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• A review of the difficulties faced when burning high-alkali coals in boilers. • The effective combustion utilization methods of high-alkali coals are summarized. • The ash deposition solutions of burning high-alkali coals are summarized. • Proposed future research directions for the utilization of high-alkali coals. High-alkali coals are distributed in many countries all over the world, mainly characterized by contains abundant alkali metals and alkaline earth metals (AAEMs) elements, direct combustion in boilers will bring serious ash deposition, fouling, and slagging (ash-related issues) on the metal heat transfer surface of the boilers. In particular, the Zhundong (ZD) coals in Xinjiang, China, as typical high-alkali coal, have a huge reserve that can effectively ensure China's coal demand in the long term. The main purpose of this paper is to clarify the problems and difficulties faced by the combustion of high-alkali coals in boilers, explore and summarize the effective utilization methods. First, the primary difficulties and problems faced by direct combustion of high-alkali coals in boilers are summarized. Second, the research progress of current efficient utilization methods and solutions of high-alkali coals combustion in boilers are summarized and reviewed, it mainly includes the following aspects: (1) pretreatment technology before combustion; (2) control and optimization of combustion parameters such as temperature and atmosphere; (3) co-combustion with inorganic additives, other coals, biomass fuels, and other fuels, respectively. The above methods have been proved can be effectively reduced and alleviated the ash-related issues, but no methods can be eliminated or solved the problems completely at present. Based on the existing research and practice results reviewed in this paper, aiming at the efficient utilization of high-alkali coals combustion in utility boilers, further feasible research directions or key points for the future are proposed. [ABSTRACT FROM AUTHOR]

This paper aims to clarify ash deposition/slagging behavior of the newly found Xinjiang high-alkali coal in China, which is notorious for large amounts of sodium (Na 2 O, 4.0–10 wt%) and calcium (CaO, 20–40 wt%). A 12-h exposure test of ash deposition/slagging was conducted in a 3 MW th pilot plant, with a number of deposition probes and heat transfer tubes being inserted along the furnace to mimic the real pulverised coal-fired boilers. The samples were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray Detector (EDX). Thermodynamic equilibrium calculations using FactSage 6.1 were also conducted to interpret the experimental observations. As has been confirmed, the Fe-rich species including magnetite and wollastonite are dominant in the fire-side deposit, due to the low melting temperature of Fe-/Ca-bearing minerals. Those particles were firstly melted and agglomerated to form large clusters, which subsequently attached to cold furnace wall by the inertial force. In convective section, the vaporisation of alkali metals, as well as their subsequent condensation and sulphation reaction to form Na 2 SO 4 and CaSO 4 are the major routes fouling the water tube. Compared to the coal samples studied in the literature, the ash deposit for Xinjiang coal grew quickly in the first 1.5 h, causing a significant decrease on heat transfer efficiency. Na 2 SO 4 , CaSO 4 and Ca–Al–Si slags are the dominant phases in the growing layer of the ash deposit. The continuous interaction between liquid phase and newly captured particles led to the formation of eutectic compounds which embed anhydrite (CaSO 4 ), thenardite (Na 2 SO 4 ), gehlenite (Ca 2 Si 2 O 7 ) and hematite (Fe 2 O 3 ) inside. The formation of these eutectics accelerated the capture of more solid particles in ash deposit, which also turned sticky and harder to remove upon the increase on exposure temperature and time. [ABSTRACT FROM AUTHOR]

Excessive water production from hydraulically fractured unconventional reservoirs is historically considered as burdensome liability in terms of costly treatment and disposal. However, this paper demonstrated produced water as a valuable asset, which is capable of producing geothermal energy, and it can actually help offset the operation costs for unconventional oil and gas producers. Harnessing geothermal energy of produced water for power generation from unconventional horizontal wells features significant advantages over traditional geothermal wells, especially in reducing capital expenditure and operational risks. This paper proposed a novel design of downhole power generation in horizontal wells by integration of thermoelectric technology with hydrocarbon production. In the design, we retrofit the horizontal wellbore to circulate cold water inside the tubing so that a cold temperature interface can be created. The produced fluids flow through the annulus of the tubing and casing to maintain hot temperature on the side of tubing. By attaching thermoelectric generators (TEG) on the outer surface of tubing, this retrofit will make it possible to capture subsurface in-situ geothermal energy of the produced fluid and directly transfer heat to electricity inside the horizontal wellbore. Furthermore, we also built the mathematical model to account for the heat conduction and convention in the wellbore and surrounding formation, which could accurately simulate the temperature distribution and power generation under variable operational conditions. Based on the model, we identified the key parameters with significant impact on power generation and established the well section criteria for best power generation performance. To discover the potential of power generation in horizontal wells, we conducted a case study in Daqing Oilfield in northeast China and compared the power generation performance with vertical well with same TVD and same power generation device installation. The results of case study indicated that horizontal well displayed exceeding power generation performance over vertical wells. In practice, this paper could contribute to enrich geothermal development methods and provide the guidelines for oil and gas producer to evaluate their horizontal assets and identify the opportunity to capitalize on geothermal power generation from horizontal wells. [ABSTRACT FROM AUTHOR]