CEE Oct-Dec 2002

ENERGY EFFICIE~CY AND CONSERVATION ANTHRACITE COAL FIRING: 100% SUCCESS Ry11ki Oka, !t!ilsubishi Heavy Industries [Jd, Mit subi shi Heavy Industri es Ltd (MHI ), succeeded in develop ing a new Mitsubi shi rluidi sc d calc iner (N-MFC) for I 00% anthraci te coal firing in the pn.::calciner for Nghi Son Cement Corp (NSCC). Stabl e and rated operation was attained one month after it\\ as first fi red. The company claims that the N-MFC has been provcu to be the most suitabl e precalciner for I 00 % anthracite coal firing. NSCC took an interest in the resu lts o f MHI 's pre-bid verifi cation ofN-MFC and awarded the semi-turnkey contract to MHI in November 1997. The production line has a capacity of 5800 tpd o f clinker. M ill then undertook total plant eng inee rin g, equipment supp ly, local fabrication, erection and commissioning work as \·Veil as techn ica l assistance to successfully deliver on time. This article describes th e special fea tures of the main equ ipment in the plant placing emphasis on the characteristics and advantages obtained from N-MFC as a re sult of operat ion and maintenance. tate-of-the-art technology is employed to minimise the operating costs and secure reliable operation and maintenance. -MFC sys tem for th e preca lciner ki l n ensures long and stable operation w ith I 00% anthracite coa l !iring. M it subi shi Exte rn al Ci rcuit (MEC) sys tem which is the p11tent or Ml-11. fo r the vertical roller mill to dry and grind raw material to save electric power. Fu lly computer co1ttrolled plant operations and quality control. Envi ronmental protection system incorporating a low-NO ki ln burner, hi gh effi c iency dedusting equipment. N- MFC is a fluidised bed type precalciner and has a retention time or approximate ly 60 sec for raw meal and coal in N-MFC. This is much longer than any other type of preca lc iners. such as gas suspension, which has a very short retention time of around 4 - 6 sec. The MFC-System Eliminates coating trouble Extends refractory use time Reduces power consumption in the coal mill system A ll ows fu el cost sav ings resulting in significantly reduced operating cost Thi s long retention time has enabled I 00% anthracite coa l firing in the precalciner maintaining the exhaus t gas temperature of approx imately 850-860°C without any operational and maintenance problems, such as coating and local overheating. The system comprises a Mitsubishi Suspension Preheater (M-SP) with low pressure-loss and high-effic iency cyclones. A 5-stage two-string preheater consisting of K-line preheater and C- 1 ine preheater, was selected to save energy and for ease of operation. During operation. the coal mill was used to produce a product fineness of app rox imately 7% res idue on 9011 sieve, as the comp lete combustion of the coal was attained wi th this fineness in the precalciner ki ln system. Raw meal fed to the preheater is calcined up to about 90% and fed to the rotary kiln. The main characteri stics of the equi pment arc as follows: Suitab ility for burning hard -to- 46 burn fuel, such as anth racite coal and oil cake. A fluidised bed of infect raw meal and coa l is formed at the bottom of the N-MFC. In thi s bed, the ·raw meal and coal arc cl ispersed and mixed uniformly, then re tai ned for approximately 60 sec to completely burn the anthracite coal wi th th e relat i vely lo\\ temperature of850- 870 °C. It is not necessa ry to raise the burning temperature and /or to lower the fi neness or the pulveri sed coal excessively to achieve comple te combusti on of the coal. since the retention time is longer than any other type of precalc iner. Hot fr esh air is used as the secondary air for fuel burning. K i In exhaust gas i s not i ntroduced into the N-MFC. The hot f resh air is recovered from th e cli nker coo l er as secondary combustion air to be mixed with the raw mea l and coa l uniforml y. TIHI S. combustion speed of the coal is remarkably enhanced in t he hi gh oxygen concent rat ion atmosphere allowing the coal to be comp letely burnt at a relatively low temperatu re. Burner name is not formed 1n N-MFC. A uniform temperature d istr ibution can be obtai ned i nside N-M FC due to th ese characteristics. As a result. local burning and coat i ng can be eliminated and a long life of the re fractory lin ing can be attained. • The operating results at NSCC prove that running costs can be minimi sed. This is ach ieved if the ·eq uipment is adopted as the precalc iner, since norma l rated operati on o r th e sys tem can be executed with only anthraci te coal without using any rue! oil, such as