REDUCTION OF 802 EMISSIONS BY AMMONIA GAS DURING UNSTAGED COMBUSTION W. Z. KHAN*

Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, K.S.A. and B. M. GIBBS

Department of Fuel and Energy, University of Leeds, Leeds, LS2 9JT, U.K. (Received: February 1995; revised: July 1995)

Abstract. The reduction of SO2 by the addition of ammonia gas has been studied in a 2 m high fluidized bed combustor having a 30 cm static bed height and a freeboard height of 170 cm. Ammonia gas was injected at 52 cm above the distributor where the temperature is ca. 700 ~ by an uncooled stainless steel tube injector. Experiments were carded out to investigate the effects of amminia gas injection on sulphur dioxide emissions at unstaged conditions of: (i) excess air level, (ii) NH3:SO2 molar ratio, (iii) fluidizing velocity and (iv) bed height. A maximum reduction of 75% in SO2 emissions was found at 40% excess air, at an NH3:SO2 molar ratio of 5.4. The onset of SO2 reduction occurred at an NH3:SO2 ratio of 1.5. However, the most effective ratio was found to be between 3 and 5. Fluidizing velocity and bed height were also found to have significant influence on SO2 reduction. It is difficult to determine how the SO2 reduction varied with operating conditions. When ammonia is added in the main combustor zone, the temperature is much higher than that required for the occurrence of sulphur dioxide-ammonia and sulphur trioxide-ammonia reactions. However, this paper points out the significance of ammonia addition in the reduction of sulphur dioxide.

1. Introduction The fluidized bed combustion of coal has the potential for integrating environmental control by the simultaneous reduction of oxides of nitrogen and the sulphur. The unstaged combustion of coal is a normal combustion condition where all the combustion air is introduced to the bottom of the combustor through the air distributor. The distributor has multihole projections to maintain a uniform distribution of air. The area above the bed where combustion is completed is called the freeboard. The combustion air fluidizes the bed of sand particles, and by changing its flow rates, the desired fluidizing velocity (susperficial gas velocity) can be set. All the coal is injected into the bed at a certain feed rate. The rate of coal feed and fluidizing air maintains the bed above stoichiometric conditions. If the air exceeds the stoichiometric requirement of coal, the combustor is in an overall excess air mode as in a conventional operation. Due to fluidization, the static bed of sand particles expands and the height of the expanded bed depends upon the fluidizing velocity. * To whom correspondence should be addressed.

Environmental Monitoring and Assessment 40: 157-170, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.

158

W.Z. KHAN AND B. M. GIBBS CONTROL VALVE

H2

,

,.

Heat Exchanger

Coil

PRIMARY

~

i

TC6

,

LI.................,~,..|:: re41 l a s h

--~]~i--:','.~::,:"-"; r~l

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'wl

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Ammonia gas injector

PG - PRESSURE GAUC~ TC - THERHDCDUPLE

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CW = COLD WATER

Fig. 1. Mainfeaturesof fluidized-bedcombustor.

Lucas 1 report that NH3 injection does not affect SO2 and SO3 emissions, whereas some other researchers2'3'4 report that ammonia reacts with SO2 and SO3 to form either ammonium sulphate and ammonium sulphite or ammonium bisulphate and ammonium bisulphite via the reactions (1) to (4) given below. NH4 HSO4

(1)

SO3 + H20 + 2NH3

---* (NHnh 504

(2)

SO2 + H20 + 2NH3

---* (NH4)2 SO3

(3)

SO2 + H20 + NH3

---* NH4 HSO3

(4)

SO3 + H20 + NH3

These reactions are low temperature reactions and occur between 200 ~ and 260 ~ and, in the case where ammonium sulphite and bisulphite are formed, below 60 ~ However, the results of Barnes 5 indicate a reduction in SO2 when ammonia gas is injected in the main combustor zone and freeboard, where the temperature is very high. It was thought that the reaction between ammonia and SO2/SO3 occurs downstream of the reaction zone or in the sample line.

REDUCTION OF SO2 EMISSIONS BY AMMONIA GAS

159

TABLE I Typical analysisof Coventrycoal Proximateanalysis(dry basis) weight(%) Ash Volatile matter Fixed carbon

6.22 33.00 60.78

Ultimateanalysis(dry basis) weight(%) Carbon 77.51 Hydrogen 4.8 Oxygen 8.5 Nitrogen 1.43 Sulphur 1.5 Moisture 5.0 Gross calorificvalue (MJ/kg) 31.185.

2. Experimental Set-Up The experimental set-up is shown in Figure 1. The set-up used in this study is composed of a fluidized bed combustor, blower, propane overhead burner, coal (Table I shows the typical analysis) hopper and feeding assembly, heat exchanger, cyclone and ash collector. The combustor was constructed from stainless steel and was 2 m high and 0.3 m square in cross-section. The bed consisted of silica sand of mean size 0.665 mm. The static bed height was 30 cm. The fluidization or primary air, supplied by a blower, was metered and introduced to the plenum chamber below the air distributor plate. The coal sized -16+3 mm was supplied from a sealed coal hopper and fed via a screw feeder into the combustor. The coal entered 42 cm above the air distributor plate. It should be noted that all the heights in the combustor are measured from the air distributor plate. The ammonia gas is metered, mixed with a known volume of nitrogen gas (a carder gas) and introduced by an injector into the freeboard area of combustor. An overhead propane burner was used to preheat the bed during the start-up of the combustor. Exhaust gases were ducted to a cyclone where carryover was removed and collected in an ash bin. The temperature of the bed, freeboard and exhaust gases were continuously monitored by Chromel Alumel thermocouples located in the bed (TCI-TC4), freeboard (TC5) and the exit (TC6) of the combustion gases. A sampling probe, located at the cyclone exit, was used for the determination of SOz in the flue gases (combustion gases). The bed temperature was controlled by an adjustable heat exchanger coil immersed in the bed. Further details of the experimental set-up and procedure can be found in Khan and Gibbs 2~

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W. Z. KHAN AND B. M. GIBBS

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