VAM Abatement in an Regenerative Thermal Oxidizer: Overview of Waste Heat Recovery Strategies

Global Methane Initiative ForumCoal Mine Methane Session

UNECE Group of Experts on Coal Mine Methane Toronto| April 16, 2018

▪ Energy Potential in VAM

▪ Overview of RTO process

▪ Strategies to Retrieve Heat from a RTO

Agenda

▪ Despite low CH4%, VAM contains a lot of energy!– Ex.: 300k scfm (500k Nm3/h) @ 1%CH4 → 47 MWt*

VAM : A lot of Energy Wasted

*Potential heat resulting from combustion of methane to CO2 and water vapor.

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Methane Concentration in VAM (%v/v)

Potential Energy Discharged by a Vent Shaft* vs CH4%(*for 500,000 Nm3/h or 300,000 scfm)

47 MWt

▪ Regenerative Thermal Oxidizer (RTO) → An effective way for releasing the energy contained in VAM.– It allows abating VAM in self-sustained operation (no need

of supplemental fuel following the initial warm-up).

The RTO Solution

Operation PrincipleCycle 1

HOT COLDheat

Operation PrincipaleCycle 2

HOTCOLDheat

▪ Proportional to VAM Flow Capacity and %CH4.

▪ Rule of Thumb: 100k scfm @ 1%CH4 → 15 MWt

Amout of Heat ReleasedInside a RTO

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Methane Concentration in VAM (%v/v)

Heat Released within a RTO* as a function of Flow Capacity and CH4%

Assumption: 97% destruction

100k Nm3/h(59k scfm)

175k Nm3/h(103k scfm)

250k Nm3/h(147k scfm)

High Temperature Inside RTO

▪ Temperature in ceramic beds and oxidation chambermay exceed >1100°C (>2000°F)

BOTTOM TOP TOP BOTTOM

>1100°C

Large Variations of Temperatureat the stack (exit)

▪ T° ramps up and drops at each flow inversion.

▪ Average T° & Amplitude depend on many factors(cycling time, flow rate, VAM concentration, etc.)

Amplitude mayexceed 200°C (360°F) every few minutes

i.e. 150°C

i.e. 350°C

▪ Need for thermal heat nearby the plant?– District Heating– Industrial process preheating– Heat Pump– …

▪ Steam generation for use in:– Power generation– Mechanical power– Process steam– …

Possible End-Uses of Energy

▪ Generating hot water is much simpler and cheaper than generating power.

▪ If there a need for thermal heat nearby the plant, this option shall be assessed in priority.

▪ Many requirements for power generation, including:– Plant should be located near electrical high voltage grid (for

exportation)– VAM %CH4 should be at least 0.6% to be technically feasible.– Drainage gas should be available to enrich and stabilize VAM

concentration.

Thermal Heat vs Power Generation

Where Can EnergyBe Extracted from RTO?

At the stack:

▪ Low-Grade Heat

(i.e ~150°C to ~300°C)

▪ Technically easy

▪ No impact on RTO process

From the oxidation chamber:

▪ High-Grade Heat (>1000°C or > 1800°F)

▪ More complexe

▪ Impacts on RTO process

▪ Hot oxidized gas forced through a heat exchanger.

▪ Only impact on RTO Process→ Flow resistance.

Extraction of Heat from Stack

Hot Water, …

▪ Temperature variation at stack must be taken into account in the design of heat exchanger– RTO process may be adapted to minimize T° variations– Large heat exchanger tank can be used to stabilize water T°.

Challenge ofTemperature Variations

Water return i.e. 60°C

Hot Water i.e. 90°C

HX-TANK

Hot Air

Colder Air

T°variation

Hot Water Generation from stack

▪ The amount of energy retrieved is proportional to VAM Flow Rate and %CH4.

▪ Higher water T° needed → less energy retrieved

At 0.6% At 0.8% At 1.0%

Water 70°C

5.6 MWt 8.7 MWt 11.7 MWt

Water 90°C

4.4 MWt 7.5 MWt 10.6 MWt

Hot water generated from a RTO processing 100k scfm of VAM (170k Nm3/h):

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Methane Concentration in VAM (%v/v)

Hot Water Generation Potential

water - 70°C

water - 90°C

Higher % of Energy Can BeRetrieved at Higher %CH4

▪ % of heat that can be retrieved at stack increases with CH4 concentration (due to higher exit T°).

At 0.6% At 0.8% At 1.0%

Heat releasedinside RTO (MWt)

9.3 12.4 15.5

Hot Water – 90°C (MWt)

4.4 7.5 10.6

% Heat Retrieved 47% 60% 68%

▪ Organic Rankine Cycle (ORC) can be added to generate Power.

Power Generation from Stack

Boiler

Turbine

Condenser

Pump

PowerGenerator

▪ ~0.7%VAM minimum required (average T° > ~200°C/~400°F).

▪ ORC’s efficiency is low (8-15%), but increases with T° (with %CH4)

▪ ORC system uses ~15-20% of power generated to operate.

Power Generation (ORC)from Stack

At 0.6% At 0.8% At 1.0%

--- 0.9 MWe 1.3 MWe

NET Power Generation (ORC) from a RTO processing 100k scfm of VAM (170k Nm3/h):

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Methane Concentration in VAM (%v/v)

Power Generation From ORC

▪ Heat exchanged from inside oxidation chamber

Options to Retrieve Heat fromOxidation Chamber

▪ High-Grade Heat (>1000°C)– i.e Power Generation

Drawbacks:

▪ Heat coil exposed to high thermal stress

→ may limit max T° in the RTO

→ may limit max %CH4 (i.e. <0.9%)

▪ Possible impact on RTO’s uptime– What if flow of working fluid is

stopped?

▪ Hot-Bypass from oxidation chamber

Options to Retrieve Heat fromOxidation Chamber

▪ High-Grade Heat (>1000°C)

▪ Lower flow resistance through RTO

– Increase RTO’s flow capacity OR Reduce Fan power consumption

Drawbacks:

▪ Design & Control of by-pass damper is critical & challenging

– Improper control may lead to thermal instabilities & overheat issues.

>1000°C

To steam

power plant →

▪ Only a part of VAM Flow can by-pass 2nd bed without compromising process’ thermal stability.

▪ % of Flow that can be by-passed increases with %CH4.

▪ Challenge: By-passing as much flow as possible while maintaining thermal stability in oxidation chamber.

The Challenge ofControlling By-Pass Flow

%CH4

Max % of Flowthat can beby-passed

▪ The amount of Power generated depends how is controlled the bypass flow (aggressive vs conservative).

Power Generationfrom Hot By-Pass

Power Generation (Hot BP) from RTO processing 100k scfm of VAM (170k Nm3/h):

At 0.6% At 0.8% At 1.0%

~0.5-1.5 Mwe ~2-3 MWe ~3-4 MWe

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Methane Concentration in VAM (%v/v)

Power Generation from Hot By-Pass

Aggressivecontrol

Conservative control

▪ Hot-Bypass can be mixed with part of stack flow to reduce gas temperature supplied to steam boiler.

Reducing Hot By-Pass Temperature

▪ Reduce thermal stress in boiler

▪ Provide flexibility to control flow rate and temperature of gassupplied to steam cycle.

Drawbacks:

▪ Reduce steam cycle efficiency(less power generated)

▪ Leakage of dampers may lead to overheat issues

To steam cycle →>1000°C

>1800°F

150°C

300°F

Lower T° (i.e. 600°C or 1100°F)

▪ Increase gas flow to power plant (bigger power plant?)

▪ Less power generated despite flow increase.

▪ For example, impact of cooling Hot-Bypass from 1050°C(1900°F) down to 600°C (1100°F):

Impact of ReducingBy-Pass Temperature

25 000 Nm3/h1050°C

PowerPlant ~2.7 MWe

25 000 Nm3/h1050°C

25 763 Nm3/h125°C

50 763 Nm3/h600°C

PowerPlant

~2.5 MWe

-5%

Power GenerationHot By-Pass vs ORC

▪ For an RTO processing 100k scfm (170k Nm3/h) at 1%CH4

*Source: Waste Heat to Power Market Assessment; Department of Energy. Office of Energy Efficiency and Renewable Energy,

2015. Study prepared by ICF International (Amelia Elson, Rick Tidball, Anne Hampson).

Hot By-Pass(Steam Plant)

ORC

Power generated, MWe 3.5 1.3

Estimated Capital Cost, M$US 6.3M$(~1800$/kW*)

3.9M$(~3000$/kW*)

Estimated O&M Cost, $US/yr 230,000$/yr(0.008$/kWh*)

140,000$/yr(0.013$/kWh*)

Risk of impacting RTO’s uptime? More likely Less likely

2.6X

1.7X

Selecting the BestHeat Recovery Strategy

▪ What is the primary income stream of the RTO? Carbon Credits or Energy?

▪ Can heat recovery affect carbon revenues (i.e. forcing dilution, reducing uptime, etc.)?

EnergyRevenuesCarbon Credit

Revenues

Selecting the BestHeat Recovery Strategy

▪ Power Generation (via hot bypass) vs Hot Water

Power Generation Hot Water

PROs

- Higher revenues ($) (typically)

- Increase RTO’s flow capacity

- Easy to implement- Less sensitive to %CH4

variations- No impact on RTO process’

stability

CONs

- Much higher capital cost- More technical challenges- More sensitive to %CH4

variations- Risk to interfere with Carbon

credits production.

- Lower revenues ($)- Additional flow resistance

(reduce RTO’s flow capacity)

Conclusion

▪ VAM Abatement releases a large amount of energy (100k scfm @1%CH4 → 15 MWt).

▪ RTO is an efficient way to release that energy.

▪ Heat can be retrieved from RTO via…– Stack → Low-grade heat, easy to implement– Oxidation chamber → High-grade heat, complex

▪ Up to or ~12 MWt or ~4MWe can be generated from a RTO processing 100k scfm (170k Nm3/h) of VAM at 1%CH4.

Conclusion

▪ Generating hot water is much simpler and cheaper, but generating power can be more appealing ($).

▪ Potential impact of heat recovery on carbon credits production (where applicable) must be assessed.

Thank you!

Dominique Kay, M.Sc.A., Eng.

R&D Director

Biothermica Technologie Inc.

+1.514.488.3881 x228

Dominique.kay@biothermica.com