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Electron beam flue gas treatment (EBFGT) technology forsimultaneous removal of SO2and NOxfrom combustion of liquid fuels

Ahmed A. Basfar a,*, Osama I. Fageeha b, Noushad Kunnummal b, Seraj Al-Ghamdi b,Andrzej G. Chmielewski c, Janusz Licki d, Andrzej Pawelec c,

Bogdan Tyminski c, Zbigniew Zimek c

a King Abdulaziz City of Science and Technology, Atomic Energy Research Institute, Radiation Technology Center, P.O. Box 6086,

Riyadh 11442, Saudi Arabiab Saudi Arabian Oil Company (Saudi Aramco), Environmental Protection Department, P.O. Box 12772, Dhahran 31311, Saudi Arabia

c Institute of Nuclear Chemistry and Technology, 03-195 Warsaw, Dorodna 16, Polandd Institute of Atomic Energy, 05-400 Otwock-Swierk, Poland

Received 22 July 2007; received in revised form 15 September 2007; accepted 18 September 2007Available online 15 October 2007

Abstract

Electron beam flue gas treatment technology was applied for removal of SO2and NOx from flue gas, emitted from combustion ofhigh-sulfur fuel oils. The detailed study of this process was performed in a laboratory by irradiating the exhaust gas from the combustionof three grades of Arabian fuels with an electron beam from accelerator (800 keV, max. beam power 20 kW). SO 2removal is mainlydependent on ammonia stoichiometry, flue gas temperature and humidity and irradiation doses up to 8 kGy. NO xremoval depends pri-marily on irradiation dose. High removal efficiencies up to 98% for SO2and up to 82% for NOxwere obtained under optimal conditions.

The flue gas emitted from combustion of high-sulfur fuel oils, after electron beam irradiation, meets the stringent emission standards forboth pollutants. The by-product, which is a mixture of ammonium sulphate and nitrate, can be used as a fertilizer as such or blended withother components to produce commercial agricultural fertilizer. 2007 Elsevier Ltd. All rights reserved.

Keywords: High-sulfur oil combustion; Flue gas purification; Electron beam; SO2 removal; NOxremoval

1. Introduction

The highest percentage of electrical energy and heat(88%[1]) is produced by the combustion of fossil fuels like

coal, oil and natural gas. Many pollutants are released inthe combustion process. These pollutants affect air quality,human health, environment, economy and contribute toclimate change. Liquid fuels are used in some limited appli-cations, but are more prevalent in certain areas of theworld such as South America, central and eastern Canada,northeastern States USA. Number 2 and No. 6 oils are themost commonly used liquid fuels. Heavy fuel oil (HFO) is a

mixture of hydrocarbons composed of residual fractionsfrom distillation and processing of crude oil. It is essentiallyan industrial fuel that is suitable for use in thermal powerplants, refineries, industrial boilers, pulp and paper indus-

try and metallurgical operations which generally pre-heatthe fuel oil.Depending on the source, the sulfur content in the HFO

could be as high as 4.0% wt. Sulfur is converted to sulfurdioxide when oil is burnt; additionally, the combustionprocess creates various forms of nitrogen oxides. Flue gasemitted from heavy fuel oil-fired boiler contains high SO2and NOxconcentrations, many times exceeding the permis-sible emission limits, which necessitates the use of add-oncontrol devices for the reduction of SO2 and NOx emis-sions. Post-combustion technologies have been maturely

0016-2361/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2007.09.005

* Corresponding author. Tel.: +966 1 4813648; fax: +966 1 4813640.E-mail address:abasfar@kacst.edu.sa(A.A. Basfar).

www.fuelfirst.com

Available online at www.sciencedirect.com

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developed and widely used. Until now, most of these tech-nologies have been performed individually, i.e. one tech-nology controls only one pollutant. Simultaneousremoval technology for multiple pollutants will becomeincreasingly useful.

Electron beam flue gas treatment technology is among

the most promising advanced technologies of a new gener-ation. It is a simultaneous dry-scrubbing of SO2and NOx,where no waste, except a useful by-product, is generated.The irradiation of flue gas with fast (300800 keV) elec-trons initiates chemical changes that make removal of sul-fur and nitrogen oxides easier. After irradiation, fastelectrons interact with the main components of flue gas(N2, O2, H2O and CO2) and generate the same oxidants(*OH, HO2, O, O3) which are produced by UV sunlightin the free atmosphere, but at concentration levels severalorders of magnitude higher. The oxidants convert NOxand SO2 to nitric and sulfuric acids which form a solidpowder of ammonium nitrate and sulfate in the presence

of ammonia which is added to flue gas before its irradia-tion. The filtered by-product is usable as an agriculturalfertilizer. Overviews of the process chemistry and modelcalculations were given by Tokunaga et al. [2], Matzing[3] and Namba [4]. The application of this method fortreatment of coal-fired flue gas has been developed fromlaboratory scale to pilot and large demonstration scalethrough research and development projects in Japan, Ger-many, Poland [5,6], China, Bulgaria and USA. Highremoval efficiencies of SO2 up to 95% and NOx up to85% were simultaneously obtained at the optimal condi-tions for electron beam flue gas treatment. Three industrial

installations have already been built at coal-fired ThermalPower Plants Chengdu and Hangzhou [7] in China andPomorzany[8] in Poland. The tests demonstrated the abil-ity of the technology for highly efficient removal of SO2

and NOx from flue gas from coal combustion process.New industrial plants are under construction in Chinaand Bulgaria. The primary objective of this paper is todemonstrate the feasibility of the electron beam flue gastreatment process for simultaneous removal of SO2 andNOx from flue gas from combustion of heavy liquid fuels.

This study was performed in a laboratory with combustionof three grades of Arabian fuels.

2. Experimental test facility

The experiments have been performed in the labora-tory-scale EBFGT installation at Institute of NuclearChemistry and Technology (INCT) in Warsaw, Poland.For this study, the installation was equipped with fuel oilcombustion unit. A schematic flow chart of the test facilityis shown inFig. 1. The flue gas emitted from fuel oil burner(2 inFig. 1), passed through the conditioning system andwas irradiated by electron beam in the process vessel. A

near-stoichiometric amount of ammonia (7 inFig. 1) wasadded to flue gas before its inlet to process vessel. By-prod-uct from irradiated gas was separated in a bag filter andpurified flue gas was exhausted through a stainless steelstack (14 inFig. 1) to the atmosphere. Flue gas flow rateat 5m3/h was controlled by butterfly valve (1 h in Fig. 1)and draft of the ID fan and was measured by an orifice.The laboratory test facility consists of the following tech-nological units: fuel oil combustion unit, flue gas condi-tioning system, electron beam irradiation of flue gas,filtration unit, monitoring system and exhaust of purifiedflue gas.

Fuel oil combustion unit. Three grades of Arabian liquidfuels were combusted: Arabian Medium Crude Oil (AM),Arabian Heavy Crude Oil (AH) and Arabian Heavy FuelOil (HFO).Table 1presents properties of these fuels.

Fig. 1. Schematic diagram of laboratory scale electron-beam flue gas treatment installation. (1) Thermostated fuel oil; (2) oil burner; (3) particulate andsoot filters; (4) orifice; (5) dosage of water vapor; (6) gas sampling point-process inlet; (7) ammonia injection; (8) process vessel; (9) electron beamaccelerator; (10) retention chamber; (11) bag filter; (12) gas sampling point-process outlet; (13) induced-draught fan (ID fan); (14) stack; (15) concrete

shielding wall; (16) concrete shielding door.

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A pressure jet oil burner, type Jet 4.5EV, produced byKorting Hannover AG (Germany), was installed in thefront wall of an air-cooled cylindrical combustion chamber(1d inFig. 1) 800 mm long and 400 mm in diameter. Thechamber was lined with high-temperature chamotte bricksto assure good combustion of sprayed oils. Flue gas tem-perature was reduced in the air cooler (1f in Fig. 1). Tem-perature of gas leaving the cooler was about 420540 Cdepending on the type of fuel burned. The flue gas was

divided into two streams: one (fixed at 5 m3/h) was drawnto the laboratory installation by the ID fan and the rest wasreleased through the stack (1g inFig. 1) to atmosphere. ForHFO burning, it was necessary to lower its viscosity tomanageable level by preheating the oil further and blendingit with 10% wt. of light fuel oil (type Ekoterm Plus). Thislight fuel oil (LO) was also burnt for conditioning the lab-oratory installations before and after combustion of Ara-bian liquid fuels.

Flue gas conditioning unit. Flue gas from the burner wasdirected to the process vessel through stainless steel pipes.These tubes were wrapped with heating tapes and thermal

insulation to keep the flue gas temperature above its dew-point to avoid condensation. The soot and fly ash wereremoved from flue gas at the inlet to the installation toassure good quality of by-product. A two-step filtrationsystem was applied. The first filter (3a in Fig. 1) wasinstalled at the inlet to the facility where flue gas tempera-ture at the cross-section was above 400 C. This filter wasmade from needled cloth with prime coat glass fiber. Thesecond filter (3b inFig. 1) was installed in the colder partof installation and was made from Goretex bag with a tef-lon coat. The water vapor dosage system (5 inFig. 1) wasapplied to increase the flue gas humidity to the desiredlevel. NH3was injected to the flue gas upstream of processvessel from gas cylinder. A temperature controller was usedto adjust the gas temperature at the inlet to the processvessel.

Electron beam irradiation of flue gas. The process vesselwas constructed as a cylinder with diameter of 200 mm andlength of 850 mm. Electron beam was introduced into theprocess vessel perpendicularly to the axis of the vessel. Itpasses through a thin titanium (Ti) entrance windowinstalled on top of the vessel. The active dimensions ofthe Ti foil were 350 120 mm2 and thickness of 50 lm.The electron beam was generated by an electron accelera-tor ILU-6. It is a single cavity high frequency, pulsed reso-

nant accelerator. Average beam power was up to 20 kW

and electrons energy 800 keV. The dose applied to the fluegas was changed by varying the repetition rate of the elec-tron pulse. The flue gas flows through the process vessel,where it was irradiated by the beam of fast electrons inthe presence of a near-stoichiometric amount of ammonia.Essential changes in flue gas composition occur as a result

of physicochemical processes subsequent to flue gas irradi-ation. The magnitude of changes in its compositiondepends on the process parameters.

Filtration unit. Gas leaving the process vessel flowsthrough a retention chamber (10 in Fig. 1) to provideenough residence time for the formation of the by-product.These salts were recovered as a dry powder using bag filter(11 inFig. 1). Goretex bags with a teflon coat were used forcollection of by-product.

Exhaust of treated flue gas. Treated flue gas leaving thebag filter was drawn through duct by ID fan and was dis-charged to the atmosphere through a stack. The measure-ment point (12 inFig. 1), designed for monitoring of SO2and NOxconcentration in the treated flue gas, was locatedat the outlet of the bag filter.

Monitoring system. The flue gas contained high concen-trations of SO2, NOx, H2O and CO2 In order to assureaccurate measurements of flue gas components the follow-ing selections were made:

Type of gas analyzers: a UV pulsed fluorescent SO2ana-lyzer Model 40 and a chemiluminescent NO/NOx ana-lyzer with molybdenum converter Model 10 A/R,manufactured by Thermo Electron Corporation (TEC,USA), were used.

Continuous emission monitoring (CEM) system oper-ates by utilizing the sample gas dilution unit. The heatedsample gas dilution and conditioning Model 900 (TEC)was applied with dilution ratio 20:1.

The gas leaving bag filter may contains un-reactedammonia. The heated ammonia scrubber from Shima-dzu (Japan) was used for selective absorption of ammo-nia gas from the sample gas without changing theconcentration of the other components.

Two extractive multi-gas monitoring systems wereinstalled for continuous measurement of SO2 and NOxconcentration in the flue gas: one at the process inlet(before irradiation, point 5 inFig. 1) and second at the pro-cess outlet (after bag filter, point 12 in Fig. 1).

The portable flue gas analyzer type Lancom II, manu-factured by Land Combustion (UK), measures the concen-trations of CO, CO2, O2, SO2, NO, NO2, NOx, andhydrocarbons in the flue gas. Such measurements were per-formed in the flue gas at outlet of oil burner and occasion-ally at process inlet and outlet to check readings of bothCEM systems. The temperature of flue gas at six essentialpoints of experimental set-up was measured and controlledusing K thermocouples. The humidity of flue gas was deter-mined by manual analytical method based on EPA method

4 using u-tubes filled with granular silica gel.

Table 1Properties of Arabian liquid fuels

Property Unit AM AH HFO

Density at 15 C kg/m3 878 890 980Gross heating value MJ/kg 43.29 43.31 43.43Kinematics viscosity at 50 C cSt 7.33 15.45 175.0Sulfur content %wt 2.81 2.90 3.10

Ash content ppmw 115 268 45

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Electron-beam energy absorbed in the flue gas, definedas an absorbed dose in units of gray (Gy = J/kg), was esti-mated using a cellulose triacetate (CTA) tape of 8 (w) 0.125 (t) mm (Fuji photo film, FTR-125) [9].

3. Results and discussion

During this investigation three types of Arabian liquidfuels were combusted. Measurement of the emission ofSO2and NOxfrom the burning of these fuels was the firsttask of this work. The measurements of flue gas composi-tion were performed at the process inlet for each grade offuel. Table 2 summarizes averaged volumetric emissionconcentrations for all tested fuels.

Observations from these measurements are as follows:

Very high level of SO2emission. All SO2concentrationswere above 1200 ppmv while the sulfur content in the

burned oils was above 2.7% wt., NOx emissions ranged from 150 to 170 ppmv, oxygen content in the analysed flue gases ranged from

3.4 to 4.4% vol., natural humidity of flue gas was higher than 8.48% vol.

To evaluate SO2 and NOx emissions under comparableconditions, the measurements were normalized at 3% vol.oxygen and dry gas basis conditions. The data indicate thatSO2 emissions from combustion of these high-sulfur fueloils were very high, about 1220 ng/J, which if releaseduntreated, will adversely impact the ambient air qualitystandards. Emissions of NO

x

were also high, in the rangeof 110120 ng/J. In order to meet air quality standards,these flue gases require post-treatment with high SO2 andNOx removal efficiencies.

The applicability of the electron beam technology forremoval of SO2 and NOx from these flue gases was themain objective of this investigation. In previous studies ofthis technology it was demonstrated that removal efficien-cies of both pollutants depend on the following processparameters: absorbed dose (D), ammonia stoichiometry(aNH3 ), gas temperature at inlet to process vessel (TinletPV),gas humidity (H) and inlet NOx concentration (NO

0x

). Theparametric study of SO2 and NOx removal efficiency was

performed for flue gas obtained from combustion of eachArabian fuel.

Effect of electron beam dose on SO2and NOxremoval.Fig. 2 presents the dose dependence of SO2 and NOxremoval efficiency. The absorbed dose is the primary factorinfluencing NOx removal efficiency. The reaction starts at

zero efficiency for zero dose and indicating a saturationat high dose. This demonstrates that NOx removal is aradiation-induced process. Higher NOx removal wasachieved with higher absorbed doses because the amountof NOx molecules removed corresponds to the amount ofactive species formed by electron beam irradiation of fluegas.

The SO2 removal is based on two different pathways:thermal process and radiation-induced process. At zerodose, SO2 removal is caused by thermal reaction of SO2and NH3in the presence of moisture. These reactions takeplace in the gas phase as well as on the surface such asthose on the filter cake of bag filter. Sulfur dioxide removal

increases sharply with increase of electron beam dose up to8 kGy and then flattens out at high doses.

Effect of ammonia stoichiometry.Fig. 3presents the SO2and NOx removal efficiency obtained in two experimentswith combustion of Arabian Medium Oil. First experimentwas performed with electron beam treatment of the flue gaswithout any additives (without NH3). Low SO2 and NOxremoval efficiencies were obtained. In the second experi-ment, gaseous ammonia was added to flue gas before itsinlet to process vessel. The NH3 stoichiometry was equalto 0.90. In this case higher removal efficiency of SO2 andNOx were obtained. Ammonia addition significantly influ-

ences SO2removal.Fig. 4 shows effect of ammonia stoichiometry on SO2

and NOx removal efficiency.Effect of gas temperature at the inlet to process vessel.

Fig. 5 presents the effect of gas temperature on SO2 andNOx removal efficiency. The figure shows that higher gastemperatures yielded higher NOx removals. This is in con-trast with the SO2removal, which decreases with higher gas

Table 2Composition of flue gas from combustion of three Arabian fuels measuredat process inlet

Parameter Unit AM AH HFO + 10% LO

SO2 ppmv 1215 1270 1250/1355a

NOx ppmv 153 168 169O2 % vol. 4.40 3.42 3.78H2O % vol. 8.48 9.06 9.58CO ppmv 0 0 36CO2 % vol. 12.4 15.5 13.6CxHy % vol. 0.18 0.16 0.17Sulfur content in oil % wt. 2.81 2.90 2.7/3.1

a SO2 concentration recalculated for combustion of HFO only.

Fig. 2. SO2 and NOx removal efficiency as a function of dose from

combustion of Arabian Heavy Fuel Oil + 10% Light Oil.

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temperature. The SO2removal efficiency is improved as the

gas temperature approaches its dew-point temperature.

Gas temperature has a significant impact on the SO2removal and a small effect on the NOx removal efficiency.

These phenomena are governed by equilibrium condi-tions for ammonium sulfate formation which depends onthe temperature; above 125 C this salt undergoes thermaldissociation. This indicates that flue gas temperature at

inlet to process vessel can be effectively used to changeSO2 removal efficiency with minimal impact on the NOxremoval.

Effect of flue gas humidity. Two experiments for electronbeam treatment of flue gas at different humidity levels werepreformed. Flue gas with natural humidity (9.62% vol) wasirradiated in the first experiment. Additional water vaporwas injected to the flue gas in the second experiment toincrease its humidity to 11.57 % vol.

Fig. 6 presents the SO2 and NOx removal efficienciesobtained in both experiments. The increase of flue gashumidity does not affect NOx removal. The SO2 removalefficiency increases markedly with the increase in humidity

of flue gas. This increase is due to the thermal reaction ofSO2with ammonia in the absence of electron beam irradi-ation. The dew-point depends on gas temperature andhumidity; this is another evidence of the occurrence of het-erogeneous reaction.

By-product analysis. In each experiment a by-productwas collected in the bag filter. The by-product (white solidparticles) was analyzed using ion chromatography 2000i/SP (Dionex, USA).Table 3 presents the chemical charac-teristics of by-products collected in the different experi-ments with combustion of the three Arabian fuels.

The major constituents of by-products were sulfate,

nitrate and ammonium ions. The contents of these constit-uents varied depending on the type of fuel (AM, AH, orHFO) used in the experiments. Ion chromatography anal-yses indicate that by-product consists primarily of ammo-nium sulfate and ammonium nitrate. The data, however,show that the amount of ammonium nitrate collected inthis process was less than the amount of NOx removedfrom flue gas. This is due to the fact that in electron beamprocess, NOx is partly converted to nitrogen gas (N2) andN2O through homogeneous gas phase reactions

NH3OH ! NH2 H2O

NH2 NO ! N2 H2O

NH2 NO2 ! N2OH2O

Experiments with N-15 labeled compounds, performedat IAERI, Japan [10], proved that NOx is partly reducedto molecular nitrogen (20%) and N2O (

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Dose: 6 kGy only for SO2removal and 12 kGy for SO2and NOx removal,

ammonia stoichiometry: 0.95, gas temperature at inlet to process vessel: 65 C, gas humidity: 11.5 % vol.

An experiment with the selected process parameters wasperformed with flue gas from combustion of ArabianHeavy Fuel Oil + 10% Light Oil. SO2 removal efficiencyof 98% and NOxremoval efficiency of 82% were obtained.The obtained SO2removal efficiency is significantly higherthan that of a conventional dry scrubber and comparableto that of the wet lime scrubber systems employed in thedesulfurization of flue gas emitted from coal-fired boilers.

Operational problems and developed solutions. Duringthe one year operation of INCT laboratory plant withcombustion of Arabian oils, no major failures werenoticed, indicating the reliability of the electron beam pro-cess. A major problem, encountered at the beginning ofthis project, was removing particulate matter and sootfrom flue gas before its introduction to the process vessel.They should be removed to assure the suitability of by-

product for agricultural application as well as to avoid

their deposition throughout the installation. Soot particlestend to be sticky and can lead to clogging of duct and otherpollution treatment equipment in the facility. A two-stepfiltration system was applied. This solution was successful.The by-product collected in bag filter was a white powderdevoid of soot and other particulates. For industrial plant,

a continuous flow collector (wet or dry) should be applied.Another difficulty was in maintaining a stable bag filteroperation due to sharp increase in pressure drop over a rel-atively short period of operation. This leads to poor releaseof by-product from the bag surface. For industrial plant itis preferable to use the electrostatic precipitator, whichshall be specifically designed for collection of sticky andhygroscopic by-product.

4. Conclusions

The investigation of the electron beam process for treat-ment of flue gas from combustion of Arabian liquid fuels

shows the following main advantages of this process overcurrently used conventional ones:

Simultaneous removal of SO2and NOxat high efficiencylevels. SO2removal efficiency up to 98% and NOxup to82% were obtained under optimal conditions.

It is a dry process which can be easily controlled andoperated. Only three parameters need to be regulated:water and ammonia dosage to the flue gas and electronbeam current (i.e. dose).

The process yields a usable and valuable by-product. Itis a mixture of ammonium sulfate and nitrate which can

be used as such as a fertilizer or can be blended withother fertilizer components to produce commercial agri-cultural fertilizers NPK or NPKS.

No wastewater is formed in the process. Economic studies show that the investment and opera-

tion costs of the process are very competitive. In addi-tion, revenue from by-product sale could lower thecosts significantly.

This is the first experimental confirmation of the appli-cability of electron beam technology for treatment of fluegas from combustion of high-sulfur liquid fuels. Duringthe study, several limitations of the process were encoun-tered as follows:

The energy consumption of the process is relatively high,especially for application to flue gas with high NOxandlow SO2 concentrations. Further reduction of energyconsumption by process improvement is required.Multi-stage irradiation and non-uniform dose distribu-tion between irradiation stages are effective on DeNOxand reduce energy consumption by 1520% [11].

Flue gas directed to the process should be cleaned offsoot and other particulates to a level lower than50 mg/Nm3 for two reasons. Firstly, the particulates

present in the flue gas will contaminate the by-product

Fig. 6. Effect of humidity on the SO2 and NOx removal efficiencies forcombustion of Arabian Heavy Oil.

Table 3

Chemical characteristics of by-product collected in bag filter in theexperiments with combustion of the three Arabian fuels

Parameter AM AH HFO +10%LO

Loss of weight duringdrying at 105 C

0.77 2.4 1.39

Water insoluble 0.33 0.005 0.30 0.005 0.96 0.05

Ion content in water soluble partSO24 78.06 3.9 74.4 3.7 74.7 3.7NO3 1.68 0.03 1.59 0.05 0.51 0.025NH4 20.055 1.1 23.79 1.1 24.3 1.1Na+ 0.29 0.0014 0.02 0.001 0.02 0.001K+ 0.009 0.0004 0.04 0.002 0.2 0.01Ca2+ 0.065 0.0033 0.16 0.008 0.2 0.001

Values of parameters are expressed in % wt.

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