Kansas refinery starts up coke gasification unit

In addition to power, these plants will produce steam and hydrogen for refinery use.

The El Dorado project

The project team at the El Dorado refinery chose gasification over alternative processes available. In other words, says Texaco project manager William Preston, this is not a demonstration unit for the Texaco gasification process.

After some preliminary engineering work, the refinery received approval for the project in March 1994. The contract was awarded to Fluor Daniel Inc. the following month. Only 2 years later, the plant started up.

The gasification power plant provides a number of economic benefits to overall refinery operation. The unit meets all of the refinery's electricity requirements and 40% of its steam needs, thus reducing utility costs. This benefit enabled Texaco to shut down one of its fired boilers that had been used for steam production.

The gasification unit is small compared to commercial utility-type gasification projects. It sits on a 300 x 400 ft plot inside the refinery.

The local utility in the El Dorado area has excess generating capacity, says El Dorado plant project manager Stephen Woods. In addition, Texaco expects further reductions in the price and marketability of coke. The capacity of the gasification unit, therefore, was based on the refinery's utility needs and coke sales.

The El Dorado refinery produces 800 tons/day coke. Although, before the gasification project, Texaco sold most of this production at a profit, about 15% (120 tons/day) was being sold at break-even prices. Taking into account the additional 10 tons/day of refinery waste that was deemed suitable for gasification, Texaco decided on a feed capacity of 170 tons/day for the gasifier.

Adding to the project's financial attraction is that the existing sulfur-recovery and tail gas treating units were inadequate to enable Texaco to process more sour crudes. The gasification unit and associated air separation plant allow Texaco to use oxygen enrichment to increase sulfur-recovery capacity.

In addition, nitrogen from the air separation plant is used to blanket and purge refinery units. This has enabled Texaco to all but eliminate the purchase of liquid nitrogen.

Gasifier feeds

Typical composition of the coke feed to the gasifier is: 90% carbon, 4% hydrogen, 4% sulfur, 1.5% nitrogen, 0.5% oxygen, and 0.5% ash. The ash consists of a variety of trace metals originating from crude oil feedstock.

Four streams make up the 10 tons/day of waste streams processed in the gasifier:

• 0.1 tons/day API separator bottoms
  

  This stream comprises sludge settled from refinery process water streams. Although the sludge contains about 75% water, the dry portion is composed of 60% ash, 35% carbon, and small amounts of hydrogen, sulfur, and nitrogen. Separator bottoms are considered a hazardous waste by the U.S. Environmental Protection Agency (EPA), and designated K051. Texaco processes in its coker the volume of separator bottoms not processed in the gasifier.

• 4.5 tons/day acid-soluble oils (ASO) from the alkylation unit
  

  Before start-up of the gasifier, these heavy polymers formed in the HF alkylation unit were sent off site for use as cement kiln fuel. ASO is classified by EPA as D001 or D018 waste.

• 0.6 tons/day primary waste water treatment sludge
  

  Primary sludge is produced upstream of the API separator from the gravitational separation of oily process and cooling water streams. These wastes are classified F037 and F038 (these designations also can include wastes from refinery tanks, ditches, and impoundments). In the past, most of this material was placed in landfills designated for hazardous waste disposal, but recent regulations forced Texaco to incinerate this stream until the gasifier started up.

• A small amount of phenolic residue
  

  In the El Dorado refinery, benzene is removed from the reformate streams and converted to cumene. The cumene is then converted to phenol. Tower bottoms from the phenol production unit also are processed in the gasifier.

Output

The syngas produced in the gasifier is supplemented with natural gas and fed to the combustion turbine. The turbine consumes the equivalent of 9 MMscfd of natural gas. About one third of the fuel requirement is met by syngas.

At its 40 mw nominal rating, the turbine's exhaust heat produces about 180,000 lb/hr of 600 psi and 150 psi superheated steam.

Oxygen for the gasifier is produced by air separation, which takes place at -300 F. The unit produces about 200 tons/day oxygen and 800 tons/day nitrogen.

Praxair Inc. operates and maintains the plant under a 2-year contract with Texaco, to allow adequate time for technology transfer. At the end of the contract period, Texaco will assume operation of the plant.

Because of the gasification unit's high carbon conversion (about 99%), the solids it produces comprise only about 1% of the original coke mass. The disposition of this slag is yet to be determined.

Options under consideration include sale of the slag as a low-grade fuel (with a heating value of 500-8,000 BTU/lb) and as a metals reclamation feedstock.

Process description

Gasification is a thermal process that chemically converts hydrocarbon feedstocks to a mixture of carbon monoxide and hydrogen, plus small concentrations of other gases. The reaction takes place in an oxygen-lean environment, in contrast to combustion. In this reducing atmosphere, the ratio of oxygen molecules to carbon molecules is less than 1.

A flow scheme of Texaco's gasification process is shown in Fig. 2 [34744 bytes].

Coke is conveyed to a storage silo, which holds 1.5 days' gasification inventory. A rod mill crushes the coke in the presence of water to produce a pumpable slurry. The wastes are combined with the slurry, and the mixture is fed to the gasifier.

The feed mixture is combined with oxygen using a mixing nozzle. Gasification takes place in the refractory-lined reactor at about 2,500 F. and 450 psi. Thermocouples constantly monitor the skin temperature of the unit.

The oxygen-starved environment of the gasifier prevents the formation of NOx and SO2. Feed sulfur instead forms H2S, which, ultimately, is converted to elemental sulfur in the refinery sulfur recovery unit. And particulates are removed in the gas cleanup process.

The syngas produced in the gasifier is water-quenched in a chamber within the gasification unit (Fig. 2 [34744 bytes]). It is this same pool of water that cools and solidifies the ash portion of the feed, producing nonleachable, glassy chips. The pool circulates continuously, thus enabling fine solids to be removed from the water.

The cooled chips drop into a closed lock hopper, which is depressurized and dumped once per hour into one of two concrete basins. After dumping, the lock hopper is pressurized and reopened to the quench chamber.

The so-called "black water" from the quench chamber and syngas scrubber contains fine particulates. This stream enters a flash vessel operating under slight vacuum. In this vessel, dissolved gases are removed from the water and piped to the sulfur recovery unit.

The water then enters a gravity settler for fines separation. The clarified water is recycled to the process, and the bottoms are disposed of with the slag.

In the syngas scrubbing section, entrained water and fines are removed from the product via a multistage water wash. The syngas is cooled in two stages: a medium-pressure steam generator, followed by heat exchange with cooling water. The water condensed out of the syngas is recycled to the process.

The syngas is then sent to an acid-gas absorber. This unit comprises a countercurrent column in which the gas is intimately contacted with an amine-based solvent.

The absorber removes virtually all of the H2S and some COS. The H2S is sent to the sulfur recovery unit and the lean solvent is returned to the absorber. Sulfur recovery from H2S is about 99%, according to Texaco literature.

Combustion

The air plant receives its feed from the compressor section of the combustion turbine. This extraction air comprises about 7% of compressor capacity. Compressor inlet air is filtered via conventional pleated filter cartridges.

In addition to natural gas and syngas, 95% of the nitrogen produced in the air separation plant and varying quantities of 600-psi steam are fed to the turbine for power augmentation and NOx reduction. The nitrogen reduces NOx formation and, because the feed gases are nearly sulfur-free, little SOx is produced during combustion.

Although small amounts of NOx and SOx are produced in the combustion process, these emissions are orders of magnitude less than would produced if the gasification feeds were incinerated.

The energy generated by combustion produces about 40 mw of electricity. Exhaust from the turbine is routed to a heat-recovery steam generator. The heat recovered by this unit is used to produce 180,000 lb/hr of steam.

Emissions reduction

Where Texaco formerly sold 120 tons/day coke to be burned as fuel off site, this quantity of coke is now gasified, thus decreasing NOx and SOx emissions. In addition, the process produces CO rather than CO2, and hydrogen rather than water, which would require treatment.

Besides the emissions reduction that results from eliminating the incineration of excess coke and refinery wastes, a theoretical decrease in pollution also is associated with electricity the local utility is no longer generating for refinery usage.

In the U.S., refiners planning to construct units that will emit air pollutants must obtain a prevention of significant deterioration (PSD) permit before construction can begin. Texaco attempted to obtain an exemption from PSD review by arguing that when incineration and utility emissions are taken into account, net emissions to the environment would decrease as a result of adding a gasifier.

The Kansas Department of Health & Environment (KDHE), however, decided to view the project as a "point source," thus necessitating a PSD. Despite this decision, the KDHE saw the benefits of the gasification project and supported Texaco in an important aspect of the permitting process.

During the process of obtaining the necessary permits for the project, KDHE had to decide whether the gasification facility should be considered a hazardous waste treatment process. If it had been designated a treatment unit, Texaco would have had to obtain a Resource Conservation and Recovery Act Part B permit, requiring significant financial, legal, and manpower resources.

This process takes some 2 years to complete. Said John Mitchell, KDHE's hazardous-waste section chief, "We decided the gasification process at this facility would be both environmentally desirable and protective, especially in comparison to alternative waste management options."

"The problem," continued Mitchell, "was the process was unique, so there was no record in the nation of any permit waiver for this approach." As a result, KDHE sought EPA's support in designating the facility a processing unit.

KDHE began negotiations with EPA in September 1994, suggesting, according to a KDHE press release, "...that EPA use its policy of regulatory flexibility because of the obvious benefits to both the business and the environment." In May 1995, EPA formally agreed with KDHE's assessment that Texaco did not need a Part B permit.

The KDHE press release stated: "Texaco and the Kansas Department of Health & Environment...set a [national] precedent for using refinery waste products for fuel."

A press release from a regulatory agency espousing the benefits of an oil industry construction project is pleasantly surprising. But even more surprising is a quote from KDHE secretary James O'Connell in the release.

"[Texaco] did exhaustive research before they came to us," said O'Connell. "It was a pleasure to go to bat for this type of activity."

Commissioning

Texaco started up the cogeneration unit in late January. Following some tests of the air separation plant, the entire gasification power plant was brought on stream in mid-June.

Operating the gasification power plant requires a total of 21 people: 18 operators (four crews of 4, plus 2 additional operators), a foreman, a maintenance planner, and a process engineer. The unit operates on a separate distributed control system from the refinery, but is tied into plant information systems.

Economics

Before start-up of the gasifier, the El Dorado refinery had been the local utility company's third largest customer, spending $12-14 million/year. The refinery now pays only a few million dollars a year for stand-by service, says Woods.

In addition, the refinery is saving more than $1 million/year in waste shipment and disposal costs, and almost $1 million/year in nitrogen costs. Steam production costs have been reduced by more than half.

Other unquantified savings result from oxygen enrichment of the sulfur plant, which will enable Texaco to process a greater proportion of high-sulfur crudes when the sweet/sour differential is attractive.

Sheffield sums up the advantages of gasification at the El Dorado plant as follows: "The benefits are financial as well as environmental. We are turning so-called waste products into feedstocks, and mining the energy in these products instead of worrying about how to dispose of them."

Texaco gasification

Texaco's gasification process is the center of a number of major projects around the world. In addition the to four refineries in Italy adding gasifiers:

• Quantum Chemical Corp. is processing wastes in a gasifier at its Deer Park, Tex., facility.

• Koch Refining Co. has a gasification project under development at its 230,000 b/d Rosemount, Minn., refinery.

• Exxon Chemical Co. signed an agreement with Texaco to use Texaco technology as the basis for gasification projects at a number of Exxon sites worldwide. (The agreement includes other Exxon affiliates.)

According to Texaco Inc.'s 1995 financial and operating statement, total installed capacity of Texaco-licensed gasification units was a little more than 2.0 billion scfd in 1995. Texaco expects this total to exceed 4.0 billion scfd by the year 2000.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.