Italian refinery gasification project to make electricity, steam, and H2 from tar

This 90 m diameter floating roof tank was built to replace storage removed to make space for the IGCC plant. The massive concrete containment walls are mandatory in Italy.

A number of diverse forces have combined to catalyze the construction of a major integrated gasification combined cycle (IGCC) plant at the Saras SpA Raffinerie Sarde (Saras) complex at Sarroch, Sardinia (see accompanying article in News). They are high sulfur fuel oil, Chernobyl, and a growing need for electricity on the island.

The project will be built, owned, and operated by Sarlux SpA, a joint venture of Saras and Enron Corp., Houston. Sarlux will be headquartered in Milan. Its plant, expected to be completed in 1999, will require an estimated investment of more than $1 billion.

A pool of banks has been selected, and "a mandate and underwriting letter" was signed this past April. It is expected that the first pay out of the project finance will take place this month. On completion, 140 people will run the plant. This head count includes top managers, technical and commercial staff, and the operations and maintenance force.

Site preparation has been completed and part of the civil works started. Snamprogetti spa began detailed engineering work this past March and has completed the engineering related to gasification and waste water pretreatment.

The plant will gasify some 18,000 b/d of tar from visbroken vacuum residue and produce low-pressure steam, hydrogen, and enough synthesis gas to fuel turbines generating 550 mw of electricity. This is a world scale size that will in the year 2000 meet an estimated 40% of the needs of the New Hampshire-sized island. It has a population now of some 1.6 million. The incorporation of the Latin word for light, lux, in the company name is therefore appropriate.

The design for the gasification section of the plant allows a feedstock containing up to 6% sulfur with about 1,000 ppm nickel plus vanadium-a real witches' brew for western European refiners. Giuseppe Citterio, Saras manager of strategic planning, says that though there are some design parameters for viscosity and gravity, they boil down virtually to the ability to pump the material.

Although some high sulfur residual fuel oil and fuel coke have been able to find markets in countries that are environmentally loose, the sales prices are crumbling and the markets disappearing.

Residual fuel oil

The Italian state-owned power company, ENEL, is the world's largest consumer of resid for power generation. But the winds of change are blowing there too. The European Union and the Italian government are proposing a 0.25% sulfur limit for residual fuel oil. Even resid from an exceptionally good "champagne" crude like Norway's Ekofisk could not meet this specification. Some sort of hydrodesulfurization and conversion would therefore be necessary and costly.

Even though the legal limit on fuel oil burned in Italy is still 3.9% sulfur, ENEL is mainly burning 1% sulfur fuel because of emission limits. At times it must therefore burn low sulfur crude or atmospheric resid.

As a result, says Citterio, the Italian market is today in essence a 1% S fuel oil market; the export market a 3% S market. Saras does not like the outlook for resid at home and abroad and therefore has embraced the IGCC project.

Chernobyl factor

Italy, like most of Europe, has been turned off by nuclear energy since the Chernobyl catastrophe in the former Soviet Union. In response, the Italian government is promoting alternate electricity power schemes and allowing higher rates for such electricity.

Hydroelectric and solar power command top rates, with combined cycle electricity following closely. Rates are known and granted, not negotiated.

Synergy

Synergy between the refinery and IGCC plant was most important in justifying the project.

First, the refinery provides a sure supply of feedstock. Vacuum resid is an alternative feed. In addition, the refinery will furnish the gas oil for the start-up of the combined cycle unit and for primary turbine fuel if the gasification unit goes down. These are critical considerations when a great deal of Sardinia, as noted earlier, will depend on this output.

The refinery will be getting back a valuable gas-hydrogen-which it needs to clean up other fuels. Hydrogen produced by the Sarlux plant will be purchased by Saras under the terms of a long term, take-or-pay purchase agreement. The IGCC plant will initially deliver up to 20,000 cu m/hr or 760 Mcf/hr of 99% pure (by volume) hydrogen at a pressure of about 45 barg to the refinery. The IGCC plant can be expanded to supply up to 40,000 cu m/hr at a later date.

It will also furnish the refinery 100 ton/hr of medium-pressure steam and 85 tons of low-pressure steam.

Paradoxically, and fortunately, the addition of the IGCC plant will reduce emissions around the refinery. The reason is that the new plant has low emissions and will supply the refinery with steam and electricity that it won't have to generate. Combined cycle efficiency in excess of 50% can be achieved. This compares to efficiencies in the mid-30% range for conventional residual oil fired units, Saras says.

The plant will be built partly on reclaimed land. The process units will require 60,000 sq m (15 acres); the offsites 19,000 sq m or 4.7 acres. Sarlux will lease the land from Saras.

Other shared activities and equipment that will enhance the joint project are flares, waste water treatment, utilization of an existing maintenance force, sulfur recovery, and even a cafeteria.

The project

The plant will consist of several main units (see the flow diagram). It includes the feedstock gasification, the process units for raw syngas and black water treatment, and combined-cycle units. The gasification unit will use Texaco quench-type gasifiers.

The gasification, the other process units, and combined-cycle units will be designed and constructed by a consortium led by Snamprogetti that includes Turbotecnica and GE Nuovo Pignone.

A separate air-separation unit will be constructed, owned, and operated by Air Liquide. Texaco quench-type gasifiers will gasify, via partial oxidation with pure oxygen, the vacuum visbroken tar to produce a low heating value synthetic fuel gas (syngas). The gas is primarily a mixture of carbon monoxide and hydrogen and will fuel the combined-cycle unit.

The volume of syngas burned in the gas turbines is more than 600,000 cu m/hr. It is made up (in volume percent) of 20-25% hydrogen, 25-30% carbon monoxide, and 35-40% water. Other gases account for about 10%. The gasifiers do not produce any sulfur or nitrogen oxides. Steam produced by the combined-cycle unit is used as a process moderator in the gasification step.

The proc ess unit will remove acid gases, primarily hydrogen sulfide and carbonyl sulfide, from the syngas stream. The clean syngas will be sent to a pressure swing adsorption unit where the syngas stream will be treated to remove and recover the refinery's hydrogen from the syngas.

The acid gases removed from the raw syngas will be processed in sulfur recovery and tail gas units to destroy the ammonia and hydrogen sulfide and to recover the sulfur. The cleaned syngas will be saturated with water, which helps increase power production and reduce NOx, preheated, and piped to the combined cycle unit.

An air-separation unit will be located remotely from the refinery and will provide oxygen to be purchased by the IGCC complex.

The oxygen plant will have storage capacity equivalent to about 12 hr production. This backup storage capacity enables Air Liquide to predict equivalent availability for the oxygen plant to be above 99.5%.

The combined cycle will consist of combustion turbine generator units with a heat recovery steam generator. The units, three GE Nuovo Pignone Frame 9E combustion turbines, will burn syngas to produce electricity. The hot exhaust gases from the combustion turbines will be directed to heat recovery steam generators to recover heat from the exhaust gases and produce steam. Steam produced by the heat recovery steam generators will be used primarily to drive steam turbine generators to produce more electricity.

The heat recovery steam generators will also provide steam to the refinery for process use. Low-pressure steam produced by heat recovery from the hot raw syngas will also be supplied to the refinery.

Gas-fired, combined-cycle plants generally emit lower levels of air pollutants than do conventional residual oil-fired power plants.

Combined-cycle efficiency in excess of 50% can be achieved, compared to efficiencies in the mid-30% range for conventional residual oil-fired units.

The gas produced in the gasification and process units has been stripped of sulfur prior to combustion, negating the need for large sulfur dioxide scrubbers and the associated solid wastes. Particulate emissions are reduced, as are emissions of NOx.

Electric power produced by the combustion turbine and steam turbine generators will be stepped up to 380 kv and supplied to ENEL through underground oil-filled cables from the combined cycle substation to an ENEL substation on the north side of the refinery.

Italy, like most of Europe, has been turned off by nuclear energy since the Chernobyl catastrophe in the former Soviet Union. In response, the Italian government is promoting alternate electricity power schemes and allowing higher rates for such electricity.

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