Effect Of Coal Quality

IMPACT OF COAL QUALITY ON BOILER OPERATION & EFFICIENCY.

Introduction: CENPEEP has demonstrated Performance testing & optimization techniques at a number of NTPC and SEB stations. • Database established has been utilized to compare Performance level of different coal fired Boilers. • Performance degradation occurs on account of deterioration in coal quality. • Degradation in Boiler efficiency level between two overhauls is inevitable. • Degradation can arrested by Firing coals for which boiler was designed.

• This presentation is based on data base of a 210 MW unit firing high ash- low grade coals.

Station Concerns were• Performance Guarantee Tests had not been conducted in this unit. Base line data to fix targets were not available. • Unit is equipped tubular air heaters and a unique burner design for low GCV coals . • No controls for secondary air dampers regulations provided. • Un burnt carbon in flyash had been consistently very low, below 0.5%. • It was thought that boiler operates at high excess air which could be reduced to a more optimum value.

Station Concerns were• On line oxygen feedback was not reliable? Whether oxygen levels can be reduced. • Fuel piping was not equipped with orifices to ensure uniform fuel / air ratio in each burner. • Five mill operation necessary to achieve full load operation. • What is the that Economical load at which 5th mill should be withdrawn from service • Mills are generally run at high PA flows, reliability of PA flows need to be established.

Methodology for Performance testing: • Assessing Mill Performance using Dirty Pitot ASME kit. • Review of Plant Performance data. • Installation of test ports and measurement sections • Calibration of station instrument during shutdown. • Restoration of individual equipment performance through an effective overhaul. • Mill performance testing prior to Boiler Performance testing. • Air heater and boiler performance testing with different mill combinations. • Boiler furnace performance testing using HVT probe.

Mills & Associated Burner System • • • • • •

Mills are generally operated at high PA flows and mills operate in temperature range of 85-95 degree C. Bias in mill outlet temperature or mill loading did not affect the combustion regime. Primary airflow distribution between various burners of the same pulveriser was within permissible limits. Lack of fuel piping orifices did not affect air distribution in any way. Combustion efficiency was observed to be excellent. With five mills in service an oxygen level of around 2% was adequate to achieve loss due to unburnt carbon in ashes of less then 0.5%.

Mills & Associated Burner System • Five Mill operation did not result in loss of draught margins observed in Tri sector air heater based units. • Auxiliary Power consumption of fans was on lower side in spite of individual PA fans dedicated for each mill. • Mill capability has not been affected by deterioration in Air temperature which occurs with deterioration in AH performance. • Mills need to be purged very often to avoid increase of Mill pressure drop due to sand pile.

Optimum Combustion Regime • With five mills in service ,an oxygen level of around 2% was adequate to achieve excellent Combustion efficiency. • Station instrument was measuring a higher level of oxygen due to air ingress. • Actual oxygen level as per test grid probe was 2.5% against a value of 4.2% read by station instrument. • Boiler Part load efficiency levels was for same level of excess air. • Part load operation ensures better loading of mills and auxiliary power consumption % is not adversely affected

Oxygen in Flue Gas at AH inlet (Test 9/10) D 2.8 C

4.2

E 2.8

F 2.9

4.2

G 2.7

2.3

B

2.8

A

2.5

Average Oxygen values in Test 9 from the fourteen probe grid in Flue Gas Duct at Air Heater Inlet 2.0 N

1.9 M

2.3 L

2.7

H

2.6

I

2.6

J

3.0

H

3.0

I

2.9

J

2.6 K

'On line' Zirconia probe

D 3.3 C

2.8

B

2.8

A

3.0

4.6

E 2.7

F 3.1

4.2

G 3.2

Average Oxygen values in Test 10 from the fourteen probe grid in Flue Gas Duct at Air Heater Inlet 2.5 N

2.1 M

2.4 L

2.6 K

Tubular Air heaters: • • •

•

Test runs are conducted in line with ASME- test procedure for Air heater performance testing. 28 point sampling grid is installed at AH inlet duct and an 30 point sampling grid at AH outlet duct. AH leakage levels was on higher side ~ 10.0 % for a Tubular air heater. Leakage is inclusive of air ingress from various expansion joints in the duct before and after air heater. This was one major area for improvement. For an ambient temperature of 35 C, the flue gas exit temperature was around 127 C. Flue Gas Temp corrected for design ambient and air heater leakage was around 170 C signifying deterioration in heat transfer .

Tubular Air heaters •

Air temperature rise across air heaters was around 310 C against a design value of 325 C, confirming deterioration in heat transfer capability of tubes.

•

Flue gas temperature at Air Heater outlet was 137 degree at an oxygen level of 4.5% enabled ESP operation at very optimum flue gas volumes. Elimination of air ingress across ducts between air heater outlet and ID discharge would reduce ID fans loading further . A flue gas temperature of 135 degree at an oxygen level of 5.5% is very bad but compared to 7.0 to 8.0% oxygen level observed in Tri sector air heaters, it looks optimum.

• •

Draught Margins • 190 Tonnes of high ash low GCV coal fired to achieve full load operation. • Five Bowl mills required to achieve full load operation. • Five Mill operation did not contribute to loss of draught margins generally observed in Trisector air heater based units. • Flue gas temperature of 137 degree at 4.50 oxygen after air heater has not been observed in units served by regenerative air heaters.

Boiler Efficiency Tests •

Efficient Boiler operation with five mills was achieved at 2.5 –Oxygen at nominal output.

•

Oxygen could be reduced to 1.5 % with no increase in un burnt carbon in ash loss at nominal output.

•

Part load operation with Four mill could be achieved without any increase in excess air at comparable efficiency.

•

Boiler Efficiency level of 85.5% for a low grade coal of 2800 Kcals/ Kg of coal is excellent.

Boiler Furnace Enclosure HVT survey using HVT probe at furnace exit and multi-point sampling grid at air heater inlet was done with different mill combinations. •

• • •

Complete combustion could be achieved within boiler furnace at an average oxygen of 2.5 % as measured at air heater inlet grid for all mill combination. Difference between average oxygen at furnace exit and 2nd pass exit was negligible. Difference between average O2 % between left and right side of furnace was very nominal. Furnace exit temperature measured on left side was around 50 degree C higher than right side.

Conclusion • • • • •

Boiler and associated auxiliary equipment’s design could be coal specific. This would enable achievement of an optimum operating regime on sustainable basis. Ability to sustain an optimum efficiency between the overhauls very much depends on equipment design & an effective overhaul. An overhaul in which equipment performance has been restored. Performance feedback of this boiler for which base line Performance level were established after more than 8 years of working highlights this

Conclusions continued

• Individual Mill- PA fan combine work more economically in units which are serviced by tubular air heater. • Ambient air is drawn by FD fans at 25 Meter level and carryover of fugitive fly ash dust to SCAPH is avoided. • Boiler is fired using a low GCV coal of 15% moisture & 44% ash. • Flame stability and efficient burnout could be achieved at Mill outlet temperature of 95C for a coal with less than 18.0% volatile matter. • Mills are designed for 77degree C, however, its operation at up to 95 degree C has not affected the reliability of Milling plant..

Conclusions continued • Tubular air heater appears to be an excellent device for high ash– low GCV coals. Deterioration in thermal performance was very marginal. • Air ingress around air heaters section was slightly worse than a bisector air heater which could be corrected to ensure reduction in ID fans power consumption. • Air heater was designed for an inlet temperature of 65 degree C to keep flue gas exit temperature above dew point. • This ability has come very handy to ensure additional heat transfer area in view of higher flue temperature observed at air heater in let, 390 C against a design value of 361 degree C.