Smelter Off-Gas Heat Recovery
Transcript of Smelter Off-Gas Heat Recovery
Smelter Off-Gas Heat RecoveryPaykan Safe
WorleyParsons Gas Cleaning
Dallas, Texas
Confidential Property of WorleyParsons Gas Cleaning2
Outline
Introduction
Waste Heat Available in Smelter Off-Gas
Common Heat Recovery Technologies in Use
Challenges Faced in Recovering Waste Heat
Potential New Heat Recovery Technology Applications in Smelters
Benefits of Process Off-Gas Heat Recovery
Conclusions
Confidential Property of WorleyParsons Gas Cleaning3
Introduction
Smelter processes generate high off-gas flows at high temperatures
- off-gas represents a major heat loss from process
Off-gas heat recovery is critical to minimizing energy consumption
and reducing operating costs.
Smelter process off-gas presents real challenges for heat recovery
Technologies have been used for heat recovery on continuous
smelting and converting processes.
However, batch processes and other lower temperature applications
have been largely overlooked for heat recovery.
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Waste Heat Available in Smelter Off-Gas
Flash Furnaces
Continuous process generating continuous off-gas flow at 1000 – 1200°C
Flow rates are generally low due to high oxygen enrichment
Waste heat boilers typically close-coupled ahead of a hot ESP
Boilers cool gas below 400°C for the ESP
Produce saturated steam for process use or superheated steam for power
generation
Despite heat removed by boiler, off-gas still contains a lot of heat at ESP exit
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Waste Heat Available in Smelter Off-Gas
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Off-Gas Temperature Off-Gas Sensible Heat
Flash Furnace Off-Gas Temperature and Heat Content at Hot ESP Outlet
40,000 Nm3/hr Off-Gas Flow Rate
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Waste Heat Available in Smelter Off-Gas
Flash Furnaces (Continued)
Flash furnace off-gas at 40,000 Nm3/hr contains 5 to 6 MW of sensible heat at
the ESP outlet and is relatively clean of dust.
The off-gas is generally taken to an acid plant - requires further cleaning and
cooling
Heat removal becomes duty of water cooling tower for the gas cooling units.
Therefore, the remaining heat after ESP is wasted in wet gas cleaning plant,
which has an operating cost to remove this heat.
If hot ESP outlet gas could be further cooled through heat recovery unit,
additional heat recovery could be achieved, and cooling duty on water cooling
tower would be reduced.
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Waste Heat Available in Smelter Off-Gas
Converters
Most operating in batch process, typically 2 blowing with 1 hot standby
Process gas flow rate roughly equal to blowing rate at 1100 to 1200°C
Gas captured by primary hood with 100-120% air infiltration, roughly doubling
gas flow rate
Gas is then cooled by spray cooling or radiative cooling below 400°C for ESP –
could replace cooling device with heat recovery vessel
Off-gas still contains a lot of heat at ESP exit
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20% Recovery
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Waste Heat Available in Smelter Off-Gas
Converter Off-Gas Heat Recovery Potential
30,000 Nm3/hr Blowing Rate – 250°C Outlet Temperature
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Waste Heat Available in Smelter Off-Gas
Converter Off-Gas Temperature and Heat Content at Hot ESP Outlet
30,000 Nm3/hr Blowing Rate
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Waste Heat Available in Smelter Off-Gas
Converters (Continued)
Converter off-gas at 30,000 Nm3/hr blowing rate contains 8 to 12 MW of sensible
heat at the ESP outlet per blowing converter and is relatively clean of dust.
The off-gas is generally taken to an acid plant - requires further cleaning and
cooling
Heat removal becomes duty of water cooling tower for the gas cooling units.
Therefore, remaining heat after ESP is wasted in wet gas cleaning plant, which
has an operating cost to remove this heat.
If hot ESP outlet gas could be further cooled through heat recovery, additional
heat recovery could be achieved, and cooling duty on water cooling tower would
be reduced.
ESP outlet gas temperature and flow much more stable due to combining
multiple vessel off-gas streams.
Single heat recovery unit required with much less severe thermal cycling
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Waste Heat Available in Smelter Off-Gas
Anode Furnaces
Operate in batch process
Process gas flow rate generally less than 10,000 Nm3/hr at 1100 to 1200°C
Gas captured by primary hood with significant air infiltration
During reduction, gas can contain up to 25% combustibles which are burned
Gas is then cooled by spray cooling to 200°C for baghouse
Off-gas still contains about 4.5 MW of sensible heat at baghouse exit
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Waste Heat Available in Smelter Off-Gas
Electric Furnaces
Used to smelt laterite nickel calcine and PGM concentrate, slag cleaning
Operate continuously with gas temperatures of 500 to 1000°C
Smelting furnace off-gas typically cooled by spray cooling and then mixed with
secondary ventilation to a baghouse
Slag cleaning furnace off-gas typically sent to a scrubber or baghouse
Smelting furnace off-gas contains 12 to 25MW of sensible heat before spray
cooling
Approximatly 6 to 18MW of sensible heat could be recovered by heat recovery
device instead of spray cooling
Alternatively, process gas could be sent directly to dryer or kiln for full heat
recovery
Even after primary cooling, the off-gas still contains a significant amount of heat
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Waste Heat Available in Smelter Off-Gas
Electric Smelting Furnace Off-Gas Temperature and Heat Content at Baghouse Outlet
310,000 Nm3/hr Off-Gas Flow Rate
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Waste Heat Available in Smelter Off-Gas
Electric Furnaces (Continued)
Electric smelting furnace off-gas at 310,000 Nm3/hr contains 19 to 24 MW of
sensible heat at the baghouse outlet and is clean of dust.
If baghouse outlet gas could sent through heat recovery unit, significant heat
recovery could be achieved.
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Common Heat Recovery
Technologies in Use
Waste Heat Boilers
Handle continuous hot, dirty, & potentially corrosive gas streams
Radiative section cools gas temp. to 700°C
Convective sections further cools gas temp. to 350 to 400°C
Can be designed to produce saturated & superheated steam
Subject to impacts from thermal cycling and corrosion
Not effective below 350C due temperature requirements of boiling at pressure
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Common Heat Recovery
Technologies in Use
Air-to-Gas Exchangers (Recuperators)
Shell-and-tube type heat exchangers
Low capital cost units
Handle dirty gases
Heat recovered in the form of preheated air used for dryers, kilns, burners.
Heat recovered is low grade (less than 400°C)
Limited other uses for recovered heat
Subject to impacts from thermal cycling and corrosion
Not effective at lower temperatures because of air heat content
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Challenges Faced in
Recovering Waste Heat
Fouling & Plugging
Due to sticky dust, metals condensing, high dust loads
Reduced heat transfer, increased pressure loss
Heat recovery units should be designed to allow streamlined flow
Often necessary to incorporate rappers or sonic horns for dust removal
Sulfatizing air recommended for off-gas in sulfide smelting
Corrosion
Result of acid condensation at lower gas temperatures
Also due to Infiltration/leakage due to thermal cycling
Critical to understand water & acid dew points of gas stream
Important to understand surface temperature profile of heat recovery system
Surface temperature should be maintained above dewpoint to avoid acid
condensation
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Challenges Faced in Recovering Waste
Heat
Thermal Cycling
Major concern for batch processes
Can cause condensation and corrosion
Can cause material fatigue due to regular expansion and contraction
Heat recovery units should be designed with expansion allowance
Units to be maintained to minimize leakages
Equipment Cost
20%+ minimum simple ROI is typically required to pursue project
Material costs increase when handling higher temperature or corrosive gas
For boiler applications, high pressure vessels & steam distribution adds cost
Higher heat transfer area required for lower temperatures which adds cost
Optimal heat recovery design provides the lowest average cost
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Challenges Faced in Recovering Waste
Heat
Parasitic Loads
Heat recovery unit increases pressure drop and ID fan power requirement
Additional power required to supply/circulate heat recovery fluid
Parasitic load costs shall be considered in heat recovery savings
Batch Processes
More susceptible to thermal cycling, corrosion, & leakage
Inconsistent heat recovery due to variable gas conditions
Lower ROI due to high differential between peak & average operating conditions
Lower ROI due to fewer operatinghours compared to a continuous process
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Challenges Faced in Recovering Waste
Heat
Low Grade Heat
Gas temperatures of below 300°C are too low to produce steam effectively and
limit usage for preheated air
Higher heat recovery cost due to low temperature gradient - more heat transfer
area required
Waste heat boilers and air-to-gas heat exchangers are generally not suitable
Availability & Maintenance Requirements
Increased maintenance due to multiple components
Pumps, fans, instrumentation, valves, etc. require regular maintenance
Maintenance requirements & availability must be considered for the selection
and design of heat recovery system
System should be as simple as practical
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) Heat Recovery & Power Generation
– System Description
Similar to boiler producing steam from water but uses low boiling point fluid as
heat transfer medium
Closed circuit process
Thermal oil may be used as intermediate heat transfer fluid
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) – Direct Heat Exchange
TurboExpander with
Generator
Cooler/Condenser
(Air or water)
Receiver
Circulation
Pump
Waste Heat
Exchanger
ORC Working
Fluid
Waste Heat
Source (flue gas)
Tout = 200 – 420°F
93 – 215°C
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) – Indirect Heat Exchange
TurboExpander with
Generator
Receiver
Circulation
Pump
Waste Heat
Exchanger
ORC Working
Fluid
Waste Heat
Source
(flue gas)
Gearbox
Thermal Oil
Circulation
Pump
Tout = 200 – 420°F
93 – 215°C
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) Heat Recovery & Power Generation
– Advantages
Well established technology for low-grade heat recovery in other applications
Works well at inlet temperatures above 175°C - suitable for installing
downstream of baghouses or ESPs
Installing downstream of baghouses or ESPs reduces risk of fouling & plugging
and minimizes thermal cycling
ORC systems can be installed downstream of boilers or air-to-gas heat
exchangers for increased heat recovery
Thermal oil heat exchangers could be used to replace spray coolers or radiative
coolers in higher temperature applications in conjunction with ORC
ORC units can produce enough electricity to run ID fans & additional equipment
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Potential New Heat Recovery
Technology Applications in Smelters
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Inlet Temperature to Heat Recovery Unit (°C)
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Heat Recovery Unit (°C):
Inlet Temperature: 250°C
Outlet Temperature: 65°C
Heat Recovery: 0.073 kWt/Nm3/hr
Normalized Heat Recovery Potential for ORC Applications
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) Heat Recovery & Power Generation
– Return on Investment (ROI)
ORC system capital cost is in range of $2,000 to $3,000 per kW power
production
Net energy conversion including parasitic loads is estimated at 10% to 20% of
thermal energy recovered – can be higher for higher temperature applications
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Potential New Heat Recovery
Technology Applications in Smelters
0%
5%
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$0.02 $0.03 $0.04 $0.05 $0.06 $0.07 $0.08 $0.09 $0.10 $0.11 $0.12
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Retu
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91% Utilization (8,000 operating hours per year)
Capital Cost Estimate = $2,000 - 3,000 per kW
Typical ORC Return on Investment (ROI) Opportunity Envelope
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Potential New Heat Recovery
Technology Applications in Smelters
Organic Rankine Cycle (ORC) Heat Recovery & Power Generation
– Key Design Issues
ORC working fluid selection (fluids: low density hydrocarbons, refrigerants)
• Criteria include flammability, corrosivity, environmental & health impacts, stability, cost,
and availability
Heat exchanger design and materials of construction
• Assess of abrasion, corrosion, fouling & plugging potential
• Material selection based on temperature, gas composition, & dust loading
Air cooling or water cooling
• Air cooling requires larger foot print
• Water cooling requires water & infrastructure availability
Power generation
• Turbo-expander or turbine technology (single or multiple units)
• Selection depends on power generation rate, fluid selection, cost, & efficiency
Optimization of process for heat recovery
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Potential New Heat Recovery
Technology Applications in Smelters
Thermal Oil Heat Exchangers
Similar to waste heat boilers, but use liquid oil as heat transfer medium
Oil can be heated up to 350°C
Well suited to preheat air or other fluid – uses two heat exchangers
Can be used as intermediate medium in indirect ORC system
Key factors in selecting the oil include cost, expecting operating range, quantities
required, and availability
Does not require certified boiler operators
Oil loop is a closed loop process
Proven in dirty gas applications
Disadvantages:
System can be expensive, depending on type of oil selected
Need for secondary heat exchanger to preheat air or other fluid
Requires proper handling and safety procedures for flammable liquids
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Potential New Heat Recovery
Technology Applications in Smelters
Typical Thermal Oil Heat Recovery System
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Potential New Heat Recovery
Technology Applications in Smelters
Thermal Storage & Power Generation
Heat is stored in thermal fluid to generate steam or boil organic fluid for power
generation or to preheat air for other processes
Economical heat recovery option in batch processes
A constant stream of thermal fluid is drawn to produce power or preheat air
Turbines can be operated steadily without large variability in heat load
Thermal fluids have been used for thermal storage in solar power generation
Closed loop process
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Potential New Heat Recovery
Technology Applications in Smelters
Typical Thermal Storage Heat Recovery System
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Benefits of Process Off-gas
Heat Recovery
Reduces energy & associated operating costs
Can allow for increased production where production is limited by
energy availability
Helps reduce GHG emissions by reducing consumption of fuels
Helps achieve sustainability in operation & promotes corporate
responsibility & public image
Current & future CO2 regulations will increase fossil fuel cost and
effective cost of CO2 emissions
CO2 emissions cost could be $10 to $30 per ton in near term & will
likely increase over time
Reducing CO2 emissions can produce significant cost savings
Confidential Property of WorleyParsons Gas Cleaning34
Benefits of Process Off-gas
Heat Recovery
$-
$500,000
$1,000,000
$1,500,000
$2,000,000
$2,500,000
$3,000,000
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Power Produced by Heat Recovery (MW)
An
nu
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O2 E
mis
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Co
st
Sa
vin
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(U
SD
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$10 per metric ton $20 per metric ton $30 per metric ton
Potential Annual CO2 Emissions Cost Savings from Heat Recovery Power Production
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Conclusions
Smelters face increasing fuel and power costs as well as tighter
GHG emission regulations
Large quantity of thermal energy is lost to the off-gas
Some conventional heat recovery technologies are in use with
mixed success
Low-grade heat is lost even with conventional technologies
New technology applications can enable heat recovery from lower
grade sources and from batch processes
New technologies can complement conventional technologies for
additional heat recovery & energy savings
Confidential Property of WorleyParsons Gas Cleaning36
Thank You!
Questions?