Optimization of Coal Blend proportions for sustained improvements in generation & efficiency

Transcription

1 Optimization of Coal Blend proportions for sustained improvements in generation & efficiency A.K. Arora & D.Banerjee CenPEEP NTPC LTD

2 Coal Blending in Power Sector Coal Blending is a compulsion for some stations for meeting environmental norms & sustenance of generation. Shortfalls in domestic coal supply Progressive degradation in coal quality Coal Consumption & Import India 442 / 48 MMT 473 / 80 MMT NTPC 141 / 12 MMT 157 / 16 MMT 2

3 Issues & Concerns - Utility Perspective Compatibility of coals Blend Proportions Optimum blending ratio Maintaining desired blend proportions Methodology of Coal Blending Safe operation -Slagging/clinkering -Fire in stack yard or Mill Impact of blending Cost of Generation Auxiliary Power consumption Environmental Emissions Operating & performance parameters 3

4 Present Practices Commonly Used Coal Characteristics Moisture ASH FC VM GCV Presently two different quality coal is mixed in such a proportion that resultant coal blend have desired ASH, VM or GCV. 4

5 Compatibility - Coal characteristics Any two coals can not be blended Two compatible coals can t be blended in any proportion Additive properties Volatile Matter Ash Moisture Gross Calorific Value Non Additive Properties Combustion Reactivity of coal (Combustibility) Ash Characteristics ( Fusion Temperature ) Grindability (HGI) Swelling characteristics of coal Compatibility With Respect to Non Additive Properties of two coals are Essential for successful Blending 5

6 Assessment of Combustion Compatibility Differential Thermo gravimetric (DTG)& Differential Scanning Colorimetric (DSC) Burning profile analysis Typical DSC Burning Profile used for analysis of compatibility of two coals

7 Burning Profile Analysis 1. Coal with similar burning profile is expected to behave similarly in a full scale boiler. 2. By comparing burning profile of an unknown coal with that of a coal with known full scale performance, compatibility can be judged. 3. Comparison of burning profile temperature provides qualitative assessment of coal reactivity

8 Compatibility w.r.t Ash fusion Characteristics Ash composition of two coal from different origin may be different. May have different ash fusion characteristics It is necessary to see that blended coal ash IDT should be higher than FEGT temperature. Ash fusion temp is a measure to assess slagging and fouling propensity of coal

9 Laboratory Study to Check Compatibility of Coal A Maharatna Company Ash fusion temperature C of individual coals CL- Low ash coal CH- High ash coal CL-2 & CL-3 have lower softening temps 1370 C & 1400 C and relatively slagging prone coal Coal Initial Deformation Softening Hemispherical Fluid CL CL CL CH >1650 >1650 CH CH >1650 Ash fusion temperature C of blended coal Softening temperatures of all the blends are well above maximum furnace temperature (1400 C C) Coal Initial Deformation Softening Hemispherical Fluid CL-1+CH CL-1+ CH CL-1 + CH CL-2+CH CL-2+ CH CL-2 + CH CL-3+CH >1650 CL-3+ CH CL-3 + CH

10 Laboratory Study to Check Compatibility of Coal Hard grove Grindability Index of coals Coal CL-1 CL-2 CL-3 CH-1 CH-2 CH-3 HGI There is wide variation in HGI values of CL-3 & CH-3 coals Blending CL-3 & CH-3 may lead to selective grinding, segregation and combustion problem.

11 Burning profile of Blended coal Indigenous low ash coal, CL-1 with lowest peak temperature (PT-439 C) is most reactive, Imported low ash coal with highest PT (514 C) and burnout temperature (598 C) is least reactive Blending CH-3 with CL-3 may result flame fluctuation, thermal instability or increase in combustible losses.

12 Safe operation Imported coal has high VM (25-45%) Care to be taken to avoid spontaneous combustion in stack yard Control Mill inlet air temperature to maintain low mill outlet temperatures Imported coal has low Ash Fusion Temp Reducing atmosphere in Furnace Clinkering & Slagging tendency Imported coal has Compatibility issues Secondary Combustion High Sulphur, SOx emissions Increase in Unburnt Loss Operation at High Excess Air

13 BLENDING METHODOLOGY

14 Comparisons of different coal blending Methodologies A Maharatna Company Blending Method Brief Description Point /Place of Blending Advantage Disadvantage Application in NTPC Blending in Beds Stacking of two coal in yard in layers -Blending during reclaiming -Homogenization at transfer point Only one system to run for bunkering coal -Entire coal to be stacked first -More running Hours for CHP & more APC -Not possible to change ratio Not tried Blending by Silo Imported coal from Silo is mixed with domestic coal carried by conveyer below Silo -On conveyer below Silo - Homogenization at transfer point - V. accurate Blending ratio can be achieved -Ratio can be varied High cost Capital Facility available NTPC not in Blending by Ground Hopper Imported is Dozed to ground Hopper Domestic coal is fed directly from track hopper wagon tippler -At common transfer point - Homogenization at transfer point Ground hopper is an additional Source of reclaiming Feeding rate is not accurate Facility available in some places

15 Comparisons of different coal blending Methodologies Blending Method Brief Description Point /Place of Blending Advantage Disadvantage Application in NTPC Blending on Moving Belt (Both coal reclaimed) Two types of coal are stacked in two yard of different stacker & reclaimers -At common transfer point - Homogenization at transfer point Blending ratio can be changed any time -Entire coal to be stacked first -More running Hours for CHP & more APC Applied some station in Blending on Moving Blelt ( Imported coal reclaimed & Domestic coal from track Hopper) Imported coal is reclaimed from yard and Domestic coal is directly fed from track hopper/ wagon tripler -At common transfer point - Homogenization at transfer point Blending ratio can be changed any time Only imported coal need to stacked Accuracy wise inferior to Silo blending Commonly used Method Another method for use of two different coals is dedicating one or two mills for firing imported coal and remaining mills on domestic coal by adopting Co-firing

16 Case Study : Compatibility of Domestic & Imported coal of a Station Stations gets coals from different mines MCL Talcher, MCL Ib, SCCL, Indonesia Tests were carried out in labs to get an qualitative idea about compatibility of coals with respect to their burning behavior. Differential Thermo gravimetric Analysis/Burning Profile & Ash Fusion Tests were carried out for Domestic, Imported and blends of 10, 20, 30 and 40% of MCL Talcher & Indonesian coals Typical Proximate Analysis of Domestic Coal of different sources & Imported Coal Source TM% ASH% VM% FC% GCV KCAL/KG TALCHER IB AREA SINGARENI IMPORTED COAL

17 Thermo gravimetric Analysis MCL Talcher Coal SCCL Coal MCL Ib Coal Indonesian Coal The four figures are the burning profiles of the four types of coals. Solid line is the weight loss on account of burning of coal and dotted line is the 1st derivative of the weight loss. The bottom peak denotes maximum weight loss, peak at lower temperature indicates more reactive coals. 17

18 Thermo-gravimetric Analysis 10% blend 20% blend 30% blend 40% blend The four figures are the burning profiles of the four different blends of coals. 10% Imported with MCL Talcher, 20% imported with MCL Talcher, 30 % & 40% blends 18

19 Analysis of DTG burning profile DTG of Imported ( Presently used) coal was compared with DTG of proven domestic coal MCL Talcher MCL Ib SCCL Indonesian BOT C DTG Peak ,480.4 MCL Talcher + % Imported 10% 20 % 30 % 40% BOT C DTG Peak , Peak temp & Burnout temperatures of imported & domestic coals are comparable & compatible DTG Profiles of blended coals of different ratio are well within the range of proven coal This results indicates that these blends when fired in boiler shall not cause any combustible loss problem

20 Ash Fusion Temperatures IDT 0 C HT 0 C Free flow temperature 0 C MCL Talcher >1540 MCL IB >1540 SCCL >1540 Imported Talcher + 10% Talcher + 20% Talcher + 30% Talcher + 40% Burning profile (DTG) temperatures of blends are in the same range as those of domestic coals being fired. Combustion reactivity of the blends is similar to domestic coal & should not cause any combustion related problems. Initial deformation temperature of both the imported coal and of 40 % blend is lower than 1300ºC. Therefore, coal blends over 30% may cause melting of ash in furnace and initiate slag formation or clinkering in reducing atmosphere. 20

21 Ash Constituents MCL Talcher MCL IB CAF Fly Ash Imported SiO 2 % Al 2 O 3 % Fe 2 O 3 % CaO % MgO % SO 3 % TiO 2 % P 2 O 5 % Na 2 O % K 2 O % Imported coal ash have Higher concentrations of iron, calcium, magnesium & sulphur which may initiate slagging Since ash content of coal is small and further when blended 10,20 or 30% its contribution to blended coal ash becomes negligible Blending higher proportion of imported coal shall increase the content to appreciable extent. Concentrations of sodium & potassium, cause for fouling deposits in boiler are also higher. Hence during optimization of blend ratio compatibility w.r.t coal ash chemistry must be followed. 21

22 Conclusion Compatibility of characteristics of two coals must be assessed before blending to avoid combustion related problems. Optimum blend ratio needs to be determined and maintained during blending (Based on AFT, Burning Profile &HGI) Proper Methodology for blending must be followed based on available infra structure Blending in transfer point before bunkering ensures good mixing in different stages & is the recommended practice For new projects provisions need be made for firing blended coals at design stage There s a need to develop models for Indian coals, that could predict impact of coal blends & coal quality variations on boiler performance 22

23 Thank You