Zhundong Coal Combustion Research Articles

Ash deposition and sodium migration behaviors during combustion of Zhundong coals in a drop tube furnace

Zhundong coalfield is one super-large coalfield recently discovered in China. However, the utilization of Zhundong coal in power plants has caused serious ash-related issues mainly due to its high-sodium feature. The ash deposition problem on convection heat exchanger surfaces is still particularly difficult to resolve and its mechanism has yet to be fully understood. This study deals with the ash deposition and alkali metal migration behaviors on convection heat exchanger surfaces between 400 and 800 °C during combustion of Zhundong coal using a lab-scale drop tube reactor. Experimental results show that the sodium content in ash deposit of Zhundong coals increases obviously as the deposition temperature decreases from 800 to 600 °C, while it is almost unchanged below 600 °C. The contents of iron and calcium in ash deposits exhibit nonmonotonic variations as the deposit probe temperature varies between 400 and 800 °C. Quartz and calcium sulfate are main crystalline phases in ash deposit of Zhundong coals. Calcium is inclined to present as calcite and lime at low deposition temperature, while high temperature facilitates calcium sulfation. Sodium of crystalline phase is found as albite and sodium sulfate at low deposition temperature. Both condensation of gaseous alkali metals and formation of low-melting minerals were responsible for the ash deposition phenomenon on convection heat exchanger surfaces involved in combustion of Zhundong coal. The sodium content in ash deposit decreases considerably with the increasing combustion temperature while the case of iron variation is opposite due to its low-volatility. In addition, the Na content in ash deposits increases obviously with the access air ratio reduced from 1.2 to 1.05, but the local weakly reducing atmosphere leads to less iron within ash deposits. Clarification of sodium migration and evaluation of ash deposition behaviors during combustion of Zhundong coal is helpful for a better exploration of the functional mechanism of ash deposit and then large-scale utilization of high-sodium coals.

Investigation on ash deposition characteristics during Zhundong coal combustion

The reserves of Zhundong (ZD) coal in China are huge. However, the high content of Na and Ca induces serious slagging and fouling problems. In this study, the ZD coal was burned in a DTF (drop tube furnace), and the ashes collected at different gas temperature with non-cooling probe were analyzed to obtain the ash particle properties and their combination mode. The results showed that Na, Ca and Fe are the main elements leading to slagging when the gas temperature is about 1000 °C during ZD coal combustion, but their mechanisms are quite different. Some sodium silicates and aluminosilicates and calcium sulfate keep molten state in the ashes collected at 1000 °C. These molten ash particles may impact and adhere on the bare tube surface, and then solidified quickly. With the growth of slag thickness, the depositing surface temperature is increased. The molten ash particles might form a layer of molten film, which could capture the other high fusion temperature particles. The Fe2O3 sphere were captured by the formed molten slag and then they blended together to form a new molten slag with lower melting temperature.

Evaluation of Vermiculite in Reducing Ash Deposition during the Combustion of High-Calcium and High-Sodium Zhundong Coal in a Drop-Tube Furnace

The additive injection technology has been effective in alleviating slagging and fouling during coal combustion. In this study, experiments were conducted in a drop-tube furnace to understand the influence of vermiculite injection on ash deposition during the combustion of Zhundong coal. The interaction between coal ash and vermiculite was investigated by inductively coupled plasma optical emission spectrometry (ICP–OES), X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive spectrometry (SEM–EDS). Results show that deposits formed at 1100 °C with vermiculite additive are more friable and polyporous than those without vermiculite additive, and these deposits can be easily removed. The mechanisms behind this phenomenon can be explained as follows: (1) The agglomeration of ash particles is inhibited by the partition of low-viscosity slices formed through vermiculite swelling after heat treatment. (2) Intermetallic reactions occur on the surface of vermiculite particles, resulting in the fo...

Release characteristic of different classes of sodium during combustion of Zhun-Dong coal investigated by laser-induced breakdown spectroscopy

Serious fouling and slagging problems are associated with the combustion of Chinese Zhun-Dong coal due to its high content of sodium (Na). Understanding the release characteristic of Na during the combustion is essential to viable utilization of this coal. In this work, coal samples were treated with a sequence of solvents: water (H2O), ammonium acetate (NH4Ac), hydrochloric acid (HCl), and the release characteristics of various classes of Na during coal combustion were investigated using the technique of laser-induced breakdown spectroscopy. The relative contribution of various Na classes to the Na release during each combustion stage was found to be similar, in the order of H2O-soluble Na > NH4Ac-soluble Na > HCl-soluble Na > insoluble Na. Sodium released during the devolatilization stage can be attributed to each of the sodium classes. After the devolatilization stage, H2O-soluble Na and NH4Ac-soluble Na dominated the Na release during both char and ash stages. Over 64% of the total Na released during combustion comes from the H2O-soluble Na, which suggests that the Na release during the combustion of Zhun-Dong coal can be reduced effectively after treatment by H2O washing.

Measurement of alkali content in Zhundong coal by LIBS method

Serious slagging and ash deposition issues occur in combustion of Zhundong coal in industrial power plants due to its high content of alkali (Na, K). In this study, Zhundong coal was treated by chemical methods to make a lot of coal samples with a large range of Na and K concentrations. The results of Inductively Coupled Plasma Spectroscopy (ICP) were used as known variables for the calibration and prediction of Na and K content using laser-induced breakdown spectrum (LIBS).The results showed the measurements of Na and K by LIBS had high sensitivity, low detection limit and low Root Mean Square Prediction Error (RMSEP). The lower element concentration the sample contained, the greater the RSD was. When the element concentration reached a certain value, the RSD of measurement became stable. A large number of repeated measurements can guarantee the accuracy of LIBS measurement. Because LIBS technology itself has the advantage of fast measurement，LIBS could be a valid method to achieve online measurement of alkali content in coal.

Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier

Chemical looping combustion (CLC) of ZhunDong coal (ZD) with hematite as oxygen carrier (OC) is an appropriate solution of how to utilize abundant ZD without severe slagging and fouling problems. At first, the combustion performance of ZD was compared to that of other three types of Chinese coals. Due to the catalytic effect of sodium in ZD, carbon capture efficiency, carbon conversion efficiency, CO2 concentration of fuel reactor and CO2 capture efficiency were all evidently higher than the ones for other types of coals at the same temperature. In addition, no NO2 was detected, and N2O and NO were the sole nitrogen contaminations in CLC of ZD. The high volatile content together with carbon gasification rate for ZD was conducive to the control of nitrogen contamination release. At last, 10h of continuous operation with ZD has been accomplished to evaluate the characteristic of hematite OC and sodium formation in hematite or fly ash. Hematite showed high reactivity in the continuous operation and the porous properties of 5h-hematite was best. High sodium content is the most prominent feature of ZD. Although there are also a large amount of other phases in ZD, due to the better catalytic effect, sodium is inferred to be the main factor accelerating the carbon conversion and improving the porous properties of OC. Moreover, it was found that there was a high melting point sodium aluminosilicate, Na2O·Al2O3·6SiO2 instead of low melting point sodium silicates formed in the CLC fly ash, mitigating the severe slagging and fouling problems.

The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: A study from ash evaporating to condensing

The high contents of sodium and calcium in Zhundong coal induce severe slagging and ash deposition in boilers. In this study, the ash deposition mechanism was investigated based on the results obtained from a full-scale boiler (350 MW) burning Zhundong coal, and a fixed bed reactor used for ash evaporating-condensing. In the full-scale boiler, the condensing and depositing of sodium and calcium sulfates play an important role on ash depositing on convection heating surfaces. Sulfates start to significantly condense and deposit at the flue gas temperature of about 850 °C on the medium and high temperature reheater surfaces. Ash evaporating tests proved that, with the increasing in temperature from 400 °C to 1200 °C, the ash evaporating process is divided into three stages: 1) 400–800 °C, 80% of sodium, and 100% of chlorine are released; 2) 800–1000 °C, all the left sodium evaporates and sulfur starts to be released with the formation of partial aluminosilicates; 3) 1000–1200 °C, all the left sulfur is released through the decomposition of calcium sulfates and then calcium starts to evaporate, while silicon oxides disappear due to the formation of new complex silicates. Ash condensing tests further proved that, the sodium in Zhundong coal was released mainly in the forms of atom, oxide, and chloride, in which sodium chloride account for about 50%. When the evaporating temperature increased higher than 1000 °C, partial alkali and alkaline earth metals were released as gaseous sulfates, and afterward condense and deposit on the heating surfaces. At last, a temperature-dependent ash deposition mechanism in Zhundong coal combustion was proposed.

06/01265 Flame suppression by aerosols derived from aqueous solutions containing phosphorus: Jayaweera, T. M. et al. Combustion and Flame, 2005, 141, (3), 308–321

Zhundong coalfield are huge in China; however, high alkali contents induce severe slagging and fouling problems in boiler. To obtain the characteristics and formation of deposits in boiler burning high alkali coal, the deposits at different positions were sampled in a 350 MW boiler burning Zhundong coal, and analyzed by X-ray Fluorescence. The results indicated that the slagging was deteriorating from bottom to top at water wall. The slag with obvious layer structure only presented at platen superheater. The deposits in horizontal flue were mainly formed from the solidified particles attachment. To further understand the formation mechanism of layer structure in slag at platen superheater, the slag was separated according to the morphology and each layer was analyzed by X-ray Diffraction. The results suggested that through the condensed CaSO4 was the main compounds for the strong thermophoresis in layer 1, the little fused particles with high sodium content played the key bonding role. For layer 2, the fused particles contain high sodium content adhered to deposit surface and formed melt surface, which capture the other particles. The Fe species captured by melt surface lower the melting temperature of the formed deposit, and then aggravated the deposit fusibility in layer 3.