Mexico’s Combined Cycle Building Boom Relies on Advanced HRSGs

Iberdrola Generación México is investing $3.5 billion in new energy facilities, the latest of which is its new 890-MW Topolobampo II combined cycle power plant.

By Ravi Krishnan

Iberdrola’s 890-MW Topolobampo II combined cycle plant is scheduled for commercial operation in January 2019. Topolobampo II is a high-efficiency natural gas-fired plant. The plant’s heat recovery steam generator is illustrated. Source: Iberdrola

Significant reforms to Mexico’s Comisión Federal de Electricidad (CFE), the state-owned conglomerate that controls the country’s power generation and transmission infrastructure, began in 2013. The new electricity market mirror global wholesale markets having unlocked the electricity sector to privatization, foreign investment, and foreign party operations through service contracts. Dr. Fatih Birol, executive director of the International Energy Agency (IEA) commented on its 2016 report on Mexico’s energy reform in its Word Energy Outlook series, “This is not a reform, it’s a revolution on an unprecedented scale.” In September 2015, Mexico’s Program for Development of the National Electric System established new market operating rules, particularly with respect to renewables integration.

Today, Mexico’s installed capacity is over 70 GW with about 75% from thermal plants, 19% non-fossil-fuel generation (principally hydropower), followed by nuclear and wind. Supply side generation is dominated by natural gas and use of that fuel continues to grow. CFE generates, distributes, and markets electricity to more than 35 million customers and is growing at the rate of about one million new customers each year.

Electricity demand in Mexico has doubled over the past 20 years. Mexico expects its electricity demand to continue to grow 2.4% per year through 2040 (2014 data baseline) which equates to 85% growth over that time period. Consequently, installed electricity generation must double, from about 70 GW to almost 160 GW in 2040. However, averages can be misleading. According to CFE, “energy demand in the Western region of Mexico will grow at an average of 4.8% annually.” Mexico expects half that capacity growth to be supplied by IPPs building gas-fired combined cycle plants, particularly in the western part of the country.

It’s not surprising that CFE expects natural gas will fuel 44% of all energy additions to Mexico through 2030. According to the IEA, there are currently over 40 private companies now active in gas and electricity projects with independent power producers (IPPs) currently generating around 30% of the nation’s electricity.

Iberdrola Generación México is the largest privately-owned electricity producer in Mexico today, with over 6,000 MW currently in service. Over the past 18 years, Iberdrola has been expanding its investment in Mexico and by 2020 the company expects to have installed more than 10,000 MW of new capacity, which equates to the electricity needs of about 20 million customers. Iberdrola currently has several major new power projects under development in Mexico including three combined cycles, two cogeneration, 325MW of new wind power, and two medium-scale (totaling 270MW) solar energy plants. With these projects, Iberdrola has invested more than US$5 billion into energy facilities in Mexico, with about US$3.5 billion in projects initiated after approval of Mexico’s energy reform in 2013.

Build the Best

CFE awarded Iberdrola a contract for the design, assembly, testing, commissioning, operation and maintenance of the 890-MW Topolobampo II (also called Noroeste) combined cycle plant in April 2016. Iberdrola must also install the power lines and electrical substation in order to interconnect the plant with the Mexican electricity grid. The plant is located midway down the west coast in the municipality of Ahome at the Topolobampo port in Mexico’s Sinaloa state and will provide reliable power to about three million residents.

The report price tag of Topolobampo II is approximately US$400 million and the plant is scheduled to enter commercial service in January 2019. The entire electricity production from the plant will be sold to CFE under a 25-year power purchase agreement with fixed capacity charges.

CFE is installing a $1 Billion underwater transmission line that will cross the Sea of Cortez west to connect Baja California Sur to replace diesel-fueled generation with efficient, combined cycle generation and to provide new renewable capacity under development in Baja California Sur and entry point into the national grid.

Iberdrola subsequently selected the popular Mitsubishi Hitachi Power Systems (MHPS) 501 J-series combustion turbine (CT), nominally rated at 327MW. The plant is arranged as a 2 x 2 x 1 combined cycle. Each CT is coupled to a CMI Energy natural circulation heat recovery steam generation configured as triple-pressure with reheat to produce steam for the plant’s single MHPS TC4F steam turbine. When operated in a one-on-one configuration, the 60-Hz plant will deliver 470MW (gross) at 61.5% thermal efficiency (LHV). When operated two-on-one the plant produces 942.9MW (gross) with a plant thermal efficiency of 61.7% (LHV) Natural gas will be supplied through the El Encino-Topolobampo line extension from Mexico’s north-northwest pipeline system.

Hotter is Better

The HRSG steam conditions are state-of-the-art (see the Table). In comparison, EDF’s Bouchain plant, commissioned in June 2016 in northern France, was recently recognized as the world’s most efficient combined cycle plant with a 62.22% thermal efficiency under ISO conditions. The plant operates at 50Hz. That plant’s CMI Energy HRSG produces design superheat and reheat temperatures of 585C, that is, 20C higher than steam produced by HRSGs connected to the latest generation of F-class CTs, yet 17C lower than Topolobampo II. At these temperatures, the correct metallurgical selections are critical.

The increase in steam temperature is made possible by moving upscale from the conventional choice of SA231 P91, which allows up to 650C at 100,000 operating hours per the ASME Code. However, at these pressures and temperatures tube wall thickness must increase due to maximum allowable stress limitations.

The tradeoff is thicker wall tubes are more susceptible to damage from thermal cycling. Austenitic stainless steel tubes are commonly found in ultra-supercritical steam boilers but use in HRSGs subject to cycling has been a concern. CMI Energy, in collaboration with research institutions in Germany and Belgium and steel suppliers have thoroughly examined the use of austenitic stainless steels in high temperature and pressure HRSG applications, particularly with respect to maximum allowable stress, thermal expansion, cycle fatigue, and weldability (including dissimilar welds).

Dissimilar welds are required when joining ferrous alloy steel headers with the austenitic stainless steel tubes.

Welding of dissimilar materials is necessary in high temperature steam applications, either on tubes upstream of the header or on connector downstream of the collector (Figure 1).

Each method has been successfully used in practice. Welding connectors (6-inch diameter) was used on the Bouchain project in close communication with GE. For the Topolobampo II project, CMI elected to weld tubes to P91 header tube stubs while keeping the header in P91 material.

“The IEA states that to accomplish Mexico’s energy reform agenda, investments of as much at $240 billion will be required through 2040…”

Welding dissimilar metal tubes was preferable on this project because there is less thermal expansion, weld stresses are lower, and the process is less expensive because there is reduced usage of expensive Incoloy transition pieces.

CMI Energy selected austenitic stainless steel SA213 S30432 tubes (commercially marketed as Super 304H) for the first few rows of the superheater and reheater tube sheets where the temperatures are the highest to take advantage of its increased resistance to stress corrosion and stress relaxation cracking and because of technical concerns related to steam oxidation above 605C with P91. These headers and critical dissimilar metal welding were performed at the CMI Welding Expertise Center in Belgium. The completed headers were then shipped to CMI Energy’s Mexican subsidiary where the superheater and reheater fabrication was completed.

The HRSG design is modular, not unlike that of most combined cycle plant designs. The modular nature of the design speeds installation and commissioning of the completed HRSG on the job site. Each Topolobampo II HRSG consists of 12 prefabricated modules that are arranged in a three wide by four deep configuration. Each HRSG has 11,575 tubes totaling 2,173 meters long. The heat transfer surface is 330,000 square meters.

Made in Mexico

With Mexican energy reform measures firmly in place, indigenous gas supply and transmission facilities under construction, and a world-leading energy growth rate predicted, Mexico has become a very attractive country in which to do business.

Pedro Joaquin Coldwell, Mexico’s Secretariat of Energy, has likened the country’s power sector reform to an “economic competitiveness reform” where access to adequate and reliable electricity will give Mexican businesses a competitive advantage that will increase jobs and overall economic growth.

The IEA states that to accomplish Mexico’s energy reform agenda, investments of as much as $240 billion will be required through 2040 plus another $130 billion investment in energy efficiency.

CMI has long experience with HRSGs with installations across the globe, including over 20 combined cycle projects in North America alone, many with multiple HRSGs, during the past decade alone.

The Mexican power market is one of CMI Energy’s largest customers, with 15 HRSGs supplied on six projects totaling over 4GW of capacity in recent history, the latest of which is Topolobampo II. Located 30 km north of Monterrey, CMI Energy’s Mexican subsidiary, CMI Energy Mexico (CEM) has been fabricating pressure parts, structures, stacks, and providing inspection and repair services since 2005.

In 2014, CMI Energy invested in an expansion of the fabrication facilities of CEM to meet the local content requirements of the North American Free Trade Agreement.

In September 2017, the HRSGs for Topolobampo II were completed by CEM, the company’s first HRSGs that were 100% manufactured in Mexico. CEM has since increased its fabrication capacity to 4 to 6 HRSGs every year, each consisting of approximately 10,000 tubes. The shop is fully ASME certified (R, PP, U, S).

Mexican energy market reforms promise to dramatically increase use of natural gas-fired combined cycle plants.

From a strategic perspective, the largest HRSG market in North American will remain in Mexico and the U.S. for the foreseeable future. From a geographic perspective, CEM has the advantage.

The company’s fabrication plant is centrally located to the U.S. (only 125 miles from the southern border of the U.S.), Canadian, and Latin America markets.