La Caridad 500MW plant reducing

Grupo México’s COE by 40 percent

Unit II of La Caridad was declared ready for commercial operation in June this year, marking completion of the copper mining company’s two-unit 500MW combined cycle power plant.

Mining site. Minera México’s copper mining and refining facilities near La Caridad in Sonora State.

In a move to cut mining production costs, Grupo México has embarked on a plan to ultimately have its own power plants provide 90-95 percent of its operating electricity requirements.

 

One of its subsidiaries, Minera Mexico, which owns copper mines in Mexico and the United States, recently commissioned the second of two Siemens SGT6-5000F combined cycle turnkey units.

 

Power output. The two 1x1 combined cycle units are site rated at 258.1MW net output at site conditions and 56.9 percent efficiency.

 

Cost savings. New plant is expected to lower the cost of electricity for mining operations by 40 percent to 6 cents/kWh from 10 cents.

 

Emissions. Gas turbine DLE combustion will limit NOx to less than 25 ppm in compliance with México and World Banks emissions standards.

 

Mining and smelting is a highly energy intensive process. Reducing the cost of electricity can therefore be a significant contributor to lowering the cost of production and thereby improving the price competitiveness of mining companies.

 

Grupo México began looking several years ago at how it might reduce the cost of energy. It developed a business case to show that it would be more economical to have its own generating facilities to supply electricity for Minera México’s copper mining and refining facilities near La Caridad, in Sonora State, in the western part of the country and for another mine some 100 km away.

 

Project scope

The company hired Black and Veatch as its owner-engineer to perform plant feasibility and siting location studies etc. followed by a Request for Proposals (RFP). Initially four companies submit competitive bids to provide equipment construction of the two plants.

 However, since building power plants is not Grupo México’s core business, it subsequently decided to instead look for a supplier that could deliver the plants on a turnkey basis and would offer a long-term service agreement.

 

Eliuth Lopez, Siemens project manager for La Caridad recalled: “We initially did submit an offer for equipment only, i.e. a reduced scope power island and later submitted an offer for the turnkey bid request.

 

“Company mining executives visited the Norte combined cyle power plant that we had just built in the state of Durango, Mexico, in August 2010 and liked what they saw.

 

This greatly improved our position with Grupo Mexico who subsequently came to Siemens for an EPC solution.”

 

Under the EPC turnkey scope of supply Siemens delivered two combined cycle units each equipped with an SGT6-5000F gas turbine and SGen6-1000A air-cooled generator, one SST-700/900 RH steam turbine and SGen6-100A-2P generator, heat recovery steam generator, and the complete electrical and SPPA-T3000 instrumentation & control equipment.

 

Notably, the gas turbine for the La Caridad I combined cycle power plant was shipped in November 2011 at the opening of Siemens’ new gas turbine manufacturing facility in Charlotte, USA. It was the first turbine to be shipped from the facility.

 

Looking to generate 90-95% of its

power requirements in-house

 

Grupo México is Mexico’s largest mining company and one of the world’s biggest copper producers.

 

The company operates mines in Arizona and in Texas where it also has mines, smelting facilities and a refinery for copper. It also specializes in infrastructure projects such as highways, hydroelectric dams and railways services.

 

One of its subsidiaries is Minera México, which owns two large copper mines in Mexico that, until recently, were powered by electricity predominantly supplied by CFE, Mexico’s state utility.

 

The utility supplied around 90 percent of Minera México’s electricity, with the other 10 percent coming from independent power producers (IPPs). In a move to cut production costs.

 

Grupo México is now embarked on a plan to have 90-95 percent of its electricity supplied from its own power plants.

Plant configuration

The SGT6-5000F gas turbine is the latest version of Siemens 60Hz F-class engine providing over 200MW of power at ISO conditions.

 

Its design features a 16-stage axial-flow compressor, combustion system comprised of 16 can-annular dry low NOx combustors, and a 4-stage reaction-type turbine.

 

The gas turbine’s power output shaft is coupled directly to the generator at the compressor end of the engine. Natural gas fuel is provided by three different sources in the US via a 105 km pipeline that interconnects with El Paso Natural Gas’s interstate pipeline system.

 

Hot exhaust gas leaving the turbine is fed to a three-pressure heat recovery steam generator for steam turbine operation. The HRSG for the first combined cycle unit was supplied by Nooter Eriksen. The second HRSG was supplied by Cerrey SA (Mexican company formerly called Combustion Engineering Monterrey).

 

High pressure (HP), intermediate pressure (IP) and low pressure (LP) sections of the HRSG contain superheater, evaporator and economizer tube bundles.

 

The HRSGs are each connected to a 2-stage kettle boiler. Steam generated in the HRSG is conveyed through piping systems to the steam turbine.

 

The SST-700/900RH steam turbine is a two-case multi-stage, reheat condensing unit with a high efficiency blade path. The higher speed HP turbine drives the generator via a gearbox, while the IP/LP turbine is directly connected to the other end of the generator.

 

A water-cooled condenser is provided to condense the steam turbine exhaust and miscellaneous drains from the steam cycle. The condenser includes a vacuum system that utilizes liquid ring vacuum pumps. The condenser is designed to allow 100 percent steam bypass of the steam turbine.

 

A forced-draft, counter-flow cooling tower provides the heat sink for the steam cycle. The cooling tower transfers heat from the circulating cooling water by means of circulating water evaporation and sensible heating of the air.

 

Circulating water pumps maintain the water flow between the cooling tower and the condenser and other cooling water users.

 

The choice of technology was determined by the power demands of the mining operation as well as efficiency. According to Siemens and the plant owners, the combined cycle station has an electrical efficiency of 56.9%.

 

Grupo México has made a concerted effort to minimize environmental impacts by optimizing the efficiency of the entire facility. Heat from the water used to cool the flash furnace in the refining process is also recovered in a waste heat recovery boiler to generate steam for feeding a small 11.5MW Siemens turbine.

 

The environmental impact of the plant was considered in the evaluation process and plant design. The gas turbine combustion system allows the plant to achieve NOx levels of ≤25 ppm complying with both Mexico and World Bank emissions standards.

 

Water for the power station will come from an underground source 2-3 km away and any effluent discharged will be treated in order to comply with the Mexican standards for discharged water.

 

Modularization

Lopez also noted that another key consideration for Grupo México was design of a plant that could be constructed in the shortest amount of time.

 

The construction time for each plant – with an 8-month stagger between Units I and II – was less than 30 months from the Notice to Proceed. This short schedule was achieved through extensive pre-fabrication and pre-assembly.

 

Lopez commented: “There are two options when constructing plants. Either you can ‘stick build it’ – where you bring all the materials to site and literally build it like a house – or you can build as much as possible off the construction site and bring it in as pre-assembled modules.”

 

Systems such as cooling units and pump and pipe systems were delivered to the construction site prefabricated and integrated into the other systems on site.

 

“It’s a case of shifting as much work as possible to a controlled environment and doing all the pre-assembly, quality control and shipping to the job site where only minor integration is then needed,” added Lopez.

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