Design of scrubbers for condensing boilersF. Haase*, H. Koehne

Lehr- und Forschungsgebiet fur Energie- und Stofftransport, RWTH Aachen, Kopernikusstr. 16, 52056 Aachen, GermanyReceived 16 April 1997; received in revised form 15 January 1999; accepted 15 January 1999

Abstract

Many fuels (oil, wood or process gases) contain components which can be found as acid-forming compounds in the flue gasafter combustion and which are absorbed to a small degree in the condensate when the dew point is reached. This acid, andtherefore highly corrosive condensate, increases the demands on the materials used for the areas affected by condensation incondensing boilers. The concepts described in the article Design of Scrubbers for Condensing Boilers are based upon use ofthe condensate for washing the flue gas. To achieve this, the flue gas is cooled below the dew point in contact with the alreadyneutralized condensate. This process step allows the wet separation of noxious matter and avoids acid corrosion of the materialsin the area of condensation, raising the choice of possible materials decisively. The review article also exemplifies the state-of-the-art for condensing boiler technology, as it gives a view of the fundamental principles of two-phase flows, of absorption ofacid-forming gases and their neutralization. Using the examples of sulfuric oxides SOx and nitrous oxides NOx, the effects ofdifferent characteristic features of the gases on the reaction steps are described and possible process steps of the wet separationin the condensate are discussed. To achieve as high a separation degree as possible between the flue gas and the condensate,good heat and mass transfer conditions must be guaranteed between the two phasesflue gas and condensate. Dispersing oneof the two phases leads to a strong increase of the interphase. Generally, fluid vaporizers (flue gas as coherent phase) and gasbubble washers (condensate as coherent phase) can be taken into consideration. Advantages and disadvantages of theseabsorbers are worked out for use as washers in combination with condensing boiler technology and the fluid-mechanicalprinciples necessary for the design of a gas bubble washer. The sometimes contrary influences of constructive parameterson pressure changes in the flue gas, as pressure loss and mass transfer conditions between flue gas and condensate, arediscussed. q 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Condensing boiler; Scrubber; Desulfurization; NOx absorption; Neutralization

Contents

Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3061. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3082. State of science and technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

2.1. Condensing boiler technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3082.1.1. Dew point and combustion efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3082.1.2. Integration of condensing boiler technology in the heating system . . . . . . . . . . . . . . . . 3112.1.3. Concepts for condensing boiler technology for sulfurous fuels . . . . . . . . . . . . . . . . . . . 312

2.2. Deposition of sulfur oxides from flue gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3142.2.1. Phase equilibria and oxidation at the absorption of SO2 . . . . . . . . . . . . . . . . . . . . . . . . 3152.2.2. Mass transfer at SO2 absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316

Progress in Energy and Combustion Science 25 (1999) 305337PERGAMON

0360-1285/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved.PII: S0360-1285(99)00002-7

www.elsevier.com/locate/pecs

* Corresponding author. Address for correspondence: Shell Research and Technology Centre, Hamburg, Deutsche Shell AG, PAE Labor,OGMPT/4, Hoh-Schaar-Str. 36, 21107 Hamburg, Germany. Tel.: 1 49-40-7565-4739; fax: 1 49-40-7565-4581.

E-mail address: frank.f.haase@ope.shell.com (F. Haase)

2.2.3. Neutralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3182.3. Wet separation of nitrogen oxides from flue gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

2.3.1. Solution of NO and NO2 in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3192.3.2. Oxidation of NO in aqueous solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3202.3.3. Absorption and reduction of NO in aqueous solution . . . . . . . . . . . . . . . . . . . . . . . . . . 321

3. Experimental apparatus and measuring techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3223.1. Design of the test boiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3223.2. Measuring techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

3.2.1. Gas analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3233.2.2. Water analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

4. Fluid-mechanical layout of a gas bubble washer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3234.1. Gas flow through the complete perforated bottom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3244.2. Drag reduction of the flue gas flowing through the water bath . . . . . . . . . . . . . . . . . . . . . . . . . 3274.3. Mass transfer between dispersed gas and liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

5. Experiments on the absorption and neutralization of SO2 and NOx in the water bath . . . . . . . . . . . . . 3295.1. Neutralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

5.1.1. Formation of Mg(OH)2 via hydration of MgO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3305.1.2. Carbonate formation by CO2 absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

5.2. SO2 absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3325.2.1. Influence of pH on the SO2 absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3325.2.2. Sulfite oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

5.3. Wet absorption of nitrogen oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3345.3.1. Oxidation and absorption of NO in the water bath . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3345.3.2. Absorption of NO2 in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336

Nomenclature

Physical symbolsA area (m2)a volume specific interphase (m21)b weir height (m)c concentration (mg/l, mol/m3)c specific heat capacity (J/kg K)d diameter (m)dS Sauter diameter (m)E specific emission (mg/MJ)F force (N)f power ratio of a radiatorFr* modified Froude numberg gravitation constant (m/s2)H enthalpy (J)h height of water column above perforated

bottom (m)H Henry constant (N m/mol)h specific enthalpy (J/kg)hfg latent heat of vaporization (J/kg)HHV higher heating value (J/kg)K equilibrium constant (mg/l)21k velocity constant (s21)l length (m)LHV lower heating value (J/kg)m mass (kg)

_m mass flux (kg/s)M molar mass (kg/kmol)_n 00 transferred mole number per time and area

(mol/s m2)N mole number (mol)P power (W)p pressure (Pa)pH pHQ heat (J)_q 00 heat flow density (J/m2 s)_Q heat flow (J/s)_Qstd standard heat demand (J/s)R gas constant (J/mol K)r reaction velocity (mol/m3 s)r specific vaporization enthalpy (J/kg)s jet length (m)T temperature (K)t time (s)t90 time until 90% of the stationary value is

reached (s)_vd average velocity in tube (m/s)_V volume flow (m3/s)

Symbolsw average gas velocity (m/s)We Weber number

F. Haase, H. Koehne / Progress in Energy and Combustion Science 25 (1999) 305337306

x mole fractiony height of perforated bottom above boiler

bottom (m)z number of holesa absorption degreeb mass transfer coefficient (mol/N s, m/s)D difference1 i Henry activity coefficient1 volume fractionh efficiency degreel airfuel ratioq temperature (8C)Dq ln logarithmic average temperature (K)r density (kg/m3)s interfacial tension (N/m)j mass fractionc mole fraction

Chemical symbolsAg silverAgCl silver chlorideAl aluminumCa calciumCa(OH)2 calcium hydroxideCaCO3 calcium carbonateCaO calcium oxideCaSO3 calcium sulfiteCaSO4 calcium sulfateCH4 methaneClO42 perchlorate ionCO2 carbon dioxideCO322 carbonate ione electronEDTA ethylene diamine tetra-acetateFe ironH1 hydrogen ionH2CO3 carbonic acidH2O waterH2O2 hydrogen peroxideH2SO3 sulfurous acidH2SO4 sulfuric acidHN(SO3)222 imido disulfonateHNO2 nitrous acidHNO3 nitric acidHO22 hydrogen dioxide ionHON(SO3)222 hydroxylamine disulfonateH3PO4 phosphoric acidHSO32 hydrogen sulfite ionK potassiumKClO4 potassium perchlorateKMnO4 potassium permanganateM1 metal cationM2SO3 metal sulfiteMg magnesiumMg(OH)2 magnesium hydroxideMgNO3 magnesium nitrate

MgO magnesium oxideMgO2 magnesium peroxideMgSO3 magnesium sulfiteMgSO4 magnesium sulfateMn manganeseMnO2 manganese dioxideMOH metal hydroxideN(SO3)332 nitrilo trisulfonateN2 nitrogenN2O dinitrogen monoxideN2O4 dinitrogen tetroxideNa2O2 sodium peroxideNa2C2O6 sodium peroxocarbonateNa2SO3 sodium sulfiteNa2SO4 sodium sulfateNaOH sodium hydroxideNH3 ammonia(NH4)2SO4 ammonia sulfateNO nitrogen monoxideNO2 nitrogen dioxideNO22 nitrite ionNO32 nitrate ionNOx nitrogen oxideNTA nitrilotriacetateO2 oxygen (molecular)O3 ozoneOH2 hydroxide ionPbO2 lead dioxidePt platinumRh rhodiumS2O422 dithionite ionS2O622 dithionate ionSiO2 silicon dioxideSO2 sulfur dioxideSO3 sulfur trioxideSO322 sulfite ionSO422 sulfate ion

Index* state of saturation0 standard conditionsaq dissolvedb basebu gas bubblec combustionc continuouslycrit criticald disperseddp dew pointfg flue gasfp flow pipeg gas sideg gaseousgran granulatedh heating mediumi component

F. Haase, H. Koehne / Progress in Energy and Combustion Science 25 (1999) 305337 307

in enteringkin kineticl lifting forcel liquid sidelim limitmax maximum valuemin minimum valueout leavingp pressurer returns interfacial tensionstd standard statewb water bath

1. Introduction

Burning fossil energy sources, the acid-forming com-ponents sulfur dioxide (SO2) and nitrogen oxides (NOx)are formed, as well as others, and are emitted into the at-mosphere. Following the mass flow of the nitrogenous andsulfurous compounds after their emission, it is necessary todifferentiate between their effects on the air and on plants,earth and water.

As the average retention times of the trace gases SO2 andNOx are only very short (hours up to several days), theirinfluence on air quality is regionally limited. SO2 and NOxare toxic to human beings at a certain concentration. Thecurrent discussion about anthropogene influences on theclimate considers SO2 and NOx not in the first place. Themajor climate influencing gases in the atmosphere are steam(H2O), carbon dioxide (CO2), ozone (O3), dinitrogen oxide(N2O) and methane (CH4). These gases cause the so-calledgreenhouse effect of the atmosphere. But SO2, NOx (andammonia NH3) lead to the formation of aerosols which aresolid or liquid parts in the air outside of clouds with adiameter between 0.001 and 100 mm. They scatter andabsorb solar irradiation and emit thermal radiation so theybehave contrary to the anthropogene heating of the earth [1].

The largest part of the emission is chemically convertedin the air before it is deposited via rain. At the chemicalconversion of sulfur dioxide, sulfuric acid (H2SO4) isformed; its salts are called sulfates (SO422). Finally, gypsumor Na2SO4 is formed. Nitrogen monoxide is oxidized tonitrogen dioxide, which forms nitric acid; its salts are callednitrates (NO32).

Before the acids formed by SO2 and NOx get into theground, the protective wax coating on tree leaves andneedles can be damaged. Nutritive substances are washedout. Also, the supply of nutritive substances of the grounddecreases. With increasing acidity of the ground, firstcalcium magnesium and potassium, and in later stages,manganese and aluminum are dissolved out of the groundparticles. They are washed out and lost to the ecosystem [2].Together with the washed out alkali sulfates and nitrates

combine with drained rainfall into surface waters or viathe sewage system into a sewage treatment plant.

Fig. 1 shows the effects of sulfurous and nitrogenouscompounds in the air, on plants, earth and water for theemissions of SO2 and NOx.

This mass flow proves that the wet deposition of SO2 andNOx in condensing boilers under the formation of sulfateand nitrates and the necessary neutralization correspond tothe processes in nature.

The wet deposition of SO2 and NOx, immediately aftertheir formation in the combustion chamber of the washer forcondensing boilers, is described here. The specialty of thesewashers is that the condensate formed by falling short of thesteam dew point is used as washing water.

Thus, an almost complete absorption of the sulfur diox-ides is possible in the condensate if the flue gas contactsimmediately neutralized condensate under good heat andmass transfer conditions. Meeting these requirements resultsin washing the flue gas and preventing the boiler parts in thecondensation area from corrosion.

Combining the condensing boiler technology (CBT) withflue gas washing increases the choice of usable materials forfuels with a very aggressive condensate and the operativeexpense is kept low at the same time. As a result of lowenergy prices, condensing boiler technology is difficult torealize economically for the fuels mentioned and a smallpower output. But the additional flue gas washing providesa motivation which should be encouraged by means oflegislation.

2. State of science and technology

2.1. Condensing boiler technology

According to their energy content, fuels are described bythe lower heating value (LHV) and the higher heating value(HHV). The LHV of a substance is the amount of heat setfree at complete combustion when reactants and productsshow the same temperature and water formed at combustionexists as steam. The difference to the gross caloric value ofthe fuel is that it considers the water as liquid, so the grosscaloric value is, by the condensation heat of the water,greater than the net caloric value (HHV=LHV hfgjh=LHV with j h mass fraction of hydrogen in thefuel). Condensing boilers are those which make the latentheat contained in the flue gas usable by condensation.

2.1.1. Dew point and combustion efficiencyThe condensation of water begins when the state of

saturation for the steam in the flue gas is reached whilecooling the flue gas. This point is called the dew point,and its temperature is the dew point temperature q dp. At thedew point the steam pressure equals the saturation steampressure of water, which isas an approximationfor

F. Haase, H. Koehne / Progress in Energy and Combustion Science 25 (1999) 305337308

a total pressure below 10 bar, only dependent on thetemperature.

For the combustion of fuels, the partial pressure of steamin the flue gas depends on fuel composition (hydrogencontent), airfuel ratio l and the humidity of the combus-tion air. Disregarding the air humidity, the dependencebetween the dew point temperature q dp and airfuel ratiol is obtained, as shown in Fig. 2 for the standard heating oilin Germany called EL (extra light). With an increase inairfuel ratio l , the flue gas must be cooled to achievesteam condensation.

By burning sulfurous fuels, the hydrophile sulfur trioxide

(SO3) in the flue gas acts as a condensation nucleus resultingin an increase of the dew point temperature. For an exactdetermination of the so-called acid dew point, knowledgeof the conversion of sulfur dioxide (SO2) to sulfur trioxide(SO3) is necessary. As an approximation, it is assumed thatthe volume concentration of SO3 is about 1% of the concen-tration of SO2 [3]. It can be seen in Fig. 3 that the dew pointincreases under these conditions for a sulfur content in theflue gas of jS 0.2 % by 72 K and lies for an airfuel ratiol 1.1 at q dp 1208C.

As a measure against acid corrosion or sooting, conven-tional heating technology avoids falling short of a flue gas

F. Haase, H. Koehne / Progress in Energy and Combustion Science 25 (1999) 305337 309

Fig. 1. Deposition of SO2 and NOx in the condensate of the flue gas as an alternative to their emission.

temperature q fg 1508C before leaving the flue gas system.By introducing condensing boiler technology into an exist-ing heating system, a large fraction of the sensible heatconnected to the high flue gas temperature of conventionalheating systems can be used.

To evaluate a heat generator, the combustion efficiency

hc supplied energy 2 energy at flue gas outletsupplied energy 1

can be used (the so called boiler efficiency takes intoaccount in addition heat losses through the jacket).

Determining the delivery energy by the HHV, oneobtains:

hc 1 2Piji hiqfg2 hiq0

1 jH2Ohfg

Hgq0 2

where hi is the specific enthalpy of the gaseous component i,

j i the mass fraction of the gaseous component i in kg/kgfuel,q 0 the flue gas temperature and hfg the latent heat of vapor-ization of water at standard temperature.

Here we assume that fuel and dry air are supplied at astandard temperature q 0, which usually is the thermody-namic standard temperature q 0 258C. The enthalpy ofthe condensed water is disregarded here. For heating oilEL, the result is dependence between combustion efficiencyh c, flue gas temperature q fg and airfuel ratio l shown inFig. 4.

When falling short of the dew point temperature, a strongincrease of the combustion efficiency h c can be observed,which is attributed to the addition of condensation heat tothe sensible heat.

The concept of flue gas washing, used with condensingboiler technology, dealt with here, aims at completeabsorption of the sulfur oxides in the condensate of the

F. Haase, H. Koehne / Progress in Energy and Combustion Science 25 (1999) 305337310

Fig. 2. Dew point temperature qdp of the flue gas from the combustion of heating oil EL versus airfuel ratio l .

Fig. 3. Increase of the dew point caused by sulfuric acid in the flue gas (calculated data) [3].

flue gas. Good mass transfer conditions between flue gas andnecessary condensate result in complete saturation of theflue gas with steam, so that for only a slight increase ofthe temperature above the dew point, condensate will vapor-ize and combustion efficiency decreases compared to a dryflue gas system. This phenomenon is shown in dashedcurves in Fig. 4 and provides the necessity of avoidingflue gas temperature above the dew point by appropriatecontrol systems.

2.1.2. Integration of condensing boiler technology in theheating system

For the layout of a heating system, the so-called standardheat demand of a building, _Qstd is used; it describes the heatpower supplied to heat the rooms to comfortable tempera-tures at a low outdoor temperature [4]. Using the standard

heat demand for the layout of the boiler (provided that thepiping losses are negligible), the boiler has to supply thepower

_Qh _Qstd _mhch qfp 2 qr

3

to the heating circuit. In the aforementioned equation, mhrepresents mass flux of the heating medium, ch the specificheat capacity of the heating medium, q fp the flow-pipetemperature of the heating medium and q r the returntemperature of the heating medium.

The values of the process parameters mh, q fp, q r and theircombinations can only be chosen within certain limits. If theflue gas is cooled below its steam dew point by the heatingmedium, the maximum return temperature is then deter-mined according to Fig. 2 depending on the airfuel ratiol chosen.

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Fig. 5. Influence of flow-pipe and return temperature on the power ratio f of a radiator.

Fig. 4. Combustion efficiency h c specific to HHV.

The value of the flow-pipe and the return temperature ofthe heating medium has a decisive influence on the heatsupply of a radiator; its heat flux _Q can be described by_Q , A Dqln

n 4

with the radiator surface A and the logarithmic averagetemperature

Dqln qfp 2 qr

ln qfp 2 qroom=qr 2 qroom 5

The value of the exponent n lies within the limits 1.1 to 1.5;for radiators, n < 1.3 is obtainted [5], for floor radiators, n