The goal of this research is to establish the applicability of an electrostatic measuring technique for monitoring the quality of a coal-milling process in direct-firing systems. Such systems are ...used in large steam boilers fired with low-rank coal where the pulverized coal is transported pneumatically from the mills to the burner nozzles via ducts with large cross-sections. The electrostatic measuring method, in connection with intrusive rod-type sensors, was studied because it provides good spatial sensitivity and cost effectiveness. A laboratory test rig was constructed, where the pulverized coal carried by ambient air was employed for the experiments emulating the pneumatic transport of coal particles in direct-firing systems. The test rig enables an extensive variation of the most influential parameters, like the mass-flow, the velocity and the size of the particles. A linear, multi-regression analysis of the results of the experiments was carried out and the appropriate regression model enabling a determination of the mean diameter of the particles using the electrostatic signal was chosen. Based on the results of the study the electrostatic measuring technique can be used for monitoring the size of pneumatically transported particles. The appropriate regression model needs to be chosen for each particular application to describe the dependency of the acquired electrostatic signal on the influential parameters of the pneumatic transport.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Fan/impact mills are commonly applied in the grinding and dilute-pneumatic transportation of lignite or brown coals with high moisture contents to the furnaces in large steam boilers. Each of the two ...to eight mills feeds pulverized coal into two or more burner nozzles. An online detection of the pulverized-coal mass flow distribution among the burners is vital for the control of the combustion process. Knowing the distribution, measures for the redistribution of the coal or, alternatively, for the adjustment of the combustion air flow according to the actual distribution can be employed. Determining the characteristics of a gas-solid two-phase flow using an electrostatic principle is a promising online method of measurement because it is robust and inexpensive. Furthermore, due to their better spatial sensitivity, rod sensors are more suitable for large rectangular ducts related to fan/impact mills than ring-, pin-, or arc-shaped sensors. Sets of 1-D and 2-D electrostatic sensor arrays with a corresponding data acquisition system were employed to determine the mass flow distribution in the cross section of the duct that feeds the pulverized lignite to the four burner nozzles. Various operating regimes for the fan/impact mill were tested. The time series of the signals from the electrostatic sensors were analyzed statistically. It was shown that the skewness, kurtosis, and autocorrelation time delay at the characteristic value can indicate different grinding qualities of the coal.
This paper presents a study of heat transfer in a rotating axial rotor. The study was done on the basis of computational fluid dynamics simulations and validated with an infrared thermocamera ...experimental setup. The influence of full turbulence vs. Menter's boundary layer method was studied in terms of heat transfer and flow phenomena. Additionally it was found out that a thin paint layer has a major influence on heat transfer phenomena and acts as insulation. For comparison of numerical results with experimental, a model of the actual rotor was established. Results were found to be in good agreement, so the effect of rotation on heat transfer was studied. Results are presented in terms of Nusselt number distribution and an empirical model of heat transfer.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Operational surveillance of all vital parts of thermal power plants is nowadays more important than any time before due to requirements for their extremely flexible operation resulting from ...intermittent behaviour of renewable energy sources. New methods for online measuring of pneumatic transport provide new possibilities for control and early fault detection of coal grinding and conveying system in direct-fired power plant boilers. Arrays of intrusive electrostatic sensors are an attractive option due to their inexpensive application and good spatial sensitivity required in large rectangular ducts of pulverized-coal systems. In this study, statistically treated electrostatic signals are used for detection of unexpected change in operating regime of coal grinding and conveying. Model-based and model-free autocorrelation reduction techniques are used to reduce the inherent autocorrelation of data. Forming batch-means of data, a model-free autocorrelation reduction technique is proposed in combination with an autoregressive-integrated moving-average (ARIMA) method. Residuals between real and ARIMA-model fitted data are entered into exponentially- weighted-moving-average (EWMA) control chart for statistical surveillance of the process. The robust and cost-effective measuring method accompanied with a simple and intuitive control scheme proves to be effective for early fault detection of the pulverized-coal preparation system. Keywords: ARIMA model, control chart, fan mill, fault detection, pneumatic transport, statistical modelling Namen raziskave je bil razviti ucinkovito in robustno metodo za sprotno spremljanje karakteristik pnevmatskega transporta premogovega prahu od mlina do sob gorilnika v realnih obratovalnih pogojih. Med drugim naj bi omogocala pravocasno zaznavanje stanja preobremenjenosti mlina. Zaradi nestanovitne proizvodnje elektricne energije s soncnimi in vetrnimi elektrarnami se zahteva vse bolj fleksibilna proizvodnja elektricne energije s termoelektrarnami. Istocasno se zahteva, da so izpusti skodljivih snovi in ucinkovitost proizvodnje s termoelektrarnami znotraj dopustnih meja. Za kakovostno in stabilno zgorevanje je nujno, da se na vseh sobah gorilnikov stalno vzdrzuje optimalno masno razmerje med premogovim prahom in zgorevalnim zrakom in da ne prihaja do obcasnih preobremenitev mlinov za premog. Preobremenitve ventilatorskih mlinov, ki se uporabljajo za mletje lignita in rjavega premoga, so pogost pojav. Posledice prepozno zaznane preobremenitve mlina se odrazajo v nedopustnem nihanju pretoka, temperature in tlaka pare, ki jo proizvaja kotel, se nekaj casa po razbremenitvi mlina. V najslabsem primeru lahko pride tudi do prekinitve proizvodnje pare. Trenutno se za zaznavanje preobremenitve mlinov uporablja merjenje temperature zmesi premogovega prahu in nosilnih plinov na izstopu iz mlina. Prehitro padanje temperature in prenizka temperatura prasne zmesi sta znak preobremenjenosti mlina. Izkusnje kazejo, da je ta metoda sicer uporabna, a obicajno nekoliko prepozno sprozi opozorilo. V raziskavi so bile uporabljene elektrostaticne merilne sonde in ustrezen elektronski sistem za zapisovanje in obdelavo njihovih signalov. Zaradi velikih presekov prahovodnih kanalov ni primerna uporaba toku nevsiljenih elektrostaticnih zaznaval, ki so vgrajena v stene kanalov in imajo zato omejeno zmoznost zaznavanja karakteristik toka oddaljenega od sten kanalov. Za reprezentativno merjenje je bilo potrebno uporabiti sonde, ki so potekale skozi kanal vzporedno ena z drugo v ravnini pravokotni na tok. Signali so bili merjeni s frekvenco 1000 Hz. Frekvenca je bila izbrana glede na predhodne preizkuse tako, da se je z zmerno kolicino izmerkov dovolj natancno posnel casovni potek signalov. Surove casovne vrste izmerkov so bile nato obdelane s statisticnimi metodami za zmanjsevanje avtokoreliranosti med sosednjimi izmerki. Zaporedno sta bili uporabljeni dve metodi: tvorjenje neutezenih povprecij podskupin zaporednih izmerkov, kot metoda zmanjsevanja avtokoreliranosti brez modeliranja in nato se metoda, ki sloni na ARIMA (Autoregressive Integrated Moving Average) modelu. Odstopki med izmerki in modeliranimi vrednostmi ARIMA modela vstopajo v EWMA (Exponentially Weighted Moving Average) kontrolni diagram, ki slui za statisticni nadzor procesa. Analiza je pokazala, da je predlagana metoda statisticne obdelave surovih izmerkov elektrostaticnih zaznaval zelo primerna za zaznavanje nenadnih hitrih sprememb pnevmatskega transporta in zaradi tega tudi za zgodnjo detekcijo preobremenjenosti mlina. Opozorilni signali predlagane metode so zanesljivi in zgodnejsi v primerjavi z opozorilnimi signali, ki se sprozijo na podlagi merjenja temperature prasne zmesi na izstopu iz mlina. Predlagana metoda vkljucuje uporabo elektrostaticnih merilnih sond, ki so vrinjene v kanal in podvrzene abraziji ter jih je zato potrebno redno menjavati. Casovni razmik med zamenjavami zaznaval je odvisen od odpornosti materiala sond in abrazivnosti premogovega prahu. Nastavitve ARIMA modela in EWMA kontrolnega diagrama niso univerzalne in jih je potrebno optimizirati za vsak objekt. Uporaba ARIMA modela in EWMA kontrolnega diagrama je pogosto predstavljena v literaturi in je uporabna za nadzor mnogih procesov. V tej raziskavi sta prvic uporabljeni v povezavi z zaznavanjem motenj v pnevmatskem transportu premogovega prahu od ventilatorskega mlina do gorilnikov. Predlagana metoda omogoca pravocasno zaznavanje in posledicno preprecevanje stanj preobremenjenosti mlinov za premog, kar omogoca stabilnejse obratovanje velikih parnih kotlov s sprotno pripravo premogovega prahu. Kljucne besede: ARIMA model, elektrostaticna merilna metoda, kontrolni diagram, ventilatorski mlin, pnevmatski transport, premogov prah, statisticno modeliranje
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