In recent years many refineries around the world have been re-equipped with “severe” processes both thermal (coking) and catalytic (hydrocaking, catalytic cracking, dewaxing), to change their product mix in favour of an increased proportion of light products – in line with market trends. In Italy, however, there has been a reluctance to invest in these technologies, with a tendency to favour less severe thermal processes such as visbreaking and thermal cracking. These processes take as input the residue from the bottom of the vacuum column and produce as output other distillates plus a residue, tar to be precise. The characteristics of tar are high viscosity, making it difficult to move, high sulphur (ranging between 3 and 6 per cent depending on the quality of the crude processed) and the presence of various metals (notably nickel, vanadium, iron).

The Italian approach depended on the capacity of the state owned utility ENEL to buy and burn the high sulphur fuel oil obtained from the tar. The tar was mixed with lighter products having very low sulphur content to produce a power station fuel with high sulphur content (1-3 per cent) but good calorific value. However, Italian and EU environmental regulations will now not allow products with sulphur content over 0.25-0.30 wt per cent to be burnt in power stations without flue gas desuphurization fitted – resulting in a sharp decline in the high (2.5-3 per cent) and medium (1-2 per cent) sulphur fuel oil market. This has forced refiners to make the substantial investment needed to change their product mix or to find new outlets for their tar.

As well as these environmental pressures and the decline in the demand for residual fuel oils, other factors also came into play in Italy in the early 1990s. These included a referendum putting an end to the national nuclear programme and a growing need for new generating capacity in several regions of the country. Against this background the Italian government passed a law in 1991 and the CIP Resolution in April 1992 providing tariff incentives to power projects able to offset these negative effects. The disposal of refinery residues by IGCC (Integrated Gasification Combined Cycle) fell into this category. Four major IGCC projects, sponsored by different refineries, were announced. Three have been implemented and are close to completion, Sarlux, Falconara (Modern Power Systems, June 1998) and the 520.8 MWe (net) ISAB Energy project, located in Priolo, Sicily, next to ERG Petroli’s ISAB refinery, the second largest in Italy. ISAB Energy was the first to enter the commissioning phase, which it did in mid 1998.

The decision to gasify

The ISAB refinery predominantly processes heavy crudes. In spite of upgrading its cracking facilities (visbreaker and thermal cracker) about 30 per cent of its output is tar, resulting in large volumes of heavy fuel oil. Three options were looked at for addressing the new market conditions:

  • Deep conversion – this entails complete redesign of the refinery with installation of catalytic processes.

  • Tar desulphurization – installation of desulphurization plant to adapt the tar to power plant requirements.

  • Tar gasification, with integrated power production.

    On economic and environmental grounds, the gasification option was chosen. Ownership of the ISAB Energy IGCC plant is 51 per cent ERG Petroli and 49 per cent Edison Mission Energy.

    Basic design features

    The project essentially started in 1991 with the assignment to Foster Wheeler of a feasibility study to establish key process features and optimize process design. In 1992-1993 Foster Wheeler received a contract to develop the basic design of the complex, with the assistance of Texaco, for the gasification unit, and Ansaldo for the power block. In parallel ISAB initiated the long and complex licensing process. About 150 permits were required, involving various local and central authorities. In 1995 a consortium formed by Foster Wheeler and Snamprogetti was awarded a lump sum turn-key contract for engineering, procurement and construction. The “notice to proceed” was finally given in July 1996, after closure of the non recourse project financing package.

    Feedstock (asphalt, visbroken tar, heavy fuel oil, etc.) is introduced into the gasifiers which are operated at a temperature above 1400°C and a pressure of about 70 barg. Gasification occurs in the presence of oxygen and high pressure steam acting as a temperature moderator. The syngas and solids (consisting of unconverted carbon and ash) are quenched by water sprays before they leave the gasifier. The solids are trapped in the quench water, leaving the syngas clean and relatively cool. Unconverted carbon is recycled back to the gasifier to achieve 100 per cent carbon conversion, using naphtha as the soot carrier. Quench grey water from the gasification is treated and filtered to recover a metal cake which contains large amounts of nickel and vanadium and is intended to be sold to metal reclaimers. The process waste water, after a further pre-treatment for ammonia removal, is sent to a municipal treatment facility.

    The particulate-free raw syngas is then further cooled through heat exchangers, generating medium pressure steam which is used to generate electricity in the steam turbines. The sulphurous compounds in the syngas (acid gas) are removed so that the sulphur emissions in the form of SO2 are minimized when the syngas is burned in the combustion gas turbine. From the acid gas removal unit, the sulphur offgas is sent to a Claus sulphur unit where 99.8 per cent of it is converted to elemental sulphur suitable for sale as a byproduct. The virtually sulphur-free, high pressure syngas is then sent to the gas turbines, via a gas expander that recovers the high delta-pressure of syngas through 10 MW of electric power and a humidifier which saturates the syngas in order to avoid high NOx formation in the combustion chamber of the gas turbines. The exhaust gas from the two gas turbines is ducted to heat recovery steam generators which produce high pressure steam to drive the steam turbine generators to produce additional power and also to act as a temperature moderator in the gasifiers. Additional syngas is combusted in supplemental burners in the heat recovery steam generators to produce additional steam for the steam turbines.

    Among the technologies adopted are:

  • Texaco gasification process. The key licence is with Texaco, designer of the gasification section. ISAB Energy will be the first IGCC to operate with asphalt, although Texaco has tested heavier feeds at the Montebello Gasification Pilot Plant. Two quench type gasifiers convert 132 t/h of asphalt produced from the visbroken tar of the refinery. The visbroken tar is first passed through a Kellogg Technology solvent deasphalting plant (ROSE). This recovers deasphalted oil (which is returned to the refinery for more processing) and the asphalt, which is fed to the gasifiers. The asphalt is very heavy and has to be pumped at a temperature of 240-290°C. It has up to 6 per cent S, 1000 ppm metals (Ni-V-Fe) and a softening point close to 120°C.

    The syngas, produced at above 1400°C by partial oxidation with pure oxygen, is cooled down to 250°C using a water stream injection to control the reaction. The oxygen is coming from a nearby air separation unit constructed, owned and operated by Air Liquide.

    The syngas goes through a heat recovery section (producing MP and LP steam) then a COS catalytic hydrolysis system and then an acid gas recovery stage.

  • Heavy metal recovery. Grey water from the gasification section is processed to remove sulphides, cyanides, thiocyanates and suspended solids including all the heavy metals. Chemical injection (caustic soda, polyelectrolite, ferrous sulphate, etc) favours precipitation of dissolved salts of heavy metals and the other compounds and concentrates the precipitate in a filter press to yield a cake with up to 28 per cent V and 9 per cent Ni.

  • Acid gas removal. The acid gas removal system uses a solution of Methyl-Di-ethanolamine (MDEA) to wash the gas and selectively remove H2S gas, which is routed to the sulphur recovery system. The process operates at a relatively low temperature to improve selectivity and avoid co-absorption of CO2 into H2S gas. The washing absorbs virtually all the H2S and a very small amount of CO2 yielding a purified fuel gas of less than 15 ppm total H2S + COS and less than 10 ppm total HCN and NH3 by volume.

  • Sulphur recovery. S recovery is based on the Claus reaction in which H2S and SO2 react to form elemental sulphur. Most Claus plants use air, but many plants installed over the last few years have chosen to use oxygen, which is more economical, especially if an oxygen plant is being installed for another purpose as ii is in this case.

    Around 50 per cent of conversion to sulphur takes place in the combustion chamber. The gas leaving the chamber is cooled to condense sulphur vapour. Heat removed from the gas is used to generate medium pressure steam. The remaining conversion of the sulphur gases to elemental sulphur occurs in two stages of catalytic reactors.

  • Combined cycle units. The power plant comprises two combined cycle gas turbine modules. Each module consists of one Siemens type V94.2 gas turbine modified for use with low calorific syngas, one heat recovery steam generator (HRSG) and one re-heat condensing steam turbine generator unit. The rating (gross) for each gas turbine is about 161 MWe per unit and for each steam turbine is around 115 MWe per unit. As already noted an additional 10 MWe is generated in the gas expander.

    Start-up progress

    Most utilities of the IGCC complex, eg cooling water, sea water desalination, compressed air, fuel gas, water demineralization, machinery cooling water etc, were successfully commissioned during the last four months of 1998 and, since then, kept in continuous operation to support the start-up of the production units.

    In October 1998 the first gas turbine was started with the auxiliary fuel, gasoil. After a few weeks of intermittent operation, to test and tune at no load, the generator was connected to the national grid in December 1998 and the power increased up to the target value. Simultaneously the corresponding heat recovery steam generator started steam production at conditions sufficient to test the steam turbine for power production. Over this period several functional tests were completed, such as load rejection, overspeed, control, alarm and emergency shutdown tests.

    Over the period March to May 1999 the same start-up procedure using gasoil was completed for the second combined cycle unit.

    In early April 1999 the first process units of the plant, the solvent deasphalting systems, were commissioned. In the middle of June the hydraulic and performance tests of the solvent deasphalting unit were successfully carried out. In the middle of July the first startup of the gasifier with low sulphur fuel took place and in August syngas derived from residues were being fed to the turbine for the first time. Commercial operation and plant test is expected before the end of the year.

    Main commercial agreements relating to the ISAB IGCC


    Table 1
    Table 2

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