatilization and char combustion processes. The effects of these two processes have important influence on the emission characteristics of particulate matter. With some different characteristics of lignite from the higher rank coal, including the high contents of organically bound cations and the apparent char−CO 2 gasification reaction upon oxy-fuel combustion, the effects of both coal devolatilization and char combustion processes on the emission characteristics of particulate matter during oxy-fuel combustion of lignite would accordingly be distinctive. A typical Chinese lignite was devolatilized in a N 2 or CO 2 atmosphere to generate N 2 −char and CO 2 −char, respectively. Raw coal and the two chars were burned in a drop-tube furnace under both air and oxy-fuel conditions. Afterward, the yields and inorganic compositions of segregated PM 10 were carefully analyzed. The results have shown that the devolatilization in the N 2 condition promotes the emission of sub-micrometer particles (PM 0.5 ) and coarse particles (PM 1−10 ) but the devolatilization process in CO 2 inhibits their emission. N 2 −char and CO 2 −char present entirely different combustion behavior and, thus, reflect the discrepant emission characteristics of PM 10 . The rough higher yields of PM 0.5 and PM 1−10 from N 2 −char combustion compared to raw coal combustion may be driven by the higher particle combustion rate and/or temperature of N 2 −char. Furthermore, the quite severe gasification reaction of CO 2 with char during char preparation results in an inhibited combustion rate of CO 2 −char, which can explain the case in which much less PM 0.5 and PM 1−10 are generated from CO 2 −char combustion than those from raw coal and N 2 −char combustion. The apparent effects reveal that more detailed work can be continued to further explore this research.