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ELSEVIER Solid State Ionics 86-88 (1996) 1013-1016

STAm IONICS

Development and testing of HCl gas sensor for flue gas monitoring

N.N. Ahmed”3b, G.M. Kale”, R.V. Kumar”‘*, D.J. Fray”

*Department of Mining and Mineral Engineering, University of Leeds, Leeds LS2 JT, UK “Department of Fuel and Energy Engineering, University of Leeds, Leeds LS2 JT, UK

Abstract

A novel solid-state sensor for monitoring HCI gas incorporating a unique combination of two solid electrolytes, strontium

cerate (SrCeO,) doped with 10 mol% neodymium oxide (Nd,O,) that can exhibit protonic conduction at temperature 2 773

K, and strontium chloride (SrCl,) which is a chloride ion conductor, has been developed. The optimum composition for the doped cerate has been found to be (0.7)SrO.CeO, containing 10 mol% Nd,O,, produced by calcining the raw materials at

1573 K and sintering the fabricated tube at 1773 K. Sensors gave reproducible electromotive force (e.m.f.) with varying partial pressure HCI gas in argon at temperatures above 773 K.

Keywords: HCl sensor; Doped SrCeO,; Proton conductor; &Cl,; Cl- ion conductor

1. Introduction

The quality of the environment depends on moni-

toring and controlling complex species from natural and anthropogenic sources. The major anthropogenic

sources of HCI emissions are coal power plants and incinerators for burning domestic, clinical or in-

dustrial wastes. Coal may contain substantial amounts of chlorine in the mineral matter. Pyrohydrolysis during combustion results in the formation of HCl gas, which unless treated is released into the atmosphere. Combustion of plastic based products in waste incinerators give off, amongst other gases, HCl in the flue which once in

contact with water from the combusted material or in the atmosphere, forms highly corrosive HCl acid.

HC1 is gaining increasing recognition as a serious pollutant which needs serious attention. It is like SO, a contributor to acid rain, and more so than SO,,

*Corresponding author.

exerts severe local effect, as it does not have to

participate in any further chemical reactions to become an acid. Regulations are being introduced in

many countries to reduce the levels of HCl emitted into the atmosphere, typically less than 30-100 ppm.

Removal of HCl can be achieved by electrostatic

precipitators and scrubbing equipment. Both in order to ensure compliance with regulations and to control the gas scrubbing systems, continuous monitoring of HC1 is likely to be widely practiced in the future. The conventional spectroscopic techniques based on 1R are expensive and require careful sampling and conditioning in special heated probes, and may be

cross-sensitive to other species especially moisture. A novel solid-state electrochemical sensor for

monitoring HCl is being developed using solid electrolytes. Although solid electrolytes have been used to develop chlorine sensors [ 11, no earlier work has been reported on solid state HCl gas sensors. Using a new approach, two solid electrolytes are interfaced together [2], without any need for a

0167-2738/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved

PII SO167-2738(96)00243-3

1014 N.N. Ahmed et al. I Solid State Ionics 86-88 (1996) 1013-1016

separate reference compartment. The two solid elec-

trolytes used are Nd,O,-doped SrCeO,, a H+ ion or a mixed conductor [3] in the form of a tube, and SrCl, a Cl- ion conductor [4], which is melted into

the cerate tube.

2. Experimental procedure

Strontium cerate was synthesised using appro-

priate amounts of SrCO, and CeO, (both 99.9% pure). The powder was ball milled in acetone, dried and calcined at 1300°C. The fine powders were

characterised by X-ray diffraction (Cu Kcu radiation). The SrCeO, was then mixed with 10 mol% Nd,O,.

Doped SrCeO, tubes were fabricated by iso-static

pressing of the powders and sintering resultant tubes

Fig. 1. The interlace between the two solid electrolytes, 10 mol% Nd,O, +SrCeO, and &Cl,: (a) SEM view of exposed surface, (b)

back scatter image.

Strontium chloride

1 Om% neodymium oxide doped in strontium cerate

+ve - v’e Pt. lead firmly attached

Fig. 2. Schematic diagram of HCl gas sensor.

at 1500°C in air. The sintered solid electrolyte tubes were 15 mm long with 3 mm inner diameter and 4

mm outer diameter. Platinum lead was attached by Pt

ink (type A4731, Engelhard) onto the outer surface of the doped SrCeO, electrolyte tube. The second

electrolyte SrCl, was melted (m.p. 1147 K) into the doped SrCeO, tube forming a compact solid plug. The interface between the two electrolytes is com- pact as shown in Fig. 1.

The solid state sensor used in this study is schematically shown in Fig. 2. The response of the

solid-state HCl sensor is monitored in HCl (g) + Ar atmosphere of known partial pressure at temperatures of 775 and 876 K. The detailed experimental set-up

is shown in Fig. 3. The e.m.f. of the sensor was

measured between 900 and 3000 ppm of HCl gas in dry argon, thus evaluating the behaviour of the

sensor e.m.f. under controlled environment. The e.m.f. was mointored using a high impedance ( > lOI ohms) digital electrometer (Keitheley 614) and recorded on a Siemens x-t chart recorder.

3. Results and discussion

For 1: 1 ratio of SrO and CeO,, a homogenous

phase of SrCeO, was observed and agreed well with reported XRD data [5]. A molar ratio of 0.7:1 was seen to produce the optimum density, fine grained material and a compact interface between the cerate and SrCl,. 10 mol% of Nd,O, doping in cerate was selected to obtain favourable electrical properties in the cerate electrolyte [6,7].

Initial stabilisation of e.m.f. in HCl-containing gas took several hours. Once the stable values were

N.N. Ahmed et al. I Solid State Ionics 86-88 (1996) 1013-1016 1015

0.3% HCI gas

in argon

I)rying Cohmm Flow Meters Gas outlet to water path Quartz apperatus for

,__> _)_ Mixing Column j Sensor holding

A

>-- A

on/off Valve

”

High Impedance DIgital Electrometer

E Earthlead Keithkey 614 + Positwe lead - Negative Lead

Fig. 3. Schematic illustration of experimental apparatus.

T .7751( I attained, the performance of the solid-state HCl gas

I sensor in varying partial pressure of HCl gas in argon showed a rapid response ( < 40 s). Several

sensors were fabricated and tested for reproducibil-

3000 ity. An example of the performance of the sensor is shown in Fig. 4. Variation of the e.m.f. as a function

m t

of the logarithm of partial pressure of HCl gas in

1S30 argon at temperatures of 775 and 876 K are plotted

emf vs ppm HCI

in Fig. 5, which shows a linear relationship, given by

tEeZZ(tZg P,,,) + 152.53 mV, (2)

Fig. 4. E.m.f. vs. ppm HCI (chart recorder data). E = 91.59 (log P,,,) + 122.04 mV , (3)

E a,

260.

2 2 0, , .96 2.16, 2.36

LO9 pHCI(Pa)

Fig. 5. Plot of e.m.f. vs. log PHc, (g),

1016 N.N. Ahmed et al. I Solid State Ionics 86-88 (1996) 1013-1016

respectively. It is therefore deduced that the number

of electrons are =2. A cell reaction with n =2 is postulated and needs to be thoroughly investigated. It is suggested that SrHCl is formed on the surface of

the SrCl> due to reaction between SrCl, and HCI.

SrHCl is known to be stable under non-oxidising

conditions and has been utilised as an electrolyte in

sensing hydrogen [3]. This may explain the long initial time required for stable e.m.f.s to be produced.

The solid state HCl gas sensor can thus be

schematically represented as

Gas Containing Gas Containing . (-)Pti HCI iSrCeOl+ 10 mol’% Nd203 /SrC121SrHCII HCI /PI(+)

The postulated anodic and cathodic reactions are

Anode 2Cl + SrHCl&SrCl, + HClcinterface) + 2e _

(4)

Cathode HCl(,, + SrCl, + 2e-&SrHCl + 2Cll.

(5)

Therefore the overall cell reaction can be written as

HC1~g~CJHC1(interface, (61

with the following Nemst equation

E = $ In P,,, -R$lna HCl(interface) (7)

where: a = thermodynamic activity; T= temperature

(Kelvin, K); F = Faraday’s constant 9.65 E +04 (C

mol- ’ ); n = number of electrons; P = partial pressure

(atm); R=8.314 J mol-‘. It is concluded that the proton conductor essential-

ly acts both as an electrode and a means of fixing the activity of HCl at the interface, resulting in the

equation:

E = !$ In PHc, + Constant. (8)

Essentially, the sensor has no separate reference

material and is, therefore, much less susceptible to drift and degradation.

4. Conclusion

A unique solid state electrochemical sensor was

fabricated by interacting doped SrCeO, and SrCl,.

SEM revealed a compact dense interface between the electrolyte. The sensor was found to be sensitive to

small change in partial pressure of HCl(g). The observed responses to these changes were rapid (<40 s) and stabilised to a constant e.m.f. for a fixed P HC, at a given temperature. A linear relationship is

obtained between the measured e.m.f. of the sensor

and logarithm of partial pressure of HCl gas in argon atmosphere, thus indicating a Nemstian behaviour.

The e.m.f. are reproducible to about 510 mV. The

postulated cell reaction suggests that e.m.f. is de- pendant upon log P,,, with n = 2.

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