Hot gas desulphurisation with dolomite sorbent in coal gasification
Introduction
In the gasification of coal or blends with other alternative fuels, sulphur is mainly present in the gas as hydrogen sulphide. This sulphur compound must be removed prior to gas combustion to comply with legislation and especially to avoid damage to the turbines. In the chemical industry, the presence of H2S leads to larger corrosion problems (pipes, compressors, etc.) and also problems with some catalysts. The scrubbing of sulphur from the gas stream has been widely studied using different agents, such as the amines used by Elcogas IGCC in Spain; however, this option requires a decrease in gas temperature that causes the inherent energy losses.
Some sorbents that allow H2S to be removed at high temperatures have been previously investigated; these include diverse metal compounds [1], [2]. Sorbent regeneration without significant degradation during a certain number of cycles is extremely important. Several Zn compounds [3], [4], [5], [6], some Mn and Cu compounds and blends [7], [8], [9], and some Fe compounds [10] have also been studied. The main problem is the sufficient duration of cycles without degradation. One very cheap oxide, CaO, that does not need to be reused has also been studied. The main sources are two abundant minerals, calcite and dolomite. The direct addition of this oxide or the calcite in the gasification bed for coal gasification or the gasification of other alternative fuels has been investigated [11] not only to clean the gas, but to produce hydrogen [12] and also the absorption of H2S in the oxides to remove them from the gas [13], [14], [15], [16], [17], [18], [19].
Calcium oxide is effective in adsorbing H2S from the gasification gases according to the reaction:but many studies with calcite or dolomite have been carried out on small samples in a thermobalance or differential reactor with 1 mg to 1 g [14], [15], [18]. In other cases, larger amounts of absorbent have been used but mixed with silica grains to form a homogeneous mixture of sorbent and silica to avoid improper random distribution of the gas flow and reduce errors in the longitudinal conversion of profiles as in articles [17], [19] with the aim of obtaining appropriate measures to develop mathematical models.
In the case of an industrial application it would be appropriate to apply the pure absorbent which is why this investigation was carried out in this way and the different behaviour investigated according to grain size and position in the absorbent column: at the top or at the bottom subjected to more mechanical pressure. Absorbent amounts of 100 g and 150 g that give bed lengths of 11.4 and 17 cm, respectively, were also used.
The absorbent chosen was dolomite as it works well under calcining (1123 K) and non-calcining conditions (1173 K) according to previous studied by Adanez [18], [19].
H2S in the gasification gases is known to be accompanied by a certain proportion of carbonyl sulphide (COS) that when burnt, yields SO2 according to the following reaction:This reaction increases the SO2 content of gases and increases the corrosive effect in gas turbines, gas motors, pipes and general industrial equipment and also increases SO2 emissions. Therefore, it should be reduced. At Elcogas, for example, this is carried out by COS hydrolysis according to the following reaction [20]:H2S is later eliminated by amine extraction.
There are various studies on COS hydrolysis especially aimed at the development of new catalysts such as rare earth oxysulphides [21], but that require operating at relatively low temperatures.
However, the ability to reduce COS at high temperatures using the same approach as H2S on earth alkaline sorbents would be of interest. This possibility has already been studied by Heesink [22] but limited to lower temperature (up to 700 °C) and with a very small amount of sorbent and that postulates the reaction:Ishida [23] also postulates the reactions:The proportion of COS to be expected will also depend on the concentrations of CO2, H2S and H2O (reaction (5) is the reverse of (3)).
To achieve a reduction in H2S and COS levels at very low values, some authors use Zn composites as absorbents [24] that carry out desulphurisation over two stages, one to reduce the high level of H2S at a low value, followed by another stage that uses an especially prepared absorbent with a high specific surface area.
In the present study, H2S was contaminated with a small amount of COS (about 1% of the H2S content) with the aim of investigating any decreases or increases under the test conditions, now operating with relatively large amounts of 100 g or 150 g of material. The contamination is sufficiently low so as not to decisively influence the principal sulphidisation reaction with hydrogen sulphide (1).
Another important reaction to consider is the reverse water-shift reaction, which influences the proportion of H2 in the outlet gases, an important aspect if hydrogen is to be produced:
Section snippets
Experimental
All experiments were performed using a dolomite from the province of Granada, Spain, that presented an essentially theoretical composition. The X-ray diffractrogram and analysis are shown in Fig. 1 and Table 1, respectively. The chemical analysis was performed by X-ray fluorescence for the aforementioned elements and by gravimetric methods for insoluble material and loss on ignition.
This dolomite can be completely decomposed at 850 °C as shown in Fig. 2, which contains the thermogram for
Influence of grain size, gas velocity and relative position in the desulphuring column (top or bottom)
By operating as described in Section 2, virtually complete (98–99%) dolomite sulphidisation is obtained when the higher grain size of 2–2.5 mm was used, which would be the most difficult to sulphurise under different test conditions. Fig. 4 shows the X-ray diffraction of sulphurised dolomite, which contains no line of CaSO4 or CaO, but rather only CaS (oldhamite) and MgO (periclase) lines.
Fig. 5 shows the breakthrough curves from various tests carried out at a gas velocity of 29.1 and 14.5 cm s−1
Conclusions
- (1)
The principal factors for the decrease in H2S content in the outlet gases of the desulphuration column in the initial stages of bed use are gas velocity, bed length and inlet H2S concentration. Lower gas velocities, lower inlet H2S concentrations and higher bed lengths result in lower H2S content in outlet gases.
- (2)
Dolomite position at the top of the desulphurant bed or bottom (under more static pressure) is another important factor that conditions its use. The behaviour at the bottom is worse
Acknowledgements
The authors would like to thank the European Commission – DG Research, Research Fund for Coal and Steel (RFC-CR-04006) for supporting this study.
The authors would also like to thank UPM-R05/11205(RFC M0500204145) and UPM-CCG06-UPM/TQ-351 (RFC M0700204178) for supporting this line of investigation of the research group.
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