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


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.



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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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



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.



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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Converter Off-Gas Heat Recovery Potential

30,000 Nm3/hr Blowing Rate – 250°C Outlet Temperature



Converter Off-Gas Temperature and Heat Content at Hot ESP Outlet

30,000 Nm3/hr Blowing Rate

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Slag

Blow

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Standby



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



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



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



Electric Smelting Furnace Off-Gas Temperature and Heat Content at Baghouse Outlet

310,000 Nm3/hr Off-Gas Flow Rate

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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.


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


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


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


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



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



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


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




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




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





– 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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40°C 65°C 90°C 200°COutlet Temperature from

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





– 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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Electricity Costs ($ / kWh)

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





– 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




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




Typical Thermal Oil Heat Recovery System




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




Typical Thermal Storage Heat Recovery System


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


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

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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


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


Thank You!

Questions?

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Smelter Off-Gas Heat Recovery