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Case study-GHG Accounting

1. Air Preheater for Stove Inlet Air

Background:

The stove in a blast furnace preheats the air up to 1250°C before it’s blown into the furnace. This high-temperature air improves the combustion of coke and the reduction of iron ore, enhancing energy efficiency and reducing fuel consumption. The stoves also help maintain consistent furnace temperatures, ensuring stable and efficient operation. Two CA fans are installed for providing combustion air to the stoves. One out of two will be in operation and another in standby condition.

Findings:

  • The air inlet temperature for the combustion air fan is at 35oC
  • The combustion air is heated up to 158oC in air preheater by the stove waste gas
  • The blast furnace has water cooling system to maintain the temperature of outer walls
  • The average heat energy extracted from the blast furnace by the primary water circuit is 40 MW
  • The cooling circuit has both primary and secondary circuit
  • The primary circuit water is leaving the furnace at 55oC and then entering the plate heat exchanger where it rejects the heat to water coming from the secondary circuit indirectly. Then the water leaves the heat exchanger at 45oC
  • The flow rate of primary water is 3250 m3/hr
  • The secondary water leaves the PHE at 42oC and enters the cooling tower, where the heat is rejected to atmosphere and the water leaves the basin at 32oC
  • Two pumps are operated out of three pumps with the third one being standby in secondary circuit. Each secondary pump consumes 400kW on an average
  • There are four fans operating in cooling tower to reject heat to atmosphere

1. Existing System

Recommendations:

Install an additional APH before the present air preheater and divert the primary water to flow through the APH.

The new APH will take away the heat in primary circuit and reduce the heat load on secondary circuit.

The reduction in heat load will reduce the secondary circuit water flow rate, thus one pump can be switched off.

Recommendations:

Following recommendations were suggested based on our analysis:

  • Install an additional APH before the present air preheater and divert the primary water to flow through the APH.
  • The new APH will take away the heat in primary circuit and reduce the heat load on secondary circuit.
  • The reduction in heat load will reduce the secondary circuit water flow rate, thus one pump can be switched off.

2. Proposed System

Benefits:

The cost benefit analysis is tabulated below.

Description                                                                             Unit Value
Operating Parameters
Annual Operating days                                                                                                        
days/annum
365
Initial Temperature of stove combustion air
0C
35
Air density
kg/m3
1.11
Volume of inlet air for stove combustion
Nm3/hr
2,76,605
m3/hr
3,21,746
Primary cooling Circuit
Volume of Water flowing for cooling Stave in primary circuit          
m3/hr
3250
Specific heat capacity of water
kCal/kgK
1
Water inlet Temperature to PHE
0C
55
Water Outlet Temperature from PHE
0C
45
Description Unit Value
Number of Primary pumps in operation                                                  
no’s
2
Primary Pump Power each
kW
210
Secondary cooling Circuit
Volume of Water flowing for cooling Stave in secondary circuit
m3/hr    
2400
Water inlet Temperature to Cooling tower
0C
42
Water Outlet Temperature from cooling tower
0C
32
Number of Secondary pumps in operation
no’s
2
Secondary Pump Power
kW
400
Present System
Heat rejected in primary circuit                                                                
MCal/hr
32,500
Heat rejected in secondary circuit
MCal/hr
24,000
Proposed System: Install an air preheater in stove combustion inlet air to reduce the heat load on primary circuit and Secondary load to minimize pump operation
Specific Heat capacity of air                                                                                                    
kCal/kgK
0.25
Effectiveness of heat exchanger
%
60%
Heat absorbed in the air pre heater
MCal/hr
19,500
Expected combustion air temperature of stove after new air preheater
0C
50.15
Proposed heat load to primary circuit
kCal/hr
13,000
Proposed heat load to secondary circuit
kCal/hr
11,000
Estimated water flow in secondary circuit
m3/hr
1100
Energy Savings
Proposed Energy Savings                                                                                              
MWh/annum
3504
Energy Cost
Rs. /MWh
5300
Annual Cost Savings
Rs. Lakhs
186
ROI
Investment for APH                                                                            
Rs. Lakhs
200
Simple Payback Period
Month
13

Case study-2

Throttling of damper valve in stock house

Stock house dedusting fans are operated to control dust emissions in stock house which ensuring compliance with environmental regulations, and safeguarding equipment while promoting worker safety. To achieve these objectives, two stock house exhaust units are employed to meet the specified criteria effectively. The actual power consumption of the motor is 588 kW at 100% open damper position.

Description                                                                             Unit Value
Present system
Number of fans in operation                            
kW
875
Actual power consumption of SH dedusting fan
kW
588
Damper opening
%
100%
Proposed system: Throttling of damper valve in stock house
Damper valve setting                                                                                                              
%
80%
Proposed energy consumption in each fan
%
10%
Expected Energy savings in each fan
kW
59
Annual energy savings
MWh
1030
Energy cost
Rs. /MWh
5300
Annual Cost Savings
Rs. Lakhs
55
Investment
Rs. Lakhs
Nil
Simple Payback
Months
Immediate

Annual energy savings

Annual Carbon Reduction

Carbon Reduction