natural gas system carbon capture and storage

Natural Gas System Carbon Capture and Storage

CHEE 462 – Technical Paper 2

Submitted by: Michael Garibaldi [260353823]

Submitted to: Pierre Bisaillon

February 14, 2014

Abstract

An efficient and cost-effective system for capturing carbon dioxide is to treat the exhaust of a

new natural gas-fueled power plant rated for 300 MW and 1000 tonnes of CO2 production per day. A

carbon tax has been put into effect which charges power plants a fee for every tonne of CO 2 that leaves in

the stack gas. It is therefore beneficial to the environment and to the plant to include a carbon capturing

technology. There are three known and tested methods that will be evaluated, each of which acts at a

different point in the combustion cycle. Post-combustion carbon capture treats the exhaust from

generation directly. Pre-combustion carbon capture decreases the carbon levels entering the combustion

reaction and indirectly lowers CO2 output. Chemical-looping combustion capture, or oxy-fuel combustion

capture, performs combustion with only oxygen and natural gas, leading to higher thermal efficiencies

and a decreased fuel consumption. This, in effect, also lowers CO2 output in the exhaust gas.

Each technology is compared with a set of selection criteria. The first five criteria are essential

and will determine if a technology is suited for the plant’s needs. These criteria are CO 2 recovery percent,

thermal efficiency, cost of capital, cost per tonne of CO2 over life and cost of solvent. The next four

criteria determine which technology is most appropriate. These include a more stringent range of CO2

recoveries (90% or above), a more narrow range for thermal efficiency, a minimum cost of CO 2 recovery

over lifetime of the plant and secondary CO2 emissions from auxiliary duties.

It is determined conclusively and supported by evidence from numerous reports that post-

combustion carbon capture is the more appropriate carbon sequestration method for this plant. Recovery

is found to be 90% but can be tuned for higher levels. Costs are also moderate and the solvent involved in

CO2 absorption can be regenerated and reused. Fuel efficiency is not enhanced however and long-term

research on alternative energy sources is supported by the findings in this paper.

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Table of ContentsAbstract......................................................................................................................1

Table of Contents......................................................................................................2

Table of Figures and Tables 3

Introduction 4

Background 5

Selection Criteria 6

Alternatives 7

Analysis 8

Comparison Tables 9

Conclusion and Recommendation 10

Potential Problem Analysis 11

References 12

2

Table of Figure and Tables Table 1: Plant Performance Data...............................................................................................................4

Figure 1: The Carbon Energy, Capture and Storage Cycle.......................................................................5

Table 2: Essential Criteria for Carbon Dioxide Capture System...............................................................8

Table 3: Desirbale Criteria for the Carbon Dioxide Capture System........................................................8

Table 4: Specifications for Plant without Carbon Capture........................................................................9

Figure 2: Post-combustion Carbon Capture Block Flow Diagram............................................................9

Table 5: Specifications for Post-combustion Carbon Capture with Amine Solvent.................................9

Figure 3: Pre-combustion Carbon Capture Block Flow Diagram.............................................................9

Table 6: Specifications for Pre-combustion Carbon Capture with Amine Solvent.................................10

Figure 4: Chemical-looping Combustion Carbon Capture Block Flow Diagram.....................................9

Table 7: Specifications for Chemical-looping Combustion Carbon Capture..........................................11

Table 8: Comparative Analysis Table for Essential Critera....................................................................13

Table 9: Comparative Analysis Table for Desirable Criteria..................................................................13

Table 10: Comparison Table for Essential Criteria.................................................................................14

Table 11: Comparison Table for Desirable Criteria................................................................................14

3

Introduction

A power plant is to be constructed in the southern regions of the Quebec province. The plant

contains a natural gas-fueled turbine that supports up to 300 megawatts of electrical output to supply a

new industrial complex nearby. Due to increasing concern over rising global carbon dioxide levels and

the threat of a probable Greenhouse Effect, regulation dictates that all novel petroleum-fueled power

plants recover substantial amounts of carbon from stack gases to prevent their release into the

atmosphere. Carbon dioxide, suspected to be the leading cause of the Greenhouse Effect, is the primary

target of the new carbon capture mandate. Environmental studies show that this recovered carbon can be

transformed into solid or liquid form and stored underground for long periods of time (up to 500,000

years) where it cannot harm natural ecosystems.

Due to the volume of carbon dioxide released in combustion reactions like those of a natural gas-

fueled power plant, complex capture systems are being designed to ensure that the vast majority of it does

not make it into the atmosphere. Three mechanisms have been proposed for the new plant: post-

combustion capture, pre-combustion capture, and oxy-fuel combustion capture. All three techniques are

proven to have better results for carbon capture than conventional plants with no capture system. These

systems can also be produced on a large scale in order to handle large volumes of exhaust year-round.

Each method is optimal for different applications, however; one of these methods is more beneficial for

this specific application than the rest. The table below is a summary of the plant performance parameters

and their values. From these specifications, an analysis will be drafted to compare each of the capture

systems and to choose which is the most suitable for this operation.

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Table 1: Plant Performance Data

Parameter Units Type - Once-Through Steam GeneratorExhaust Gas Flow Rate kg/hr 114000Power Output MW 300CO2 Production tonnes/day 1000CO2 Recovery % 90.00Inlet Temperature °C 182Inlet Pressure kPa(g) 0.0 ± 0.04 Inlet Composition

N2 mole % 71.613Ar mole % 0.871O2 mole % 2.595

CO2 mole % 8.616H2O mole % 16.298SO2 ppmv 24SO3 ppmv < 2NO ppmv 57

NO2 ppmv 3Particulates mg/nm3 < 10

Product Stream Temperature °C 40Product Stream Pressure kPa(g) 45Life expectancy years 20Plant availability % 95

Long-term operation% of design

capacity 85

5

Background

The carbon cycle begins with mining, or the extraction of fossil fuels from below the Earth’s

surface. These fossil fuels are the product of millions of years of sedimentation and other geological

processes and are rich in energy. The energy contained in these fuels can be recovered by their

combustion, or their burning – most effectively when in a controlled environment such as a combustion

engine. To burn these fuels, however, oxygen is required. This leads to the formation of carbon-oxides, or

most predominantly, carbon dioxide (CO2). Carbon dioxide is released in the form of a gas and typically

rises into the atmosphere, where it blends into the other atmospheric gases. This is a natural process

which has always occurred. Figure 1 below gives an illustrative example of this process.

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The difference today however, is the volume at which carbon dioxide is produced. In 1992, total

world oil production equaled 60 million barrels per day. This number has grown by millions of barrels

each year since 1992 due to increasing demand. As consumption increases, the burning of fossil fuels

such as oil, natural gas and coal leads to an equivalent number in thousands of megatons of carbon

dioxide released per day. The planet is equipped with several mechanisms to mitigate carbon dioxide

build-up in the atmosphere. These are ocean sequestration and carbon capture by plants, bacteria and

fungus. There are also several man-made sequestration techniques in place such as direct-air capture for

industrial carbon dioxide use. But even when all of these processes are combined, they do not absorb

carbon quickly enough, leading to rising levels of carbon dioxide in the atmosphere. Over time, the rising

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Figure 1: The Carbon Energy, Capture and Storage Cycle

levels will theoretically result in the entrapment of solar energy in the form of heat, gradually increasing

the mean temperature of the Earth. Furthermore, the world’s oceans, which typically account for the

largest amount of carbon sequestration, are nearing a saturation level. After this saturation is reached, the

rate at which CO2 builds up in the atmosphere will increase exponentially. This is known as the

Greenhouse Effect, the consequences of which are dire for life everywhere.

Leading environmental agencies across the world have set CO2 reduction quotas to combat the

Greenhouse Effect. Rewards are given to companies which can boast significant amounts of CO 2

production and release into the atmosphere. The energy industry faces the greatest challenge in checking

its carbon emissions. Oil-, natural gas- or coal-powered electrical plants currently face a carbon tax for all

exhaust that is released into the atmosphere. This has led to widespread technological improvement across

the petroleum and energy industry. The new technologies sequester carbon dioxide from the source, from

the exhaust streams of combustion plants. The carbon is trapped in a saturated liquid solution and is

converted into solid calcium carbonate, which is then stored underground. The carbon may also be

recovered in gaseous form and pumped deep into the Earth, or put into use such as in enhanced oil

recovery. This allows for carbon sequestration organizations to sell carbon dioxide back to the oil

industry, making additional incentive for cleaning flue gas.

In a power generation plant, fuel is burned at a high flow rate to create steam in a separate stream.

This steam drives a turbine by a pressure differential which in turn generates electrical energy from

mechanical energy. Three major methods for industrial carbon capture are known: post-combustion

capture, pre-combustion capture and oxy-fuel combustion capture. Post-combustion capture involves

feeding all exhaust gases generated by a combustion reaction to a treatment unit. The unit typically is

composed of an absorber and a stripper. This process requires a solvent, usually an amine solution, that

efficiently absorbs CO2 from exhaust and which in most cases can be regenerated and reused. Pre-

combustion capture, on the contrary, utilizes the same units as post-combustion capture except in this

case, the unit is placed before the combustion reaction. In this way, the feed fuel is reformed so that

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carbon is separated and only hydrogen is combusted. On average, however, pre-combustion capture

necessitates more fuel to achieve generation efficiencies equivalent to non-capture processes. The last

process, oxy-fuel burning incorporates an oxygen carrier which selectively binds oxygen from air. The air

is released to the atmosphere and the oxygen is then dissociated from the carrier in another reactor where

it is used in the combustion reaction with the fuel. This results in an oxygen-rich combustion reaction and

higher efficiency. This type of process often includes a CO2 recycle to maintain high temperatures for

steam generation which in turn also decreases emissions.

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Selection Criteria

The tables below indicate the criteria by which each alternative will be compared so a selection of

the appropriate technology can be made. The criteria are divided into essential criteria, or those which

must be met for an alternative to be considered as suitable, and desirable criteria, which define ranges of

specifications that are optimal for the plant design.

Essential Criteria

Carbon dioxide recovery percentage is most relevant to the design of the new plant. It is therefore

necessary that the carbon capture system effectively reduces carbon output. A capture system’s

effectiveness is measured by the volume of carbon found in the product stream of the treatment unit. Also

of importance is that thermal efficiency is not reduced substantially by the addition of a carbon capture

system. A large reduction in thermal efficiency can render the capture system more detrimental than

beneficial for the plant. Typical thermal efficiencies for gas-powered plants are in the range of 55-60%.

Studies on various carbon capture systems set the minimum bound for thermal efficiency at 42.4%, after

which point capture becomes inefficient and fuel is wasted.

Capital expenditure is also considered as a necessary limit to a carbon capture system

implemented in a power plant. For the purposes of this plant, a limit of $100,000,000 CAD is the

maximum cost of capital allowable. This number is derived from a cross-study of carbon capture

installations which gives the typical expenses of such a project including costs for equipment, shop

fabrication, site installation, engineering, project management and fees for technology licenses. With a

planned life expectancy of 20 years and assuming 95% plant availability and operation at 85% of design

capacity, the cost per tonne of CO2 recovered can also be measured. This calculation is also of importance

in determining the desired system. The value of $300.00 per tonne of CO2 is the high-end estimate put

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forward by environmental and carbon recovery papers for the cost of industrial carbon removal. Lastly,

the cost of solvent is associated with selection. It is not uncommon that a solvent can be recycled and

reused after undergoing regeneration. In the case of oxygen carriers as well, replacement is needed only

on a monthly or discontinuous basis. It is in fact, always more cost effective to purchase pure O 2 than

purchasing a complex capture system if the cost of solvent is greater per tonne than the cost of oxygen. A

caveat to this, however, is that pure O2 is fed on a continuous basis, as it cannot be reused or regenerated;

therefore the use of a solvent which regenerates is almost always preferred to pure O 2, as purchase

frequency for such a solvent is drastically decreased.

Table 2: Essential Criteria for Carbon Dioxide Capture System

Parameter Units CO2 Recovery % > 0.00Cost of Capital $CAD < 100,000,000.00

Cost Over Life$CAD/tonne CO2

product < 300.00Thermal Efficiency % ≥ 42.40

Cost of Solvent Less Than Cost of Pure Oxygen $CAD/tonne solvent < 65.00

Desirable Criteria

Of the alternatives available for carbon capture, that which has the highest carbon dioxide

recovery is desired. A higher thermal efficiency is also preferred, as it indicates less consumption of fuel,

which in turn reduces emissions. Similar to the essential criteria, a lower cost per tonne of CO2 is favored.

The value of $175.00 per tonne of CO2 recovered is the low-end estimate provided in cost analysis reports

by environmental regulatory affairs. Of final importance is the ratio of CO2 emissions from secondary

generation sources to the amount of CO2 recovered. The heating and cooling systems required for solvent

and oxygen carrier regeneration produce carbon emissions which are typically not recovered. It is

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practical to assume that any capture system which produces more than a quarter of the carbon that is

recovered loses value as an alternative.

Table 3: Desirable Criteria for Carbon Capture System

Parameter Units

Cost Over Life$CAD/tonne CO2

product < 175.00Thermal Efficiency % ≥ 60.00CO2 Recovery % ≥ 90.00

Auxiliary Generation CO2

Emission Low tonnes CO2/day < 250

Alternatives

No Carbon Capture System

The conventional power plant does not feature a carbon capture system. Exhaust gas created by

combustion is released directly to the atmosphere. This alternative therefore features the least carbon

recovery and is not being considered as a viable alternative, but remains as an important reference point

for comparison. Table 4 below gives the exhaust gas composition and CO2 discharge from the

combustion reaction. For all other alternatives, the gas composition described below serves as the inlet

conditions to each of the carbon capture systems. Note that a configuration such as this one would require

a high carbon tax, typically above the $300 CAD per tonne of CO2 range. Such charges would render this

plant configuration extremely fiscally inefficient.

Table 4: Specifications for Plant without Carbon Capture

Parameter Units Reboiler Duty GJ/hr 0

Fuel Gas Consumption for Reboiler Duty m3/hr 0Cooling Duty GJ/hr 0

12

Total Power Consumption kW 0CO2 Recovery % 0.00Outlet Composition


CO2 mole % 8.616H2O mole % 16.298SO2 ppmv 24SO3 ppmv < 2NO ppmv 57

NO2 ppmv 3

Particulates mg/nm3 < 10Cost of Capital $CAD 0.00Cost Over Life $CAD/tonne CO2 0.00Thermal Efficiency % 60

Cost of Solvent$CAD/tonne

solvent 0.00CO2 Capture tonnes/day 0CO2 Emission (Solvent Regeneration Duty) tonnes/day 0CO2 Emission (Electrical Load) tonnes/day 0Net CO2 Emission tonnes/day 1000Net CO2 Capture tonnes/day 0CO2 Emission per Tonne Captured 1

Post-combustion Carbon Capture with Amine Solvent

Post-combustion capture features an absorber column that brings the carbon dioxide in the

exhaust gas into contact with a solvent, and a stripper which recovers the solvent for reuse. Figure 2

illustrates the basic block flow diagram for a post-combustion carbon capture system. The solvent

typically used in this process is an amine solvent, as carbon dioxide is highly miscible while the other

components of the gas are not. The schematic requires a reboiler for the regeneration of solvent, and a

condenser for the recovery of CO2 product. Both the condenser and the reboiler have duties that

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necessitate additional electrical energy to be added to the system. The addition of this electricity creates

CO2 emission from secondary power generation.

While the drawing in Figure 2 does not indicate an exhaust recycle stream, it is possible to

incorporate a recycle to enhance CO2 recovery. The system described has a 90% CO2 recovery however

and achieving higher levels would require additional reboiler and condenser duties, which may create

difficulty in justifying a recycle stream.

Because the amine solvent can be regenerated, costs for materials are low following the initial

capital cost for fabrication, engineering and installation. This makes post-combustion capture a fiscally

conservative option over the 20-year lifetime of the project. It is also worth noting that the cost per tonne

of CO2 is the sum of two values: one being the cost derived from the capital cost of the capture process

and the other from the cost of CO2 handling operations (approximately equal to $30.00 per tonne of CO2

for this alternative and all that follow). Table 5 lists all relevant specifications for this CO 2 capture plant

configuration.

Figure 2: Post-combustion Carbon Capture Block Flow Diagram

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Table 5: Specifications for Post-combustion Carbon Capture with Amine Solvent

Parameter Units Reboiler Duty GJ/hr 130Fuel Gas Consumption for Reboiler Duty m3/hr 4080Cooling Duty GJ/hr 230Total Power Consumption kW 2575CO2 Recovery % 90.00Outlet Composition (Treated Gas)


CO2 mole % 1.05H2O mole % 6.99

Cost of Capital $CAD 83,100,000.00Cost Over Life $CAD/tonne CO2 67.00Thermal Efficiency % 60Cost of Solvent $CAD/tonne solvent < 50.00CO2 Capture tonnes/day 1000

CO2 Emission (Solvent Regeneration Duty) tonnes/day 207CO2 Emission (Electrical Load) tonnes/day 40

Total CO2 Emission due to Auxiliary Systems tonnes/day 247Net CO2 Capture tonnes/day 753

CO2 Emission per Tonne Captured 0.247

Pre-combustion Carbon Capture

A pre-combustion carbon capture such as the one described in Figure 3 reforms the natural gas

feed so that all carbon is removed prior to burning. This makes the addition of more fuel a necessity, but

ensures very high carbon capture results. The required electrical duty for secondary systems such as the

15

reboiler and condenser is also about 50% higher for pre-combustion capture. Like in post-combustion

capture however, the amine solvent is reusable. The system is slightly more complicated than the post-

combustion system on the other hand and therefore requires a slightly higher cost of capital before start-

up. Table 6 lists details pertaining to the pre-combustion carbon capture system’s performance.

Figure 3: Pre-combustion Carbon Capture Block Flow Diagram

Table 6: Specifications for Pre-combustion Carbon Capture with Amine Solvent

Parameter Units Reboiler Duty GJ/hr 195

Fuel Gas Consumption for Reboiler Duty m3/hr 4610.4Cooling Duty GJ/hr 345Total Power Consumption kW 3862.5CO2 Recovery % 90.50Outlet Composition


CO2 mole % 1.00

16

H2O mole % 7.00Cost of Capital $CAD 88,000,000.00Cost Over Life $CAD/tonne CO2 65.00Thermal Efficiency % 47


solvent < 50.00CO2 Capture tonnes/day 1000

CO2 Emission (Solvent Regeneration Duty) tonnes/day 310.5CO2 Emission (Electrical Load) tonnes/day 60Net CO2 Emission tonnes/day 370.5Net CO2 Capture tonnes/day 629.5


Chemical-Looping Combustion Carbon Capture

This system is a type of oxy-fuel combustion capture process. The key elements are an air

reactor, a fuel reactor and an oxygen carrier. The air reactor is fed ambient air which is stripped of its

oxygen by an oxygen carrier, or a solvent with a very high affinity for oxygen molecules. The air is

released with the exhaust gas as its composition is the same as atmospheric air except it is depleted of

oxygen. The oxygen carrier is pumped into the fuel reactor where high temperatures lead to the

dissociation of the oxygen molecules from the carrier. The oxygen molecules are then used in the

combustion reaction, creating CO2 and H2O. The pure oxygen input leads to much higher thermal

efficiencies for combustion reactors, resulting in more steam generation with less fuel. Also of major

significance is the CO2 recycle stream, which uses carbon exhaust gases to help satisfy thermal energy

requirements, thereby reducing the need for fuel and drastically reducing carbon in exhaust gases. Figure

4 depicts a block flow diagram for the typical chemical-looping combustion capture system. Table 7

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contains performance and specification data pertinent to the system that would support the power plant

discussed in the introduction.

While some estimates put the cost of capital for a chemical-looping combustion capture system as

high as $90,000,000 CAD, the novel technology described by this paper is estimated to have only a

$31,000,000 CAD capital expenditure. This is a newer, more efficient and simpler system than previously

described by many papers in the early 2000s. The combination of novel oxygen carriers and a newly

designed CO2 recycle have the potential to cut costs drastically. Note that electrical costs are higher than

in post-combustion capture by more than 5% due to an increased energy requirement for condensing. This

is reflected in the $70.00 per tonne CO2 cost over lifetime.

Figure 4: Chemical-looping Combustion Carbon Capture Block Flow Diagram

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Table 7: Specifications for Chemical-looping Combustion Carbon Capture

Parameter Units Reboiler Duty GJ/hr 136.5Fuel Gas Consumption for Reboiler Duty m3/hr 4284Cooling Duty GJ/hr 241.5Total Power Consumption kW 2703.75CO2 Recovery % 99.00Outlet Composition


CO2 mole % 0.96H2O mole % 7.01

Cost of Capital $CAD 31,000,000.00Cost Over Life $CAD/tonne CO2 70.00Thermal Efficiency % 90


solvent 100.00CO2 Capture tonnes/day 1000

CO2 Emission (Solvent Regeneration Duty) tonnes/day 217.35

CO2 Emission (Electrical Load) tonnes/day 42Net CO2 Emission tonnes/day 259.35Net CO2 Capture tonnes/day 740.65


Analysis

Table 8: Comparative Analysis Table for Essential Criteria

Criteria Units ValueNo CO2

CapturePost-combustion

Pre-combustion

Oxy-fuel Combustion

CO2 Recovery % > 0.00 0.00 90.00 90.50 99.00Cost of Capital $CAD < 0.00 83,100,000.00 88,000,000.00 31,000,000.00

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100,000,000.00

Cost Over Life$CAD/

tonne CO2 < 300.00 0.00 67.00 65.00 70.00Thermal Efficiency % ≥ 42.4 60 60 47 90

Cost of Solvent Less Than Cost of Pure Oxygen

$CAD/tonne

solvent < 65.00 0.00 < 50.00 < 50.00 100.00

Data for cost of capital, cost over life, cost of solvent, thermal efficiency and CO 2 recovery is

compiled and compared with essential criteria in Table 8. An additional column has been added for a

plant without CO2 capture as a reference. Each of the proposed alternatives fit within the criteria given in

the Selection Criteria section. The chemical-looping combustion capture, or oxy-fuel combustion

alternative promises highest CO2 recovery (99%) and lowest capital expenditure ($31,000,000 CAD)

when compared to all options. Additionally, thermal efficiency is maximized under this alternative

technology. It also has an acceptable cost over life ($70 per tonne of CO 2). The cost of solvent per tonne

as compared to the cost of pure oxygen per tonne appears to indicate that the chemical-looping

technology does not pass in this particular regard, however. An exception is made for this category, as the

cumulative expense for oxygen over the lifetime of the plant far outweighs the cumulative expense for

solvent, or oxygen carrier. While the cost of the oxygen carrier is $100.00 per tonne, this material is

replenished only on a monthly basis, whereas the pure oxygen is part of a continuous flow. Therefore,

assuming that the plant will require one tonne of pure oxygen per day to produce one equivalent tonne of

CO2, the pure oxygen stock will require replacement on a daily basis. With pure oxygen amounting to

$65.00 per tonne, this accumulates a much larger cost than that of the oxygen carrier in just a single

month.

Post- and pre-combustion carbon capture technologies both pass in all essential selection criteria

as well. The notable difference between these two alternatives and the oxy-fuel combustion alternative is

capital cost, which is almost three times larger for pre-combustion carbon capture. Another key

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observation is that the thermal efficiency of a pre-combustion capture system is 47% as compared to 60%

for post-combustion capture. The CO2 recoveries for post- and pre-combustion capture are 90% and

90.5%, respectively; however, the large increase in fuel demand does not justify the only slight advantage

in recovery that pre-combustion capture offers. Additionally, recovery for the post-combustion capture

can be enhanced by several percent only by adding a recycle stream for the treated off-gas, while

expending less auxiliary electrical energy on reboiler and condenser duties.

Table 9: Comparative Analysis Table for Desirable Criteria

Criteria Units ValuePost-combustion

Pre-combustion

Oxy-fuel Combustion

Cost Over Life$CAD/tonne CO2 product < 175.00 67.00 65.00 70.00

Thermal Efficiency % ≥ 60.00 60 47 90CO2 Recovery % ≥ 90.00 90.00 90.50 99.00

Auxiliary Generation CO2

Emission Low tonnes CO2/day < 250 247 370.5 259.35

Having passed each of the essential criteria however, the three alternatives are then compared

with the desirable criteria also discussed in the Selection Criteria section. The pre-combustion capture

alternative falls short of desired criteria; the CO2 emissions generated by auxiliary power sources is much

too large as compared to the recommended level of no more than 250 tonnes of CO 2. Thermal efficiency

is also below levels that would be considered most cost effective and fuel efficient for pre-combustion

carbon capture. This eliminates pre-combustion capture as a viable alternative for the new natural gas

plant. The emissions for the chemical-looping combustion technology are also slightly higher than the

recommended level. Additionally, the secondary CO2 emissions for post-combustion capture are lower

than the 250 tonnes of CO2 limit. This gives the post-combustion option a slight advantage over the oxy-

fuel option, as it meets both essential and desirable criteria. The tables of comparison in the next section

21

provide a visual representation of how the alternatives rank according to essential (Table 10) and

desirable (Table 11) characteristics.

Comparison Tables

Table 10: Comparison Table for Essential Criteria

Criteria ValueNo CO2

CapturePost-combustion

Pre-combustion

Oxy-fuel Combustion

CO2 Recovery > 0.00 FAIL PASS PASS BEST

Cost of Capital<

100,000,000.00 PASS PASS PASS BESTCost Over Life < 300.00 BEST PASS PASS PASSThermal Efficiency ≥ 42.4 PASS PASS PASS BEST

Cost of Solvent Less Than Cost of Pure Oxygen < 65.00 BEST PASS PASS

PASS (with exceptions)

Table 11: Comparison Table for Desirable Criteria

Criteria Units ValuePost-combustion

Pre-combustion

Oxy-fuel Combustion

Cost Over Life

$CAD/tonne CO2 product < 175.00 PASS BEST WORST

Thermal Efficiency % ≥ 60.00 PASS WORST BESTCO2 Recovery % ≥ 90.00 WORST PASS BESTAuxiliary Generation CO2

Emission Low

tonnes CO2/day < 250 BEST FAIL FAIL

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Conclusion and Recommendations

Post-combustion carbon capture effects only the exhaust gas that leaves the combustion reaction.

Unlike oxy-fuel combustion capture and pre-combustion capture, post-combustion capture with an amine

solvent has the most negligible impact on standard power production with a once-through steam generator

with natural gas as a fuel. The ability to extend CO2 recovery through the addition of an exhaust recycle

also gives the post-combustion alternative flexibility to increase recovery that the other two options

simply do not offer. The capital cost for such a system falls between the two other alternatives, at about

$83,100,000 CAD, accounting for all possible expenses. The recommended configuration for CO2

sequestering in this case is post-combustion carbon capture with an amine solvent. Also of relative

importance is the determination that cost estimates per tonne of recovered CO2 are drastically

overestimated by previous studies ($175-$300 per tonne given when actual cost is $67.00 per tonne). The

time to construct this system from the approval stage to the final plant initiation stage is estimated to be

twenty months.

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Potential Problem Analysis

The secondary CO2 emissions generated by reboiler and condenser duties can also be recovered if

the exhaust from these units is treated. Instead of capturing 1000 tonnes of CO2 per day from the plant,

the absorber and stripper would need to have capacity increases of about 1180 tonnes of CO2 per day.

This challenges the current view that 1000 tonnes of CO2 per day is the maximum practical size for a

plant of this specification.

Thermal efficiency of steam generation is neither enhanced nor decreased when post-combustion

carbon capture is implemented. This means that fuel will be consumed at the same rate as it had been

before the new emission regulatory measures and carbon tax were put into effect. Carbon dioxide

emissions will be drastically decreased and all sequestered carbon will be stored so it has the least effect

on the global climate. However, natural gas consumption will not decrease following implementation of

the capture system, raising the issue of how to deal with a depleting natural resource that cannot be

regenerated.

Carbon sequestration is not a long-term fix for the global energy crisis. It will certainly allow

industries to continue powering their electrical grids for up to another two-hundred years, but it will

become increasing difficult as reserves decline. Therefore, the energy industry must rely on research

towards new alternative energy sources in order to meet energy demands in the future. With technologies

such as post-combustion carbon capture however, the Greenhouse Effect can be avoided with the aid of

new pollution control measures including a high carbon tax.

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