Chapter 2

43

Chapter 2

2. CO

Capture from fossil fuel power plants:

Introduction of a model for assessing the energy

requirements of CO

chemical absorption capture

technologies

2.1. Introduction

In chapter 1 was analyzed the different approaches in order to mitigate the environmental impact produced by the energy production systems.

From the economic point of view, economists consider that market-incentive policy instruments could be the least-cost solution to the problem attaining environmental standards. In this context, it appears that economic-incentive instruments possess the inherent advantage of cost-effectiveness and also provide inducements to technological innovations. Nevertheless, engineers have been always critics with an absolute economic vision, focusing their studies to broad the environmental problems on direct regulations, “command –and-control” strategies, by means of increasing energy efficiency of power generation systems, switching to less carbon-intensive fuels and

44

capturing the CO2 emitted from power plants, arguing that the technological solutions

should be an indispensable tool of any energy policy.

Although the use of charges and permits is not entirely absent in operational control strategies, their adoption and implementation has been quite restricted and they have been limited to a supplementary role in the direct regulatory approaches1_.

The CO2 capture technologies have been widely analyzed in the literature under

chemical and economic points of view, leaving their energy impact on the power plant performance in a second plan. In this chapter, after a review of the state of the art of the different CO2 capture technologies for fossil-fuel power generation systems, the

author presents an analysis of the energy required to operate CO2 capture systems.

2.2. CO

capture

2.2.1 The basis for CO

capture in fossil-fuel power generation

The main application of CO2 capture is likely to be at large point sources. Fossil fuel

power plants contribute approximately to one-third of the world’s CO2 emissions,

attaining the highest density of CO2 emissions in terms of mass per area per time.

The aim of CO2 capture technologies is to capture the CO2 presents at the flue gas

before reaching the atmosphere, producing a concentrated stream of CO2 ready to be

transported and stored. Besides the economic analysis, the CO2 capture systems

demand significant amount of energy for their performances. This is such that power plants with capture technology require more fuel per kilowatt-hour generated, reducing net plant efficiency, around 6-14% points2_{. Indeed, it results in an increase in most} other environmental emissions per kilowatt-hour of electricity generated, also producing a proportional large amount of solid waste or by-products relative to the same type of base plant without capture. In addition, there is an increase in the consumption of chemicals, as ammonia and limestone use by De-NOx and De-SO2

emission control technologies. These factors restrict the emissions reduction requirements to a not-always-available thermodynamic plant performance.

45

In order to reduce the intensive energy cost, the researches must be conducted to archive higher levels of system integration, increasing efficiency and reducing the capture energy requirements. Advanced plant designs, that further reduce energy requirements for the CO2 capture, should also reduce overall environmental impact and

the fuel input.

2.2.2. State of the art of CO

capture systems for fossil fuel power plants

interaction parameters, such a partial pressure of the CO2 to be captured, total pressure

of the feed gas, inlet temperature of the feed gas, degree of removal, sensitivity to impurities such as acid gases and particles; purity of the desired CO2 product, utilities

available, economic considerations (capital and operating cost of the process, including cost of additives necessary to overcome fouling and corrosion when applicable), required plant life, size and weight, location, and environmental constraints3_{. The} possibility of integrating the removal plant within the overall plant, utilizing, for example, low-grade heat for regeneration, can also influence the choice of CO2 capture

technology4_.

Capture techniques can be retrofitted to existing conventional air-based fossil-fuel – fired power systems or integrated into new power-generation facilities- The main approaches to capture the CO2 generated from the combustion of primary fossil fuel

are based on solvent wet scrubbing with chemical or physical absorbent, solid dry scrubbing with physical or chemical absorbents gas membrane separation or cryogenic methods. These technologies can be performed in post- or pre-combustion processes see, figure. 2.1.

46

Post-combustion CO₂ Removal from flue gas

Separation Task: CO₂/N₂ Pre-combustion fuel decarbonization Separation Task: CO₂/H₂ Oxy-fuel combustion Separation Task: -Air separation unit O₂/N₂ -CO₂separation CO₂/H₂O Absorption -Chemical solvents -Physical solvents Adsorption -Pressure swing/Temperature swing

-Solid Sorbents (Zeolites, activated carbon, silicates)

Cryogenic -Liquefaction/Hybrid processes/Distillation Membranes -Polymeric/Ceramic/ Ion transport

CO

Capture Systems

CO

Capture technologies

Figure 2.1: Main technologies for CO2 capturing systems.

2.2.2.1 Post-combustion processes

Post-combustion processes capture CO2 from flue gases produced by combustion of

fossil fuels or biomass. Instead of being discharged directly to the atmosphere, the flue gas is passed through equipments which capture the CO2, while the remaining flue gas,

with a low CO2 concentration, is discharged to the atmosphere.

Flue gas from combustion approaches atmospheric pressure, which indicates that, in post-combustion processes a large volume of gases should be treated involving the use of large scale equipments. Because of the low CO2 partial pressure, absorption

processes based on chemical solvents are currently the preferred options for post-combustion captured5_{. The future of post –combustion techniques are based on} emerging capture technologies, as improved solvents, novel contacting equipments and improved design of processes. However, they are not yet in such an advanced stage of development.

47

The energy required in CO2 post-combustion capture processes could be taken from

the power process, for instance, in the form of low-pressure steam. This energy requirement reduces the overall efficiency of the process, see figure. 2.2

Figure 2.2: Post-combustion process

2.2.2.2. Pre-combustion processes

At the Pre-combustion processes, the carbon of the fuel is removed prior the combustion, trough a process called decarbonization. This method converts, in a first stage, chemically fossil fuels into a synthetic gas (syngas) composed mostly of H2 and

CO. Thee are two ways to do this conversion; (i) adding steam, in which case the process is called “steam reforming”, or (ii) adding oxygen to the primary fuel, what is called “partial oxidation”. In both cases, the heat value from the primary fuel will be transferred to the produced H2.

This is follow by a CO-shift reactor that converts the CO in CO2 and more H2. The

CO2, with a high partial pressure, can be then captured, usually by physical or

chemical absorption. The resulting high-pressure stream is a hydrogen-rich fuel, which can be used in many applications.

A partially oxidation process for solid fuels is the Integrated Gasification Combined Cycle (IGCC) power plant, further explains in chapter 3. This process, shows in figure 2.3, is one of the most promising pre-combustion technologies for CO2 capture. The

Power Process CO₂ Capture Flue Gas Air Fuel W_{out Energy}

Exhaust gases with low CO₂ concentration

48

high-pressure flue gas implies lower volume to be treated with a high CO2

concentration. This results in smaller equipment sizes, lower energy penalty and capital costs comparing to the post-combustion processes6_{. However, the introduction} of the shift reaction in the process cannot be made without energy losses.

Figure 2.3: Gasification process

Furthermore, steam reforming processes for fossil gas fuels use a conventional steam reformer furnace where the methane is allowed to react with steam to produce H2-rich

fuel7,8

. There are different methods for reforming natural gas into a H2-rich fuel which

enable pre-combustion capture of CO2. An air-blown auto-thermal reforming reactor

(ATR) integrated with a gas turbine combined cycle, is shown in figure .2.4.

Figure 2.4: Gas turbine combine cycle with auto-thermal reforming

Gasification reforming Air Solid Fuel ASU O2 CO shift: CO(g) + H2O(g)



H2(g) + CO2(g) CO2

Capture Combined_cycle CO₂ to transport and stored

Exhaust gases with low CO2 concentration H₂ Wout ATR CO/H2 Gas shift CO₂

Capture Combined_cycle

Compressed air MP-steam

HP steam

Air CO₂ to transport and stored H₂+ CO₂++ H₂+ CO++ Gas Fuel H₂+

Exhaust gases with low CO₂ concentration

49

2.2.2.3. Oxy-Fuel combustion capture

Oxy-fuel combustion can be seen as a form of pre-treatment, where nearly pure oxygen is used for combustion instead of air. The resulting flue gas is mainly CO2 and

H2O, requiring simplest treatments for the CO2 capture. The higher energy

requirements take place at the Air Separation Unit, (ASU), where the oxygen in air is separated for the combustion process. In order to obtain a combustion gas with suitable properties, the oxygen from the air separation unit is mixed with recirculated flue gas, see figure. 2.5. This technology will be further explains in chapter 3.

Figure 2.5:O2/CO2 firing process

2.2.3. CO

Capture technologies

CO2 capture systems apply many of the well known technologies used for gas

separation in industrial processes. A summary of these technologies is given below in table 2.1.

Capabilities Adsorption Membrane Absorption Cryogenic Feed Pressure Low to high Medium to

high Low to high

Medium to high

CO2 pressure Low Low Low

Low to medium CO2 Purity Medium to high Low to medium Medium to high High CO2 recovery Medium to

high Low High High

Table 2.1: CO2 separation technologies

Power Process Condensing Water Flue Gas Air Fuel W_out ASU Water O₂ O₂ Recycled CO₂/H₂0

50

2.2.3.1. Separation with sorbents/solvents

2.2.3.1.1 Chemical absorption

Chemical absorption processes make use of the reversible nature of the chemical reaction of an aqueous alkaline solvent with CO2. A by-product is originated, which

afterward will be heating, to broke the bound between the chemical solvent and the CO2, producing a CO2-rich stream and a regenerated chemical solvent. Heat energy is

consumed to regenerate the chemical solvent so that the net power output of the plant is reduced. Figure 2.6 illustrates the general schema of an absorption process. After the absorption, the sorbent loaded with the capture CO2 is transported to a different vessel,

where it released the CO2 after being heated, after a pressure change or after any other

change in the conditions around the sorbent.

A B S O R P T IO N S T R IP P IN G Pre-treatment of Exhaust gases Post-treatment of Exhaust gases Clean gas CO₂ Flue gas

Low Pressure Steam/ kg CO₂avoided

Figure 2.6: Chemical absorption process

The main absorption parameters determining the technical and economic operation of a CO2 absorption system are:

o The flue gas flow rate; which determines the absorber size.

o The CO2 content in the flue gas; CO2 usually have a low concentration and a

low partial pressure in the flue gas. Under these conditions it needs a very powerful absorber, which will require a high energy to be regenerated.

51

o Energy requirements; The energy consumption of the process is the sum of the thermal energy needed to regenerate the chemical solvent, the electric energy required to operate pumps, blowers and compressors.

o The Cooling requirements; for cooling the flue gas and the stripper product. A suitable choice of solvent for chemical absorption processes depends on high CO2

loading capacity, low heat of desorption, low by-products formation and low decomposition rates.

The Chemical absorption is suitable for medium till low partial pressure of CO2, and

for a relative low temperature, where the solubilities are governed mainly by stoichiometric relations. Amine-based are the principal commercial chemical solvents used to separate CO2 from exhaust gases including, (MEA), diethanolamine (DEA)

and methyldiethanolamine (MDEA). To date, all commercial plants that capture CO2

from power plant use processes based on chemical absorption with MEA solvent. The purity from an amine-based chemical absorption process could reach 99.9% by volume9_.

Furthermore, salts of strong alkalis with weak acids offer various possibilities for chemical absorption process10_{. Typically are employed aqueous solutions of a salt} containing Na or K11_{. The principal technologies are based on hot K}

2CO3 solution used

for the removal of CO2 from high-pressure gas stream. For processes at ambient

temperature Na2CO312and K2CO3 or CO313,14 solutions can be used. This technology

will be further analyzed in the section 2.3 in this chapter.

2.2.3.1.2. Physical absorption

CO2 can be physical absorbed in a nonreactive solvent according to Henry’s law and

then regenerated using pressure reduction or heat. The absorption capacity of organic and inorganic solvents for CO2 increases with increasing pressure and with decreasing

temperatures. Therefore, the physical absorption methods do not appear likely to compete for flue gas CO2 capture, since partial pressure of CO2 is low and the flue gas

52

temperature is high. Under these conditions chemical absorption processes are more applicable.

However, this type of technology could be a very efficient for high-pressure CO2-rich

streams, as those encountered in pre-combustion processes of advanced power generation systems, such as IGCC15_{. The physical solvents commercially used are} Selexol16_{, Rectisol}17_{, and Morphysorb}18_{. The physical solvents are used to remove CO}

2

and H2S in many coal gasification applications, albeit not yet on the scale required for

advanced power generation systems.

2.2.3.1.3. Dry solid absorption

Dry gas/solid scrubbing uses a solid instead of a liquid scrubbing medium. In this process the solid sorbent does not circulate between vessels because the sorption and regeneration are archived by cyclic changes, in pressure or temperature.

Depending of the nature of the gas/solid interaction the CO2 can be physically or

chemically absorbed and may also be regenerated.

For solid chemical absorption, the CO2 undergoes in a chemical reaction with an active

compound presents on the solid to form a new product. Hence, chemical absorption must involved heterogeneous gas/solid chemical reactions occurring on the surface of the solid.

Absorption capacities and kinetics are influenced by pore size, pore volume, surface area, and the affinity of the solid for weakly bounding with CO2. Some examples are

developed in the next section.

For solid physical absorption, the CO2 is sorbed onto the surface of the solid without

undergoing to form new species.

2.2.3.1.4. Physical adsorption

An adsorption process consists of two major steps: adsorption and desorption. Strong affinity of an adsorbent for removing the undesired component from a gas mixture is essential for an effective adsorption step. The stronger the affinity, however, the more

53

difficult it is to desorb the gas impurity and the higher the energy consumed in regenerating the adsorbent for reuse in the next cycle. Several modes of operation are used to release or regenerate the adsorber gas from the solid. Pressure or temperatures changes are used during regeneration to repeat the adsorption cycle. In Pressure swing adsorption (PSA); the gas mixture flows through a packed bed of adsorbent at elevate pressure, insolating the solid, and then desorbing the sorbed gas by lowering the system pressure. In Temperature swing adsorption (TSA); the gases are adsorbed at a lower temperature, the solid is isolated, and then temperature is raised during the regeneration step to release the trapped gas. Cycle time for regeneration is typically much shorter for PSA (order of seconds) versus TSA (hours)19

.

The main advantage of physical adsorption over chemical or physical absorption is its simple and energy efficient operation and regeneration, which can be achieved with a pressure swing or temperature swing cycle.

In the adsorption process for flue gas CO2 recovery, molecular sieves or activated

carbons are used. Desorbing CO2 is then done by the PSA or TSA. The primary

adsorption material under consideration is zeolite. The concerns over this technology are scale up in the need to develop CO2 specific adsorbent materials20.

However, adsorption is not yet considered attractive for large-scale separation of CO2

from flue gas, because the capacity and selection of available adsorbents is low.

2.2.3.2. Cryogenic Separation

The cryogenic separation separates a gas component from the others gas components of a gas stream. The separation can be made producing a phase change (liquefaction or solidification) of the component that wants to be separated, thereby condensing it and removing it as a liquid/solid from the gas mixture3_{, see figure 2.7. The condensation is} fulfilled by a series of compression, cooling and expansion steps.

Cryogenic separation for CO2 is only practical for high pressure with high CO2

concentration gases, such as in pre-combustion capture processes and Oxy-fuel combustion. Cryogenic separation produces directly liquid CO2 which is favorable for

54

certain transport and storage options. A negative aspect is its high energy demand for cooling and compressing.

Figure 2.7: Cryogenic separation process

2.2.3.3. Membranes separation

Membranes are specially manufactures materials that allow either the selective transport (diffusion) or selective exclusion of a desired component. The flow of gas through the membrane is usually driven by the pressure difference across the membrane. Therefore, high-pressure streams are usually preferred for membrane separation. Indeed, the low CO2 partial pressure difference that characterizes fuel gas

provides a low driving force for gas separation membrane.

Membranes can separate CO2 from a gas stream by size exclusion or by chemical

affinity. Processes for CO2 removal from natural gas at high pressure and with a high

CO2 concentration are commercially used.

There are many different types of membrane materials (polymeric, metallic, ceramic) that may find application in CO2 capture systems. Nowadays, the removal of CO2 with

membranes applies commercially available polymeric gas separation membranes, resulting in higher energy penalties on the power generation efficiency compared to a standard chemical absorption processes21_{. Also the remove efficiency is lower than for} standard chemical absorption processes.

POWER CO2+ Vapour CO2 Water POWER CO2+ Vapour CO2 Water

55

Indeed, the development of a membrane separator for the selective removal of the CO2

presents in a gas stream, is still in the developing phase, having a huge economic cost. They have not yet been applied for the large scale and demanding conditions in terms of reliability and low cost required for CO2 capture systems. Improvements can be

made if more selective membranes become available.

2.2.3.4. The new perspectives of CO2 separation technologies

The future of separation technologies should be based on overcoming the limitations of the existing solvents, namely, the high energy consumption for solvent regeneration and the equipment size. These two parameters withstand the power plants to efficiency penalties and to added costs.

In order to overcome these requirements various different solvents are being investigated. Aqueous NH3 solutions have been proposed for scrubbing CO2 from fuel

gas with higher removal efficiency and loading capacity that MEA22,23_{. Other studies} propose NH3 scrubbing technologies to remove CO2 in flue gas along with the acid gas

pollutants, SO2, NOx, HCl and HF. The CO2 could be removal in a regenerable

process, with thermal energy consumption for the CO2 regeneration significantly less

than in MEA processes24_{. Aqueous NH}

3 solutions are further analysed in the next

section.

Ionic liquid salts have been also proposed as an effective candidate for CO2 scrubbing

processes. The ionic liquids could facilitate the sequestration of gases without solvent losses25

.

Still other researches are focused on new capture system as, electrochemical pumps for separation of CO2 from the fuel gas (proton pumps)26 or from the flue gas (Carbonate

ion pumps)27_{. Also methods to prevent the oxidative degradation of MEA by} de-oxygenation of the solvent solution have been investigated28 _.

56

2.3. Proposal of a methodology for assessing the energy requirements

of CO

chemical absorption capture technologies

Chemical absorption technologies are the most mature application demonstrated commercial technologies. Nowadays, it is the only technology used to implement large-scale capture of CO2 from fossil-fuel energy combustion systems. The capture

efficiency is higher than 90% and produces CO2 with a purity of 99%9,29. As mentioned

at the outset of this chapter, the energy required for the solvent regeneration is one of the main goals to be focused on, because it tends to translate into an important efficiency penalty the thermodynamic performance of the power plant as well as, it dictates its net size and, hence, the net cost of power generation with CO2 avoidance

technologies.

A chemical absorption process consists of two major steps, adsorption and desorption (sorbent regeneration). The absorption process, that takes place in the scrubber, governs the technical flexibility of the process. While the regeneration at the stripper unit controls its energetic and economic viability.

On the one hand, strong affinity of an absorber for removing the CO2 from the flue gas

mixture is essential for an effective absorption. But in the other hand, the stronger the affinity the more difficult is to desorp the CO2 in the stripper and the higher the energy

consumed in regenerating the absorber for its reuse in the capture cycle. Therefore, a carefully balance must be set up among absorption and desorption stages.

In the next section, an analytical model is described in order to account for the energy consumption of the chemical absorption processes.

The central question examined is the connection between abatement capability and its energy cost in the power generation performance. The analysis will evidence that the widespread application of CO2 capture needs additional technical effort and will

establish that further developments in this area must be constrained by reducing its energy requirements.

57

2.3.1. Methodology

The model proposed will estimate the required energy consumption of different chemical absorption CO2 capture processes, in order to produce a high concentrated

pressurized CO2 stream.

The capture process can be divided in three main stages, see figure 2.8; pre-treatment of the flue gas, in order to reach the desirable scrubber conditions, the scrubber/stripper processes, and the post-treatment of the produced pure CO2 stream. The energy

consumption of the whole process will be the sum of the thermal energy needed to regenerate the solvent and the energy required in order to deliver a high-pressurized CO2-pure stream ready to be transport and storage. Electrical energy for compression,

pumping and blowing the circulated solvent flow rate or to operate the flue gas, are not considered in the model.

Clean gas Pre-treatment of Flue gases T_range(100-600)°C Flue gas Capture process Post-treatment of CO2stream CO₂ For sequestration CO₂ SCRUBBER T_range(25-80)°C CO₂ STRIPPER T_range(80-140)°C

Figure 2.8: Principal model elements for the chemical absorption capture technology

The parameters used for the model are; the sorbent absorption capacity, depending on the CO2 concentration in the flue gas (lower CO2 concentration will required stronger

absorption capacity), the scrubber/stripper temperatures, depending of sorbent type, and the CO2 desirable temperature and pressure for transport and storage.

58

2.3.1.1 Description of the chemical absorption capture process

The flow diagram of the chemical absorption process used for the model is shown in figure 2.9. The flue gas is cooled down at typical absorption temperatures, between 40-60ºC. Next, it is led to the scrubber, where the flue gas enters at the bottom of the absorber column and flows upwards, counter-current to the sorbent flow. The flue gas needs to overcome a substantial pressure drop as it passes through a tall absorber column. Hence, the cooled flue gas must be pressurized using a blower before it enters to the absorber.

In the scrubber, the solvent reacts chemically with the CO2 contained in the flue gas

producing weakly bonded carbamates compounds. The rich solvent, which contains the chemically bound CO2, is then pumped to the top of the stripper via a heat

exchanger.

The regeneration of the chemical solvent is carried out in the stripper at elevated temperatures, between 80-150ºC and at pressures not very much higher than the atmospheric pressure. Heat is supplied from the reboiler to maintain the regeneration conditions, providing the required desorption heat for removing the chemically bound CO2 and for steam production, which acts as a stripping gas. Steam is recovered in the

condenser and fed back to the stripper, whereas the CO2-rich stream product gas leaves

the stripper ready to be pressurized for sequestration. The ‘lean’ solvent, containing no CO2 is pumped back to the absorber via a lean-rich heat exchanger and over a cooler to

bring it down to the scrubber temperature, while the “clean” gas is vented to the atmosphere.

59

Figure 2.9: Process flow diagram for CO2capture from flue gas by Chemical

absorption.

2.3.2. Tools for a thermodynamic simulation of the capture process

2.3.2.1. Flue Gas Pre-treatment

The flue gases from a combustion power plant are usually quite hot, depending of the power system ranges from 60 °C to more than 600°C. Absorption being an exothermic process is favoured by low temperatures. Thus, it is desirable to cool down the flue gas till scrubber temperatures in order to improve CO2 absorption and minimize sorbent

losses. A direct contact cooler can be used in order to reduce the flue gas temperature to acceptable levels, which also acts as a flue gas wash with additional removal of fine particulates. The direct contact cooler consists of a packet tower where the cooling fluid is pumped water at 25°C passing through a cooler. The flue gas temperature directly affects the volumetric flow rate of the flue gas stream, which is a key determinant of the various pre-treatment equipments sizes (e.g., direct contact cooler, flue gas blower and absorber).

Scrubber T_scb P_scb Feed Gas Stripper T_strp P_strp CO₂ Product gas Low pressure steam

rich solvent cool lean

solvent hot Exhaust gases

clean

rich solvent hot Lean Solvent cool blower cooler condenser pump reboiler Flash Tank

60

The heat duty to be removed from the flue gas at the cooler pre-treatment depends on the temperature and the mass flow of the flue gas together with the desirable absorption temperature require for the absorption process, in eq. 2.1.

) T T ( C m Q f _{p scb f} . − = (2.1)

where Q (<0) is the heat duty for cooling the hot flue gas, the mf is the mass flow of the flue gases, Cp is the heat capacity of the flue gases, Tscb is the scrubber temperature and Tf the flue gas temperature.

The capture processes involving sorbents working at high temperatures can avoid partially this energy consumption, and therefore achieving high integration levels within the power plant.

2.3.2.2. Scrubbing/Stripper process

Since the absorption of CO2 might be a spontaneous reaction in order to facilitate the CO2 capture. At the scrubbing process is fulfilled an exothermic reaction, thus no energy is demanded. The stripper process is the desorption process, where a high concentrate CO2 stream is produced, the energy required for the sorbent regeneration is quite high and one of the most relevant energy consumption in the whole process. This energy is usually supply by the low/medium pressure steam flow produced in a steam turbine and supplied by a reboiler. In any case, this required heat can be divided in three main contributions30_.

2.3.2.2.1. Sensible Heat

The sensible heat is the energy used for heating the scrubber solution to the stripper temperature, shown in eq. 2.2. For the capture with aqueous solutions it is important to consider the water presents in the solution, which gives the limitation to the absorption and represents also a relevant consume of latent energy.

61 v 2 CO sol p . sol s ∆H m ) (T C m strp en T S = − (2.2)

where; Ssen is the sensible heat required, msol is the solution mass flow, Cp is the specific heat of the solution, Tsol is the rich solvent hot solution temperature, Tstrp is the stripper temperature, mCO2 is the CO2 mass flow recovered and ∆Hv is the steam reboiler vaporization heat coming in to the stripper at the stripper temperature.

2.3.2.2.2. The CO2 desorption energy

The recovery of CO2 from the chemical solvent needs energy to reverse the adsorption reaction. This energy reaction includes the enthalpy reaction and in the solution case, also, the solution reaction of the dissolution in water, in eq. (2.3).

v sol v r des ∆H ∆H ∆H ∆H S = + (2.3)

where; Sdes is the desorption energy required, ∆Hr is the enthalpy reaction, ∆Hsol is the solution reaction and ∆Hv is steam reboiler vaporization heat coming in to the stripper at the stripper temperature.

2.3.2.2.3. Steam supplied for the stripping of CO2

The CO2 recovered in the stripper must come out along with the help of a vapour stream. The mixture is composed of 1 mol vapour/mol CO2. The vapour required for stripping is shown in eq. 2.4, considering that the whole process occurs at constant temperature and pressure, the steam goes from the saturated vapour to the saturated liquid point at the stripper temperature. This heat is also supplied by the reboiler

2 CO Steam strp m m S = (2.4)

62

where; Sstrp is the steam supplied for the stripping of CO2, msteam is the steam mass flow required from the reboiler at the stripper temperature, and mCO2 is the CO2 mass flow recovered.

The total vapour required in the stripper is the sum of the equations (2.2-2.4):

strp des sen

T S S S

S = + + (2.5)

2.3.2.3. CO2 liquefaction for transportation and storage

From the different optimal conditions for CO2 transport, in the model is proposed the transportation in pipeline at ambient temperature and 140 bar pressure.

After the stripper, the vapour-CO2 mixture is cooled down to 25 °C in order to separate water from CO2 before the liquefaction plant. For the model proposed here, it is considered that the vapour condensation occurs in a direct contact cooler with water at 25°C.

In order to liquefy the pure CO2 stream produced; three stages intercooler compression is used, based on the Linde method, see figure 2.10.

Figure 2.10: series of cooling and compression for the CO2 sequestration

The energy compression of the CO2, in eq. 2.6, is the energy required to compress the produced CO2 to sequestration parameters for an efficient transport and storage, with compressor efficiency about 80%.

CO₂ (p_i,t_i) n i f P P x 1       n i f P P x 1       n i f P P x 1       (p_f,t_f) CO₂

63 c p CO . c T C m W 2 η ∆ = (2.6)

where Wc is the compression energy, mCO2 is the CO2 mass flow recovered, Cp is the

CO2 specific heat, ∆T is the temperature different in each stage and ηc is the

compression efficiency. The compression is carried out in different stages; the number of stages is given in eq. 2.7, every stage implicates a compression and an intercooling process. i f P P n= (2.7)

where n is the number of stages and Pf, Pi are the final and initial pressures with Pf/Pi

the pressure ration.

2.3.3. Main assumptions for the capture process

The model is proposed in order to calculate the energy consumption of the six principal chemical absorbers used on CO2 absorption capture technologies;

Alkaloamine solutions, Ammonia solutions and dry solid regenerable sorbents. The treated flue gas comes from a natural gas cogeneration power plant31

(at 150°C, and 4,9%wt CO2 concentration), in order to produce a high concentrated pressurized

CO2 stream (25 °C, 140 bar) ready for sequestration.

For obtaining a detailed evaluation of the process, the required energy consumption is calculated due to a methodological analysis with the data found in the handbook from Perry,32 _{under “ideal process” conditions. This avoids deliberately the use of software} to perform the computation cycle.

For maintaining the calculation into practicable limits, some assumptions need to be made;

64

o The total amount of CO2 coming into the system is sequestrated, and no

recirculation of CO2 takes place.

o The absorption reaction is simplified in a stoichiometric reaction with the absorber

o There is no sorbent degeneration (by thermal or oxidative degradation).

o There are not sorbent losses.

o There are not corrosion problems.

o There are not presences of acid gases in the flue gas, such as SO2, NO2, HCL,

HF or O2 that induce a high absorbent make up rate

o There are not presences of fly ashes or soot in the flue gas, as they might be plug the absorber.

o The specific heat of the solution coming in to the stripper is considered to be the specific heat of the solvent33_.

The unwanted interactions of the sorbent with acid flue gas compounds, like SO2, SO3,

and NO2, lead to the formation of heat stable salts, and hence, in a absorption loss

capacity of the solvent and in a risk of an undesirable formation of solids in the solution34,35_{. It would also result in an extra consumption of chemicals to regenerate the} solvent and the production of a waste stream such as sodium sulphate or sodium nitrate.

Therefore, it is considered that the plant will be equipped with DeNOx unit, electrostatic precipitator, ceramic or bag house filters and in the case of coal combustion with a flue gas desulphurization unit, in order to provided a clean flue gas to the absorber.

However, the energy costs do not yield information regarding the reaction rate (kinetics), but rather the final equilibrium chemical state. A reaction may be thermodynamically favorable but may require a very long time to archive its final chemical state. For this methodology, reactions are assumed to be sufficiently fast kinetic to have practical application in a CO2 capture processes.

65

2.3.4. Application of the model to the Chemical CO

absorption with an

Aqueous Alkaline Solvent for flue gases

The chemical absorption with aqueous alkaline solvent is suitable for medium till low partial pressure of CO2, and for relative low temperatures. From the aqueous alkaline

solvent, amines are the most wide commercial chemical absorbers used to separate CO2 from exhaust gases including; monoetanolamine, (MEA), diethanolamine (DEA),

Triethanolamine (TEA) and methyldiethanolamine (MDEA). The TEA and DEA are not taking into consideration for this analysis. DEA due to its high corrosion problems and TEA due to its low absorption capacity30,36_.

MEA has a high CO2 reactivity, which produces very stabile carbamate ions with a

high heat demand for regeneration; however, it is still so far the most commercial chemical solvent method used for CO2 capture. Furthermore, the main advantage using

MDEA is its low corrosion problems, as well as its high load capacity.

The model has been applied; according to the schema describe at the figure 2.9, in order to calculate the energy required for recovering the CO2 presented in the flue gas

described above, using MEA and MDEA solutions as chemical sorbents, under the simplifications exposed at section 2.3.2. The chemical reaction for MEA and MDEA has been described as follow37,38

.

MEA: CO2 + 2R-NH2↔R-NH3+ + R-NH-COO

-MDEA: CO2 + R2-NCH3↔R-NH3+ + R-NH-COO

-With: R: -CH2-CH2OH

The process parameters are summarized in the table 2.2. The model results for the MEA and MDEA processes are illustrated in the figures 2.11 and 2.12, where the main thermodynamic parameters and energy consumptions are summarized.

66

Figure 2.11: Analysis of the energy required by the CO2 capture process using MEA

solution as solvent

Treated flue gas 0.9511 kg/s 40°C / 1 bar

1 kg/s Flue gas 150°C / 1 bar

Capture process

Total Energy required = 2,6 MJ/kg CO₂ CO2 SCRUBBER 40°C CO2 STRIPPER 103°C Flue gas Pre-treatment

2,3 Total capture energy:

1,1 Desorption energy;

1.1 Steam supplied for stripper:

0,15 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required

2,3 Total capture energy:

1,1 Desorption energy;

1.1 Steam supplied for stripper:

0,15 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required %molar

CO20,03

N2 0,75

O2 0,12

H2O 0,08

Sorbent capacity: 1 mol CO2/mol MDEA

MDEA %molar

N20,77

O20,14

H2O 0,08 Liquefaction plant for CO2

transportation in pipeline 140 bar/25 °C Heat duty -140 kW 340 kJ/kg CO2 Heat duty –420 kW 0.048 kg/s 100% CO2

Figure 2.12: Analysis of the energy required by the CO2 capture process using MDEA

solution as solvent

Treated flue gas 0,9511 kg/s 40°C / 1 bar Heat duty -140 kW 1kg/s Flue gas 150°C / 1 bar Capture process 340 kJ/kg CO2 Heat duty –420 kW 0.048 kg/s 100% CO2

Total Energy required = 3,7 MJ/kg CO₂ CO2 SCRUBBER 40°C CO2 STRIPPER 140°C MEA

Flue gas Pre-treatment

Liquefaction plant for CO2

transportation in pipeline 140 bar/25 °C

3,4 Total capture energy:

1,9 Desorption ene rgy;

1,1 Steam supplied for stripper:

0,3 Sensible heat:

MJ/kg CO2

Scrubber /stripe r energy required

3,4 Total capture energy:

1,9 Desorption ene rgy;

1,1 Steam supplied for stripper:

0,3 Sensible heat:

MJ/kg CO2

Scrubber /stripe r energy required %molar N20,77 O20,14 H2O 0,08 %molar CO20,03 N2 0,75 O2 0,12 H2O 0,08

67

Table 2.2: Operating parameters for CO2 capture with MEA/MDEA solvent30,32,34,35 ,36.

From the different results obtained with the model for assessing the energy requirements of the different chemical absorption processes, only the one concerning the MEA sorbent can easily find confront in the literature. The theoretical minimum heat requirement to reverse MEA reaction with CO2 is about 1.9 MJ/Kg CO2. Different

studies are found in literature giving an energy demand between 3.9 and 4.2 MJ/kg CO2 captured39,40, which agree with the resulting data of our model.

As already said, nowadays, all the commercial CO2 capture plant processes are based

on chemical absorption by MEA36

. On closer examination, however it is seen that this decision is entirely economic and that an end-of-pipe strategy is completely divorced from the thermodynamic performance of the power plants. However this does not mean that there is no way by which the CO2 capture process might be integrated into

the energy process of the power plants9_.

2.3.5. Application of the model to the Chemical CO

absorption with Aqua

Ammonia solution

Aqua Ammonia solutions are considered as the best candidates for replacing MEA in CO2 chemical absorption capture technologies41. Its increasing presence in the

literature is based fundamentally on the following advantages42,43,.44

; the ammonia solution has the greater CO2 load capacity among the studied sorbents. This is justified

by its low molecular weight. It does not involve sorbent degradation, producing no equipment corrosion problems45_{, and furthermore, aqua ammonia solutions can be used} for simultaneous removal of CO2, SO2, and NOx in a single process. Anyway, the most

Sorbent Hr°[T] Lean/amine solution Capture temperature Regeneration temperature Stecho. Sorbent capacity [°C] [kJ/kg CO2] [%wt] [°C] [°C] [mol CO2/mol Amine] MEA 100 1519 30 40 140 0.5 MDEA 100 1105 35-50 40 103 1

68

important advantage to the process is that the thermal energy consumption for the CO2

regeneration is significantly less than the MEA processes46_.

Still the scrubbing process can be seen as a technique to produce fertilizers. The mayor by-products from the aqua ammonia processes include ammonium sulphate, ammonium nitrate and ammonium bicarbonate that can be utilized as fertilizers. This gives an alternative at the CO2 capture processes, where the regeneration does not take

place and the by-products are commercial products.

The model has been applied; according to the schema describe at the figure 2.9, in order to calculate the energy required for recovering the CO2 presented in the flue gas

described above, using aqua ammonia solutions as chemical sorbents, under the simplifications exposed at section 2.3.2. Three potential reactions could be responsible for liberating CO2 during the thermal regeneration23,24,47.

2NH₄HCO₃(aq) ↔ (NH₄)₂CO₃(aq) + CO₂(g) + H₂O NH₄HCO₃(aq) ↔ NH₃(aq) + H₂O + CO₂(g) (NH₄)₂CO₃(aq) ↔ 2NH₃(aq) + H₂O + CO₂(g)

The process parameters are summarized in the table 2.3, the model results are illustrated in the figure 2.13, where the main thermodynamic parameters and energy consumptions are summarized. The major contributor to heat saving comes from the stripper heat, which for ammonia solution is considered as not necessary47

Sorbent Hr°[T] Lean amine solution Capture temperature Regeneration temperature Stecho. Sorbent capacity [°C] [kJ/kg CO2] [%w] [°C] [°C] [mol CO2/mol NH3] NH₄HCO₃ 100 1455.5 30 27 82 1

69

Treated flue gas 0.9511 kg/s 27°C / 1 bar

1 kg/s Flue gas 150°C / 1 bar

Capture process

Total Energy required = 2 MJ/kg CO₂ CO2 SCRUBBER 27°C CO2 STRIPPER 82°C Flue gas Pre-treatment

1,7 Total capture energy:

1,5 Desorption energy;

-Steam supplied for stripper:

0,23 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required

1,7 Total capture energy:

1,5 Desorption energy;

-Steam supplied for stripper:

0,23 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required %molar

CO20,03

N2 0,75

O2 0,12

H2O 0,08

Sorbent capacity: 1 mol CO2/mol NH3

NH₃aq. %molar N20,77 O20,14 H2O 0,08 340 kJ/kg CO2 Heat duty –384 kW 0.048 kg/s 100% CO2

Liquefaction plant for CO₂ transportation in pipeline 140 bar/25 °C

Heat duty -156 kW

Figure 2.13: Analysis of the energy required by the CO2 capture process using Aqua

ammonia solution as solve

2.3.6. Application of the model to Chemical CO

absorption with solid dry

regenerable sorbent

Solid-based CO2 capture processes based on dry scrubbing use a solid instead of a

liquid sorbent. The solid sorbent should have high chemical reactivity and high attrition resistance48_{. The absorption must be a heterogeneous chemical reaction} occurring on the surface of the solid to form new chemical species such a carbonate or bicarbonate. The systems involving gas/solid interactions can also operate in a cyclic mode of alternating reaction-regeneration cycles in the same vessel.

The operation can occur in a thermo-gravimetric analyzer (TGA), in a circulated fluidized-bed transport reactors for steady state operation, or using multiple fixed-bed reactors, depending on gas velocity and reactivity sorbents. A wide range of temperatures for alternating absorption and regeneration reactions has been analyzed10_. The main advantages of solid absorption processes are listed next49_.

70

o The avoidance of liquid waste, namely, no energy is waste to heat or cool down the inert solvent.

o Solid sorbent may be disposed without undue environmental precautions50

o The by-products can be used as commercial products

For the model the carbonation and the regeneration take places in different unit, as shown in figure 2.14, where there is an absorption unit, carbonizer, and a regeneration unit, decarbonisation. Such a process requires the based sorbents to be recycled many times to reduce the sorbent make up flow. However, several studies on the reversibility of the carbonation and calcination reaction have shown that the recarbonation is far from reversible in practice51_{. Two different processes for dry regenerable sorbent are} analyzed, depending on its performed temperature rage.

C a rb o n iz e r D ec a rb o n is a ti o n Flue gas Water Q_steam CO2gas Separator cooler Treated Flue gas cooler XHCO3(s) XCO3(s)

71

2.3.6.1. Low temperature capture for dry regenerable sorbent in flue gases

The model employs alkali compounds for dry chemical reaction absorption of CO2 forming carbonate composes at low temperature, suitable for fuel gas capture. It is followed the same simplifications exposed in section 2.3.2., taking into account the specials characteristic of a solid sorbent, see table 2.4 for thermodynamic parameters. The carbonation reaction is as followed:

X2CO3 (s)+ CO2 (g)+H2O (g) ↔ 2XHCO3 with X = Li, Na, K, etc.

The figures 2.15 and 2.16 illustrate the model results of the K2CO3 and Na2CO3 dry solid sorbents for the CO2 absorption process, where the main thermodynamic parameters and energy consumptions are summarized.

Figure 2.15: Analysis of the energy required by the CO2 capture process using K2CO3

solid as sorbent

Treated flue gas 0.9511 kg/s 80°C / 1 bar

1 kg/s Flue gas 150°C / 1 bar

Capture process

Total Energy required = 3,6 MJ/kg CO₂ Flue gas Pre-treatment

3,3 Total capture energy:

3,2 Desorption energy;

-Steam supplied for stripper:

0,09 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required

3,3 Total capture energy:

3,2 Desorption energy;

-Steam supplied for stripper:

0,09 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required %molar

CO₂0,03 N₂ 0,75 O2 0,12

H2O 0,08

Sorbent capacity: 1 mol CO2/mol K2CO3

K₂CO₃ %molar N20,77 O₂0,14 H₂O 0,08 0.048 kg/s 100% CO2

Liquefaction plant for CO₂ transportation in pipeline 140 bar/25 °C Heat duty –88.8 kW CO2 ABSORPTION 80°C CO2 DESORPTION 100°C 340 kJ/kg CO₂ Heat duty –384 kW

72

Treated flue gas 0.9511 kg/s 60°C / 1 bar

1 kg/s Flue gas 150°C / 1 bar

Capture process

Total Energy required = 3,5 MJ/kg CO₂ Flue gas Pre-treatment

3,2 Total capture energy:

2,9 Desorption energy;

-Steam supplied for stripper:

0,28 Sensible heat:

MJ/kg CO₂ Scrubber /striper energy required

3,2 Total capture energy:

2,9 Desorption energy;

-Steam supplied for stripper:

0,28 Sensible heat:

MJ/kg CO₂ Scrubber /striper energy required

%molar CO₂0,03 N₂ 0,75 O₂ 0,12 H2O 0,08

Sorbent capacity: 1 mol CO2/mol Na2CO3

Na₂CO₃(s) %molar N₂0,77 O₂0,14 H₂O 0,08 0.048 kg/s 100% CO₂ Liquefaction plant for CO₂ transportation in pipeline 140 bar/25 °C Heat duty –114 kW 340 kJ/kg CO₂ Heat duty –384 kW CO₂ ABSORPTION 60°C CO₂ DESORPTION 100°C

Figure 2.16: Analysis of the energy required by the CO2 capture process using

Na2CO3 solid as sorbent

Sorbent Hr°[T]

Max capture temperature

Min regeneration

temperature Sorbent capacity

[°C] [kJ/kg CO2] [°C] [°C] [g CO2/g Sorbent]

Na2CO3 150 2920.6 60 100 0.42

K2CO3 100 3210.27 80 100 0.32

Table 2.4: Operating parameters for CO2 capture with Alkali compounds10,11,12,32, 52

Sodium and potassium carbonate aqueous solutions have a number of problems in practice. The solutions tend to react only relatively slowly with carbon dioxide and the heat requirements for regeneration of the solution is large compared to the various alkanolamine based processes20_.

2.3.6.2. High temperature capture by dry regenerable sorbent in flue gases

Different sorbents have been proposed to high temperature (>200°C) CO2 capture by chemical solid absorption. The high temperature capture is very suitable to emissions

73

capture in pre-combustion processes, where CO2 can be removed from fuel gas at the most convenient stage of the process. The analysis involves alkaline earth metals oxide reacting with CO2 to form alkali earth metal carbonate53,54:

XO + CO2 ↔ XCO3 with X= Mg, Ca, etc

The alkaline earth metal reactions, depending of the specific metal can occur over a wide and elevates rage of temperature (approximately 773-1173 K). With a high temperature materials this can be applied to a CO2 capture in process as IGCC, with a stream at elevate temperatures.

Also reactions involving alkaline metals to form metals carbonate, for example Li, Zr, Si, etc., can be used for high temperature absorption reactions. However, long-term stability of the adsorption capacity of Li4SiO4 in cyclic operation has not been verified55_.

Li2ZrO3 + CO2 ↔ Li2CO3 + ZrO2 Li4SiO4 + CO2 ↔ Li2CO3 +Li2 SiO3

CaO has more than twice the CO2 capture capacity than Li4SiO4 and also a stronger affinity for CO2 as indicated by the large heat of reaction, meaning that higher capture temperatures are flexible but also that higher regeneration temperatures are required. The minimum regeneration temperature is the temperature where the equilibrium CO2 pressure reaches 1 bar, so the atmospheric pressure regeneration is theoretically possible in pure CO256.

The solid sorbent shown at the table 2.5, together with its corresponding parameters has been used for the model. The figure 2.17, illustrates the model results of the CaO high temperature solid sorbents for the CO2 absorption process, where the main thermodynamic parameters and energy consumptions are summarized.

74 Sorbent Hr°[T]

Max capture temperature

Min regeneration

temperature Sorbent capacity [°C] [kJ/kg CO2] [°C] [°C] [g CO2/g Sorbent]

CaO 780 4587.38 700 900 0.79

Table 2.5: Operating parameters for CO2 capture with CaO solid sorbent53,54,32.

Treated fuel gas 0.9511 kg/s 500°C / 1 bar No heat duty 1 kg/s Fuel gas 500°C / 1 bar Capture process

Total Energy required = 6 MJ/kg CO2

ABSORPTION PROCESS 500°C REGENERATION PROCESS 900°C Flue gas Pre-treatment

5,5 Total capture energy:

4,5 Desorption energy;

-Steam supplied for stripper:

1 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required

5,5 Total capture energy:

4,5 Desorption energy;

-Steam supplied for stripper:

1 Sensible heat:

MJ/kg CO2

Scrubber /striper energy required %molar

CO20,03

N2 0,75

O2 0,12

H2O 0,08

Sorbent capacity: 1 mol CO2/mol CaO

CaO (s) _%molar N20,77 O20,14 H2O 0,08 0.048 kg/s 100% CO2

Liquefaction plant for CO₂ transportation in pipeline 140 bar/25 °C

340 kJ/kg CO2

Heat duty –384 kW

Figure 2.17: Analysis of the energy required by the CO2 capture process using CaO

solid sorbent at high temperature

2.4. Fossil fuel harmful pollutant control technologies

Since the early 1960s, there has been a growing worldwide awareness that energy production from fossil fuels as coal is accompanied by the release of potentially harmful pollutants as particulate matter, sulphur dioxide (SO2) and nitrogen oxides (NOx) into the environment. To address these environmental concerns, the emissions of these substances have been limited to admissible levels by national governments. In order to provide lowest cost energy with an acceptable impact on the environment, more efficient combustion technologies and more advanced flue gas cleaning systems have been developed. It has resulted in the increase of the net generating efficiency and

75

in the sharp diminution of the emissions of these pollutants per produced energy unit. The release of harmful compounds to the atmosphere has been lowered by using advanced flue gas cleaning technologies. It follows a list of the main pollutants with their respective cleaning technologies.

2.4.1. NO

Control technologies

NOx emissions have been identified as contributors to acid rain, ozone formation, visibly degradation and human health concerns. NOx refers to the cumulative emissions of nitric oxide (NO) and nitrogen dioxide (NO2) generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both air and fuel. NOx emissions from PC combustion are typically 90 to 95% NO and the rest is NO2.

Depending on the fuel used, the combustion conditions, the air ratio and the flame type in the burner, a considerable mass of nitrogen oxide might be produced during the combustion process. Three main mechanisms of NOx formation have been described: thermal, prompt and fuel NOx. The formation of thermal NOx was firstly described by Zeldovich in 1947 and refers to the NOx formed through high temperature oxidation of the nitrogen found in the combustion air. The formation of thermal NOx increases exponentially when flame temperature reaches about 1500ºC. Therefore, it is typically controlled by reducing the peak and average flame temperatures. This can be accomplished by utilizing primary measures (in-furnace) as mixing burners, staged combustion or by recirculating flue gas. The formation of prompt NOx has been found to occur in low-temperature flames. During the first part of the combustion, the carbon bearing radicals from the fuel react with nitrogen forming N radicals that further react with O2 forming larger amounts of NOx than one would predict from the Zeldovich thermal mechanisms.

The major source of NOx emissions from nitrogen containing fuels such as coal is however the conversion of the fuel-bound nitrogen to NOx during combustion. More than 80% of the total NOx emissions in PC units are formed via fuel-NOx mechanism.

76

The formation of fuel-NOx is strongly dependent on the fuel/air stoichiometry but it is relatively independent of variations in combustion zone temperature. Therefore, this conversion can be controlled by reducing oxygen availability during the initial stages of combustion. Primary NOx control techniques such as staged combustion and controlled fuel-air mixing provide a significant reduction in NOx emissions by controlling the stoichiometry in the initial devolatilization zone. Nevertheless, the above mentioned primary measurements can not reach NOx reductions over 50-60% and additional NOx reduction methods have to be utilized in the postcombustion zone in order to achieve the restrictive emissions legislation. The selective catalytic reduction (SCR) system provides the highest NOx removal efficiencies (70 to 90%) by using a catalyst and a reductant (ammonia gas, NH3) to dissociate NOx to nitrogen gas and water vapor. This technique is usually preferred to other NOx reduction systems in the post-combustion zone as the SNCR (selective non catalytic reduction) that can only reach 20 to 40% NOx reduction in large utility units.Table 2.6 summarizes the various options with their limits level.

Control techniques NOx reduction potential [%]

Overfired air (OFA) 20-30

Low NOx Burners (LNB) 35-55

LNB+OFA 40-60

Reburn 50-60

Selective non Catalitic reduction (SNCR) 30-60

Selective Catalitic reduction (SCR) 75-85

LNB with SCR 50-80

LNB with OFA and SCR 85-95

Table 2.6: Potential reduction of NOX control technologies

2.4.2. SO

reduction technologies

SO2 is a major constituent of acid rain and its inhalation by humans results in important health damages. Coal contains significant amount of sulphur that during combustion about 95% is converted to SO_{2. A number of technologies for SO}2 control are in use.

77

Commercialized processes include wet (natural and forced), semi-dry (slurry spray with drying) and completely dry processes. Of these, forced wet SO2 scrubber and natural wet SO2 scrubber have been the dominant technologies for the control of SO2 from PC utility power plants in Europe and US respectively. This process can reach 95 to 99% desulfuration, as illustrated in table 2.7. Post-combustion removal includes Wet and Dry Flue Gas Desulphurization (FGD and DFGD) or Spray-dry scrubbing. FGD is the current state-of-the art technology used for removing SO₂ from the exhausts in power plants.

Wet Flue Gas Desulphurization (FGD) utilizes a variety of slurry of sorbent materials to scrub the gases in order to accomplish SO₂ removal with efficiencies approaching 99%. These reagents include limestone (CaCO₃), lime (CaO), sodium (NaOH) and related variants to absorb and neutralize the SO₂ in the flue gas.

Control techniques SO2 Reduction potential [%]

Pre combustion removal Physical cleaning

Chemical and biological cleaning

30-50 90 Post-combustion removal

Wet Flue gas Desulphuration (FGD) 80-98

In situ capture

Dry Sorbent Injection (DSI) 50

Table 2.7: Potential reduction of SO2 control technologies

2.4.3. Particulate matter control technologies

Particulate matter (PM) inhalation has been associated with acute respiratory distress in humans57_{. When coal is burned in conventional boilers, a portion of the ash is} carried out of the furnace disperse in effluent gases. The coal-ash leaving the furnace along with the flue gas can range from 60 to 80% in PC units PM composition and emission levels are a complex function of coal properties, boiler firing configuration, operation and pollution control equipments. In the combustion of solid fuels, dust and ashes that are included in the exhaust gases as small particulate are produced. PMs

78

control is mainly possible with post-combustion methods, like electro-filters, cyclones and ceramic filter, with quite good results, see table 2.8.

The methods relying on gravitational or centrifugal forces are effective for removal of particles larger than 10 µm radius. Efficient collection of submicrometer particulate requires devices that depend on electrical force, impaction, interception, or inertial diffusion; these are the capture mechanism at work in electrostatic precipitators, fabric filters, and high-energy wet scrubbing58

.

Control Techniques Reduction potential

Electrostatic precipitator (ESP) 99% (for 0.1>d(µm)>10) 99% (for 0.1<d (µm)<10)

Filters (or baghouse) As high as 99.9%

Wet scrubber 95-99%

Cyclone 90-95% (d(µm)>10)

Table 2.8: Most used PM controls (mainly post-combustion methods)

2.5 Conclusions

The concept of CO2 capture in power generation and heating processes is still in many ways in a developing phase. The more commercial CO2 capture technologies are the chemical absorption analyzed in this chapter, which have been treated as an end-of-pipe device without considering further thermodynamic improvements. The results reached with the proposed model point out the energy consumption required to capture the CO2 emissions for the different CO2 chemical absorption technologies. Taking in to account that no energy required for transport or storage is considered in the model. The extra energy consumed for capturing the CO2 causes a significant reduction in the power plant energy efficiency, which claims for an agreement between the desirable amount of CO2 to be capture and the permitted energy penalty for an acceptable thermodynamic performance of the plant.

Anyway, the results obtained with the model alone do not give any information about the economic or environmental effects of the capture. Indeed, in chapter 6, a utility