New Technolgical Researches On Reductive Desulfurization Of Coal Water Slurry

So far, many problems still need resolution for the technique of coaldesulfurization after combustion (i.e., SO2 removal). Take for example, theproblems emerged from the limestone-based process, which has been widelyapplied in our country included high investment in construction, high costand tedious in operation, large floor space, susceptible to corrosion andplugging, intensive water consuming, etc. Moreover, the plaster generated ishard to handle. Therefore, the sulfur presented in coal should be removedprior to combustion. So much coal has to be used at present at relatively lowefficiency, so the environmental effects and energy waste are very severe.As a result, clean coal technology is being applied and popularized at anunprecedented rate in China. CWS is a kind of clean liquid fuel made of lowgray and sulfur clean coal after washing and choosing. Although CWS isconsidered as a clean coal technology in China, with the increase of rawcoals for pulping, the emission of sulfur dioxide is growing, indeedexceeding to the environmental requirements. Therefore, as a clean fuel,sulfur removal of CWS before combustion can save lots of costs of flue gasdesulfurization (FGD). Therefore, desulfurization for CWS before combustion is playing an important and practical meaning for air pollutioncontrol in modern times.This research work utilized chemical reduction and electrochemicalreduction for desulfurization process of coal water slurry (CWS). At first,we took sodium borohydride (NaBH₄) as the initial reductant, andinvestigated reductive desulfurization process for CWS prior to combustion.And then we converted the chemical reductive process into electrochemicalreductive process to realize the filtrate recycling. It is significant to providethe basis of science and experiments on CWS boilers in practice. The mainlyresearch contents and results are as follows:(1) The present work aimed at the investigation of a mild, rapid, effectiveand recycling desulfurization method of NaBH₄ reduction for coal waterslurry (CWS). Processing parameters such as reductant concentration ofNaBH₄, treatment time, particle size of coal, reaction temperature, shakingrate and initial pH were taken into consideration. The optimum runningparameters have been determined as follows: 2.0 mM (0.8 g/L) of NaBH₄concentration, 140 meshes of coal particle size, 10 min of total treatmenttime,100 rpm of shaking rate and 30°C of reaction temperature. The totalsulfur reduction of CWS can reach about 43% operating at above parameters.Moreover, sulfate sulfur reduction can reach about 47.8%, pyritic sulfur can be removed by 55.4%, and organic sulfur could be removed by 23.9%. Thesulfur content of CWS was decreased by the treatment and combustionperformance got improved. After treatment, the calorific value was slightlyincreased and the ignition temperature was reduced.(2)The investigated work was an innovative process of sodiumborohydride reduction using ultrasound-assisted metal boron hydrides(NaBH₄, KBH₄) reduction. The ultrasound used for reductive desulfurization(RD) of CWS can greatly improve the process conditions (CWSconcentration, coal particle size, chemical reagents concentration, etc.) andchemical efficiency. Under the ambient pressure and temperature conditionsof ultrasonic time (10 min), ultrasonic frequency (20 KHz), deoxidizerconcentration (10 mM), CWS concentration (60 g/L) and coal particle size(≤120 mesh), the desulfurization efficiency of CWS can reach about 42%(NaBH₄) and 45% (KBH₄), respectively. Addition of Ni2+ catalyst (1 mM),desulfurization efficiency up to the maximum value of 57% was greatlyimproved. Significantly, the nitrogen (N) of CWS was reduced by 11.5%.The decrease of S and N greatly cut down the SOx and NOx emissions fromCWS combustion. After the processing, the ash content and volatile matterwere decreased contributing to the slight improvement of combustioncharacteristics.(3)The present work investigated an alternative desulfurization process forcoal water slurry (CWS) using sodium metaborate (NaBO2) electroreductionunder the non-isolated system. Desulfurization efficiency improved with theincreases of NaBO2 concentration, electrolytic time, and electrolytic voltage, while with the decreases of coal particle size and CWS concentration. Up to50% desulfurization efficiency was obtained in the≤109μm (140 mesh)coal under the optimized conditions of CWS (50 g/L), NaBO2 (13.2 g/L),NaOH (1 g/L), electrolytic voltage (3.5 V), electrolytic time (4 h), andanode/cathode (C/Pb). Furthermore, the possible reaction mechanism ofelectrochemical desulfurization (ED) was that NaBO2 was converted intoNaBH₄ by means of the electrochemical reduction (ER) on the cathode,which induced transformation of pyritic sulfur (PS) and organic sulfur (OS)in the coal into H2S and S2-, and S2- in the solution was further oxidated intoSO42- (SS) on the anode. Among different type sulfur species, PS and OSwere most efficiently removed, with the reduction of 100% and 46.7%,respectively. These observations indicate this ED method is effective inremoving PS and OS of CWS. The increase of SS (highest valence S)caused by anodic oxidation, could be easily removed by filtering andleaching. Namely, the S releasing from coal particles is mainly in the formsof H2S (volatilization) and SS (filtration), so only the residues of OS in thetreated coal may produce SO2 during the process of CWS combustion. Theproduced H2S could easily be collected by the alkaline liquors. Aftertreatment the calorific value of CWS was slightly reduced due to oxidation.(4)This electrochemical reductive desulfurization (ERD) method for CWSwas an offshoot of the previous work, utilized cathodic reduction to changesome chemical compounds into strong reducing substances which convertsthe variety forms of sulfur (S) in coal into H2S and S2-, thus releasing S fromcoals. Meanwhile, it was an effective way to enhance S removal from coal by adding divalent metal catalysts (Ni2+, Mn2+, etc.). According to theatmospheric conditions of 12 g/L of NaBO2, 3.0 V of electrolytic voltage, 4 hof electrolytic time, 50 g/L of CWS,≤140 mesh of coal particle size, 1.0 g/Lof NaOH, and 1 mM (about 0.13 g/L) of NiCl2 catalyst, the total sulfur (TS)of CWS can be dramatically removed by 58.5%. The ERD described herewas a predominant desulfurization process, such as mild (slight improvementof the combustion characteristics), high desulfurization efficiency,convenient operation conditions and low expense (reuse of the filtrate andby-production of hydrogen). Moreover, the realization of the filtraterecycling and the evaluation of the hydrogen releasing rates of NaBH₄ werefurther helpful in cost reduction.