Utilization Of Zhundong Coal Research Articles

The correlations of chemical property, alkali metal distribution, and fouling evaluation of Zhundong coal

The utilization of Zhundong coal has encountered severe ash-related problems principally due to its high-alkali feature. However, the evaluation of fouling tendency and correlations of chemical properties of Zhundong coals based on a large database has been seldom performed. The present work aimed to reveal the correlations of chemical properties, distributions of alkali and alkaline earth metals (AAEMs) and iron, and the fouling propensity of Zhundong high-alkali coals based on datasets of >250 samples. The fuel ratio of the majority (∼95%) of Zhundong coals lay in the range of 1.0–2.45. A quantified correlation between fuel ratio and volatile matter content was obtained. The ash composition of Zhundong coal gathered in a small area of the ternary diagram of Fe2O3-alkali metal-alkaline earth metal oxide components. The principal sodium of water-soluble form in Zhundong coals varied notably from 45% to 95%, more than half of the potassium was insoluble form, the majority (∼90%) of alkaline earth metals were in forms of ammonium acetate-soluble and HCl-soluble, and ∼67% of iron was HCl-soluble form. The quantitative correlations based on plentiful datasets could improve the comprehension of correlations among the coal chemical properties, fouling behaviors and its reaction activity.

Release and transformation characteristics of Na/Ca/S compounds of Zhundong coal during combustion/CO2 gasification

Zhundong (ZD) coal from northwest China is a high quality steam coal with reserves of more than 390 billion tons. However, the utilization of ZD coal is limited due to the high content of alkali and alkaline earth metals. This study aimed at revealing the release and transformation mechanism of Na/Ca/S compounds during combustion/gasification of ZD coal. The results demonstrate that Na was primarily influenced by temperature, mostly releases at 600–800 °C. The transformation of Ca compounds was affected by both temperature and atmosphere. The high temperature of the combustion process could accelerate the decomposition of CaCO3 and CaSO4, and the high content of CO2 during gasification prolonged the decomposition of CaCO3. The transformation of S was primarily influenced by atmosphere. SO2 could react with CaO and form CaSO4 during the combustion process. While S compounds were mainly released as S (g) and H2S (g) during gasification process. There was a significant interaction among Na/Ca/S compounds during combustion, original CaSO4 in coal could adsorb Na compounds with SO2 at 600–800 °C and then reacted with aluminosilicates, by this reaction, Na could be fixed above 1000 °C.

Correlations of chemical properties of high-alkali solid fuels: A comparative study between Zhundong coal and biomass

The utilization of Zhundong coal has given rise to serious ash-related issues mainly because of its high-alkali characteristic, partly similar to biomass with high-potassium content. The chemical properties of such coals and biomass may have marked impacts on reactivity and ash fouling. However, the correlations of different chemical properties of high-alkali solid fuels have been seldom investigated, and then only on a few samples. Here, a comparative study of chemical properties of Zhundong coal and biomass was conducted based on a large database. Correlations between carbon content and ash yield and carbon content and fixed carbon were noted. The fuel ratios of nearly all types of agricultural biomass lay within a range of 0.15–0.30, approximately an order of magnitude lower than those of Zhundong coals, much lower than those of Zhundong coals. There were no significant associations between chemical property and wheat-growing region. In most cases, the ash composition, even for a single biomass type, showed considerable variation, with the K2O content of wheat and corn straw ash being particularly variable. Most Zhundong coals had medium and similar ash fusion temperatures regardless of the changing ash compositions. Contrary to biomass, the Na2O content in Zhundong coal ash was an order of magnitude higher than K2O content. The ash composition of a particular biomass type usually fell in a small region of the ternary diagram of acidic-alkali metal-alkaline earth metal oxide components.

A study on air-cooling waste heat recovery from molten slag of slag-tap boilers

For a better utilization of Zhundong coals which have high fouling and slagging tendency, the slag-tap boiler has attracted much attention. To avoid the high sensible heat loss of discharged molten slag, an air-cooling waste heat recovery system is proposed. Energy and economic analyses are conducted to investigate the effectiveness of heating the desulfurized flue gas by hot air and the influences of partially substituting the secondary air by hot air on heat transfer of air preheater and thermal efficiency of boiler. A case study is performed by referring to a typical 50 MW cyclone boiler with nine types of low fusion temperature coals. The results show that for coals with low ash content, the temperature increment of desulfurized flue gas can be over 7 ℃. While for coals with high ash content, the flue gas temperature can be heated to more than 70 ℃, and the surplus hot air can be sent to the furnace. When the hot air is introduced to partially substitute the secondary air, an instantaneous impact on the air preheater will give rise to a decrement of quantity of heat transferred and increments of temperatures of exit flue gas and hot secondary air. The variations of these thermodynamic parameters become smaller with increasing hot air temperature. After introduction of hot air, the thermal efficiency of boiler can increase, resulting in a decrease of fuel consumption rate. In addition, the heating surface area of air preheater can be reduced.

Effect of ashing temperature on physical-chemical features of high-sodium ashes of Zhundong coals

ABSTRACTAsh analysis provides useful information for better understanding the serious ash-related issues during utilization of Zhundong coals, while the influence of ashing temperature on exploration of ash properties of high-sodium coals is still unclear. Here, we focused on the effect of ashing temperature on physical-chemical properties of high-sodium ashes. The ash yields of high-sodium coals decrease obviously with an increase of ashing temperature, while the decrease extent is closely related to content and occurrence mode of alkali metals. Partial properties of high-sodium ashes of Zhundong coals are strongly influenced by the ashing temperature, and an appropriate ashing temperature is necessary.

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.

Influence of sodium on deactivation and regeneration of SCR catalyst during utilization of Zhundong coals

AbstractZhundong coals have a super huge reserve and many excellent characteristics. However, the sodium content in Zhundong coal is extremely high, which can accelerate the deactivation of the V‐W‐TiO2 selective catalytic reduction (SCR) catalysts. In the present work, the solution impregnation method was adopted to investigate the effect of alkali metals deposition on the SCR catalysts, while various approaches were employed to regenerate the poisoned catalysts. Experimental results indicate that sodium deposition significantly deactivates the catalytic performance of SCR catalysts and sodium hydroxide (NaOH) presents the most serious deactivation. The catalytic ability of sodium poisoned catalyst is only slightly recovered by water washing, whereas it could be improved to approach that of the fresh catalyst by acid solution washing. The regeneration effect of sulfation with SO2 is between those of water washing and acidic solution washing. Further characterizations validated that the deactivation is mainly caused by the damage of surface acid sites. Acidic solution can remove more than 75 wt% of the deposited sodium and provide new acid sites on catalyst surface. Therefore, acidic solution washing is a satisfactory method to regenerate poisoned SCR catalysts for power plants firing high‐sodium coals. © 2016 Curtin University of Technology and John Wiley & Sons, Ltd.

Transformations and Roles of Sodium Species with Different Occurrence Modes in Direct Liquefaction of Zhundong Coal from Xinjiang, Northwestern China

In this work, to better understand the thermal behavior of sodium species in the utilization of Zhundong coal (ZDC) from Xinjiang, northwestern China, the transformations of sodium species in direct liquefaction of ZDC were investigated in a bench-scale autoclave. The effects of sodium species with different occurrence modes on the product distributions in ZDC liquefaction were also examined. The results show that the transformations of sodium species strongly depend on the occurrence modes and the liquefaction temperatures. Below 370 °C, ammonium acetate-soluble sodium species (AS-Na) could be partly transformed into water-soluble ones (WS-Na). However, when the temperatures further increased, the reactions between AS-Na and silicates to form hydrochloric acid-insoluble sodium species (HIS-Na) predominated and the residue formed in this process was firmly adhered to the bottom of the autoclave. The retention ratios of sodium species over all temperature ranges are significantly higher (>89 wt %) than tho...