Article pubs.acs.org/EF
Characterization of Ash Particles from Co-combustion with a Zhundong Coal for Understanding Ash Deposition Behavior Jingying Xu, Dunxi Yu,* Bin Fan, Xianpeng Zeng, Weizhi Lv, and Jun Chen State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074, China ABSTRACT: 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 3000), including size, shape, area, and chemical composition, can be obtained. The advantages of CCSEM were well-demonstrated by Gupta et al.9 Examples of its application in coal science can be found in a number of studies.10−15 The CCSEM system used in this work consists of a FEI Quanta 200 SEM and an EDAX energy dispersive Xray spectrometer. The validity of this technique has recently been wellestablished.16,17 Due to large uncertainties associated with CCSEM analysis of very small particles, the ash samples were also characterized by a Malvern Mastersizer 2000 size analyzer to obtain particle size distributions over a broader size range.
generated ash samples were subjected to detailed analyses. The obtained data were correlated to the ash deposition behavior in co-combustion with Zhundong coals in power plant boilers.
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EXPERIMENTAL SECTION
Fuel Samples. A bituminous coal (denoted as PD coal) and a Zhundong sub-bituminous coal (denoted as ZD coal) currently used in Plant A were used as raw materials in this work. Pretests showed that the presence of very fine coal particles in the samples could cause some feeding problems. Therefore, both coal samples were sieved to produce size-cuts of 63−100 μm, which were used to prepare coal blends with various blending ratios. The properties of the component coals are presented in Table 1.
Table 1. Properties of the Component Coals PD Proximate Analysis (wt %, ad) moisture 3.54 ash 30.38 volatile matter 33.27 fixed carbon 32.81 Ultimate Analysis (wt %, ad) C 46.81 H 3.64 O (by difference) 12.69 N 2.72 S 0.22 Ash Composition (wt %, normalized) Na2O 4.25 MgO 2.19 Al2O3 28.30 SiO2 57.51 P2O5 0.85 SO3 1.60 K2O 1.11 CaO 2.25 Fe2O3 1.94
ZD 7.25 3.56 40.13 49.06 65.77 3.95 14.6 4.36 0.51
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RESULTS Ash Particle Size Distribution. Figure 1 depicts particle mass size distributions (PMSDs) of all ash samples. They are
4.27 10.58 8.70 6.95 0.71 26.73 0.51 37.69 3.86
ZD coal sample has a higher moisture content than PD coal sample but has a far lower ash content (3.56 wt %). PD coal ash is dominated by SiO2 and Al2O3 (totaling higher than 85 wt %), suggesting the abundance of silicates/aluminosilicates in the coal sample. In contrast, ZD coal ash is dominated by CaO, SO3, and MgO with SiO2 and Al2O3 only accounting for a minor fraction (totaling less than 16 wt %). The striking feature is that CaO and MgO are much more abundant in the ZD coal ash than in the PD coal ash. These are typical characteristics of low-rank coals such as lignite and sub-bituminous coals.4 Inorganic elements in low-rank coals are usually distributed more evenly as water-soluble/ion-exchangeable species and fine mineral grains.5 They tend to partition into very small ash particles that are enriched in alkali and alkaline elements and have significant effects on ash deposition.6,7 Coal blends were prepared by physically mixing PD and ZD coal samples with five blending ratios: PD:ZD = 90:10 (P90Z10), PD:ZD = 70:30 (P70Z30), PD:ZD = 50:50 (P50Z50), PD:ZD = 30:70 (P30Z70), and PD:ZD = 10:90 (P10Z90). For comparison, PD and ZD coal samples were also tested, respectively. The corresponding cases are denoted as PD100 (100% PD coal sample) and ZD100 (100% ZD coal sample). Facility and Test Methods. The drop tube furnace (DTF) used in this work is the same one as described elsewhere.8 Briefly, the furnace tube has a length of 2 m and an inner diameter of 56 mm. It is electrically heated and the working temperature can be well-controlled. During the experiments, coal particles are fed through a Sankyo Piotech microfeeder (model MFEV-10) and entrained by a primary air
Figure 1. Particle mass size distributions of the ashes from coal blends and their component coals.
based on particle volume faction size distributions determined by the Malvern size analyzer. By assuming that ash particles have a constant density, the mass fraction size distribution is equal to the volume faction size distribution. Therefore, the PMSD can be obtained by multiplying the total ash mass in 1 kg coal blend sample by the mass fraction size distribution. As shown in Figure 1, for PD100, P90Z10, P70Z30, and P50Z50 (denoted as low-ZD-loaded fuels with the proportion of the ZD coal not exceeding 50 wt %), the PMSDs of their resulting ashes are very similar to each other. These PMSDs are generally bimodal, with an apparent coarse mode centered on 100 μm and a less-apparent fine mode around 15 μm. However, for P30Z70, P10Z90, and ZD100 (denoted as high-ZD-loaded fuels with the proportion of the ZD coal higher than 50 wt %), the differences in their ash PMSDs are clearly observed. The absolute mass of particles 30 μm decreases upon increasing the proportion of the ZD coal in the fuel. Generally, these ash samples are 679
dx.doi.org/10.1021/ef401545d | Energy Fuels 2014, 28, 678−684
Energy & Fuels
Article
shown in Figure 3, for low-ZD-loaded fuels, the resulting ash samples have nearly the same specific surface area, consistent
trimodally distributed. The trimodality of the ash PMSDs for P10Z90 and ZD100 is more evident. Besides the two large modes (locate at 15 and 100 μm, respectively) as observed on the PMSDs for low-ZD-loaded fuels, an additional submicrometer mode can be readily identified. Figure 1 also shows that the ash samples of low-ZD-loaded fuels have narrower PMSDs than those of high-ZD-loaded fuels. Strikingly, they do not contain particles of 0.7) of B/A, suggesting higher deposition propensities. Particularly, this value for the ZD100 ash even exceeds 1.2. It can be seen that, for high-ZD-loaded fuels, the value of B/A increases upon increasing the proportion of the ZD coal in the fuel. According to the ranges of the value of B/A for various degrees of deposition propensity (i.e.,
