Application of K2CO3 catalysts in the coal gasification process using nuclear heat

Application of K,CO, catalysts in the coal gasif ication process using nuclear heat* Helmut Kubiak, van Heek

Hans-Jiirgen

Bergbau- Forschung GmbH, D-4300 Essen 13, GFR

Abteilung

Schrijter, fiir

Alfred

Ph ysikalische

Sulimma Chemie,

and Karl-Heinrich

Franz-Fischer-

Weg 61,

Conventional gasification processes use coal not only as feedstock to be gasified but also for supply of energy for reaction heat, steam production, and other purposes. With a nuclear high temperature reactor (HTR) as a source for process heat, it is possible to transform the whole of the coal feed into gas. This concept offers advantages over existing gasification processes: saving of coal, as more gas can be produced from coal; less emission of pollutants, as the HTR is used for the production of steam and electricity instead of a coal-fired boiler; and a lower production cost for the gas. However, the process has the disadvantage that the temperature is limited to the outlet temperature (950°C max) of the helium cooling gas of the HTR. Therefore the possibility of catalytic steam gasification was examined. Model calculations based on experimental results show that use of S4 wt% relative to coal of K2C03 catalyst increases the throughput of a large scale nuclear gasification plant by ~65%. while gas production costs decrease by ~15%. Corrosion by catalysts is not significant at low concentration (~5 wt%) and low temperature ((900°C). (Keywords:

coal; gasification;

STEAM GASIFICATION NUCLEAR HEAT

potassium;

catalysis;

OF COAL USING

The flow scheme in Figure 1 shows the connection between the nuclear reactor and the gasification loop.‘s2 In the primary circuit of the HTR the working fluid and heat carrier, helium, is heated up to 950°C. In an intermediate heat exchanger heat is transferred to a secondary circuit, in which helium also is used as fluid. The secondary helium provides the gas generator and a steam generating and superheating system with high temperature heat. The gas generator is the central component in the gasification loop. In this apparatus the process heat is transferred from the secondary circuit to the gasification process by metal heat exchanger tubes. Superheated steam is used as the gasifying agent and as the fluidizing medium for the fluidized bed of coal particles. It is assumed that future gasification plants will use bituminous coals, of which Germany has large reserves. Therefore the German Leopold long flame coal (Gasflammkohle) was the reference coal used in this study. The product raw gas has to be treated in a heat-recovery system and in a gas cleaner. The effluent product gas consists mainly of H,, CO, CO, and CH,. It can be used directly as an energy carrier or can be converted to synthesis gas or by methanation to SNG. The key problems to be solved before the gas generator can be technically realized include feasibility, e.g. design and material questions, and working out of acceptable operation data before a larger plant can be planned. As far as technical feasibility is concerned, there are reasons why the equipment is likely to be larger than conventional gas * Presented at International Symposium, ‘Fundamentals of Catalytic Coal and Carbon Gasification’, held at Amsterdam, The Netherlands, 27-29 September 1982 001&2361/83/020242~4.S3.00 @ 1983 Butterworth & Co. (Publishers)

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heat)

generators. At 800°C gasification conversion rates are still relatively low and necessitate large reaction volume and suitable catalysts. Secondly, the introduction of a heat exchanger involves a certain additional volume. Figure 2 shows a design which incorporates these aspects. The gas generator is a horizontally mounted cylindrical pressure vessel, up to 50m long, 7 m in diameter and consisting of 4-7 modular units. The lower portion contains the fluidized bed to which steam is passed through apertures in the bottom. Coal is fed to one end of the fluidized bed by a jet feeder, traverses the fluidized bed in plug flow (as tests in a flow model show) giving carbon burn-off up to 95x, so that an ash-rich fraction is obtained. Heat is transferred by heat exchanger

I

I

He I

Figure 7 Combination of HTR and steam gasificationof coal. 1, HTR; 2. heat exchanger;3, steam generator;4, gas generator;5, gas cleaning; 6, methanation

K-JO3

catalysts

in coal gasification:

H. Kubiak

et al

Raw Gas

Ash

Steam Section

Figure 2

A-B

Cooling

Water

Helium

Gas generator

Table 7

Gasifier heat balance -

ljeat consumed = tjeat transferred =07

QC q*k(Tl.p*

where Secondstep

v

=h-FB

= reaction heat = reactivity of carbon = gasification temperature P = carbon density of fluidized bed V = volume of fluidized bed h = overall heat transfer coefficient F = heat transfer area 8 = log temperature difference Q k T

.

Figure 3

Cascade model for gasifier calculations

tubes immersed vertically into the fluidized bed from above. Catalysts can be introduced by dry mixing into the crushed coal, by impregnation or by solution in the steam. The design work is being conducted in colla~ration with Mannesmann. A key feature is the tube bank for helium distribution, the mounting and insulation, the type of thermal insulation of the pressure vessel and the distributor plate. For the hot components suitable high temperature alloys are under test and development.3 THROUGHPUT

OF THE GASIFIER

For model calculations of the gasifier a cascade programme was developed,4 the principle being shown in Figure 3. For each step the heat and materials balance must be calculated on the basis that the consumed heat is equal to the transferred heat. Table I shows this

equilibrium equation. The parameters have to be determined or calculated by experiments in test facilities, e.g. in the laboratory or in the semi-technical gasification plant.2*4*J INFLUENCE

OF CATALYST REACTIVITY

In laboratory experiments the minimum catalyst concentrations needed for a sufficient reactivity increase in the gasifier have been determined.‘s6 The experiments were performed with Leopold char {containing 10 wt% ash and 35 wt% volatile matter) and various amounts of K&O, added to the char by solution in high pressure steam. Figure 4 shows the reaction rate r relative to values of the non-catalysed reaction rate rO as a function of the K/C ratio. A K/C-ratio >0.012 mol mot-’ (7 wt% K,CO, on car x 3.5 wt% on coal) is needed to increase reactivity. This minimum value is due to the fact that a certain amount of the catalyst is inactivated by the ash. Our greater concentrations lead to increased reaction rates. As carbon is gasified during the passage of the char

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K2C03 catalysts in coal gasification:

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16

of the enrichment. Therefore, in the last part of the gasifier the specific coal is gasified higher though temperature and carbon concentration decrease. In Table 2 the main data for gasification with and without catalyst are listed. The increase of coal throughput with catalyst is x65%, accompanied by higher steam decomposition and lower gasification temperature. Gas production costs are x 15% lower when K,CO, catalyst is used. In the calculations it was assumed that the catalyst will not be recycled but deposited together with the ash. I

0

I

I

I

5

IO

15 KPCOJ (wt %

Burn off,

length

I

I

I

20

25

30

I of the gasifier -

600

Figure 5 Calculated without catalyst ( -). gasified

of the gas generator

process data for the gasifier with (---) A, temperature; B, specific coal

and

through the horizontal gasifier, the concentration of catalyst in relation to carbon increases, thus increasingly promoting gasification. IMPROVEMENT CATALYST

Main data for steam gasification Table 2 catalyst (35% KzC03 initially)

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with and without

Large scale plant

OF THE PROCESS BY USE OF

In the model calculations the variation of the reactivity caused by the catalyst enrichment in each cascade step must be considered. In Figure 5 the calculated values of temperature and specific coal throughout are shown in relation to gasifier length. The values for the noncatalysed process are included. In the first part of the gasilier, pyrolysis of coal takes place and is taken into account in the model calculation. The pyrolysis range in the first part of Figure 5, characterized by lower temperature but higher specific coal throughput, is longer with catalyst than without catalyst because of the higher reaction rate caused by the catalyst, which leads to a higher demand for coal heating in the pyrolysis range of the gasifier. Further, the specific coal gasified with catalyst is higher than that without catalyst although the temperature is lower. The catalytic effect increases along the gasifier because

244

OF HEAT-

The technical feasibility of the nuclear gasification concept is dependent on the availability of a metallic material suitable for the heat-exchanger tubes of the gasifier. Such a material had to be developed with high mechanical and chemical stability to withstand the severe loadings in the gasifier atmosphere. With this aim Bergbau-Forschung a few years ago started a joint research project with Mannesmann Research Institute in Duisburg.3 It was found after 10000 h tests in a gasification atmosphere that high-temperature alloys with a chromium content of at least 26 wt% form a dense chromium oxide scale that diminishes further corrosion. The oxide scale formation is a result of the relatively high steam level in the gasifier. These corrosion tests were performed without catalysts. In connection with the use of catalysts for promoting gasification additional corrosion tests with various catalyst concentrations were started in 1981. Up to now it is recognized that in general K,CO, catalyst can destroy the protective chromium oxide scale

Figure 4 Measure of reaction rate by K,COa catalyst (Leopold char with 10 wt% ash); fo=O.l 5% C min-‘, 700°C; 4 MPa. A, With catalyst; 8, ~3.5 wt% or without catalyst

Length

CORROSION BEHAVIOUR EXCHANGER ALLOYS

without catalyst

with catalyst

Thermal power of HTR Outlet temperature Number of gasifiers

(MW) (“C)

3000 950 12

3000 950 12 (33 m)

Coal throughput

(t

360 2.7’ 808 95 223 1092

594 4.46 753 95 369 1802

Gasification temperature Carbon burnoff Steam decomposition Raw gas production

h-l) HO6 t yr-‘)

i”o ’ (wt%) (%)

(IO3 m3 (r.t.p.) h-l) IlO9

m3 (s.t.p.) 819

yr-‘) Efficiency of gasification (including methanation) SNG production

(%) (lo3 m3 (s.t.p.) h-l) (lo9 n-l3 (s.t.p.) yr-‘)

98.4 300

13.5

98.4 495

2 25

3.71

K2C03 catalysts in coal gasification: H. Kubiak et al.

on the alloy by formation of potassium chromate. Consequently, internal oxidation may occur after prolonged operation. However, these detrimental effects are dependent on temperature and catalyst concentration. Experiments have shown that the additional corrosion attack caused by the catalyst is negligible at < 900°C and at a concentration < 5 wt% K,CO, on the coal. This was confirmed in a 10000 h test. Further experiments are under way that take into account the rise in the concentration of the catalyst at the end of the gasifier.

the framework of joint work involving BergbauForschung GmbH, Essen; Gesellschaft fur Hochtemperaturreaktor-Technik mbH, Bensberg; HochgemperaturReaktorbau GmbH, Mannheim; Kernforschungsanlage Jiilich GmbH; and Rheinische Braunkohlenwerke AG, Kijln on the Prototype Plant Nuclear Process Heat (PNP) project, sponsored by the Federal Republic of Germany, Bundesministerium fur Forschung und Technologie, and the Land Nordrhein-Westfalen.

CONCLUSIONS Model calculations show that by use of K,CO, catalyst in the range of 3-4 wt% on coal the throughput of a large scale nuclear gasification plant is increased by %65%. The gas production costs decrease by ~15%. Therefore, the use of catalysts is an attractive possibility for improving coal gasification process using HTR heat. The corrosion attack by catalysts is not severe at lower concentrations ( < 5 wt%) and lower temperature ( < 9OOC). The exact limitations have to be determined by appropriate corrosion testing that is now in progress. ACKNOWLEDGEMENTS The work described in this Paper was performed within

REFERENCES Jtintgen, H. and van Heek, K. H. ‘KohlevergasungClrundlagen und technische Anwendung’, Thiemig Taschenbuch Bd. 94, KarlThiemig-Verlag, Munchen, 1981 van Heek, K. H., Hewing, G., Kirchhoff, R. and Schroter, H. J. Paper presented at the Symposium on Nuclear Heat for High Temperature Fossil Fuel Processing, Institute of Energy, London, 28 April, 1981 Schroter, H. J., Weber, H. and Schendler, W. GWF - Gas/Erdgas 1981, 122, 75 Kubiak, H. ‘ModeIlierung der allothermen Wasserdampfvergasung von Kohle’, Thesis University of Essen, 1982 Leonhardt, P., Sulimma, A., van Heek, K. H. and Jiintgen, H. Fuel 1983,62,200 Sulimma, A. and van Heek, K. H. Steinkohlevergasung unter Einsatz von dampfgelosten Katalysatoren. Forschungsbericht BMFT-FB-T 82-073, Fachinformationszentrum Karlsruhe, 1982

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