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. . . . . . .

. . . . . . .el proce. . . . . . .s . . . . .stion og on ni. . . . . . .. . . . . . .. . . . . . .

7. Summary and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3958. Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396

* Corresponding author.

Contents lists available at ScienceDirect

.e lsevier .com/locate/pecs

Progress in Energy and Combustion Science 35 (2009) 385397E-mail address: fredrik.normann@chalmers.se (F. Normann).5.3. Low-NOx burner technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3915.4. Flue-gas recirculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3915.5. High-temperature NOx reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

6. Secondary measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3926.1. SCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3936.2. SNCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3946.3. Absorption of NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3946.4. Phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . .2. Methodology . . . . . . . . . . . . . . . . . . . .3. The oxy-fuel process . . . . . . . . . . . . .

3.1. Limits for NOx in the oxy-fu4. Nitrogen chemistry . . . . . . . . . . . . . .

4.1. NOx in combustion processe4.2. Influence of oxy-fuel combu4.3. Influence of CO2 conditionin

5. Primary measures . . . . . . . . . . . . . . .5.1. Fuel-staging . . . . . . . . . . . . . . .5.2. Air-staging . . . . . . . . . . . . . . . .0360-1285/$ see front matter 2009 Elsevier Ltd.doi:10.1016/j.pecs.2009.04.002. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386ss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388n nitrogen chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388trogen chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390Emission control of nitrogen oxides in the oxy-fuel process

Fredrik Normann*, Klas Andersson, Bo Leckner, Filip JohnssonDepartment of Energy and Environment, Chalmers University of Technology, SE412 96 Goteborg, Sweden

a r t i c l e i n f o

Article history:Received 29 December 2008Accepted 10 April 2009Available online 21 May 2009

Keywords:Oxy-fuelO2/CO2CO2 puricationNOxEmissionsBoiler

a b s t r a c t

The interest in oxy-combustion as a method to capture carbon dioxide has increased drastically duringrecent years. The oxy-fuel process offers new process conditions and may take advantage of innovativetechniques as well as of new ways to apply conventional measures for emission control. The presentwork reviews available techniques for controlling both the emission of nitrogen oxides (NOx) to theatmosphere and the content of NOx in the captured carbon dioxide. The results indicate that for a rstgeneration of oxy-fuel power plants, conventional primary NOx control should be sufcient to meettodays emission regulations, if based on emission per unit of fuel supplied. However, there are severalopportunities for new methods of NOx control in oxy-fuel plants, depending on future emission andstorage legislation for carbon capture schemes. Improved understanding of the behaviour of nitric oxideand nitrogen dioxide during compression and condensation of carbon dioxide is needed, as well asimproved knowledge on the inuence of the parameters of oxy-combustion on nitrogen chemistry.

2009 Elsevier Ltd. All rights reserved.journal homepage: wwwProgress in Energy and Combustion ScienceAll rights reserved.

1. Introduction

The emission of carbon dioxide (CO2), as a cause of globalwarming, is of current concern for the power industry. Forcentralized power generation systems, where most of the fossilfuels for electricity generation are consumed, capture and subse-quent storage of CO2 is under consideration with signicant effortsmade in research and construction of pilot and demo-scale plants.Oxy-fuel (O2/CO2) combustion is one of the technologies consideredas a promising and near-term option to capture CO2 from powerplants. In addition to reduction of CO2 emissions, an oxy-fuel powerplant offers new conditions for emission control of other pollutantssuch as NOx, which is the subject of the present investigation.

briey described (more information in Refs. [13]) and aspects ofthe process that are important for NOx control are introduced.Furthermore, the basic principles of nitrogen chemistry aresummarized (more information in Refs. [4,5]), and it is discussedhow these are affected by the oxy-fuel process. Finally, solutions forNOx reduction are presented and discussed.

The NOx-control technologies are divided into primary andsecondary measures. Primary measures are techniques appliedduring combustion (A in Fig. 1), whereas secondary measures areapplied downstream of the combustion process (B through E inFig. 1). The techniques are conceptually described and evaluatedqualitatively with respect to present development, requirement ofadditional equipment, critical design parameters, power

plants in Refs. [68]), and the possible impact of oxy-fuel

F. Normann et al. / Progress in Energy and Combustion Science 35 (2009) 385397386The design of oxy-fuel power plants may be divided into twogenerations. A rst generation uses technology that is proven in air-red power plants as far as possible in order to gain a rapid andsecure introduction of the oxy-fuel technology. In the rst gener-ation, the operational data of oxy-combustion are controlled toimitate air combustion, and conventional ue-gas cleaning equip-ment is used. In a second generation, the acceptance for innovativetechniques is higher and new parameters available for optimizationof oxy-fuel combustion can be utilized to nd new optimal oper-ational conditions. For example, the possibility to control theconcentration of oxygen (O2) in the oxidizer (not possible incombustion with air) could lead to lower gas ows through theboiler (other furnace dimensions) and higher ame temperatureswith inuence on ash behaviour and combustion efciency.Furthermore, the second generation of oxy-fuel power plants willbe designed when capture and storage of CO2 is an established partof the energy system and, thus, with legislation adapted for oxy-fuel conditions.

The aim of this review is to evaluate and summarize the tech-nology status of NOx-control systems for the oxy-fuel process,including aspects relevant to a second generation of oxy-fuel powerplants. The areas investigated are.

1 Applicability of conventional NOx reduction measures to theoxy-fuel process.

2 Chemistry and performance of new NOx reduction measures inoxy-fuel combustion.

3 Need for research on NOx control for second-generation oxy-fuel power plants.

2. Methodology

The main part of the work is based on a literature review, butsome theoretical evaluations are made on the behaviour of NOx incombustion systems. As a background, the oxy-fuel process isFig. 1. Principles of the oxy-fuel process with wet ue-gas recirculation. Letters indicate locaTable 1. FGC ue-gas condenser.combustion is evaluated. For innovative reduction measures (notfeasible in air-red plants), the performance and design areobtained from published simulations of the reduction measureinvestigated. It is difcult to compare the operational performanceof existing techniques with the performance of innovativeprocesses, for which only theoretical data are available. Neverthe-less, the published process calculations are well established andwith this background the possibilities of the new processes can beillustrated.

As a complement to literature experience, modelling of gas-phase nitrogen chemistry is carried out to illustrate the effect onthe reduction measures of oxy-fuel combustion. Gas-phase reac-tions are dominating nitrogen chemistry [9] and the modelling ofthese reactions gives a good approximation on the behaviour of thesystem with respect to NOx reduction. The reacting system ismodelled as a plug-ow reactor at atmospheric pressure and witha predened temperature prole. The reaction calculations areperformed with the SENKIN [10] code of the CHEMKIN-II [11]software, and the rate-of-production analysis is carried out withthe CHEMKIN-PRO [12] software. A detailed kinetic mechanismdeveloped for modelling of NOx reduction by staged combustion,reburning and selective non-catalytic reduction (SNCR) is used forthe calculations. More information on the kinetic reaction mecha-nism and on the associated thermodynamic library used are givenin Ref. [13] and references therein.

3. The oxy-fuel process

The oxy-fuel (O2/CO2) combustion process is outlined in Fig. 1and a rough overview of property values of the ue gas throughoutconsumption, additives, catalysts, and by-products.The reduction measures are evaluated by two approaches,

differing in their level of maturity. For conventional reductionmeasures, the performance and design of todays power plants areemployed (more information on NOx control in air-red powertions of possible emission control. Representative properties of the ue gas are given in

nd Cthis process is presented in Table 1. Pulverized coal is burned inoxygen (>95% purity)mixedwith recycled ue gas (RFG) in order tomoderate the ame temperature. Due to the absence of atmo-spheric nitrogen (N2), the ue gas mainly consists of carbon dioxideand water vapour (H2O). The ow of recycled ue gas controls theproperties in the combustion process, such as temperature, burneraerodynamics, and residence time. Particles are separated from theue gas before recycling, to limit enrichment in the furnace anderosion. Water is separated from the ue gas in a ue-gascondenser (FGC). Generally, it is suggested to extract the ue-gasfor recycle from a position upstream of the FGC, i.e. wet-recycle(shown in Fig. 1). The wet-recycle is thermodynamically favourablecompared to a dry recycle of ue-gas, which is extracted down-stream of the FGC (the temperature is determined by the temper-ature in the dust separator instead of the temperature in the FGC). Adry recycle would be required if there are any restrictions on thelevel of moisture or SO2 in the RFG. Before the CO2 in the ue-gas isdischarged from the power plant, it has to be condensed for cost-effective transportation and storage. The condensation takes placeat a pressure between 15 and 40 bar and a temperature between20 C and 55 C, depending on process conditions. Aftercondensation, the CO2 is compressed to supercritical conditions.

For typical design parameters (oxygen purity and combustionoperation), the exhaust ue gas consists of w95% CO2 on a drybasis. The remaining part is mainly excess oxygen from combus-tion, nitrogen and argon (Ar) from impurities in the oxygen, and, toless extent, pollutants, such as nitrogen oxides (NOx) and sulphuroxides (SOx) (w0.10.2%). The purity of the oxygen supplied isa question of energy consumption and investment in the air-separation unit. Besides purity of oxygen, the other importantsource of impurities in the CO2 is air ingress. For safety reasons,coal-red furnaces often operate at slight sub-atmospheric pres-sure and have to be made gas-tight to avoid air ingress. However,even for a gas-tight or slightly pressurized oxy-fuel boiler there arestill possibilities for air ingress, e.g., in the particle separation, in therecycle fan, and in the coal feeding system. Air ingress will decreasethe CO2 concentration below 95%. The requirement on the nal CO2purity is not clearly dened in carbon capture and storage schemes,and the need for emission control in the oxy-fuel process is,therefore, uncertain. The CO2 condensation separates most of theO2, N2 and the Ar, which are not condensed at the given conditionsbut vented to the atmosphere. Control of impurities (i.e. NO , SO

Table 1Rough overview of properties of the ue gas in an oxy-fuel power plant (see Fig. 1 forstream numbers).

Stream 1 2 3 4 5 6 7

Pressure (bar) 1 1 1 30 30 100 1Temperature (C) 300 300 20 20 30 20 20Mass Flow 1 1/3 1/3 1/3 1/4 1/4 1/25Volume Flow 1 1/3 1/7 1/200 1/900 1/1000 1/40Phase Gas Gas Gas Gas Liquid Fluida Gas

a Supercritical uid.

F. Normann et al. / Progress in Energy ax x

and mercury) is required for emissions into the environment andpotentially for CO2 storage. The impurities can be controlled invarious parts of the process (as indicated in Fig. 1).

A In the furnace.B In the ue-gas recycle loop, to limit the enrichment of impu-rities in the furnace.

C During the conditioning of the CO2, to limit the concentrationof impurities in the stored CO2 and/or in the ventilated gas.

D After the CO2 separation, to limit emissions in the ventilatedgas.

E After the CO2 separation, to limit impurities in the stored CO2.In oxy-fuel combustion the once-through residence time of gasin the furnace (sonce) must be distinguished from the effectiveresidence time (stot), which is the cumulative residence timedepending on the number of times the gas passes through thefurnace. For isomolar combustion, this can be calculated as the totalow through the furnace divided by the fresh inlet ow of gas(excluding the recycle), which is illustrated by the followingrelationships,

sonce VCFO2 FI FRFG(1)

stot FO2 FI FRFGFO2 FIsonce VCFO2 FI

(2)

where FO2 is the volumetric ow of oxygen, FI the ow of othergases introduced with the oxygen (mainly N2 and Ar), and FRFG theow of recirculated ue gas through the volume of the combustionchamber (VC). For oxy-fuel combustion, FI approaches zero (frombeing around four times FO2 in air combustion), and the totalresidence time is signicantly increased under otherwise similarconditions. The total residence time is most critical for the nitrogenchemistry, and the once-through residence time (or volume) hasa subordinate impact on NOx reduction and could, therefore, bereduced in the design of an oxy-red furnace. The combustionprocess depends, however, on the once-through residence time,which is also affected by the amount of RFG (or oxygen concen-tration), Eq. (1). Normally, in oxy-fuel combustion of the rstgeneration, FRFG is set to obtain similar temperature and heat-transfer conditions as in air combustion (30% O2 in the oxidizer),which results in similar furnace volumes for air and oxy-fuelcombustion.

3.1. Limits for NOx in the oxy-fuel process

The oxy-fuel process has two outlet streams; the emitted ventstream and the CO2-rich stream to be stored. The amount of NOxallowed in the two streams should probably be limited, eventhough there are no clear regulations at present. Such legislationcan be based on either environmental requirements or on bestavailable techniques (BAT) [14]. The BAT approach to emissionlegislation is based on an assessment of the available techniques foremission control. A denition of BAT is given in the EU directive forintegrated pollution prevention and control [15].

Emission of NOx to the atmosphere has been a major environ-mental concern since the beginning of the 1970s, and today thereare regulations on emission levels worldwide [14]. Emission limitsare usually dened as concentration limits (e.g. in Europe as mg/Nm3 ue gas [16]). In oxy-ring, nitrogen is excluded from thecombustion and the ue-gas volume (Nm3) is decreased witharound 80% compared to air ring. Furthermore, the main part ofthe ue gas is extracted for storage and only around 10% of thegenerated ue gas is emitted to the atmosphere, see Table 1. Thesame emission formation generates, thus, a higher concentration ofpollutants in oxy-fuel combustion, and emission limits based onconcentration in ue gas from air ring are not comparable to thoseof oxy-ring without adjustments. Standards based on emissionper unit energy produced (e.g. mg/MJ electricity), which encour-ages emission reduction by efciency improvements, are incorpo-rated in the USA. The energy penalty associated with O2 productionand CO2 compression and the efciency of the power processinuence the emission per unit of energy produced. With state-of-the-art efciencies, the emission must be reduced by around 20%per unit of energy supplied in order to reach same emission per

ombustion Science 35 (2009) 385397 387unit of energy produced in an oxy-fuel power plant as in a

corresponding air-red unit. The emission per unit of fuel supplied(e.g. mg/MJ fuel) is a similar unit, but it is not affected by the ef-ciency of the power plant. Therefore, if a comparison betweenprocesses is focused on the emissions only, the latter unit ispreferred.

The requirement for the purity of the gas to be stored is not anemission limit in the same sense as that applied to a vented stream.Storage of CO2 offshore was allowed in 2006 according to anamendment to the London Convention Protocol (on dumping ofwaste in the marine area) [17] and by a similar amendment to theOSPAR Convention (on the protection of the North-East Atlantic) in

process that may be of importance for nitrogen chemistry are listed

F. Normann et al. / Progress in Energy and C3882007 [18]. Both conventions state that the gas to be stored shouldconsist overwhelmingly of CO2. No quantitative limits are dened.The feasibility of co-storing NOx (and other pollutants) with the CO2depends on the understanding of reservoir mechanics andgeochemistry as well as the public acceptance for dumping of NOx.A review of on-going work on the determination of these limits isgiven by Bachu [19].

4. Nitrogen chemistry

Nitrogen oxides or NOx include nitric oxide (NO) and nitrogendioxide (NO2). NO is the most important nitrogenoxygen productof combustion, but it is further oxidized to NO2 at lower tempera-tures. Considerable formation of nitrous oxide (N2O) can take placeat lower combustion temperatures, as in uidized-bed combustion,but it is not included in the present work. Nitrogen chemistryduring air ring has been thoroughly studied in several reviews, forinstance Refs. [4,5]. As indicated above, the conditions are changedin the oxy-fuel process compared to an air-red power plant, bothwith regard to the combustion process and the ue-gas train. Theprincipal changes, inuencing the NOx control, are summarized inTable 2.

4.1. NOx in combustion processes

The nitrogen chemistry in combustion processes is summarizedin Fig. 2a. NOx are formed along three routes: thermal and promptformation from N2 and oxidation of fuel-bound nitrogen. ThermalNO evolves from the recombination of N2 and O2, described by theextended Zeldovich mechanism [20];

N2 O4NON (3)

N O24NO O (4)

N OH4NO H (5)

Reaction (3) has high activation energy, which controls thereaction rate of thermal-NOx formation; temperatures above1500 C are required to initiate the reaction. The Zeldovich

Table 2Principal changes in the oxy-fuel process compared to an air-red power plant withrespect to NOx control (FGR ue-gas recirculation).

Relevant to primary measures Relevant to secondary measures

Lowered concentration of N2 Increased pressureRecycling of ue gases (NOx) Decreased temperatureIncreased concentration of

combustion productsIncreased concentration of combustionproducts

Changed residence time(depending on FGR)

Reduced mass ow

Changed temperature conditions

(depending on FGR)in Table 2. The external recycle of ue gas in the oxy-fuel processaffects nitrogen chemistry, since it increases the concentration ofcombustion products and inuences the residence time and thetemperature prole in the furnace. In air-fuel combustion, N2 isdominant in the combustion gas, whereas in the oxy-fuel case withrecirculation, CO2 plays that role. The higher thermal capacity ofCO2 (the ratio of molar heat capacity for CO2 and N2 is around 1.6)tends to decrease the combustion temperature, which can becompensated for by higher oxygen concentration (reduced CO2mass ow). Besides the heat capacity difference, N2 is transparentto thermal radiation, and the gas emissivity is higher in an atmo-sphere with elevated concentrations of CO2 and H2O. The impor-tance of increased gas radiation in a coal-red ame is, however,limited because of the dominance of particle radiation [22].

Chemically, the replacement of N2 with CO2 is, obviously, ofimportance for the prompt and the thermal-NOx formation. Fig. 3presents the inuence of N2 concentration on the equilibriumconcentration of NO and on the reduction of 1000 ppm NO duringa residence time of 5 s at two oxygen stoichiometries. At lowtemperatures, the equilibrium concentration is always low and noformation of NO occurs. At higher temperatures, both the equilib-rium concentration and the reaction rate increases. When theconcentration of N2 and the stoichiometric oxygen/fuel ratio (l) arehigh, as during air ring, NO is formed, but when the concentrationof N2 and l are low, NO is reduced. The optimal temperature withrespect to NOx reduction depends on the combustion conditions,but it is higher in oxy-ring than in air ring. At temperaturesabove 1600 C, reduction of NO is strongly dependent on thenitrogen concentration (oxygen purity and air ingress). Thismechanism is reversible and reduces NO when the equilibriumconcentration of NO is low enough, given that the temperature issufciently high for the Zeldovich mechanism to be active. PromptNO [21] is initiated by the reaction between N2 and hydrocarbonradicals forming a volatile-N (an intermediate gaseous compound)species, which is oxidized to NO or reduced back to N2. Prompt NOis of minor importance and occurs predominantly under fuel-richconditions. The conversion of fuel-bound nitrogen contributes tomost of the NO formed during combustion of coal. Fuel-N is splitinto volatile- and char-N during the devolatilization. The volatile-Ntransforms into either NO or N2, while char-N reacts through a setof heterogeneous reactions along with the oxidation of char. Thesplit between volatile- and char-N varies with type of coal anddepends on qualities like nitrogen content, coal rank and volatility,but it also depends on combustion conditions, such as heating rateand temperature. The NO formed can also be reduced to N2. Similarto the prompt formation, NO can be reduced by hydrocarbons tovolatile-N under fuel-rich conditions (known as reburning).Furthermore, if present in sufcient quantities, char could beimportant as a catalyst for the reduction of NO, either directly, or byreaction with carbon monoxide (CO) or hydrogen (H2).

The volatile-N formed from fuel-N, N2 or from NO mostlyconsists of hydrogen cyanide (HCN) and ammonia (NH3). Thesespecies react in a series of reactions, but nally they end up as NHand N, as shown in Fig. 2b. The subsequent reactions, whichdetermine if NO or N2 is formed, are highly dependent on the O/OH/H radical pool, which is related to the ame conditions (e.g. stoi-chiometry, temperature and composition) [4].

4.2. Inuence of oxy-fuel combustion on nitrogen chemistry

The properties of the combustion process differ between air andoxy-combustion. The new conditions of the oxy-combustion

ombustion Science 35 (2009) 385397dependence is, however, lower at sub-stoichiometric conditions.

The elevated concentration of CO2 in oxy-fuel combustion alsoinuences the radical pool of the combustion process, primarilythrough Reaction(6), [23,24];

CO2 H4CO OH (6)Glarborg and Bentzen [25] investigated the effect of increased

CO2 concentration in a small-scale methane ame and detectedsignicantly increased concentrations of CO, see Fig. 4, caused byReaction (6) and, to some extent, by reactions of CO2 with hydro-carbons. They also concluded that Reaction (6) will raise theconcentration of hydroxyl radicals (OH), which could promote theformation of NO from volatile-N (cf. Fig. 2b). It has been suggested[26,27] that the concentration of CO in oxy-coal combustion couldalso be affected by the heterogeneous Boudouard reaction;

to the increased CO concentration in oxy-fuel combustion, it issmall compared to the contribution of the homogeneous reactions.

the compressor (Stream 4 in Fig. 1 and Table 1), the pressure is

Fig. 2. (a) Overall mechanisms of NO formation and reduction. (b) Reaction-path diagram, illustrating the decisive steps in volatile-N conversion to NO or N2 (shaded area in Fig. 2a)[4]. The dashed line indicates multiple reactions. Vol-N denotes intermediate gaseous compounds containing nitrogen, e.g., HCN or NH3.

F. Normann et al. / Progress in Energy and Combustion Science 35 (2009) 385397 389CO2 CS42CO (7)This was further investigated by Mackrory [28] who concluded

that, even though the contribution of Reaction (7) is of signicanceFig. 3. Inuence of N2 concentration on the NOx reduction potential as obtained bymodelling of gas-phase chemistry. i 0% N2, ii 1% N2, iii 10% N2. Full lines equilibrium concentration, dashed lines 5 s residence time. l stoichiometric ratio.Initial concentration of NO 1000 ppm.4.3. Inuence of CO2 conditioning on nitrogen chemistry

The purpose of the ue-gas train in an oxy-fuel power plant isdifferent from an air-red plant. In an oxy-fuel power plant, thefocus is on the conditioning of CO2 rather than on the treatment ofthe vented gas. Therefore, new conditions that may be of impor-tance for the nitrogen chemistry are created; see Table 2. Repre-sentative physical properties of the CO2 conditioning are presentedin Table 1. The most important effect is that the reduced mass ow(elimination of N2), high pressure and low temperature drasticallyreduce the volume ow throughout the ue-gas train. As a conse-quence, the later in the ue-gas train the NOx reduction is per-formed, the smaller is the ue-gas volume that has to be treated,and the size of the treatment equipment is reduced in a corre-sponding manner.

The elevated pressure and the low temperature required in theCO2 condenser also affect the chemistry of importance for NOxcontrol. The oxidation rate of NO by the O2 present in the ue gas,according to

2NO O242NO2 (8)is favoured by low temperature and high pressure [29]. This is

a well known reaction, describing NO2 formation in the industrialproduction of nitric acid (HNO3) by the Ostwald process [30] atpressures around 10 bar. Fig. 5 illustrates the oxidation rate of NO toNO2 in a ue-gas stream at representative pressures, calculatedwith the reaction scheme of Skreiberg et al. [13]. Downstream ofFig. 4. Experimental data on outlet CO concentration from a ow reactor for oxidationof CH4 under fuel-lean conditions with N2 (closed symbols) or CO2 (open symbols) asbulk gas. Source: Ref. [25].

introduced into the furnace to ensure burnout of the fuel. The goalis to convert the volatile-N to N2. In accordance with Fig. 2b, this isachieved by creating a sub-stoichiometric zone when introducingthe reburning fuel. Often, natural gas is used as reburning fuel, butany hydrocarbon fuel may sufce. Current development of fuelstaging under air-red conditions is focused on optimization of thecontrolling parameters (i.e. stoichiometry, temperature and resi-dence time), investigating fuels for reburning, and, developingadvanced reburning concepts with additives like ammonia toenhance NOx reduction [32,33].

Staged combustion has been experimentally investigated for

F. Normann et al. / Progress in Energy and Combustion Science 35 (2009) 385397390sufcient (around 30 bar) for a large part of the NO to be convertedto NO2 within the ue-gas train, which is not feasible in an air-redpower plant. NO2 can be controlled by other techniques than thoseavailable for NO, and new measures are enabled (further discussedin Chapter 6). A complete oxidation to NO2 is feasible only at veryhigh pressures and long residence time, which is a limitation of thetechnique.

5. Primary measures

Primary measures aim at adjusting the combustion parametersin order to reduce the formation of NOx from fuel-bound nitrogenand N2, or to reduce the NOx formed inside the furnace. Conven-tional primary measures for NOx control that could be applied inoxy-fuel combustion are illustrated in Fig. 6. These are: fuel staging(reburning), air-staging (staging), and low-NOx burner technology.In addition, the ue-gas recirculation in oxy-fuel combustion offersan opportunity for NOx reduction, and a technique using high-temperature oxy-fuel combustion to reduce NOx has been proposed[31].

Fig. 5. Inuence of pressure on the conversion of NO to NO2. Calculated by gas-phasechemistry applied to a plug-ow reactor, NOinitial 400 ppm and, O2,initial 3%,temperature 20 C.5.1. Fuel-staging

Fuel staging, or reburning, (Fig. 6a) is away to reduce the formedNOx to N2 by reaction with hydrocarbon radicals, according toFig. 2a. The reduction is carried out by introduction of fuel down-stream of the primary combustion zone. No additional oxidizer isadded in the reburning zone, and the ame is partly sub-stoichio-metric yielding hydrocarbon radicals, which reduce the NOxformed in the primary combustion zone to volatile-N species.Downstream of the reburning zone, additional oxidizer is

Fig. 6. Sketches of conventional primary measures for NOx control: (a) fuel-staging (reburnil is the stoichiometric ratio.pulverized-coal combustion in an O2/CO2 environment. Fig. 7shows results of a staging experiment by Maier et al. [35] per-formed in a once-through (O2/CO2 combustion without ue-gasrecirculation) 20 kW electrically heated test unit. The results showthat staged combustion is as effective for NOx reduction in oxy-fuelas in air combustion. Furthermore, Maier et al. detected increasedconcentrations of CO in the oxygen-lean zone during oxy-fuelconditions, which can be seen as a result of Reactions (6) and (7), asdiscussed in Section 4.2. Possible consequences are increasedcorrosion if the atmosphere at the furnace walls is reducing anda requirement to redesign the burners to counteract this issue. Liuet al. [36] investigated two levels where the secondary oxidantThe formation of OH radicals by Reaction (6) at elevatedconcentrations of CO2 could affect fuel staging negatively duringoxy-fuel combustion by promoting the formation of NO from thevolatile nitrogen species produced by the reburning mechanism[25]. It is also discussed by Park et al. [34] that elevated CO2concentration can suppress the reburning mechanism. Fuel staginghas not yet been applied in experimental NOx studies related tooxy-fuel ring. There is, however, some related work where thereburning mechanism has been studied for reduction of NOx in theprimary zone (further discussed in Section 5.4). Based on thesestudies, the reduction potential can be expected to be similar to thatduring air ring, despite the possible negative impacts mentioned.

5.2. Air-staging

In air-staged (or simply staged) combustion (Fig. 6b) the aim isto inhibit the formation of NOx by dividing the furnace into twozones. In the primary zone, only a part of the oxidizer is introducedwith the fuel to form an initial oxygen-lean combustion zone. Theremaining oxidizer is added as over-re air (over-re oxidizer) inthe secondary combustion zone to complete the fuel burnout. Byreducing the supply of oxygen in the primary zone, which is criticalto NOx formation, less NOx (and more N2) is formed from the fuel-bound nitrogen (see Fig. 2). The peak temperature of combustion isdecreased by the sub-stoichiometric conditions. The major draw-back of this procedure is the risk for low combustion efciency andcorrosion. The research on air-staging is mostly focused on devel-oping further understanding of the process for optimization ofcombustion parameters and ring strategies.ng), (b) air-staging in furnace (overre), and (c) air-staging in burner (low-NOx burner).

(over-re air) ports should work well also in oxy-fuel combustion.

oxy-fuel processes. In contrast, Becher et al. [38] and Hohenwarterand Giendl [39] have investigated burners operating with high O2concentration (up to 50% O2), where the combustion temperature iscontrolled by lowering the stoichiometric oxygen/fuel ratio. Theinuence on NOx formation during the employment of theseburner concepts was, however, not investigated. Tan et al. [40]studied the performance of burners for NOx reduction usinga combination of experiments and CFD modelling. They concludedthat burners should be specically designed for oxy-fuel combus-tion and take advantage of the pure O2 stream to minimize NOxformation.

5.4. Flue-gas recirculation

Recirculation of ue gases is used in air-red power plants tolower the emission of NOx by decreasing the concentration of O2 inthe combustion zone and to lower the combustion temperature tosuppress the formation of fuel-NOx as well as thermal-NOx. It wasconcluded by Andersson et al. [41], Mackrory [28] and Okazaki andAndo [42] that a more important effect of the ue-gas recirculationin oxy-fuel combustion is reduction of NOx that is reintroduced tothe ame. Nozaki et al. [43] came to a similar conclusion afterdetecting elevated concentrations of HCN and NH3 in the earlystages of the ame, which indicates that NOx is reduced via

nd Combustion Science 35 (2009) 385397 3915.3. Low-NOx burner technology

Low-NOx burners (Fig. 6c) are a well proven technology for NOxcould be introduced in a 20 kW test unit and came to similarconclusions: staging of the oxidizer is an efcient method forcontrolling emissions of NOx, and the conventional design of OFA

Fig. 7. Measured axial emission prole during staged combustion with 27% O2/CO2(once-through) and air combustion at a burner stoichiometry of 0.75, overall stoichi-ometry of 1.15 and a residence time in the reduction zone of 3 s. Filled symbols:unstaged combustion [35].

F. Normann et al. / Progress in Energy acontrol in pulverized-coal combustion. Staging of the oxidizer is thecentral technique applied in low-NOx burners. The combustionzones are created by dividing the oxidizer ow into secondary andtertiary streams surrounding the main combustion zone where thefuel is converted. New boilers in countries with strict NO-regula-tions all employ low-NOx burners. Low-NOx burners are easilycombined with air-staging, fuel staging, or secondary measures toachieve further reduction of NOx. The development of low-NOxburners is an on-going process with the following commonobjectives [37]:

- maximizing the rate of volatile evolution and total volatileyield from the fuel in the early reducing part of the ame,

- creating an initial oxygen-lean zone to minimize the conver-sion of fuel-N to NOx,

- optimizing the residence time and temperature in the reducingzone with respect to fuel-N to NOx conversion,

- maximizing the residence time of char in fuel-rich regions tominimize conversion of char-N to NOx, and,

- avoiding high-temperature peaks to minimize formation ofthermal NOx.

In connection with the design of oxy-fuel burners it has beencommonly suggested that the oxygen concentration in the recir-culation gas should be around 30% to achieve stable combustionwith conventional burners, see review by Buhre et al. [1]. Also,burners specically designed for oxy-fuel combustion have beenproposed: Toporov et al. [27] designed a burner to sustaincombustion at O2 concentrations below 21% for membrane-basedFig. 8. Experimental data on NOx emissions under air-fuel and oxy-fuel conditionsreburning reduction when recycled as illustrated by Fig. 2. Theeffect of the increased reduction is seen in several experimentalinvestigations (in test furnaces of a fuel power between 0.1 and2.5 MW) [40,41,4448] on the differences in NOx emissionsbetween oxy-fuel combustion with recycle and air combustion.Fig. 8 presents data from previous work, showing the maximumreduction in NOx emission when switching from air to oxy-fuelcombustion. The emission of NOx from oxy-fuel combustion relatedto unit of fuel supplied is typically reduced to around 30% of theemission during air ring, mainly due to the reduction by reburningin the fuel-rich regions generated in the ame during recirculationof ue gas. It should be noted that these experiments were per-formed in a temperature range where the inuence of the thermal(maximum reduction achieved). Full line: NOx emission during oxy-fuel combustion is34% of the emission during air ring. The dashed line denotes equal emission.

Fig. 9. Performance of high-temperature NOx reduction. Calculation of CH4O2/RFGcombustion in a plug-ow reactor with a mixture of HCN (60%) and NH3 to representfuel-N (0.6 wt% d.a.f.). The circles indicate the inlet concentration of NO and the arrowsa temperature drop of 100 C. Maximum temperature is 1500 C and 2000 C in thereference and high temperature cases, respectively. The stoichiometric ratio is 1.1,except during the rst 2 s in the staged combustion case when it is 0.8. Source:Ref. [31].

nd Combustion Science 35 (2009) 385397mechanism (Reaction (35)) is low for both NOx formation anddestruction.

The importance of ue-gas recycling is emphasized bycomparison with once-through oxy-fuel experiments [35,36,49](>0.1 MW), which do not show any substantial difference in theemission of NOx between oxy-fuel and air-red cases. Instead,once-through experiments show increasing conversion of fuel-N toNOx with increasing concentration of oxygen and combustiontemperature. In order to decrease the concentration of O2 in theoxidizer without reducing the combustion efciency, Liu andOkazaki [50] proposed an oxy-fuel combustion process with heatrecirculation. By heating the ue gas before recycling it to thefurnace, the concentration of O2 required in the oxidizer for stablecombustion was lowered from 30% to 15% and, thereby, theconversion of fuel-N to NO was lowered from 0.08 to 0.04 (it was0.3 for air combustion).

5.5. High-temperature NOx reduction

In a second-generation oxy-fuel power plant, new concepts ofcombustion chambers can be considered. In this context, Normannet al. [31] have proposed a combustion process that uses highcombustion temperatures to reduce nitrogen oxides in oxy-fuelcombustion, taking advantage of the reversed Zeldovich mecha-nism. The furnace is designed for an oxidizer with high concen-tration of oxygen (low ue-gas recirculation) and with burnerspromoting rapid mixing to achieve high combustion temperature;18002000 C is required. This design would achieve more intensecombustion and lower volume ows through the furnace, whichincrease combustion efciency and result in a more compact andcost-effective boiler. In the proposed process, the concentration ofnitrogen should be kept low in the combustion atmosphere (w1%)through high purity oxygen and low levels of air ingress to decreasethe equilibrium concentration of NO (cf. Fig. 3). Themajor feature ofthe technique is melting of the ashes, which has to be handled bythe furnace design. Boilers (air red) with wet ash drainage existand a similar design should be applicable. The high-temperatureNOx reduction is proven in principle by modelling, employingdetailed reactionmechanisms with NOx reduction efciencies up to95%, but experimental validation remains to be done.

Fig. 9 presents a comparison between NOx emissions in high-temperature combustion and conventional combustion (the refer-ence case, 1500 C) calculated with a detailed kinetic mechanism[31]. The inlet concentration of NO (marked with a circle in Fig. 9) iscalculated from the outlet concentration of the furnace adjusted forthe addition of oxygen to the RFG. The difference between the inletand maximum concentrations of NO arises from the conversion ofvolatile-N to NO, which is fast at the time scale of the diagram. Thediagram shows that the reduction during the high-temperaturecombustion is improved compared to the reduction at conventionaltemperature [31]. Staging of the oxidizer reduces the emissionfurther. Fig. 10 shows the major reaction paths obtained by a rate-of-production analysis, integrated for a time scale of 0.1 ms to 2.5 s.The calculations are based on the cases in Fig. 9. The rst 0.1 ms isleft out as it is dominated by the oxidation of fuel-N. It is shownthat the reaction route NO to N2 via N and NO to N2 dominatesthe reduction of NO, which is caused by the reverse Zeldovichmechanism (Reactions (13)), as opposed to air-red conditionswhere this mechanism forms NOx from air-borne nitrogen.

6. Secondary measures

Secondary measures aim at converting or capturing NOx in theue-gas downstream of the combustion process. Generally,

F. Normann et al. / Progress in Energy a392secondary measures have the disadvantage of requiring additionalFig. 10. Normalized net production of major species during the reduction of NO inFig. 9.

equipment and additives to control the emissions. On the otherhand, secondary measures do not interfere with the combustionprocess and usually perform better in terms of NOx reduction thanprimary measures. The two most common secondary measures areselective catalytic reduction (SCR) and SNCR, Fig. 11. The oxy-fuelprocess also has the possibility to remove NOx as NO2. NO2formation is favoured by high pressure, and the NO2 removalprocesses must be located in the high-pressure part of the ue-gastrain (Streams 46 in Fig. 1). In addition to SCR and SNCR, threeoptions to capture NO2 are presented, Fig. 12: absorption in water,

furnace. The low-dust SCR is placed outside the recycle loop andhas lower catalyst degradation compared to a high-dust option. Thelow-dust option is normally not used in air-red power plants dueto the high cost of installing a high-temperature dust separation.However, in oxy-fuel combustion ue-gas recycle at highertemperature gives an incentive to install a high-temperature dustseparator. The ue gas of an oxy-fuel power plant will have higherconcentration of sulphur dioxide (SO2), which could be oxidized tosulphur trioxide (SO3) in the SCR. To prevent SO3 formation, a ue-gas desulphurization unit (FGD) prior to the SCR might be required,which would necessitate a tail-end SCR. The tail-end option differsfrom the low-dust option in that it is placed downstream of an FGDwhere the ue gas is cooled and must be re-heated before enteringthe SCR, and this has a negative impact on plant efciency. In thetail-end (vent), option the SCR unit is placed downstream of theCO2 condenser and only a fraction of the initial volume is processed.In this case the NOx following the CO2 stream is not reduced. TheNO part of the NOx leaves the CO2 condenser with the vent gas,while NO2 stays with the CO2, and, consequently, it may be of

Fig. 12. Sketches of NO2 processes, located downstream of the rst compression inFig. 1. (a) Absorption in water, (b) co-storage with CO2, (c) distillation.

F. Normann et al. / Progress in Energy and Combustion Science 35 (2009) 385397 393co-storage with CO2, and distillation.

6.1. SCR

Selective catalytic reduction is based on reduction of NOx byammonia (or urea) to form nitrogen and water according to Reac-tion (9),

4NO 4NH3 O244N2 6H2O (9)The reduction occurs over a catalyst, usually at 300400 C, but

the temperature window can be expanded depending on thecatalyst used, see [51]. The ammonia is injected into the ue-gasupstream of the catalyst. Catalyst technology is important for theprocess, and much of the on-going research is focused onimproving the catalyst [51]. There is, however, no work presentedon the inuence on the catalyst activity from the changed ue-gascomposition during oxy-fuel combustion. The selectivity of theprocess is high, allowing for an NH3NOx stoichiometry near unity.Even distribution of the NH3 and efcient reactions are importantto avoid ammonia slip (unreacted ammonia in ue-gas after theNOx reduction unit) with possible downstream effects, such asbuild-up of ammonium salts on heat-transfer surfaces or emissionof ammonium chloride and gaseous ammonia.

The ue-gas train of an oxy-fuel power plant offers newconditions to consider when deciding where to locate the SCR unit.The cost of an SCR unit depends on the ue-gas volume [52], anda location in the latter part of the oxy-fuel ue-gas train is, there-fore, benecial, as seen by the ue-gas conditions in Table 1: thetreated volume decreases throughout the ue-gas train and issmaller than in an air-red power plant. The reduction of NO byReaction (9) is not thermodynamically favoured by an increasedpressure and, the SCR unit should not be placed in the pressurizedpart of the ue-gas train. Fig. 11 shows the four basic locations forthe SCR reactor in the ue-gas treatment train: High-dust, low-dust, tail-end and tail-end vent. The high-dust option is the mostcommon technique in air-red power plants. In the oxy-fuelprocess, a high-dust unit would be placed in the recycle loop wherethe ue-gas ow is larger but the total residence time (stot) isincreased by the recycle (cf. Eq. (2)), which decreases the requiredvolume of the SCR unit. The gas recycle will also have the conse-quence that the N2 formed from the ammonia added and from theNOx reduced may be converted to NOx when returned to theFig. 11. Positions for SNCR and SCR (high-dust, low-dust and, tail-end) in the oxy-fuel process. FGD ue-gas desulphurization.

performance is often found at a molar ratio between 1.5 and 2.5.The SNCR system is more difcult to control during load variationthan the SCR, and the risk for ammonia slip is higher. The slip ofammonia is mainly caused by insufcient mixing or if applied withinappropriate temperature or concentration proles in the ue gas.

It is known that the NH3NOx reaction is sensitive to changes inthe radical pool, and the effect of the ue-gas composition andadditives has been thoroughly investigated; see review by Javedet al. [53]. For example, water slightly increases the optimumtemperature [54], whereas presence of hydrogen could reduce theoptimum to as low as 700 C [55]. The inuence of oxy-fuelcombustion on the NH3NOx reaction mechanism has not been

6.3. Absorption of NOx

F. Normann et al. / Progress in Energy and Combustion Science 35 (2009) 385397394importance to control the oxidation of NO (see Fig. 5). The tail-end(vent) option also requires re-heating of the ue gas to attain thedesired temperature for reaction, but the ue-gas ow is small andheating to some extent is probably required anyway before thestack.

6.2. SNCR

Selective non-catalytic reduction operates by the same overall

Fig. 13. Comparison between NO reduction by SNCR during air and oxy-fuel condi-tions. Calculated by gas-phase chemistry applied to a plug-ow reactor, residence time0.1 s, NOinitial 400 ppm, O2,initial 3% and, NH3:NO 2:1. Based on modelling of gas-phasechemistry.reaction as SCR (Reaction (9)), but the SNCR arrangement does notinclude a catalyst. Instead, the temperature must be higher for thereactions to be active (>800 C). At too high temperatures(>1100 C), the ammonia tends to oxidize to form NOx instead ofreducing it. Thus, the reduction takes place in the temperaturewindow between 800 and 1100 C, which is normally foundsomewhere along the gas path of a pulverized coal-red boiler. TheSNCR principle is, therefore, simply applied by injection and thor-ough mixing of the reducing agent (ammonia) in the ue-gasstream within this temperature window. The SNCR is not asselective towards reduction of NOx as the SCR system, and the NH3to NOx molar ratio should be above unity. The efciency of thereduction tends to increase with the NH3 to NOx ratio up toa certain level depending on operational condition: the optimum

Fig. 14. Mechanism for absorpAbsorption of NOx in water has been thoroughly studied due toits importance for industrial production of nitric acid; see [30] andreferences therein. The mechanism of NOx absorption presented byCounce and Perona [56] is shown in Fig.14. NO is oxidized in the gasphase before it is absorbed and nally bound in the liquid as HNO3.The oxidation of NO to NO2 is the critical step for the absorption. Asdiscussed in connection with Fig. 5, the low temperature and highpressure in the CO2 condensation step of the oxy-fuel processfavour NO2 formation. Furthermore, these conditions increase theconcentration of nitrogen tetroxide (N2O4) at the expense of NO2,which is known to enhance the absorption [57]. The initialconcentration of NOx in the gas and the concentration of HNO3 inthe absorbing liquid are important design parameters. The acidstrength is of great importance for the equilibrium between the gasand the liquid phase, and thus for the outlet concentration(achievable reduction) of NOx, while the concentration of NOxmainly inuences the absorption rate. The concentration of NOx ininvestigated. However, as previously mentioned, an increased CO2concentrationwill inuence the radical pool and this will affect thetemperature window. To illustrate the effect of CO2, the reductionof NO by SNCR is calculated for both air and oxy-fuel conditionswith the detailed kinetic mechanism from Ref. [31]. In the lattercase, N2 in the ue gas simply is replaced by CO2. The result isshown in Fig. 13. The efciency of SNCR is similar in oxy-fuel and aircombustion, but the optimum temperature is shifted to highertemperatures and the temperature window is increased.

An SNCR arrangement must be placed inside the recycle loop tond the required temperature of the process. Similar to the casewith a high-dust SCR unit, the total residence time is increased bythe recycle of ue gas (cf. Eq. (2)), and a large part of the N2 formedfrom both NOx and NH3 will be reintroduced to the ame witha chance of reforming NOx, but in SNCR even more NH3 is addeddue the lower selectivity. The inuence of this excess of NH3 on theemission depends on the combustion conditions and on theperformance of the SNCR in reducing NOx.tion of NOx in water [56].

the ue gas is lower than in an acid production plant, but therequirement on concentration of HNO3 is not necessarily high forue-gas cleaning. Absorption in water has been proposed forsecondary NOx control in air-red power plants [58]. Therequirement of a strong oxidizer (e.g. ozone or hydrogen peroxide)or an efcient catalyst to oxidize NO to NO2 at atmosphericpressure has, however, prevented these processes from commer-cial breakthrough.

Absorption of NOx in the oxy-fuel process (Fig. 12a) has beenstudied by Allam et al. [59], White et al. [60,61], and Kuhnemuthet al. [62,63]. They conclude that absorption of NOx in water ataround 30 bar and 30 C is a feasible technology, achieving reduc-tion efciencies of up to 90%. Fig. 15 shows the absorption of NOxfrom an oxy-fuel ue-gas [63]. The oxidation of NO to NO2 isstrongly dependent on pressure and this is a condition for theabsorption of NOx (cf. Fig. 5). The absorption is limited by equilib-rium, and in the calculated case the reduction limit is just below200 ppm (at 30 bar). The performance of the downstream CO2condenser is negatively affected by pressures above 30 bar where

the case of simultaneous SO2 removal, NO2 is required in relatively

F. Normann et al. / Progress in Energy and Chigh concentration, and, therefore, the formation of NOx in thepreceding system must be sufcient [63]. Initial small-scaleexperiments have been performed on the SO2 absorption withpromising results [61]. However, the technique cannot be consid-ered mature, as can the absorption of NO2.

6.4. Phase separation

Phase-separation is a common industrial method for separatingthe components of a mixture. The separation is based on thedifference in composition between the liquid and the gas phase ofthe mixture. In practice, there are two principal methods: single-stage partial vaporization without reux (ash distillation), andmulti-stage distillationwith reux (distillation). Flash distillation isused for separating components that boil at widely different

the purity of the nal CO2 decreases [64], and it is, therefore, notpractical to further increase the pressure solely for the NOxabsorption.

Common for the processes investigated is that they includesimultaneous removal of SO2 based on the Lead-chamber process[65] for sulphuric acid (H2SO4) production. In this process, the NO2formedworks as a catalyst for the rapid oxidation of SO2 to SO3. SO3is highly soluble in water and is transformed to sulphuric acid ina separate absorption column upstream of the absorption of NOx. InFig. 15. Absorption of NOx from an oxy-fuel ue-gas at different pressures (20 C).Source: Ref. [63].temperatures, while the more rigorous distillation column isneeded for separating components of comparable volatility. In theoxy-fuel process, phase separation is performed in connectionwiththe CO2 condenser to separate gaseous O2, N2 and Ar. The boilingpoint of NO2 (Tb,30bar z100 C) is higher than the boiling point ofCO2 (Tb,30bar z0 C) and NO2 is, thus, condensed in the CO2condenser. In contrast, NO has a lower boiling point than CO2(Tb,30barz100 C) and is emitted with the vented gases from theCO2 condenser. An option to control the atmospheric emission ofNOx, which is not available for air-red power plants, is to co-storethe NO2 with the captured CO2 (Fig. 12b). To avoid emission of NO itis important that the residence time of the ue-gas at elevatedpressure is sufcient for the oxidation of NO to NO2 to go tocompletion (see Fig. 5). This option is achieved without extraequipment as the CO2 condenser is required anyway, but the CO2 tobe stored is polluted by NO2, which might not be acceptable.

To prevent pollution of the CO2 stream, Allam et al. [66,67] havesuggested a process with a distillation column operating at a highertemperature downstream of the CO2 condenser (Fig. 12c) to sepa-rate NO2 from the CO2 stream. In this column, NO2 remains in theliquid phase while CO2 is separated as a gas. The CO2 is thencondensed again and compressed to the transportation pressure.This process has also been studied by Kuhnemuth et al. [63] andGonshorek [68], of which the latter indicates on-going constructionof a pilot plant. Common for all studies is that also the CO2condenser consists of a distillation column, instead of a ash tank.The separation of CO2 and NO2 is efcient and a high reduction ofthe NOx emission is achieved (up to 97%) [63], but compressionwork in addition to that needed for the condensation of CO2 isrequired. SO2 has a boiling point in between CO2 and NO2 (Tb,30barz100 C) and can be separated with the NO2 in the second columnat the cost of even further increased compression work.

7. Summary and discussion

The performance of the NOx-control measures reviewed in thepresent work is summarized in Table 3 as independent processes(combinations are not considered). Overall, the conditions forcontrolling NOx emission in the oxy-fuel process are better and thenumber of parameters available for process optimization is higherthan in an air-red power plant, both in terms of primary andsecondary measures. Primary measures have proven a reduction ofaround 65% of the emission (per unit of fuel supplied) during oxy-combustion compared to air-combustion, utilizing technologydesigned for air ring (see Fig. 8). High-temperature NOx reductionis an example of how the combustion and emission levels could befurther improved if the features of oxy-fuel combustion are opti-mized. Secondary measures normally require additional equip-ment, but the volume ow of ue gas throughout the oxy-fuelprocess is heavily decreased compared to an air-red power plant(see Table 1), which directly reduces the investment cost of theadditional equipment. Furthermore, in the high-pressure part ofthe ue-gas train, the oxidation rate of NO to NO2 increases anda new set of secondary measures applicable to NO2 is enabled.

Primary and secondary measures can be used separately or inseries. For a rst generation of oxy-fuel power plants, conventionalprimary measures should be enough to achieve the present emis-sion limits if based on unit of fuel supplied. The secondarymeasures are favoured by a combination with primary reduction ofNOx, with one exception; the lead-chamber process requires NOx tocapture SO2. Even co-storage of NOx with CO2, which is basicallyfree of cost, benets from primary measures in terms of improvedpublic acceptance and less need for safety measures during trans-port and storage. Based on this, it is probable that the combustion

ombustion Science 35 (2009) 385397 395process will be designed for NOx reduction also in a second

el prbin

Primary Measures

nd CReburning Conventional technology

Staging Conventional technology

Low-NOx burner Conventional technology

Flue-gas recirculation Included in the oxy-fuel process

High temperature Improved combustion efciencySmall and compact furnace

Secondary MeasuresSCR Conventional technology

SNCR Conventional technology

Absorption Simultaneous removal of SOxPlaced in high-pressure part

Co-storage Included in the oxy-fuel process

Distillation Simultaneous removal of SOxPlaced in high-pressure part

a Based on practical experience under air-red conditions [52].b Based on experiments under oxy-fuel conditions (Fig. 8).Table 3Summary of the performance of measures for NOx control when applied to the oxy-fumultiplication of measures. The achievable reduction for High temperature is in com

Advantages

F. Normann et al. / Progress in Energy a396generation of the oxy-fuel process. However, the optimization maybe different from that of present pulverized fuel plants and couldfocus more on combustion efciency. The constraints on fuel-oxidizer mixing to lower the combustion temperature in order tocontrol thermal-NOx formation are not as important. With low N2concentration in the furnace, high combustion temperatures couldeven be promoted to reduce NOx by the thermal mechanism.Furthermore, if any of the secondary measures prove to be efcientand cost-effective for NOx control in the oxy-fuel process, thedemand on primary reduction could be lowered, i.e., during thedesign of the combustion process less consideration is required onNOx emission.

As the design of oxy-fuel power plants matures, secondary NOxcontrol in connectionwith the compression and the CO2 condenserare of interest, due to the benet from the lower volume ow. Thetail-end (vent) SCR unit is a favourable option to control NOx in thevent stream; the concentration of NO is high and the treated ow issmall. If required, distillation and absorption of NO2 are promisingtechniques to control NOx in the CO2 stream, although both havethe disadvantage of creating a by-product that must be disposed.The most advantageous alternative to control emissions of NOx inthe oxy-fuel process is co-storage with the captured CO2. It is,however, uncertain to what extent co-storage of NO2 will beallowed, although the concentration in the storage gas is small. Thismay also depend on the type of storage employed.

8. Future work

The present study has demonstrated several methods to reduceNOx emission in the oxy-fuel process. However, an economicalevaluation or a system study to nd out the optimum organisation

c Based on modelling of the oxy-fuel process [31,63].ocess. The achievable reduction is for the measure concerned and not for addition oration with staging as described in Section 5.5.

Disadvantages/tendencies Achievable reduction

(Natural gas consumption) 60%a

High-temperature corrosion

Reduced combustion efciency 40%a

High-temperature corrosion

Reduced combustion efciency 60%a

High-temperature corrosion

65%b

Melting of ashes 90%c

Requires low N2 concentrationHigh-temperature corrosion

Requires catalysts 95%a

Ammonia consumptionExtra unitsAmmonia slip

Ammonia consumption 50%a

Ammonia slip

Extra units 90%c

Waste (weak nitric acid)

Pollution of the CO2 95%c

Power consumption 95%c

Extra unitsWaste (liquid NOx)

ombustion Science 35 (2009) 385397of NOx capture still remains to be done. The survey has lead to theconclusion that more knowledge is required in three key areas tooptimize NOx control in future oxy-fuel power plants.

- Legislation. NOx control is driven by emission legislation.Regulations adapted to CO2 removal processes are, therefore,required both with respect to emissions to the atmosphere andfor the storage gas in order to nd appropriate design solutionsfor the oxy-fuelCO2 capture system.

- Oxidation of NO to NO2. The differences in behaviour of NO andNO2 makes it essential to further investigate the oxidation ofNO under the conditions of the CO2 conditioning. The perfor-mance of the CO2 condenser in separating NO and NO2 is alsocrucial.

- The combustion process. NOx control has constrained the designof air combustion for a long time. It is, therefore, of importancenot to imitate air combustion in future developments but toutilize the opportunities given by oxy-fuel combustion to ndthe optimal performance, both with respect to emissions andcombustion efciency.

Acknowledgements

This work is nanced by Vattenfall AB. Our colleague, Mr DanielKuhnemuth, is acknowledged for his valuable input on the removalof NO2.

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Emission control of nitrogen oxides in the oxy-fuel processIntroductionMethodologyThe oxy-fuel processLimits for NOx in the oxy-fuel process

Nitrogen chemistryNOx in combustion processesInfluence of oxy-fuel combustion on nitrogen chemistryInfluence of CO2 conditioning on nitrogen chemistry

Primary measuresFuel-stagingAir-stagingLow-NOx burner technologyFlue-gas recirculationHigh-temperature NOx reduction

Secondary measuresSCRSNCRAbsorption of NOxPhase separation

Summary and discussionFuture workAcknowledgementsReferences