Effect of HCl on the Formation of Calcium Silicates in
Sand Beds in Fluidised Bed Boilers
JATTA PARTANEN∗, PETER BACKMAN AND MIKKO HUPA
Åbo Akademi
Process Chemistry Group
Turku, Finland
* On leave from Fortum Engineering Oy, Vantaa
ÅA/PCG/CMC/2002
CaO(s) + SiO2(s)
→ CaO
.
SiO2(s)
(CaSiO3(s))
slow at T < 1000 °C
ÅA/PCG/CMC/2002
EXPERIMENTAL WORK WITH A TGA
A
ÅA/PCG/CMC/2002
THERMOGRAVIMETRIC APPARATUS
(TGA)
Cl
ÅA/PCG/CMC/2002
HF
He
• Approximately 205 mg of quartz sand (Merck, pro analysi) was mixed with 40 mg
limestone powder.
• Experiments of varying duration were made at 850oC.
• The samples were examined by scanning electron microscopy (SEM) and energy
dispersive spectrometry (EDS) to verify the chemical changes in them.
• On the following page SEM-EDS results are shown for a sample prepared at 850oC,
but after being in a gas containing 5% O2 in nitrogen and no HCl for 90 min. the TGA
did not show any change in the sample's mass during this experiment, and the SEMEDS results clearly show that no calcium silicates were formed in the absence of HCl.
• This supports the claim that CaO and SiO2 do not react with each other under FBC
conditions through a direct solid-solid reaction.
ÅA/PCG/CMC/2002
200 µ m
EDX line profile analyses
Counts
1000
900
800
Si
700
600
Cl
Ca
500
400
300
200
100
0
0
40
80
12 0
160
2 00
240
Point number
Linear analysing direction
EDX line profile analyses of Si, Ca and Cl for a sample of SiO 2 and CaO
treated in O 2 and N2 at 850oC. Reaction time: 90 min.
ÅA/PCG/CMC/2002
Temperature and sample mass versus time
Gas atmosphere during reaction step: 5% O2 and 2000 ppm HCl in N2
1800
3600
5400
7200
250
Weight, mg
CO2
240
N2
900
800
700
600
500
400
300
200
100
0.2%HCl, 5.0%O2 and N2
230
220
210
0
1800
3600
Time, s
ÅA/PCG/CMC/2002
5400
7200
Temperature, °C
0
Temperature and sample mass versus time
Gas atmosphere during reaction step: 5% O2 and 2000 ppm HCl in N2
0
1800
3600
5400
7200
250
900
N2
800
0.2%HCl, 5.0%O2 and N2
Weight, mg
240
700
CaO(s) + 2HCl(g)
600
CaCl2(s,l) +H2O(g)
230
500
1 or 2
400
220
300
200
or
210
100
0
1800
3600
5400
7200
Time, s
1. CaCl2(l) + SiO2(s) + 0.5 O2(g) => CaSiO3(s) + Cl2(g)
or
2. CaCl2(l) + SiO2(s) + H2O(g) => CaSiO3(s) + 2HCl(g)
ÅA/PCG/CMC/2002
Temperature, °C
CO2
CaCl2 + SiO2 + H2O(g) = CaSiO2 + 2HCl(g) (HSC 4.1)
1000000
c(HCl), ppm
100000
CaCl2
10000
c(H2O) = 25%
1000
CaSiO3
c(H2O) = 1%
100
10
1
0
200
400
600
Temperature, oC
ÅA/PCG/CMC/2002
800
1000
200 µm
EDX line profile analy ses
Counts
800
700
Si
600
Ca
500
Cl
400
300
200
100
0
0
40
80
120
160
200
240
Point number
Linear analysing direction
EDX line profile analyses of Si, Ca and Cl for a sample of SiO2 and Ca O
reacted with HCl, O 2 and N 2 at 850 oC. Rea ction time: 10 min.
ÅA/PCG/CMC/2002
100 µm
ED X line profile analyses
6000
Si
5000
Ca
Counts
4000
Cl
3000
2000
1000
0
0
40
80
120
160
200
240
P oint number
Linear analysing direction
EDX line profile analyses of Si, Ca and Cl for a sample of SiO2 and CaO
reacted with HCl, O2 and N2 at 850o C. Reaction time: 90 min.
ÅA/PCG/CMC/2002
More information:
Partanen, J., BACKMAN, P., and HUPA, M.,
The Effect of HCl on the Formation of Calcium Silicates in Sand
Beds in Fluidised Bed Boilers, Combustion and Flame 2002
ÅA/PCG/CMC/2002