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Gasification of sulphate soap for the lime kiln
Bioresource Technology 46 (1993) 159-165
GASIFICATION OF SULPHATE SOAP FOR THE LIME KILN Karl Saviharju Technical Research Centre of Finland (VTT), Laboratory of Fuel and Process Technology, PO Box 205. FIN 02151 Espoo, Finland
&
Tuomas Timonen Finnish Energy' Economy Association (ETY), Finnish Recovery Boiler Committee, SF-02151 l:spoo, tblland
The successful evaporation of black liquor requires that soap be separated through acid from a weak and/ or intermediate liquor. Conventionally, sulphate soap has been utilized either by cooking it to raw tall oil and by selling the end-product forward to distilleries or by burning it in recovery boilers. However, both these methods are problematic in the present sulphate-pulp mills. Tall-oil production has an indirect effect on the mill's sulphur balance either by increasing sulphur emissions or by causing additional costs involved in reducing SO2 with separate scrubbing systems. Furthermore, tall-oil production is today uneconomic, since its price has collapsed. Neither is burning of sulphate soap in recovery boilers justified owing to the surplus in the energy bahmce of the mill. Further, at many mills, the recovery boiler is the bottleneck of production and cannot be loaded any more. If the alkaline solids could be removed, about 4-8% of Na + K in dry solids, crude soap could be fired directly in lime kilns. Without the removal, low-melting compounds are formed in kilns, resulting in unacceptable operational problems. A typical analysis of crude soap is presented in Table 4 below. The above two targets, reduction in sulphur emissions and cost-savings by replacing purchased fuel, can be achieved by soap gasification. Sulphur emissions would fall automatically if sulphuric acid and highsulphur oil were withdrawn from use. In addition, the tendency to use chlorine-free pulp bleaching will cause problems in many mills when the sulphuric acid used in tall-oil cooking is reduced. Many mills save a considerable sum of money in their fuel costs by using synthetic gas to replace heavy fuel oil normally used in mills. The heat input from soap is quite well in balance with the 20-30 MW based on LHV, required for the lime kiln in Finnish mills. The project 'Processing of sulphate soap to lime-kiln fule" was carried out by the Laboratory of Fuel and Process Technology of VTT and was financed by the ETY-Finnish Recovery Boiler Committee and the Ministry of Trade and Industry through the JALO
Abstract
Sulphate soap is" a by-product of pulp mills utilized as a raw material for the chemical industry. However, this results i, an increase in sulphur input of several kilograms S(): per ADt pulp into the mill. Another increasingly interesting alternative is to utilize soap in the lime kiln of the mill. This has a positive effect, in addition to sulphur problems, on the energy balance of the mill. The crucial problem is the high Na + K content of the soap, from 4 to 8%, which can result in plugging of the lime kiln. The operational problems can be avoided by gasi~,ing the soap and by separating the inorganic materials fi'om the product gas before the kiln. This paper describe~" research work on the gasification of crude sulphate soap carried out at VTT over the years 1991 and 1992. This work will be continued in 1993 by focusing on specified problems, after which commercial applications ~hould be available. A detailed research report will be published after this" second stage of research has been completed. Key words: Gasification, sulphate soap, lime kiln, sulphur emissions, black liquor.
INTRODUCTION Fhe development of the recovery process and its equipment is today driven by investment cost, energy efficiency, and environmental requirements. The interest of Finnish sulphate-pulp mills in the gasification of sulphate soap for the lime kiln is based on two factors: (i) the possibility of reducing sulphur emissions from the soap treatment; and (ii) the possibility of replacing fuel purchased from elsewhere by the plant's own biomass product. Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
159
K. Savihar]u, T. Timonen
160
research programme. The pilot tests were carried out in the VTT test plant located at the ,~iinekoski Mills of Metsii-Sellu Oy. During the first two years, in 1991-92, the suitability of sulphate soap for a lime-kiln fuel was studied both theoretically and experimentally. The planned further work will focus on practical problems and is discussed at the end of this paper. PRODUCT GAS QUALITY Product gas to the lime kiln has to fulfil two criteria: (i) the amount of Na + K in the gas must be low enough to prevent agglomeration in the lime kiln; and (ii) the sum of chemical energy and sensible heat must be high enough to produce the required temperature. The L H V of sulphate soap as received is approximately 20 MJ/kg, which is enough to produce a good fame. The problem is the high alkaline content of soap. Na ÷ K of dry solids may range from 4 to 8%. The acceptable value requires further work, the required reduction being approximately 50-90%. According to Kiiskilii (1985), the stoichiometric adiabatic-flame temperature for the product gas should exceed 1750°C. This figure is based on wood-waste gasification. Table 1 indicates how this is achieved with different types of gasifier. Product-gas cooling in some options is thought to be required for alkaline and unreacted carbon recovery. The calculations show that water-washing of product gas for N a + K recovery is not possible. The
remaining possibilities are either-solid phase recovery or smelt recovery with minor product-gas cooling. GASIFIER OPTIONS Because the economic benefit is limited, the equipment cannot be complicated. Possible options are presented in Table 2. The entrained-flow gasifier has to operate clearly above the melting point of the salts to avoid slagging and to achieve a high reactivity in a short residence time. Salts may be recovered through a slagging cyclone or an agglomerating separator. Owing to the short residence time, the sulphur reduction is poor for non-reduced sulphur in soap. Below the melting point, a pyrolyzer-type reactor seems to be the most promising owing to the low carbon residue in char and the long residence time required for high conversion. The gasifier may be equipped with a burner, or it may be a fluidized-bed reactor. The advantage of the burner is a simple construction and the disadvantage an uneven temperature profile and difficult control of the local gas atmosphere. GASIFICATION BEHAVIOUR OF SULPHATE SOAP Gasification behaviour was studied with a heating-grid pyrolyzer and an atmospheric thermobalance. The main problem was the char reactivity because the volatiles from soap were reacting fast compared with char. Figure 1 shows how soap is pyrolyzed and gasified in a thermobalance. Fast pyrolysis starts at 300°C and is completed at 500°C. Figure 2 shows how the amount of
Table 1. Adiabatic-flame temperature of the product gas (65% dry solids in soap, stoichiometric combustion in lime kiln)
Gasification temperature (°C)
Gasification air of ' stoichiometric (%)
Product-gas cooling
Adiabatic-flame temperature (°C) (rain 1750°C required)*
900 700 500 900
43 35 30 38
1870 1880 1880 1870
700
32
900
43
700
35
900 700 500 500 500
43 35 35 ** **
Without cooling Without cooling Without cooling With gasification air down to 700°C With gasification air down to 700°C Steam generation to 500°C Steam generation to 500°C Quench to 600C Quench to 60°C Quench to 60°C Without cooling Quench to 60°C
*From Kiiskilfi (1985). **Pyrolysis.
1880 1680 1790 1390 1510 1610 2100 2100
161
Gasification of sulphate soap for the lime kiln Table 2. Sulphate-soap gasifier options
Residence time Seconds
Minutes
Pyrolyzer with a mechanical separator for the inorganics Unreacted char with inorganics into a recovery boiler Entrained-flow gasifier with a hot cyclone or a slagging-type separator for the inorganics
Fluidized-bed gasifier with a mechanical separator for the inorganics Char with inorganics into a recovery boiler or into a dissolver
Gasification temperature T< 700°C (below the melting point)
T> 800-900°C (above the melting point)
110
'Recovery-boiler'-type gasifier lnorganics as smelt into a dissolver
SCHEMATIC DIAGRAM OF PRESSURIZED HEATED GRID UNIT
100 90
Weight
80 A 70
/ - -,
Temperature
~ 6o DATALOGGINGUNIT
•~ 50
[ ]
GASCHROMATOGRAPHICANALYSES
~ 4O 30
/ / 111 / ~"~(2} .........
HEATEDGRID
(3)
20
PRINTER
10 I
0
1
I
100
I
I
I
200 t (min)
300
I~ q
I
400
Fig. 1. Dry sulphate soap in the thermobalance. The pyrolysis temperature was 675°C and the steam-gasification temperature 600°C with 15% H20 and 5"8% H 2 in He. The gsa atmosphere during the holding time on the grid at 675°C was 7% CO in He.
DIRECT GU~ENT
~II~.Y
~
Fig. 3.
UNIT
The heated-grid test assembly.
40 t-
o
:
DRY SULPHATE SOAP I t : 600 K/s A "i" : 700 ... 800 Kh 35 . . . . . . . • . . . . . . . ~p= 21~r p= lbllr
.c_
,
~=10$
~=20$
30 U
._c n~ < -rO
i
!
=
~
!
500
550
600
650
700
25
20
450
750
PYROLYSIS TEMPERATURE, "C
Fig. 2. Dry sulphate soap (pine) on a heated grid. The holding time at the final temperature was 10 s and the heating rate, T, 600 K/s.
char (carbon + inorganics) is affected by the pyrolysis temperature. The data have been generated with a heated grid (Fig. 3) and with dry sulphate soap from pine pulping. The amount of inorganics in char is
16-17% calculated from original dry solids for both fast and slow pyrolysis. The analysis of the chars in Fig. 2 is presented in Table 3 and the analysis of the soap in Table 4. There was no difference between fast and slow pyrolysis for the amount of char. This may depend on the experimental procedure applied (behaviour of tars). If the gasifier is of the pyrolyzer type, the heat of unreacted char is lost from the lime kiln. This means a loss of approximately 10%, which is sufficient for reduction and for heating the char to bed temperature in a recovery boiler. Because oxygen is reacting with the volatiles, the char will react mainly with steam. The steam-gasification reactivity of the chars was measured in He with H: as an inhibiting gas. The reactivity, r, was calculated from: R =
dm/dt m
(11
K. Saviharju, T. Timonen
162
Table 3. The analysis of the chars measured with ion chromatography
tso % " t i m e in minutbes for 50 % char con~ndon
4n(t~%)
0 Oi
Pyrolysis temperature, °C
500
600
700
Carbon (by difference)(%)
50 45 4.9 65
48 47 4.8 59
33 62 4.6 49
Na2CO3"(%)
Na2SO4 (%) S from the input sulfur (%)
;
................ ---~-_o .....
....
=
',
;
" ....... i ........
- ....... - .......
i ....... ~ .......
i ....... 7 .......
........
: ........
! .......
, .......
........
i .......
i .......
-1 ....
"All CO 3 calculated as Na2CO 3. -2 . . . . . . .
÷. . . . . . .
-3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- .......
~.,z.
- -~.-.-.-r . . . .
-
.......
Table 4. The ultimate analysis of the soap used in laboratory tests
Pulping Moisture (%) In dry solids, C(%) H (%) N(%)
S (%) Na (%) L H V , MJ/kg -- dry solids -- as received Char (N 2 atmosphere at 675°C) -- slow pyrolysis (10 K/min) -- fast pyrolysis (700... 800 K/s) Inorganics after the steam gasification (%) NaeCO3 {from Na) (%) Carbon (from slow pyrolysis)(%) Volatiles (%)
Pine
. . . . . . .
:T.=~
. . . .
38"4 10.4
70.9 9'6 <0-1 0-7 7-1
10.6
10.8 11 11.2 11.4 10 000/Temperature (K)
PYROLYSIS: T=675"C : 10 s
Atmosphere = N
31-8 18'7 23'0 23.0 16-3 16-3 6'7 77-0
Fig. 4.
GASIFICATION: H20=15% Soap H2 = 0 % ~ 5,8% 11% Ea, k J/tool 201 R-square 0.887 n 33
11.6
B.L. ell. 207
0.977 22
Steam-gasification reactivity of black liquor and sulphate soap.
mPa.s 10,000
where m = mass of unreacted organics and t = time. The results are shown in Fig. 4 as the time required for 50% carbon conversion. 'Slow pyrolysis' means pyrolysis in the thermobalance, the heating rate being 10 K/s, and 'fast pyrolysis' that on the heated grid, the heating rate being 6 0 0 - 8 0 0 K/s. The char was gasified with a piece of the grid. The soap was from the Afinekoski mill. Gasification data for black liquor from the same mill are also shown in the figure. The steamgasification reactivity of the chars from the soap and from the black liquor is the same. One possibility is that the char from soap is caused by black liquor associated with soap. The further gasification tests with black liquor indicate that the holding time before gasification (Fig. 1) may have a negative effect on reactivity, although the pyrolysis has been fast. GASIFICATION TESTS The gasification-test facility consists of the reactor equipped with an air-atomized burner. The diameter of the reactor is 450 mm and its height 1800 mm. Soap is gasified without preheating because it would increase the viscosity up to approximately 150°C. The viscosity behaviour plotted against the shear rate (Fig. 5) caused many plugging problems in pump-
3,000
1,000
300
100:
i I
o
1oo
20o ~o 400 Shear r a t e , - / s
500
~oo
110"C 9 ~ ' C 70"C 50~C 150°C
Fig. 5.
Viscosity of sulphate soap.
ing, and poor atomization because viscosity was increasing with a high shear rate. Atomization requires further work. Some test results are presented in Tables 5, 6 and 7. A typical feature of the test runs was a high amount of tars in product gas, comparable with that of wood (see Kurkela and Stfihlberg (1992)) and non-existence of HaS. The quality of the product gas is sufficient for
Gasification of sulphate soap for the time kiln lime-kiln use and Na separation looks promising. With a high-pressure-loss cyclone, good sodium capture was achieved at 550°C, which is well below the melting point of the inorganics. T h e sulphur balance of the gasifier remained open because of sampling and analyzer problems.
Table 5.
Date
Sulphate soap Moisture (%) Dry matter (%) C J N S Na LHV, MJ/kg Dry matter As received Density kg/m 31
163
FUTUREPLANS
The nical have tions
Finnish Recovery Boiler Committee, the TechResearch Centre of Finland, and Mets~i-Sellu Oy planned to study jointly the following open quesduring 1993:
Gasification (pyrolyzer) test results
29 April 1992
7 May 1992
Hardwood & so]?wood 34-3
f lardwood & softwood 36"6
73.5 9.9 <0.1 0.7 6-2
73.9 9.7 <0.1 O.7 6.8
31-8 20-1 845
3 1-8 19.3 ~.}15
655 742 680 23.5
870 780 (}90 21.0
58 30 12 9"7
38 38 24 7.7
0.32 0.36 1.4 195
0"37 0.40 1-0 150
Reactor Temperatures (°C) Burner 600 mm downstream 200 mm downstream Air flow (g/s) Air distribution (%) atomization axial flow tangential flow Soap flow (g/s) Excess air faclor total soap without unreacted char Atomization air flow/flue flow Fuel input (kW) Residence time (s) Volumetric fuel input (based on LHV) (kW/m 3)
I>roduct gas Product-gas composition (dry)(%) CH~ C,tt, C_,tt,, It, H,S CO, CO O,-Ar N, Total H20 from the H~ balance (%) tars (g/m3n) LHV, MJ/m3n dry gas without tars dry gas with tars wet gas without tars wet gas with tars Total enthalpy (wet gas)(MJ/m3n) without tars with tars
2.6 780
2.9 600
6.3 3'4 0'3 4-3 0 9"6 7-2 1"2 68'2
5"8 2.5 0'2 6"3 0 11 "0 5-3 0-9 68-{}
100.5
100.0
21 25
18 17
5.7 7' 1 4-5 5"6
5.{1 5"8 4-1 4"8
5.6 6.7
5.1 5.8
164
K. Saviharju, T. Timonen Table 6. Mass and energy balances
29.4.92 Mass balance Incoming flows (g/s) Dry solids Moisture Gasification air
Table 7. Tar and char in the product gas
7.5.92
6.4 3'3 23.5
4-9 2.8 21.9
Total Outcoming flows (g/s) Product gas Char Imbalance
33-2
28.7
31"4 1.5 0'3
26"9 1"2 0"6
Total Energy balance (based on HHIO Incoming fows (kW) Soap Gasification air
33.2
28"7
2t7 0
167 0
Total Outcoming flows (kW) Chemical energy in product gas Sensible heat in product gas Chemical energy in char Sensible heat in char Heat of reduction Imbalance
217
167
162 40 17 1 0 - 2
120 33 13 1 0 0
Total
218
167
(i) sodium capacity of lime kilns; (ii) testing different atomization possibilities for a gasifier; (iii) the size distribution of particulates from the gasifier; (iv) the sulphur balance of the gasifier; and (v) evaluation of the technology and costs of a possible demonstration plant on the basis of the test results.
Compound Gas chromatography (mg/g) naphthalene 2-methylnaphthalene 1-methylnaphthalene
biphenyl acenaphthylene acenaphthene dibenzofurane fluorene phenanthrene anthracene methylphenanthrenes methylanthracene phenylnaphthalenes fluoranthene benz(e)acenaphthylene pyrene tetramethylphenathrene triphenylene chrysene benzo(b)fluoranthene benzo(e)pyrene benzo(a)pyrene perylene indeno( 1.2.3cd)pyrene benzo(ghi)perylene coronene Identified compounds (mg/g) Unidentified compounds (mg/g) All eluated compounds (mg/g) S (%) Na"(%) K"(%) Ash (%) Char + uneluated (%)
Amount 1.22 0"36 0.39 0"62 2.79 0.37 0-69 0.23 29.8 4.27 10"8 6"83 15"1 11"0 6"55 10"9 1.98 8"45 9"05 2.64 2.35 3"77 1-01 1-14 1"22 0"38 134 86 220 2"93 26.5 3.8 69 9
"Via ash with AAS. economy of such units can be calculated and investment decisions made. The most important open questions are related to good atomization of soap and the sodium capacity of lime kilns.
In 1993, the effect of soap gasification on the fuel balance of a mill will be studied in another ETY project called 'Potential to increase electricity production by means of IGCC technology in the Finnish Pulp and Paper Industry'. The study will also include an assessment of the national significance of soap gasification. The project will be carried out jointly by the Industrial Thermal Engineering Committee of ETY, EnsoGutzeit, Kymmene, and the United Paper Mills. We hope that, at the end of 1993, we shall have at our disposal sufficiently promising research results to be able to implement a demonstration-scale plant at a Finnish pulp mill.
The authors wish to thank the J A L O Programme, the Recovery Boiler Committee, and VTT for financial support and essentially Mets/i-Sellu Oy for co-operation and for the possibility of carrying out the tests at A~inekoski. Thanks are also due to those who have worked on the project; Reino Flinkman, Jaana Komulainen, Eeva Kuoppala, Jarmo Korhoren, Paterson McKeough, Antero Moilanen, Mirja Muhola, Anja Oasmaa and Minna Pyykk6nen.
CONCLUSION
E T Y - F I N N I S H RECOVERY BOILER
Gasification of sulphate soap looks interesting from a mill point of view because of the energy and sulphur balances. It also looks feasible from the gasification point of view, but further work is required before the
ACKNOWLEDGEMENTS
COMMITTEE The purpose of the ETY-Finnish Recovery Boiler Committee is to promote safe and economic utilization and development of recovery boilers and processes
Gasification of sulphate soap for the lime kiln related closely to these. T h e Committee carries on and supports research work and projects relating to the safe operation or improved economy of recovery boilers. All companies using recovery boilers in Finland, manufacturers of recovery boilers, the insurance companies, and the Pulp and Paper Research Institute belong to the Committee. In addition to the members, various research institutes, VTT, and universities are participating in the research work and projects of the Committee.
165
REFERENCES Kiiskil~i, E. (1985). Biomass gasification in an Ahlstr6m Pyroflow gasifier replaces oil in lime kilns. In New Alternatives of Biomass Conversion in the 1990s, Espoo, VTT Symposium, Vol. 75, pp. 76-89. Kurkela, E. & Stfihlberg, E (1992). Air gasification of peat, wood and brown coal in a pressurized fluidized-bed reactor. Carbon conversion, gas yields and tar formation. kt~el Processing Technologq', 31. 1 21.
GASIFICATION OF SULPHATE SOAP FOR THE LIME KILN Karl Saviharju Technical Research Centre of Finland (VTT), Laboratory of Fuel and Process Technology, PO Box 205. FIN 02151 Espoo, Finland
&
Tuomas Timonen Finnish Energy' Economy Association (ETY), Finnish Recovery Boiler Committee, SF-02151 l:spoo, tblland
The successful evaporation of black liquor requires that soap be separated through acid from a weak and/ or intermediate liquor. Conventionally, sulphate soap has been utilized either by cooking it to raw tall oil and by selling the end-product forward to distilleries or by burning it in recovery boilers. However, both these methods are problematic in the present sulphate-pulp mills. Tall-oil production has an indirect effect on the mill's sulphur balance either by increasing sulphur emissions or by causing additional costs involved in reducing SO2 with separate scrubbing systems. Furthermore, tall-oil production is today uneconomic, since its price has collapsed. Neither is burning of sulphate soap in recovery boilers justified owing to the surplus in the energy bahmce of the mill. Further, at many mills, the recovery boiler is the bottleneck of production and cannot be loaded any more. If the alkaline solids could be removed, about 4-8% of Na + K in dry solids, crude soap could be fired directly in lime kilns. Without the removal, low-melting compounds are formed in kilns, resulting in unacceptable operational problems. A typical analysis of crude soap is presented in Table 4 below. The above two targets, reduction in sulphur emissions and cost-savings by replacing purchased fuel, can be achieved by soap gasification. Sulphur emissions would fall automatically if sulphuric acid and highsulphur oil were withdrawn from use. In addition, the tendency to use chlorine-free pulp bleaching will cause problems in many mills when the sulphuric acid used in tall-oil cooking is reduced. Many mills save a considerable sum of money in their fuel costs by using synthetic gas to replace heavy fuel oil normally used in mills. The heat input from soap is quite well in balance with the 20-30 MW based on LHV, required for the lime kiln in Finnish mills. The project 'Processing of sulphate soap to lime-kiln fule" was carried out by the Laboratory of Fuel and Process Technology of VTT and was financed by the ETY-Finnish Recovery Boiler Committee and the Ministry of Trade and Industry through the JALO
Abstract
Sulphate soap is" a by-product of pulp mills utilized as a raw material for the chemical industry. However, this results i, an increase in sulphur input of several kilograms S(): per ADt pulp into the mill. Another increasingly interesting alternative is to utilize soap in the lime kiln of the mill. This has a positive effect, in addition to sulphur problems, on the energy balance of the mill. The crucial problem is the high Na + K content of the soap, from 4 to 8%, which can result in plugging of the lime kiln. The operational problems can be avoided by gasi~,ing the soap and by separating the inorganic materials fi'om the product gas before the kiln. This paper describe~" research work on the gasification of crude sulphate soap carried out at VTT over the years 1991 and 1992. This work will be continued in 1993 by focusing on specified problems, after which commercial applications ~hould be available. A detailed research report will be published after this" second stage of research has been completed. Key words: Gasification, sulphate soap, lime kiln, sulphur emissions, black liquor.
INTRODUCTION Fhe development of the recovery process and its equipment is today driven by investment cost, energy efficiency, and environmental requirements. The interest of Finnish sulphate-pulp mills in the gasification of sulphate soap for the lime kiln is based on two factors: (i) the possibility of reducing sulphur emissions from the soap treatment; and (ii) the possibility of replacing fuel purchased from elsewhere by the plant's own biomass product. Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain
159
K. Savihar]u, T. Timonen
160
research programme. The pilot tests were carried out in the VTT test plant located at the ,~iinekoski Mills of Metsii-Sellu Oy. During the first two years, in 1991-92, the suitability of sulphate soap for a lime-kiln fuel was studied both theoretically and experimentally. The planned further work will focus on practical problems and is discussed at the end of this paper. PRODUCT GAS QUALITY Product gas to the lime kiln has to fulfil two criteria: (i) the amount of Na + K in the gas must be low enough to prevent agglomeration in the lime kiln; and (ii) the sum of chemical energy and sensible heat must be high enough to produce the required temperature. The L H V of sulphate soap as received is approximately 20 MJ/kg, which is enough to produce a good fame. The problem is the high alkaline content of soap. Na ÷ K of dry solids may range from 4 to 8%. The acceptable value requires further work, the required reduction being approximately 50-90%. According to Kiiskilii (1985), the stoichiometric adiabatic-flame temperature for the product gas should exceed 1750°C. This figure is based on wood-waste gasification. Table 1 indicates how this is achieved with different types of gasifier. Product-gas cooling in some options is thought to be required for alkaline and unreacted carbon recovery. The calculations show that water-washing of product gas for N a + K recovery is not possible. The
remaining possibilities are either-solid phase recovery or smelt recovery with minor product-gas cooling. GASIFIER OPTIONS Because the economic benefit is limited, the equipment cannot be complicated. Possible options are presented in Table 2. The entrained-flow gasifier has to operate clearly above the melting point of the salts to avoid slagging and to achieve a high reactivity in a short residence time. Salts may be recovered through a slagging cyclone or an agglomerating separator. Owing to the short residence time, the sulphur reduction is poor for non-reduced sulphur in soap. Below the melting point, a pyrolyzer-type reactor seems to be the most promising owing to the low carbon residue in char and the long residence time required for high conversion. The gasifier may be equipped with a burner, or it may be a fluidized-bed reactor. The advantage of the burner is a simple construction and the disadvantage an uneven temperature profile and difficult control of the local gas atmosphere. GASIFICATION BEHAVIOUR OF SULPHATE SOAP Gasification behaviour was studied with a heating-grid pyrolyzer and an atmospheric thermobalance. The main problem was the char reactivity because the volatiles from soap were reacting fast compared with char. Figure 1 shows how soap is pyrolyzed and gasified in a thermobalance. Fast pyrolysis starts at 300°C and is completed at 500°C. Figure 2 shows how the amount of
Table 1. Adiabatic-flame temperature of the product gas (65% dry solids in soap, stoichiometric combustion in lime kiln)
Gasification temperature (°C)
Gasification air of ' stoichiometric (%)
Product-gas cooling
Adiabatic-flame temperature (°C) (rain 1750°C required)*
900 700 500 900
43 35 30 38
1870 1880 1880 1870
700
32
900
43
700
35
900 700 500 500 500
43 35 35 ** **
Without cooling Without cooling Without cooling With gasification air down to 700°C With gasification air down to 700°C Steam generation to 500°C Steam generation to 500°C Quench to 600C Quench to 60°C Quench to 60°C Without cooling Quench to 60°C
*From Kiiskilfi (1985). **Pyrolysis.
1880 1680 1790 1390 1510 1610 2100 2100
161
Gasification of sulphate soap for the lime kiln Table 2. Sulphate-soap gasifier options
Residence time Seconds
Minutes
Pyrolyzer with a mechanical separator for the inorganics Unreacted char with inorganics into a recovery boiler Entrained-flow gasifier with a hot cyclone or a slagging-type separator for the inorganics
Fluidized-bed gasifier with a mechanical separator for the inorganics Char with inorganics into a recovery boiler or into a dissolver
Gasification temperature T< 700°C (below the melting point)
T> 800-900°C (above the melting point)
110
'Recovery-boiler'-type gasifier lnorganics as smelt into a dissolver
SCHEMATIC DIAGRAM OF PRESSURIZED HEATED GRID UNIT
100 90
Weight
80 A 70
/ - -,
Temperature
~ 6o DATALOGGINGUNIT
•~ 50
[ ]
GASCHROMATOGRAPHICANALYSES
~ 4O 30
/ / 111 / ~"~(2} .........
HEATEDGRID
(3)
20
PRINTER
10 I
0
1
I
100
I
I
I
200 t (min)
300
I~ q
I
400
Fig. 1. Dry sulphate soap in the thermobalance. The pyrolysis temperature was 675°C and the steam-gasification temperature 600°C with 15% H20 and 5"8% H 2 in He. The gsa atmosphere during the holding time on the grid at 675°C was 7% CO in He.
DIRECT GU~ENT
~II~.Y
~
Fig. 3.
UNIT
The heated-grid test assembly.
40 t-
o
:
DRY SULPHATE SOAP I t : 600 K/s A "i" : 700 ... 800 Kh 35 . . . . . . . • . . . . . . . ~p= 21~r p= lbllr
.c_
,
~=10$
~=20$
30 U
._c n~ < -rO
i
!
=
~
!
500
550
600
650
700
25
20
450
750
PYROLYSIS TEMPERATURE, "C
Fig. 2. Dry sulphate soap (pine) on a heated grid. The holding time at the final temperature was 10 s and the heating rate, T, 600 K/s.
char (carbon + inorganics) is affected by the pyrolysis temperature. The data have been generated with a heated grid (Fig. 3) and with dry sulphate soap from pine pulping. The amount of inorganics in char is
16-17% calculated from original dry solids for both fast and slow pyrolysis. The analysis of the chars in Fig. 2 is presented in Table 3 and the analysis of the soap in Table 4. There was no difference between fast and slow pyrolysis for the amount of char. This may depend on the experimental procedure applied (behaviour of tars). If the gasifier is of the pyrolyzer type, the heat of unreacted char is lost from the lime kiln. This means a loss of approximately 10%, which is sufficient for reduction and for heating the char to bed temperature in a recovery boiler. Because oxygen is reacting with the volatiles, the char will react mainly with steam. The steam-gasification reactivity of the chars was measured in He with H: as an inhibiting gas. The reactivity, r, was calculated from: R =
dm/dt m
(11
K. Saviharju, T. Timonen
162
Table 3. The analysis of the chars measured with ion chromatography
tso % " t i m e in minutbes for 50 % char con~ndon
4n(t~%)
0 Oi
Pyrolysis temperature, °C
500
600
700
Carbon (by difference)(%)
50 45 4.9 65
48 47 4.8 59
33 62 4.6 49
Na2CO3"(%)
Na2SO4 (%) S from the input sulfur (%)
;
................ ---~-_o .....
....
=
',
;
" ....... i ........
- ....... - .......
i ....... ~ .......
i ....... 7 .......
........
: ........
! .......
, .......
........
i .......
i .......
-1 ....
"All CO 3 calculated as Na2CO 3. -2 . . . . . . .
÷. . . . . . .
-3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- .......
~.,z.
- -~.-.-.-r . . . .
-
.......
Table 4. The ultimate analysis of the soap used in laboratory tests
Pulping Moisture (%) In dry solids, C(%) H (%) N(%)
S (%) Na (%) L H V , MJ/kg -- dry solids -- as received Char (N 2 atmosphere at 675°C) -- slow pyrolysis (10 K/min) -- fast pyrolysis (700... 800 K/s) Inorganics after the steam gasification (%) NaeCO3 {from Na) (%) Carbon (from slow pyrolysis)(%) Volatiles (%)
Pine
. . . . . . .
:T.=~
. . . .
38"4 10.4
70.9 9'6 <0-1 0-7 7-1
10.6
10.8 11 11.2 11.4 10 000/Temperature (K)
PYROLYSIS: T=675"C : 10 s
Atmosphere = N
31-8 18'7 23'0 23.0 16-3 16-3 6'7 77-0
Fig. 4.
GASIFICATION: H20=15% Soap H2 = 0 % ~ 5,8% 11% Ea, k J/tool 201 R-square 0.887 n 33
11.6
B.L. ell. 207
0.977 22
Steam-gasification reactivity of black liquor and sulphate soap.
mPa.s 10,000
where m = mass of unreacted organics and t = time. The results are shown in Fig. 4 as the time required for 50% carbon conversion. 'Slow pyrolysis' means pyrolysis in the thermobalance, the heating rate being 10 K/s, and 'fast pyrolysis' that on the heated grid, the heating rate being 6 0 0 - 8 0 0 K/s. The char was gasified with a piece of the grid. The soap was from the Afinekoski mill. Gasification data for black liquor from the same mill are also shown in the figure. The steamgasification reactivity of the chars from the soap and from the black liquor is the same. One possibility is that the char from soap is caused by black liquor associated with soap. The further gasification tests with black liquor indicate that the holding time before gasification (Fig. 1) may have a negative effect on reactivity, although the pyrolysis has been fast. GASIFICATION TESTS The gasification-test facility consists of the reactor equipped with an air-atomized burner. The diameter of the reactor is 450 mm and its height 1800 mm. Soap is gasified without preheating because it would increase the viscosity up to approximately 150°C. The viscosity behaviour plotted against the shear rate (Fig. 5) caused many plugging problems in pump-
3,000
1,000
300
100:
i I
o
1oo
20o ~o 400 Shear r a t e , - / s
500
~oo
110"C 9 ~ ' C 70"C 50~C 150°C
Fig. 5.
Viscosity of sulphate soap.
ing, and poor atomization because viscosity was increasing with a high shear rate. Atomization requires further work. Some test results are presented in Tables 5, 6 and 7. A typical feature of the test runs was a high amount of tars in product gas, comparable with that of wood (see Kurkela and Stfihlberg (1992)) and non-existence of HaS. The quality of the product gas is sufficient for
Gasification of sulphate soap for the time kiln lime-kiln use and Na separation looks promising. With a high-pressure-loss cyclone, good sodium capture was achieved at 550°C, which is well below the melting point of the inorganics. T h e sulphur balance of the gasifier remained open because of sampling and analyzer problems.
Table 5.
Date
Sulphate soap Moisture (%) Dry matter (%) C J N S Na LHV, MJ/kg Dry matter As received Density kg/m 31
163
FUTUREPLANS
The nical have tions
Finnish Recovery Boiler Committee, the TechResearch Centre of Finland, and Mets~i-Sellu Oy planned to study jointly the following open quesduring 1993:
Gasification (pyrolyzer) test results
29 April 1992
7 May 1992
Hardwood & so]?wood 34-3
f lardwood & softwood 36"6
73.5 9.9 <0.1 0.7 6-2
73.9 9.7 <0.1 O.7 6.8
31-8 20-1 845
3 1-8 19.3 ~.}15
655 742 680 23.5
870 780 (}90 21.0
58 30 12 9"7
38 38 24 7.7
0.32 0.36 1.4 195
0"37 0.40 1-0 150
Reactor Temperatures (°C) Burner 600 mm downstream 200 mm downstream Air flow (g/s) Air distribution (%) atomization axial flow tangential flow Soap flow (g/s) Excess air faclor total soap without unreacted char Atomization air flow/flue flow Fuel input (kW) Residence time (s) Volumetric fuel input (based on LHV) (kW/m 3)
I>roduct gas Product-gas composition (dry)(%) CH~ C,tt, C_,tt,, It, H,S CO, CO O,-Ar N, Total H20 from the H~ balance (%) tars (g/m3n) LHV, MJ/m3n dry gas without tars dry gas with tars wet gas without tars wet gas with tars Total enthalpy (wet gas)(MJ/m3n) without tars with tars
2.6 780
2.9 600
6.3 3'4 0'3 4-3 0 9"6 7-2 1"2 68'2
5"8 2.5 0'2 6"3 0 11 "0 5-3 0-9 68-{}
100.5
100.0
21 25
18 17
5.7 7' 1 4-5 5"6
5.{1 5"8 4-1 4"8
5.6 6.7
5.1 5.8
164
K. Saviharju, T. Timonen Table 6. Mass and energy balances
29.4.92 Mass balance Incoming flows (g/s) Dry solids Moisture Gasification air
Table 7. Tar and char in the product gas
7.5.92
6.4 3'3 23.5
4-9 2.8 21.9
Total Outcoming flows (g/s) Product gas Char Imbalance
33-2
28.7
31"4 1.5 0'3
26"9 1"2 0"6
Total Energy balance (based on HHIO Incoming fows (kW) Soap Gasification air
33.2
28"7
2t7 0
167 0
Total Outcoming flows (kW) Chemical energy in product gas Sensible heat in product gas Chemical energy in char Sensible heat in char Heat of reduction Imbalance
217
167
162 40 17 1 0 - 2
120 33 13 1 0 0
Total
218
167
(i) sodium capacity of lime kilns; (ii) testing different atomization possibilities for a gasifier; (iii) the size distribution of particulates from the gasifier; (iv) the sulphur balance of the gasifier; and (v) evaluation of the technology and costs of a possible demonstration plant on the basis of the test results.
Compound Gas chromatography (mg/g) naphthalene 2-methylnaphthalene 1-methylnaphthalene
biphenyl acenaphthylene acenaphthene dibenzofurane fluorene phenanthrene anthracene methylphenanthrenes methylanthracene phenylnaphthalenes fluoranthene benz(e)acenaphthylene pyrene tetramethylphenathrene triphenylene chrysene benzo(b)fluoranthene benzo(e)pyrene benzo(a)pyrene perylene indeno( 1.2.3cd)pyrene benzo(ghi)perylene coronene Identified compounds (mg/g) Unidentified compounds (mg/g) All eluated compounds (mg/g) S (%) Na"(%) K"(%) Ash (%) Char + uneluated (%)
Amount 1.22 0"36 0.39 0"62 2.79 0.37 0-69 0.23 29.8 4.27 10"8 6"83 15"1 11"0 6"55 10"9 1.98 8"45 9"05 2.64 2.35 3"77 1-01 1-14 1"22 0"38 134 86 220 2"93 26.5 3.8 69 9
"Via ash with AAS. economy of such units can be calculated and investment decisions made. The most important open questions are related to good atomization of soap and the sodium capacity of lime kilns.
In 1993, the effect of soap gasification on the fuel balance of a mill will be studied in another ETY project called 'Potential to increase electricity production by means of IGCC technology in the Finnish Pulp and Paper Industry'. The study will also include an assessment of the national significance of soap gasification. The project will be carried out jointly by the Industrial Thermal Engineering Committee of ETY, EnsoGutzeit, Kymmene, and the United Paper Mills. We hope that, at the end of 1993, we shall have at our disposal sufficiently promising research results to be able to implement a demonstration-scale plant at a Finnish pulp mill.
The authors wish to thank the J A L O Programme, the Recovery Boiler Committee, and VTT for financial support and essentially Mets/i-Sellu Oy for co-operation and for the possibility of carrying out the tests at A~inekoski. Thanks are also due to those who have worked on the project; Reino Flinkman, Jaana Komulainen, Eeva Kuoppala, Jarmo Korhoren, Paterson McKeough, Antero Moilanen, Mirja Muhola, Anja Oasmaa and Minna Pyykk6nen.
CONCLUSION
E T Y - F I N N I S H RECOVERY BOILER
Gasification of sulphate soap looks interesting from a mill point of view because of the energy and sulphur balances. It also looks feasible from the gasification point of view, but further work is required before the
ACKNOWLEDGEMENTS
COMMITTEE The purpose of the ETY-Finnish Recovery Boiler Committee is to promote safe and economic utilization and development of recovery boilers and processes
Gasification of sulphate soap for the lime kiln related closely to these. T h e Committee carries on and supports research work and projects relating to the safe operation or improved economy of recovery boilers. All companies using recovery boilers in Finland, manufacturers of recovery boilers, the insurance companies, and the Pulp and Paper Research Institute belong to the Committee. In addition to the members, various research institutes, VTT, and universities are participating in the research work and projects of the Committee.
165
REFERENCES Kiiskil~i, E. (1985). Biomass gasification in an Ahlstr6m Pyroflow gasifier replaces oil in lime kilns. In New Alternatives of Biomass Conversion in the 1990s, Espoo, VTT Symposium, Vol. 75, pp. 76-89. Kurkela, E. & Stfihlberg, E (1992). Air gasification of peat, wood and brown coal in a pressurized fluidized-bed reactor. Carbon conversion, gas yields and tar formation. kt~el Processing Technologq', 31. 1 21.