Flex Fuel Technology


Due to the size and complexity of the project, Generac’s chosen method was design-assist, allowing them to get involved earlier in the process.
Due to the size and complexity of the project, Generac’s chosen method was design-assist, allowing them to get involved earlier in the process.
All photos: Doosan

Diesel- and natural gas-fueled generators have been the go-to solutions for industrial and commercial standby power requirements for decades. Diesel gen-sets traditionally have dominated the standby generator market for applications greater than 150 kW due to a lower capital cost, noted Laura Unger, vertical marketing manager with Generac. These diesel-based solutions continue to advance through increased power density provided by electronic engine injection technology. However, she cautioned, diesel-based solutions are now being critically evaluated by many customers. “Driven by fuel reliability and maintenance concerns, current market trends have seen a significant expansion of natural gas-supported generator solutions. Many customers simply don’t want to deal with onsite diesel either going bad or running out.”

Sites that are difficult to access, far from sources of diesel fuel, or in harsh environments pose challenges for diesel-powered generators. Conversely, natural gas generators are ideal for use in oil and gas exploration, site preparation and production, and industrial plants.

Doosan Portable Power offers a line of natural gas-powered mobile generators that combine the ability to operate on wellhead natural gas with rugged features and reliable performance in remote locations and harsh environments. Its patent-pending Onboard Scrubber System improves machine performance by removing excess dirt and water from wellhead gas. Its natural gas generators are also equipped with an automatic dual-fuel switch, allowing the generator to operate on natural gas as well as propane from an external tank.

Where natural gas is readily available and affordable, coal plant retirements and replacement of these assets with natural gas-fired power plants are taking place. But the switch has more to do with CO2 emissions and less to do with supply, largely because there is increased focus on burning a wider range of fuels and burning them more cleanly than previously possible.

Natural gas generators are ideal for use in oil and gas exploration, site preparation and production, and industrial plants.Natural gas generators are ideal for use in oil and gas exploration, site preparation and production,
and industrial plants.

While much of that focus is centered on reducing CO2 emissions, utilizing natural gas produces lower emissions of combustion pollutants such as NOx and VOCs. Michael Welch, industry marketing manager for Siemens, said, “The challenge to decarbonize energy production requires the fossil fuel mix to change. While we are already achieving low levels of these emissions through the use of dry low emissions (DLE) technology, we are working to reduce these emissions still further.”

Where natural gas is not so readily available—for dis­tributed power applications in developing countries, for example—fuel oils have traditionally been used as the fuel for power generation. Siemens has been expanding its DLE fuel capabilities on smaller gas turbines to enable lower carbon fuels such as liquid petroleum gas (LPG) to be used in these situations, looking to make use of the existing domestic and commercial LPG infrastructures in these regions. LPG has a much lower environmental footprint than fuel oils for all types of emissions.

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Another area of development is in burning fuels containing hydrogen. Hydrogen is a “zero carbon” fuel, meaning there are no CO2 emissions when it’s combusted. Therefore, there is a lot of interest in burning pure hydrogen or hydrogen/natural gas blends to reduce the carbon footprint of energy production.

Recent advancements in burner technology and manufacture are now enabling Siemens to offer DLE systems with up to 60 percent hydrogen by volume blended into natural gas (and 100 percent hydrogen in non-DLE combustors on some gas turbine models). Welch said Siemens is continuing research and development towards its 100 percent hydrogen goal.

“While expanding the fuel capabilities, we also have to bear in mind that our customers may want to switch between fuels for security of supply purposes or may have a fuel source that is inherently variable in composition,” Welch explained.

Siemens has been working on not only expanding the types of fuel it can combust but also the range of fuels that can be accepted in a single burner design. For example, it now has DLE burner designs on some of its small and medium gas turbines that can run on natural gas, a ‘lean’ gas (such as a biogas with a high CO2 content), and ‘rich’ gases such as LPG—and switch between them automatically on load without needing to stop the turbine or modify any hardware.

Bi-fuel gen-sets combine the power density and capital cost benefits of diesel engines with the extended run time of natural gas. Using a compression-ignited diesel engine as the prime mover, bi-fuel generators start up on diesel fuel in a normal manner. As the generator picks up load, bi-fuel delivery systems introduce natural gas into the combustion air while reducing the amount of diesel fuel.

Under typical load conditions, bi-fuel generators will operate on a ratio of 25 percent diesel and 75 percent natural gas, with no reduction in power. Though this technology has historically been available as an in-field adaption to standard diesel generators, the combined complexity of electronic injection technology and the demands of the current EPA regulations make original equipment manufacturer development processes and certifications a must-have.

When evaluating the flexibility of generator fuel and various gen-set configurations, Unger said that spark-ignited generator technology is one of the key technologies. “Spark-ignited generators offer numerous advantages compared [with] diesel solutions. The most noticeable advantage is the extended run time offered by an endless supply of natural gas.”

Other advantages include: no fuel permitting requirements, reduced preventive maintenance costs, less risk of environmental contamination, and cleaner engine emissions. Though this technology has been around for years, the market is seeing various equipment suppliers expanding their product offerings with higher power density, larger kW, rich-burn gas engines.

When evaluating advancements in spark-ignited generator technology, the move to larger rich-burn engines that are optimized for standby power applications has significantly increased engine power density and lowered capital costs while improving load step responsiveness. Rich-burn engine technology operates with an actively controlled mix of air and fuel that allows for more fuel to interact with more air, creating more power. This type of technology is currently seeing widespread adoption up to the 750-kW power rating.

The Museum of the Bible application has eight 400 kW natural gas units located in a rooftop mechanical room.The Museum of the Bible application has eight 400 kW natural gas units located in a rooftop mechanical room.

The environmental benefits of flex fuel generators are clear. “No matter where in the world you are, more environmentally friendly fuels can be used to produce the energy needed for economic growth,” Welch stated. The reduction in ‘local’ pollutants such as NOx and particulate matter also has a positive impact on human health by improving local air quality.

But, he added, there are also economic benefits. “Increasing fuel flexibility means that gases previously flared or vented by industry can be considered as potential fuels. These fuels are essentially free, reducing the annual fuel costs of industry and helping improve the affordability of energy in local communities, as well as helping to reduce CO2 emissions by displacing fossil fuel usage and the CO2 (and other pollutants) produced by flaring. As energy markets become more digitalized, we could see real-time fuel switching to make use of fuel price variations to minimize the costs of energy production.”

Looking forward, Welch predicted that improved fuel flex technology may act as an enabler for other technologies, such as those that produce biomethane or biohydrogen, to help meet the deep decarbonization targets to which governments aspire.

Current advantages of bi-fuel gen-sets include lower capital costs because the diesel engine is retained; extended run times per tank of diesel fuel; and minimized onsite fuel storage and maintenance issues.

The reduced consumption of diesel fuel by the engine under bi-fuel operation means that run times per tank of fuel are significantly extended, Unger elaborated. “Run times with bi-fuel operation can be measured in days rather than hours. This can be very important, since replenishment supplies may be difficult to obtain during widespread, extended outages associated with blackouts or major weather events.”

Because natural gas is the predominant fuel, smaller diesel tanks are a viable option. That’s noteworthy because the risk of fuel going bad and the cost of fuel maintenance is significantly reduced in smaller fuel tanks. Unger points out that if the natural gas supply is interrupted for any reason, the controls will automatically direct the unit back to 100 percent diesel without interruption of operation.

Generac’s installation at the Museum of the Bible demonstrates the combined impact of integrated generator paralleling.Generac’s installation at the Museum of the Bible demonstrates the combined impact of integrated generator paralleling.

Innovative Applications
As OEMs develop enhanced fuel technologies that maximize customer value, these technologies can be leveraged further, utilizing an integrated approach to generator paralleling that connects generators together and combines their output without using external equipment.

“Parallel power solutions have always offered the standby generation marketplace significant advantages,” Unger stated. “However, the traditional implementation of these solutions has been limited to mission-critical applications and large kilowatt projects. This is largely due to the constraints in implementing paralleling solutions using external switchgear. These constraints include costs, space, issues of single source responsibility, and a significant level of complexity.”

To access the benefits of parallel generation while removing the cost and complexity limitations, generator manufacturers have developed integrated generator paralleling. For example, three 500 kW gensets operating in parallel offer a similar cost as a single 1,500 kW unit—but with the advantage of built-in redundancy.

This technology allows bi-fuel and rich-burn, spark-ignited engines to be optimized at power ratings that create the best fit for the application and then scaled up for capacity. Generac’s installation at the Museum of the Bible demonstrates the combined impact of these technologies. As Unger explained, this application has eight 400 kW natural gas units located in a rooftop mechanical room, stacked in two rows of four because of spatial constraints. “Through the combined technologies of high-power density rich-burn generators and integrated paralleling, the customer was able to locate the generators on the roof while benefiting from the flexible fuel choice provided by natural gas.”

Case Study: Museum of the Bible
The Museum of the Bible, located just three blocks from the U.S. Capitol in Washington, D.C., opened to the public in 2017. A 1920s-era refrigerated warehouse was completely renovated to house the 430,000-square-foot museum complex with the goal of becoming one of the most technologically advanced and engaging museums in the world.

To achieve that goal, a unique set of needs had to be met. From a backup power generation standpoint, there were numerous considerations because of the size of the structure. For safety considerations, fire alarms, security, and egress lighting must be supported by backup power generation. In this application, the climate control system, vital to the preservation of the museum’s relics, which require specific temperatures and humidity set points, also necessitated backup power. Kenz Meliani, electrical engineer at SmithGroupJJR, said, “A large part of the climate control mechanical equipment was required to be on backup power.”

Due to the size and complexity of the project, Generac’schosen method was design-assist, allowing them to get involved earlier in the process. They partnered with Kelly Generator & Equipment Inc., Clark Construction, Ennis Electric, Southland Industries, and SmithGroupJJR.

The original design by SmithGroupJJR specified three 1 MW diesel-fueled generator units on the penthouse level of the building, each with a dedicated load. But because the owner didn’t want to deal with maintenance issues associated with diesel fuel, the risk of a spill or a leak, the loss of parking spaces to accommodate storage in the adjacent parking lot, or the challenge of installation on the penthouse level, Kelly Generator recommended a Generac 3.2 MW modular power system consisting of eight paralleled 400 kW natural gas generators, a solution that fit with the maximum height requirements. To take advantage of the room height available, Kelly mounted four units on a steel structure over the other four.

They used a grated structure to facilitate airflow provided by four 50,000 CFM overhead fans because natural ventilation was not an option to supply ample air flow for the eight generators. Life safety systems needed to be upgraded to meet current codes and regulations as well. Generac was able to guarantee at least one of the eight generators would be serving life safety loads within ten seconds for up to 400 kW.

With Generac’s paralleling system, there is no single point of failure. The design team found another benefit by choosing Generac’s digitally integrated paralleled system. Paralleling in the past always required a separate switchgear lineup and often a separate controller lineup, but these units were plug-and-play with everything included.

High Energy with Low Emissions
A number of industries are using their process off-gas to generate the power (and heat) needed to produce their end product. Welch said, “Amongst the many units we’ve installed to operate on ‘non-standard’ fuels, we have units in refineries to run on high hydrogen process off-gases, in the petrochemical industries such as propane dehydrogenation (PDH) to work on propane and butane-rich waste gases, and in the ethanol industry to run on the biogas by-product from the fermentation process.”

Siemens is currently working on a project with Braskem in Brazil that features two SGT-600 gas turbines with third-generation DLE technology that will run on residue gas with high concentrations of hydrogen. The Braskem project plan includes a fully integrated, redundant design. The plant’s state-of-the-art technology solutions will combine high energy efficiency and extreme operational reliability with low emissions.

Braskem is the largest petrochemical company in Latin America. It recently joined with Siemens to modernize a cogeneration power and steam plant at its complex in Sao Paolo, where Siemens will be responsible for implementation and operation of the plant for 15 years, with performance guarantees for reliability, availability, efficiency, costs, maintenance, and emissions included in the agreement.

In addition to the SGT-600 gas turbines, the plan calls for an E-house, as well as an extension of the existing high-voltage substation, three reciprocating compressors, an advanced load-shedding system, and associated software for plant control. These elements are part of an energy-as-a-service concept, which means that the customer will receive the energy without having to invest in, build, and operate the plant on its own.

The project involves the complete overhaul and technological update of the existing cogeneration plant, which provides steam and power to the petrochemical complex’s cracking unit. The unit has an ethylene production capacity of 700,000 metric tons per year and produces raw materials for the chemical and plastic sectors.

According to Siemens, the optimized design leads to an increased efficiency of the ethylene plant. They estimate that the upgrade project will reduce the cracking unit’s water consumption by 11.4 percent and CO2 emissions by 6.3 percent, helping Braskem meet its sustainability goals by reducing the facility’s overall energy consumption in an amount equivalent to that of a city with one million inhabitants, estimated Luís Pazin, southeast region chief industrial officer for Braskem Chemicals.

Dan Simpson, head of global solutions for Siemens Gas and Power, Oil & Gas, explained that the integrated and redundant design of the facility and use of Siemens equipment, coupled with the adoption of a build, own, and operate business model, will result in 100 percent plant availability and reduced energy consumption.

The power output of an SGT-600 turbine is 24 megawatts. For this application, each turbine will provide 19 MW of power and 80 tons per hour of steam. In addition, they will feature third-generation DLE technology and run on residue gas with high concentrations of hydrogen. The DLE technology will reduce CO2 emissions, and NOx levels from the turbines will be low at just 25 parts per million. A load-shedding system ensures safe operation of the plant by managing all loads, depending on the available power supply. Deployment is currently underway, and completion of the project is expected in early 2021.

There are many more applications being looked at today, Welch indicated—particularly those using surplus renewable energy to generate hydrogen to store and use at a later time for power generation. He sees the industry moving in this direction, using flex fuel technology to suit each application and reduce emissions while increasing efficiency and power output.

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