NOx formation and selective non-catalytic reduction (SNCR) in a fluidized bed combustor of biomass
NO x formation and selective non-catalytic reduction (SNCR)in a ?uidized bed combustor of biomass
Shiva Mahmoudi *,Jan Baeyens,Jonathan P.K.Seville
University of Warwick,School of Engineering,Coventry,CV47AL,UK
a r t i c l e i n f o
Article history:
Received 30January 2010Received in revised form 13March 2010
Accepted 16April 2010Available online 15May 2010Keywords:
Biomass combustion Flue gas treatment NO x removal
Selective non-catalytic reduction
a b s t r a c t
Caledonian Paper (CaPa)is a major paper mill,located in Ayr,Scotland.For its steam supply,it previously relied on the use of a Circulating Fluidized Bed Combustor (CFBC)of 58MW th ,burning coal,wood bark and wastewater treatment sludge.
It currently uses a bubbling ?uidized bed combustor (BFBC)of 102MW th to generate steam at 99bar,superheated to 465 C.The boiler is followed by steam turbines and a 15kg/s steam circuit into the mill.Whereas previously coal,wood bark and wastewater treatment sludge were used as fuel,currently only plantation wood (mainly spruce),demolition wood,wood bark and sludge are used.
Since these biosolids contain nitrogen,fuel NO x is formed at the combustion temperature of 850e 900 C.NO x emissions (NO tNO 2)vary on average between 300and 600mg/Nm 3(dry gas).The current emission standard is 350mg/Nm 3but will be reduced in the future to a maximum of 233mg/Nm 3for stand-alone biomass combustors of capacity between 50and 300MW th according to the EU LCP standards.NO x abatement is therefore necessary.In the present paper we ?rstly review the NO x formation mechanisms,proving that for applications of ?uidized bed combustion,fuel NO x is the main consideration,and the contribution of thermal NO x to the emissions insigni?cant.
We then assess the deNO x techniques presented in the literature,with an updated review and special focus upon the techniques that are applicable at CaPa.From these techniques,Selective Non-catalytic Reduction (SNCR)using ammonia or urea emerges as the most appropriate NO x abatement solution.
Although SNCR deNO x is a selective reduction,the reactions of NO x reduction by NH 3in the presence of oxygen,and the oxidation of NH 3proceed competitively.
Both reactions were therefore studied in a lab-scale reactor and the results were trans-formed into design equations starting from the respective reaction kinetics.An overall deNO x yield can then be predicted for any operating temperature and NH 3/NO x ratio.We then present data from large-scale SNCR-experiments at the CFBC of CaPa and compare results with the lab-scale model predictions,leading to recommendations for design and operation.Finally the economic impact is assessed of implementing SNCR-technology when applying an NH 3SNCR or urea SNCR to the CFBC at CaPa.
a2010Elsevier Ltd.All rights reserved.
ABBREVIATIONS:BFBC,Bubbling Fluidized Bed Combustor;CaPa,Caledonian Paper,(Ayr,Scotland);CFB(C),Circulating Fluidized Bed (Combustor);deNO x ,NO x Removal;FBC,Fluidized Bed Combustor;FBN,Fuel-Bound Nitrogen;FGR,Flue Gas Recirculation;I.D.,Internal Diameter (m);NSCR,Non-selective Catalytic Reduction;SCR,Selective Catalytic Reduction;SDF,Solid-Derived Fuel;SNCR,Selective Non-catalytic Reduction.
*Corresponding author.Tel.:t442476522122;fax:t442476418922.E-mail address:S.mahmoudi@https://www.sodocs.net/doc/4114434683.html, (S.
Mahmoudi).
A v a i l a b l e a t w w w.s c i e n c e d i r e c t.c o m
h t t p ://w w w.e l s e v i e r.c o m /l o c a t e /b i o m b i o e
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
0961-9534/$e see front matter a2010Elsevier Ltd.All rights reserved.doi:10.1016/j.biombioe.2010.04.013
1.
Introduction
1.1.
Background
Renewable energy is an important imperative and use of biomass appears to be the only carbon-based sustainable route,with a high potential in the immediate future [1e 3].World energy supplies have been dominated by fossil fuels for decades,representing approximately 80%of the world demand of over 400EJ/year.Biomass accounts for about 14%of the world’s primary energy demand,despite much being wasted through inef?cient use and unsustainable exploitation [3].Biomass is versatile and widely available,but is a complex and dif?cult fuel.In particular,the presence of alkali,chlorine and ash to a large extent,and nitrogen and sulphur in some types of biomass,are major sources of concern in its thermal application.Whereas the former group of contaminants is linked to corrosion (Cl),erosion (ash),dust formation (ash)and agglomeration (eutectic compounds with low melting points mostly due to the presence of alkali and earth-alkali),the latter is the origin of emissions of NO x and SO x ,both recog-nised as sources of acid pollution.
Among the technologies used for biomass combustion,?uidized beds (bubbling and circulating)have been proven to be most appropriate,due to their excellent heat and mass
transfer,and their high ef?ciency and ?exibility as far as the versatility of the biomass is concerned.
In sourcing biomass,it is clearly important to avoid deforestation,and not to compete with https://www.sodocs.net/doc/4114434683.html,mon sour-ces include crops speci?cally grown for energy and waste biomass such as saw-dust,sewage sludge,refuse-derived biosolids,wood bark,husks,wood trimmings,etc.
Biomass is composed primarily of ?brous cellulose held together with the polyphenolic natural glue,lignin (15e 30%of the wood constituent).Cellulose has the formula (C 6H 12O 5)n and is a polymer of glucose,C 6H 12O 6.Its approximate percentile formula is C 29H 48O 24.A typical hardwood can be represented as C 32H 45O 22,being quite similar to cellulose.All dry biomass species have energy contents in the range of 16e 20MJ/kg.
Typical compositions of such biomass are given in various sources and are illustrated in Table 1.Although the contents of N and S are rather low,the formation of NO x and SO 2during combustion cannot be avoided,making deNO x and deSO x techniques necessary.The maximum concentration of NO x and SO 2can be estimated from the concentration of N and S in the biomass when considering that the stoichiometric air required for combustion is about 4.0e 4.5Nm 3/kg of dry biomass (db).With 20%excess air,the average requirements are about 5Nm 3/kg db.
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Using the N and S contents for sawdust from Table1,the combustion of1kg of dry sawdust with5Nm3/kg db of air will produce maximum emissions of NO x(as NO2)and SO x(as SO2) around4070and1430mg/Nm3,respectively.Similarly,when burning1kg of dry sewage sludge with4Nm3/kg db of air,the potential emission of NO x will be10times higher,SO2reach-ing6g/Nm3.It is demonstrated below that a large fraction of N and S,about70%,remains captured within the char and/or ash.The Dutch emission guidelines applied for burning biomass fuels are given as an example in Table2.
Clearly deSO x and deNO x measures are necessary to meet these stringent standards when burning biomass fuels.The present paper focuses upon NO x only.
1.2.Objectives of the paper
CFB-combustion emits signi?cant quantities of nitrogen oxides(NO x),collectively representing NO and NO2.At atmo-spheric pressure and25 C,NO is an odourless and colourless gas and NO2is a pungent reddish-brown gas.NO x produced from combustion processes is typically made up of95%NO and5%NO2[7].
NO x is predominantly formed during combustion although a few industries do emit it from process operations,such as nitric acid production[8].If the NO x emissions of the CFBC are to be reduced,it is important to understand fully the mecha-nisms by which NO x forms.
2.Description of NO x formation mechanisms
Three reaction pathways cause the formation of NO x in combustion processes:fuel NO x,formed from the oxidation of fuel-bound nitrogen;thermal NO x from the reaction of atmospheric nitrogen and oxygen at high temperatures;and prompt NO x formed from the reaction between atmospheric nitrogen and fuel-derived hydrocarbon fragments[9,10].
2.1.Thermal NO x
Thermal NO x is formed by a reaction mechanism involving oxygen and nitrogen radicals.The formation rate is mainly a function of temperature and contact(residence)time.Due to the high temperatures required to break the triple bond in the nitrogen molecule,thermal NO x is only formed in signi?cant quantities at temperatures above1800K(see Section3)[11].The reaction mechanism established by Zel-dovich[12]assumes that O*-radicals attack N2molecules,and that N*-radicals subsequently form NO with O2.The amount of NO produced is affected by the amounts of N2and O2
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present in the combustion environment,along with the temperature of combustion.
N2tO*4NOtN*(Rate controlling reaction due to high E A) (R1)
N*tO24NOtO*(R2)
N2tO242NO(Overall representation)(R3)
The formation reactions of thermal NO x are characterized by their high activation energy[13].The NO formation rate is presented in Section3below.
2.2.Fuel NO x
The oxidation of fuel-bound nitrogen is a major source of NO x emissions generated when burning nitrogen-bearing fuel. Table1presents the concentrations of N in various biomass species and in bituminous coal and illustrates that several potential biomass species can be considered as fuels with a high fuel-bound nitrogen(FBN)content.The general form of fuel nitrogen consists of nitrogen atoms bonded to carbon or to other atoms.These bonds break more easily than the diatomic N2bonds,and fuel NO x formation rates are much higher than thermal NO x formation rates.Fuel NO x is much more sensitive to stoichiometric conditions.For this reason a preventative thermal treatment such as?ue gas recircula-tion does not effectively reduce NO x emissions generated from fuel-bonded nitrogen[14].The formation of fuel NO x can be described as:
C(N)/I(N)(R4) I(N)tO(or O2,OH)/NOt.(R5)
where C(N)represents nitrogen in the char,and I(N)repre-sents nitrogen-containing intermediate species such as CN, HCN,NH and NH2.Under the reducing conditions around the burning particle,FBN converts to intermediate nitrogen species which are readily oxidised to form NO.Typically 20e40%of FBN is converted to NO x in combustion processes [15].This will be described in detail in Section4.
2.3.Prompt NO x
Prompt NO x is formed by the reaction of fuel-derived hydro-carbon radicals with atmospheric nitrogen under fuel-rich conditions to yield?xed nitrogen species such as NH3and HCN which are then oxidised to NO in the lean zone of the ?ame.The quantity of HCN formed increases with the concentration of hydrocarbon radicals.The prompt NO x production increases at?rst,then peaks,and?nally decreases due to a de?ciency in oxygen.Prompt NO x contributes mini-mally to the total NO x emissions during combustion and is therefore only considered when the most stringent NO x emissions are to be met[16].
3.Thermal NO x in combustion processes
It is important to quantify the in?uence of the combustion temperature,the concentration of oxygen and the residence time on the NO x formation.The formation of thermal NO x has been clari?ed by Zeldovich and its formation rate is a function of temperature and residence time[17,18].The thermal NO x emissions can be estimated using the Zeldovich mechanism based on the overall reaction(R3)and intermediary reactions (R1e2).
A simpli?cation of the occurring mechanisms can be made, assuming that oxygen radicals are in equilibrium with O2and that the concentration of N radicals does not change signi?-cantly with time.The fundamentals of the Zeldovich mecha-nism and its associated reaction rate equations have been dealt with in several textbooks[8,19,20]and will not be repeated in this paper.
The simpli?ed Zeldovich mechanism is however used to determine the impacts of temperature,residence time,and oxygen concentration upon thermal NO x production in CFBC. For a common CFBC,the O2concentration at the bottom of the riser is21vol%,and exhaust concentrations are normally set at approximately6vol%,yielding an average13.5vol%O2.The residence time of the gases in the riser of the CFBC is commonly between3and4s.Fig.1presents the results of the calculations.
Fig.1shows that the temperature and reaction time signi?cantly affect thermal NO x generation:NO concentra-tions are low at common CFBC-temperatures(<<1250K).
Fig.2assesses the in?uence of the oxygen concentration at a?xed reaction time of4s,typical of a CFBC riser.
Again,concentrations are very low at common operating temperatures(w1100K).Thermal NO x emissions are insig-ni?cant,since below10à2ppm.
For the CFBC at Caledonian Paper(see Section6:Experi-mental),operating at T?1075e1150K,gas residence time?3e4s and O2?13.5vol%,thermal NO x does not contribute to overall NO x emissions.
4.Fuel NO x in combustion processes
Many fuels contain organically bound nitrogen,ranging from 0.1to0.2wt%in crude oils(higher wt%in the residual re?nery fractions);up to1.6wt%in coal,0.6e1.5wt%in wood and 4e6wt%in wastewater treatment sludge.
As an illustration,the possible paths of coal-N are shown in Fig.3.
Pershing and Wendt[22]performed experiments with pulverized coal and showed that75%of the total NO x emission was fuel NO x at adiabatic?ame temperatures up to2480K, with a further increase of10%between2480and2580K.
The degree of fuel e air mixing and the equivalence ratio signi?cantly affect the conversion of fuel-N,with increasing conversion obtained when mixing and equivalence ratio increase[23].Small temperature differences do not affect the
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production of fuel NO x ,in contrast to thermal NO x which is highly sensitive to temperature.
For a CFBC application,where mixing is excellent,it may therefore be assumed that nearly all NO x emissions are of fuel-origin.
5.Review of deNO x techniques and applicability to a CFBC
5.1.
Introduction
There are various ways to reduce NO x emissions in order to meet the legislation:(i)temperature reduction,(ii)reduction of nitrogen in fuels,(iii)creation of a combustion environment that restrains NO x formation and (iv)end-of-pipe (post-combustion)treatment for removing NO x prior to liberating it into the atmosphere.The two broad categories for the control of NO x emissions are hence (i)combustion modi?cations and (ii)?ue gas https://www.sodocs.net/doc/4114434683.html,bustion modi?cations limit the formation of NO x during the actual combustion process by controlling both the oxygen level at the peak temperature,and
the residence times in the combustion zone.End-of-pipe ?ue gas treatment is used to signi?cantly remove the NO x formed during the combustion stage by converting it to N 2.Fig.4summarises the different NO x abatement techniques.
https://www.sodocs.net/doc/4114434683.html,bustion control,air and/or fuel staging,?ue gas recirculation
Combustion control systems are commonly applied,and various options are possible,each resulting in a more or less signi?cant reduction of NO x -levels formed during combustion.These options often combine several measures.A ?rst option consists of reducing the excess air or surplus oxygen present for combustion and/or reducing the combustion temperatures [8,22],although reducing the oxygen level,can lead to incomplete combustion and the amount of unburned carbon in the ash may increase.Ultimately the minimum level of air is limited by the acceptable tar or CO emissions from the stack [24].The CFBC at Caledonian Paper (see Section 6)operates with an excess air level of 20%,yielding stack oxygen concentrations of 6%,as legally imposed.
Staging the introduction of combustion air has been used to help control NO x emissions for many processes burning a wide range of different fuels.Splitting the combustion air stream creates a fuel-rich primary zone and fuel-lean secondary zone.FBN is converted into molecular nitrogen in the primary zone and fuel NO x is suppressed.Typically staged air burners can reduce NO x emissions by 30e 60%compared to the level achieved with a single-stage injection [10].Caledo-nian Paper applies a staged air system (Fig.5),with 20%air introduced at a secondary stage into the riser of the CFBC,and 80%introduced through the bottom distributor.Additional literature ?ndings on air staging in ?uidized bed combustion are summarised in Table 3.
Fuel staging (Reburning)involves the injection of a proportion (10e 20%)of fuel above the combustion zone,creating a fuel-rich secondary combustion zone where NO x formed from the primary combustion zone is reduced through decomposition.The ?rst combustion stage is very lean,which results in low thermal and prompt NO x formation.
Fuel
Fig.1e NO formation in function of reaction time at various
temperatures.
Fig.2e NO formation in function of O 2concentration at various temperatures and at a ?xed reaction time of 4s,typical of a CFBC riser.
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staging was ?rst applied on an industrial scale in 1980,resulting in a 50%NO x reduction.Research has demonstrated that the ef?ciency of reburning is controlled by two kinetic pathways involving the reaction of hydrocarbon species with NO forming HCN,which in the presence of O*or OH*radicals convert to molecular nitrogen.Reburning using many different fuels has been shown to be effective [32],but its application to biomass has not yet been fully detailed.This technique is widely applied in conventional fuel burners but is dif?cult to implement in CFBC using solid fuels.The current construction of the Caledonian Paper CFBC does not permit fuel staging.Recent literature is summarised in Table 4.
The technique of Flue gas recirculation (FGR)can signi?-cantly reduce thermal NO x production.Recycling ?ue gas into the combustion zone reduces ?ame temperatures and overall excess air.It is possible to install internal and external recir-culation paths [10].The implementation of FGR in the CFBC at Caledonian Paper would reduce ?ue gas temperatures and also
reduce boiler output.It can therefore not be used in the present plant layout.Recent literature is summarised in Table 5.
5.3.Flue gas treatment
Although a signi?cant reduction of NO x can be achieved by means of combustion modi?cations,generally up to 50%as the sum of different measures,this alone is often insuf?cient to comply with the stringent emission standards and/or cannot be applied in existing combustors.Additional abate-ment is required and is therefore achieved by the use of end-of-pipe ?ue gas treatment technologies.
5.3.1.Selective catalytic reduction (SCR)
Selective catalytic reduction (SCR)is the most advanced and effective method for reducing NO x emissions and can do so by up to 80e 90%.SCR entails the reaction of NO x with NH 3within a heterogeneous catalytic bed in the presence of O 2at
Fig.4e Overview of deNO x techniques.
Fig.3e Possible fates of nitrogen contained in coal [21].
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temperatures normally in the range of 523e 673K.The predominant reactions are [43]:
4NO t4NH 3tO 2/4N 2t6H 2O (R6)
6NO 2t8NH 3/7N 2t12H 2O (R7)
NH 3is chemisorbed on a catalyst and reacts with NO x in the gas phase.Many catalysts with varying operating temperature windows may be used.The performance of SCR is affected by temperature,NH 3/NO x ratio,oxygen concen-tration,catalyst loading and the type of catalyst support used [43].Depending on the process parameters,various catalysts have been studied for NH 3-SCR including noble metals,metal oxides and zeolites.The most common catalyst is vanadium pentoxide,V 2O 5,supported on titanium dioxide,TiO 2.No catalysts have been reported to be active at temperatures
above 873K or below 523K.It is also possible to achieve SCR by using hydrocarbons as a reducing agent (HC-SCR).However,at temperatures above 773K all of the hydrocarbons are consumed by combustion reactions.Overall the application of SCR to CFBC is problematic due to high risks of poisoning by SO 2and vapours of volatile metals,alternating oxidising and reducing atmospheres,and the low operating temperatures of 150e 180 C after the boiler and de-dusting [44].In addition,the capital costs associated with SCR systems are high.Recent literature is reviewed in Table 6.
5.3.2.Selective non-catalytic reduction (SNCR)
SNCR is a simple process,referred to as “thermal deNO x ”,and involves the reduction of NO x to N 2in the presence of oxygen by reaction with amine-based reagents,either ammonia (NH 3)or urea,CO(NH 2)2at 1073e 1273K,the higher temperature being needed for urea.Exxon developed the SNCR process and ?rst applied it in 1974[56].Taking NH 3as the reagent the reaction scheme is as follows [57]:
Table 3e Literature ?ndings on air staging.Author
Conditions
Results
[25]2m tall CFB-combustion chamber
The excess air ratio has no signi?cant effect on the NO x emissions for all biomass species tested
[26]
Large-scale pulverized coal ?red laboratory furnace
NO x emissions decrease as the distance between the staged air injector and the combustor exit decreases,since this reduces the residence time of the gases in the primary lean-air zone
[27]Laboratory natural gas-?red furnace
Flow rate of natural gas was 5Nm 3/h.Air staging led to a 33%reduction in NO x emissions compared to non-staged air injection [28]30kW bench scale CFB-combustor for coal combustion,and co-?ring coal and biomass Air staging reduces the NO x emissions by 30%
[29]Coal combustion carried out in a 6.2m tall and 0.161m I.D.CFB
NO generated in the primary combustion zone could be effectively reduced in the secondary combustion zone
[30]Fluidized bed combustor (870 C)
50%reduction in NO x emissions achieved using 35%air staging combined with ammonia injection
[31]
Bubbling ?uidized coal combustor (30cm ?30cm)At 800 C bed temperature,a reduction of 33%in the NO emission is measured if
25%of the combustion air is injected into the freeboard.NO reduction increased as the proportion of secondary combustion
air increased
Fig.5e Experimental set-up of lab-scale experiments.
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4NH3t6NO/5N2t6H2O(R8-a)
4NH3t4NOtO2/4N2t6H2O(R8-b)
8NH3t6NO2/7N2t12H2O(R9)
Taking urea as the reagent the reaction scheme is as follows:
H2NCONH2t2NOt?O2/2N2tCO2tH2O(R10)
The reagent ammonia or urea can be injected directly into the?uidized bed or riser.The SNCR process ef?ciency relies upon temperature,reagent/?ue gas mixing,reagent/NO x ratio and reaction time[58].SNCR systems reduce NO x emissions by 30e90%but the performance is highly variable for different applications.A typical SNCR system involves reagent storage, multi-level reagent-injection equipment,and associated control instrumentation.The SNCR reagent storage and handling systems are similar to those for SCR systems.However,because of higher stoichiometric ratios required at equivalent ef?ciency,both NH3and urea SNCR processes require larger quantities of reagent than SCR systems to achieve similar NO x reductions[14].The capital cost of SNCR is low compared to that of SCR systems since there is no cata-lyst,and overall operating costs are similar[43].Some recent literature is given in Table7.The addition of aluminium-based catalysts was suggested for the full and rapid decomposition of urea.
5.3.3.Non-selective catalytic reduction(NSCR)
NSCR for NO x is associated with three-way catalysts of auto-motive vehicles.Under the correct conditions a95%reduction in NO x is achievable.Regarding application to a CFBC,there are two key problems with this technology.Firstly three-way catalysts do not perform well in lean combustion conditions. Secondly,in order to achieve high NO x conversion,the air/fuel ratio must be close to the stoichiometric value,thus making it is necessary to maintain an accurate control of the air/fuel ratio.This is not practical for CFB combustors[43].For these reasons NSCR is not an option as a deNO x technique for a CFBC.
5.3.4.Pulsed corona discharge
The pulse corona-induced plasma process has been shown to remove NO x from?ue gases with deNO x ef?ciencies of60% being reported[69].The major issues associated with this technology are the energy consumption required to achieve adequate reduction and the formation of undesirable by-products[70].The technology is still in the development stage, and hence not immediately applicable to CFBC.
5.3.5.Electron beam?ue gas treatment
This is a promising new technology which enables simulta-neous abatement of SO2and NO x,generating no waste except a useful by-product[71].The method involves?ue gas irradi-ation with fast(300e800keV)electrons,which interact with the main?ue gas components(N2,O2,H2O and CO2)and generate oxidants(OH*,O*,O3).These oxidants react with SO2 and NO x forming sulphuric and nitric acids,respectively, which in turn react with NH3,added to the?ue gas prior to irradiation,thus forming ammonium nitrate and sulphate.
Table5e Literature on FGR.
Author Conditions Results
[37]An integrated?uidized bed reactor using hot?ue gas
recycling over a wide range of temperatures
NO x reductions up to30%are measured
[38]A waste incinerator model is used to compare the
effect of FGR For an FGR ratio of20e25%,the NO x formation was reduced by approximately 20%
[39]Electrically heated combustor under low recycling
ratio with three kinds of coal The NO reduction ef?ciency increased with fuel equivalence ratio and recycling ratio.In the fuel-rich region,the reduction ef?ciency reached as high as80%at a fuel equivalence ratio of1.4
[40]Electrically heated up-?ow-tube combustor fed with
volatile bituminous coal,recycling ratios0e0.4Over60%recycled-NO was reduced at equivalence ratio>1.4.Less reduction occurred at a lower recycling ratio
[41]Combustion of semi-dried sludge in a semi-pilot-scale
FBC(150mm diameter,9m high)
NO x reduction was achieved using?ue gas recycling technique
[42]Laboratory furnace?red by a gas swirl burner of
industrial type 30%NO x reduction for hard coal using20%recirculation of?ue gas
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The product can be applied as an agricultural fertiliser.The
most important reactions for NO x are as follows [72]:
NO tO*tM /NO 2tM (R11)
O*tO 2/O 3tM (R12)
NO tO 3tM /NO 2tO 2tM
(R13)
After oxidation,NO 2forms HNO 3:
NO 2tOH*tM /HNO 3tM (R14)
HNO 3then reacts with NH 3to form NH 4NO 3.
Although associated with high capital and operating costs,the technique has been applied on industrial boilers with results of 80%deNO x and 70%SO 2removal.It also destroys volatile organic compounds [73].The technique has not yet been applied to biomass CFBC.
6.
Experimental investigation
6.1.
Introduction
It is proposed to use the SNCR abatement method to reduce NO x emissions from the CFBC at Caledonian Paper and achieve the required reduction of at least 50%NO x ,or even higher in the event of peak emissions and when the CFBC operates within
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the ideal temperature window for SNCR.Two possible reagents for SNCR are NH 3and urea (CO(NH 2)2).Both reagents are tested on the CFBC at Caledonian Paper to ?nd out which is most suitable.To determine the fundamentals of the NO x reduction,lab-scale experiments were conducted and are reported below.
6.2.DeNO x using NH 3:lab-scale experiments
To investigate the kinetics of NO x reduction by NH 3,experi-ments were carried out in the set-up of Fig.5.A square Incoloy reactor of 0.1?0.1?0.37m 3was thermally insulated and ?lled with Incoloy-?bre wool (voidage close to 0.99).Air was fed into an electrical preheater and brought to the required temperature T 1.At the lowest temperature of investigation (916K),the air ?ow was set to obtain a residence time of 4s,similar to the residence time in the CaPa CFBC.This air ?ow (Nm 3/h)was thereafter unchanged,thus yielding slightly lower residence times at higher temperatures.
The exit temperature T 2was kept at the same value by limiting reactor heat loss through heating its outside surface.A tracer residence time distribution measurement proved that no dead area was present.The ?ow rate of NO 2,fed from a preheated NO 2-cylinder (w 70 C),was set at the required inlet concentration,detected by a Testotherm 350-M-XL monitor.NH 4OH was injected after the air preheater and the dosage was set by the progression of a piston in a liquid-container of 2,5,10or 30ml,respectively.
The ef?uent was cooled to about 180 C and the NO 2concentration was again measured by the Testotherm monitor.This monitor has a detection range of 0e 300ppm NO and 0e 500ppm NO 2with an accuracy of 0.1ppm for both NO and NO 2.
The dosage of NH 4OH was gradually increased and expressed as NH 3/NO 2-ratio,varying from 0.25to 1.71.
Experimental results are plotted in Fig.6as function of temperature and NH 3/NO 2-ratio.
The experimental results demonstrate that the conversion of NO 2increases with increasing temperature at lower NH 3/NO 2-ratios where the stoichiometry of the reaction is ach-ieved.At NH 3/NO 2beyond w 0.8,the conversion starts to level off to achieve its maximum in the NH 3/NO 2-range of 0.9e 1.2.
Higher NH 3/NO 2-ratios do not improve the conversion,which on the contrary starts to decrease,as a result of possible oxidation of excess NH 3and/or NH 3slip.
It was therefore decided to study NH 3-oxidation in air in the same experimental layout,as reported in Section 6.3.
The experimental results enabled us to model the reaction,using results for NH 3/NO 2<0.8to avoid the interaction of possible secondary reactions (NH 3oxidation).
As previously demonstrated for catalytic oxidation reac-tions by Everaert and Baeyens [74],the reaction can be assumed to be of the zeroth order in oxygen,due to excess oxygen being present.In the SNCR reactions,the same excess levels of oxygen are present and can be considered as constant throughout the reaction (and thus included in the pre-expo-nential factor,A),when compared to the low concentrations of NO x being present (<1mg/Nm 3).This zeroth order was hence also previously proposed for SNCR-rates [75].
The reaction rate towards both NO 2and NH 3can be rep-resented by ?rst-order kinetics.The reaction rate is therefore expressed as:R NO 2?
d C NO 2
?àk ?C NO 2?C NH 3
(1)C NO 2?e1à%NO x removed T?C
NO 2
(2)
When the NH 3/NO 2ratio is not equal to 1(NH 3/NO 2s 1)the following kinetic equation can be derived by solving eqn.(1):1
C
NH 3àC
NO 2
?"ln C NH 3àC NO 2tC NO 2 ?C
NO 2
C NO 2?C
NH 3
#?k ?t (1-a)
When the NH 3/NO 2ratio is equal to 1,eqn.(1)can be simpli?ed to yield (1-b),1
NO 2
à
1NO 2
?àk ?t (1-b)
By using the above equations for the known operating residence time,the reaction rate constant k at different temperatures is determined.If the Arrhenius equation is applied to k ,both the pre-exponential factor A and activation energy E A are determined:ln k ?ln A à
E A (3)
The activation energy of the NO 2-reduction was determined as E A,red ?24,000J/mol and the pre-exponential factor,A red ,is 4.9?104m 3s/https://www.sodocs.net/doc/4114434683.html,ing these values of E A,red and A red ,the NO x reduction at any temperature and/or residence time can be predicted.Similar reaction rate equations have been pre-sented for SCR deNO x systems [53e 58],however,with a substantially lower value of the activation energy,reported at 14,800J/mol,with A ?1792s à1[75].
Using the equation above,the effect of the secondary reactions is not taken into account and conversions of 100%will be predicted at an NH 3/NO 2ratio in excess of w 1.2.
Clearly only a combination with NH 3-oxidation can predict the correct shape of the overall conversion curve of Fig.6.
6.3.
NH 3-oxidation:lab-scale experiments
The same experimental layout was used,but without NO 2addition.Exit concentrations of NO and NO 2were
monitored
Fig.6e Laboratory scale NO x abatement against NH 3/NO 2-ratio for varying temperature.
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
1402
at different inlet concentrations of NH 3and at various temperatures.The reactions occurring are:4NH 3t5O 2/4NO t6H 2O
(R15)
4NH 3t7O 2/4NO 2t6H 2O (R16)
Average NO 2and NO concentrations from repeat experi-ments (in triplicate)are illustrated in Fig.7NO representing about 75%of the total NO x formed.
To model the results and expressing the total NO x formed as NO 2,a ?rst-order reaction rate is found to represent the results:
d C NO 2
?k ?C NH 3
(4)
or
ln "
C NH 3àC
NO 2
NH 3
#?àk ?t (4-a)Using the Arrhenius equation for k ,yields an activation energy of the NH 3-oxidation as E A,ox of 158.835J/mol and a pre-exponential factor A ox of 1.61?106s à1.
Predicted quantities of NO x formed by NH 3-oxidation are illustrated in Fig.8.
Considering NO 2-levels present in CFBC-exhaust gases (300e 600ppm,i.e.600e 1200mg/Nm 3),additional NO x formation from NH 3-oxidation is signi?cant only for temper-atures in excess of 800 C.
https://www.sodocs.net/doc/4114434683.html,bined reduction and oxidation reactions
Combining the previous NO x reduction and NH 3-oxidation models,an overall picture is established for any initial NO x
concentration,a set of NH 3/NO 2ratio,a given operating temperature and a speci?ed reaction time.
Since NH 3-oxidation proceeds to a minor extent at high temperatures only,the calculation sequence is as follows:(1)Calculate deNO x ef?ciency (conversion)at the operating T
and NH 3/NO 2ratio.
(2)De?ne the amount of unreacted NH 3.
(3)Determine the NO x formed by oxidation of residual NH 3.(4)Combine the residual NO 2and formed ΔNO 2to obtain
a global conversion.This approach is illustrated in Figs.9e 11as:(i)predicted NO 2concentration assuming the deNO x reaction proceeds separately (Fig.9),(ii)predicted NO 2concentrations formed by the residual NH 3amount (not consumed in deNO x )(Fig.10),and (iii)?nally the predicted overall deNO x ef?ciency (Fig.11).
Having established the overall deNO x removal,the NH 3slip can also be predicted and is given in Fig.12.eNH 3TSlip ?
unreacted NH 3
NH 3
(5)
The overall predicted NO x -removal ef?ciencies are in very good agreement with previous literature ?ndings,and a maximum is achieved within the same NH 3/NO 2-ratios [76,77].
Predicted unreacted NH 3amounts at higher NH 3/NO 2-ratios also con?rm previous ?ndings,e.g.Robin [78],where a 30%NH 3slip was detected at NH 3/NO 2>1.3.
6.5.NO x and deNO x in the CFBC of CaPa
6.5.1.The CFB at Caledonian Paper:principles and operating conditions
The CFBC at Caledonian Paper has a maximum capacity of 58MW th .Dimensions and operating data for the CFBC,
e.g.
Fig.7e NO and NO 2formed from NH 3-oxidation.
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
1403
particle/gas ?ow,thermodynamics,reaction kinetics,riser temperature,transport velocity and residence time in the riser,have been presented by Van de Velden et al.[5].The CFBC operates with sand and ?y ash as bed material.The particle size of the bed material is approximately 200m m.The riser operates at a super?cial velocity of 4.4m s à1and a solid circulation ?ux of 67kg m 2s à1,considered to be in the core-annulus ?ow mode of the CFB [79,80].The isothermal operation is due to the high solid circulation used [5,79].
To record NO x emissions,the Caledonian Paper CFBC has a continuous IR probe located in the stack.NO x emissions are recorded as NO 2,as all NO is oxidised to NO 2prior to measurement.
6.5.2.Injection of NH 3in SNCR tests
In the NH 3SNCR process,NH 4OH-solution at 25%is pumped from the storage tank to injector lances,penetrating the riser
about 2m above the recycle L-valve.Upon entry into the riser,NH 3solution vapourises and reacts with the NO x in the ?ue gas.The molar ratio is de?ned as the ratio of the number of moles of injected NH 3with respect to the moles of initial NO x (expressed as the measured amount of NO 2)present in the ?ue gas.
The NO x emissions were continuously monitored using the online stack monitoring.During the experiment,the process was operated in a steady state,i.e.feed rate,composition,pressure,air ?ow rate all being constant.The operating temperature varied between 1041and 1203K.The CFBC was operating at full capacity and at four different temperatures.The initial NO 2-reading was close to 500mg/Nm 3(Fig.13).
The results of NO x abatement against NH 3/NO 2ratio for varying temperatures are plotted in Fig.14
.
Fig.8e Concentration of NO x formed at different
temperatures.
Fig.9e Predicted residual NO 2concentration for lab-scale
experiments.
Fig.10e Predicted amount of NO 2formed from residual NH 3-oxidation.
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
1404
From Fig.14,it is clear that temperature affects the NO x removal by NH 3.An increase in temperature from 1041K to 1152K results in an increase in NO x removal.This is because the rate of reaction between NO x and NH 3increases,resulting in higher removal ef?ciencies within the constant reaction time.The results at 1203K almost mirror those at 1152K and are not presented in the ?gure.The highest measured removal was 84%.
For all temperatures,the NO x removal increases with increasing NH 3/NO 2ratio up to a point,and then removal decreases with further increased NH 3/NO 2ratio.The initial increase in NO x removal is explained by the higher amount of NH 3available to reduce NO x .However,above a NH 3/NO 2ratio of approximately 1,the NO x removal rate decreases because both reactions of reducing NO x ,and the competing oxidation
of NH 3to NO x take place.These results mirror those obtained in the lab-scale set-ups,and can be predicted by the model equations developed above,as illustrated in Fig.15.
The laboratory-determined model equations predict the maximum removal within 10%accuracy.Results at lower or higher NH 3/NO 2-ratios differ more signi?cantly.
The reasons for these deviations are various and include:(i)A residence time distribution of the gaseous reactants [80],neglected in the calculations and assumed to be a constant 4s.In the CaPa core-annulus riser ?ow mode,the gas velocity in the core exceeds the super?cial velocity due to a restriction of free cross-sectional area by the occurrence of the annular down?ow of solids (near the wall).This increased velocity reduces the residence time of the bulk of the gas in the riser,and will be lower than the assumed 4s,based upon the assumption of pure gas plug ?ow.A shorter residence time will reduce the deNO x ef?ciency,with a calculated 5%reduction per 10%reduction in real residence time;
(ii)A possibly non-uniform distribution of injected NH 3
leading to higher amounts of NH 3slip than predicted;and/or
(iii)A variation of (NO 2)0during the experiments,accepted
throughout at a constant 500mg/Nm 3.Research is continuing concerning the residence time distribution in a CFB and the non-uniform NH 3-distribution.This will be presented in a separate
paper.
Fig.11e Predicted overall deNO x
ef?ciency.
Fig.12e Predicted NH 3slip.
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
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6.5.3.Injection of urea in SNCR tests
Ammonia is supplied in a 25or 33wt%solution at 200e 250V /m 3
.Urea is supplied in powder form at approximately 300V /ton,but has the advantage of being an inert,non-corrosive particulate solid.In a urea SNCR system,powdered urea can be injected with the fuel at the bottom of the riser,or it can preferably be dissolved and injected into the riser as in the case of NH 4OH.The predicted consumption can be found from reaction (R 10),which shows that 1mol of urea reduces 2mol of NO x Therefore a urea/NO x molar ratio of 0.5is required,compared to a molar ratio of 1for NH 3.
Separate urea injection tests were conducted in the CFBC at Caledonian Paper.The ?rst tests used urea powder which was mixed with the fuel on the fuel feed belt (pre-mixed at the point of injection by the screw conveyor system).The second tests used urea solution which was sprayed directly onto the fuel on the fuel feed belt.Both tests used a urea/NO 2molar ratio of about 0.5.As the fuel and urea entered the CFBC,the NO x -reading was recorded and continuously monitored.Table 8shows a summary of average results from repeat experiments.
In comparison to the results of NH 3injection,urea results are rather poor.Table 8shows a reduction in NO x of 8e 12%for powdered urea,and 24e 44%reduction for urea solution.The
low urea-NO x removal ef?ciencies are due to operating at a temperature below the optimum temperature for urea SNCR,which is between 1198K and 1273K.The Caledonian Paper CFBC operation temperature lies outside the optimum temperature window for urea.The very low value obtained with powdered urea could moreover be due to the fact that the solid urea particles may not have suf?cient residence time in the CFBC to sublimate,reducing the effective time of contact with NO x in the riser.Extrapolating the results to a tempera-ture of 1273K predicts a deNO x ef?ciency of over 80%.
7.Economy of the abatement of the SNCR application
Ammonia is supplied in a 25or 33wt%solution at 200e 250V /m 3.Urea is supplied in powder form at approximately 300V /ton,but has the advantage of being an inert,non-corrosive particulate solid.In a urea SNCR system,powdered urea can be injected with the fuel at the bottom of the riser,or it can preferably be dissolved and injected into the riser as in the case of NH 4OH.The predicted consumption can be found from reaction (R 10),which shows that 1mol of urea reduces 2mol of NO x Therefore a urea/NO x molar ratio of 0.5is required,compared to a molar ratio of 1for NH 3.
The experimental results demonstrate that the ef?ciency of NO x removal 70%when using ammonia,against 20%only when using urea.Despite the 0.5/1ratio for urea,against 1/1for ammonia as shown in reactions (R 10),the very poor deNO x urea-ef?ciency increases its required dosage to about twice that of ammonia.
To reduce the current NO 2-level from 500mg/Nm 3to the present standard of 350mg/Nm 3,the hourly required dosage will be 20.4L of NH 4OH (33wt%),against 21.1kg/h of urea.
At the respective costs of both reactants,and for an annual operation of 8000h,the respective operating costs will be 40,800V /yr for NH 4OH against 50,700V /yr for urea.
Higher temperatures could signi?cantly improve the removal ef?ciency when using urea and will consequently
Fig.13e Flow-sheet of the CaPa plant [5]
.
Fig.14e CFBC NO x abatement against NH 3/NO 2ratio for varying temperatures (CaPa).
b i o m a s s a n d b i o e n e r g y 34(2010)1393e 1409
1406
reduce the operating costs,but such temperatures are inpractical for CaPa.
8.Conclusion
Although concentrations of N in biomass are rather low,combustion transforms the fuel-bound N to concentrations of NO x that exceed the current and future emission standards.This paper reviews the formation mechanisms,the different ways to reduce NO x emissions by appropriate preventive actions during combustion,or by end-of-pipe technologies.SNCR appears to be the most appropriate of the secondary measures to reduce NO x emissions in the case of ?uidized bed combustion.
The study of SNCR using NH 3is performed by both lab-scale experiments using NH 3to de?ne the reaction kinetics,and by investigation of a large-scale 58MW th CFBC,where the use of injecting urea was also studied.
From the lab-scale results,the simultaneous reduction and oxidation reactions of NH 3,NO 2and O 2were investigated and transformed into kinetic expressions with associated activa-tion energy and pre-exponential factor of the Arrhenius equation for the rate constant.
NO 2e NH 3reduction is followed by NH 3-oxidation,thereby limiting the overall deNO x ef?ciency to around 80e 85%.
Model predictions generated by the above approach are in fair agreement with experimental results.
Large-scale measurements con?rm the existence of a maximum deNO x ef?ciency around 84%when using NH 3.When using urea,the deNO x ef?ciency is a maximum of 44%only within the studied temperature range.
Ammonium hydroxide (NH 4OH)appears to be the preferred option for CaPa,the application being about 20%cheaper than when using urea at the current operating temperature.
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