Waste Heat




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Table 1 : Waste heat and emissions for provision of natural gas to low pressure consumers

Table 1 : Waste heat and emissions for provision of natural gas to low pressure consumers

3. Employment of natural gas or synthesis gas in the sponge iron process cycle

Among the various applications of sponge iron, its application as an alternative for gas purification or conversion is considered.

3.1 Direct employment of natural gas

Beside the employment of various industrial gases such as biomass gas, synthesis gas or water gas a direct and indirect employment of natural gas is also possible for the reduction process:

Investigations are necessary to determine the influence of the main and subsidiary components of natural gas on the reduction process. The following oxidation process of the sponge iron with water vapour produces pure hydrogen:

The applicability of sponge iron is determined by its cycle behaviour. A low tendency to sintering (for example at low temperatures) and minimal side reactions (for example carbonization and pore clogging of the Fe pellets through carbon containing gas) are the preconditions for good cyclability of the sponge iron. With not too high carbon precipitation, regeneration of the sponge iron with steam at a high temperature can provide a higher number of cycles of the sponge iron. The higher stability of methane as main component of natural gas compared to CO in the lower temperature range of 400 - 600 °C leads to lower precipitation of carbon and therefore conforms to the above-mentioned pre-conditions. The following kinetic factors can contribute to the optimisation of the reaction process and are the basis for further enquiry:

- the employment of catalysts

- raising the speed of the reaction through the conversion of the pellets into powder.

Optimation of the sintering process and further improvements in the basic material for the structural substances to maintain porosity are also undertaken.

3.2 Employment of natural gas according to the steam fission method

For an indirect employment of natural gas a conversion of the gas should be carried out according to the steam fission method (4). Synthesis gas generation from natural gas in the presence of steam is based on a coupling of exotherm and endotherm gasification reactions. A number of different processes are available for the purification of synthesis gas from the components H2S, COS and C02. In the presence of catalysts, after a desulphuration process natural gas can be cracked. The gas produced contains a predominate proportion of methane which with steam at high temperature can be converted to CO and H2 in the presence of high temperature resistant Ni catalysts. Part of the product gas is burnt with addition of air or oxygen and the gas mixture thus heated to above 1200°C. At this temperature the methane is converted down to a proportion of 0.2-0.3 vol.-% with too large a proportion of CO the desired CO/H2 relation can be set by a partially catalysed conversion. The CO/H2 mixture produced can be employed for reduction of the iron oxide at temperatures over 800°C. Extended conversion of CO to C02 through the reduction process and production of pure hydrogen through the subsequent oxidation will thus be possible.

3.3 CO-conversion

In the low temperature range from 400-600°C the employment of the gas mixture mentioned is not possible or not satisfactory because of unstable behaviour and the disproportion of CO which results in heavy precipitation of carbon and clogging of the pores of the pellets. Therefore conversion of this gas should be carried out. The sponge iron reactor can offer a favourable alternative to CO shift conversion. The conversion of this gas can be carried out with steam at 200 -400°C with catalysts. Conversion catalysts consist of sponge iron Fe304 and Cr oxide mixtures which are used at 350-400°C. with them the CO content can be reduced to 3-4 vol.-%. In this phase the employment of the gas mixture generated and the CO residue's disturbance of the reduction process should be investigated. The gas generated will be employed for the reduction after condensation of the superfluous water. A second catalytic conversion can be introduced in the case of the cycle behaviour of the sponge iron being influenced by the CO residue. In this phase effective low temperature catalysts can be used, for example G66 from Girdler on the basis of Cu-Zn oxide, whose operating temperature is about 190-260°C. At this temperature there is a small amount of CO in the water gas equilibrium. As mentioned, completely pure production of hydrogen can be carried out through the subsequent oxidation process with steam.

(1) ÖSTAT, "Energieversorgung Österreichs: Jahresheft 1994", Österreichische Staatsdruckerei, Wien, 1994.

(2) R. Frischknecht, P. Hofstetter, I. Knoepfel, R. Dones, E. Zollinger, "Ökoinventare für Energiesysteme: Grundlagen für den ökologischen Vergleich von Energiesystemen und den Einbezug von Energiesystemen in Ökobilanzen für die Schweiz", ETHZ, PSI, Schlußbericht des BEW/NEFF-Forschungsprojektes 'Umweltbelastung der End- und Nutzenergiebereitstellung', 1994.

(3) G. Mauschitz, "Emissionen von Methangas und Kohlendioxid aus der Bereitstellung und Verfeuerung fossiler Energieträger für die österreichische Brennstoffversorgung", Technische Universität Wien, 1995.

(4) K. Wissermel, H. J. Arpe, Industrielle Organische Chemie, pp. 17-30, VCH, Frankfurt, 1988.

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