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Turbines whirring … churning … roaring: pushed by wind, steam, water, or fuel. They’re the workhorses of global electrification. One way or another, turbines make almost all the electrical power on Earth. In the transition from a hydrocarbon-fueled economy to a next generation suite of renewables, the ever-spinning rotors have established themselves as a fitting constant, spanning both energy eras.

Natural gas, as a fuel for turbines or any power generation, is still hard to argue against, but changes are in the wind. Although not renewable, of course, new natural gas reservoirs still seem to pop up here and there, and the nation’s pipeline makes distribution of methane or liquefied gas very easy. As for its efficiency, gas, of course, can make both combined heat and electricity (or, at a central power plant, enables impressive combined-cycles efficiency).

Regarding emissions, when best-available technology is used, the results are considered quite acceptable, if not perfect. Nevertheless, with each passing year, the inherent drawbacks of gas make it a bit less appealing, especially in comparative terms with the fair-haired darlings, green and renewable. Also, for some time now, natural gas has been facing ever-tougher nitrogen oxide (NOx) restrictions in southern California, East Texas, and other choice power markets. Per-MMBtu (one thousand, thousand British Thermal Units) rates have crept up for a decade-plus—in some areas, spiking frighteningly.

Vulnerabilities remain, too, for securing reservoir supplies that are remote and must be protected by US forces, bought with dwindling dollars, and transported a long way home. Gas market volatility and manipulability are chronic issues. And, increasingly, tariff and policy incentives favor renewable alternatives—steering fueling choices even further that way.

One way or another, the luster of natural gas for power generation, though far from gone, is rapidly fading.

Refueling, Retooling, for Syngas and IGCC
Thus, beginning a dozen-plus years ago and gaining steam in a hurry of late, there’s been almost a global mania for eliminating dependency on fossil fuels, especially ones coming from not-always-friendly petro states and, better still, developing one’s own domestic renewables.

Also on the wish list of desired fuel traits are these two: Next-generation prime movers must remain affordable and their bad emissions minimal. We’re talking, of course, about both gaseous and liquid fuels for power generation, and maybe new liquid ones for transportation: ethanol, butanol, and biodiesel (then again, maybe not—given reported advances in all-electric cars).

In any case, quests for new fuels for vehicles and power generators are both underway.  In some ways, they’re independent challenges; in others, interrelated. To run turbines with something other than natural gas, several tracks are in progress, notes Sumanta Acharya, a professor of mechanical engineering who heads a turbine research center at Louisiana State University (LSU), in Baton
Rouge, LA.

In the “lead” for years, alternative fuel-wise, he says, has been the push to develop coal-derived syngas. Under the Bush administration this was prioritized as our national energy policy. In the past decade, hundreds of millions of Research and Development dollars were spent on a concept that produces hydrogen (H2) rather cleanly from coal, then burns the gas cleanly, again, to make power.

As he explains, coal is first super-heated at high pressure, without much oxygen; the resulting gases that are emitted—after they’re cooled and cleaned—are ready for burning. Heat input to this “integrated gasification combined-cycle” (IGCC) process makes both the gas fuel and cogenerated electricity. Useful byproducts include ammonia, methanol, sulfur, and some others. The ultimate vision here is to supplement natural gas with an alternative one, and perhaps even someday replace it.

As for the present status of this ambitious undertaking, at this writing the US Department of Energy (DOE) is pushing, Acharya says, to build a prototype “zero-emissions demo plant.” If IGCC technology proves itself on cost and quality values, we might then see a proliferation of zero-emission, hydrogen-fueled power plants. They would become the backbone of America’s next-generation energy solution. And as their prime feedstock, they’d rely on the continent’s enormous coal reserves.

Though not renewable, of course, that supply is deemed sufficient “for about three centuries of projected demand,” says Acharya, citing commonly acknowledged figures.

Besides taking coal as a raw material, the IGCC process can also accept heavy oils, petroleum coke, biomass, and waste fuels—and render them all cleanly into gas. Assuming—correctly, in Acharya’s view—that the cost of gasification can be brought down enough to be competitive, H2 will emerge as the clean, abundant, cheap, and entirely domestically generated next mainstay fuel.

“Over the last few years, these things have started to look quite cost-competitive, particularly when natural gas prices were high,” he says.

On the negative side, though, coal mining remains nasty, dangerous work, and environmentally disruptive. So, despite synfuels’ comparative virtues, they’re perhaps not the perfect ideal yet. Although synfuels have lost just a tad of the allure they enjoyed initially, they remain—at least until the Obama administration makes a change—a central DOE fuel objective, Acharya notes. And, important research and innovations aimed at reducing the production cost are still ongoing, globally.

Microturbines Fueled Onsite … By Pyrolysis
Hydrogen derived from this coal process would be aimed primarily at running centralized electric plants; but what about alternative turbine fuels specifically for distributed power? Here, another similar gasifying process, called pyrolysis, has lately been introduced near the Alpine region of northern Italy to power an Ingersoll Rand (IR) microturbine in an onsite, renewable power role, IR reports. 

Pyrolysis—more apt for woody waste, which is abundant in many parts of the world and easier to get at than coal—promises potentially far-reaching usability. Though not as widely known as ethanol or methane digester gas, notes IR business development and marketing manager, James Watts, wood pyrolysis gas “is something of an alternative to biomass” as a fuel. An onsite pyrolysis plant offers the appeal of renewable distributed energy—without being tethered to a landfill or wastewater treatment plant for its renewable fuel.

“The beauty of pyrolysis, [compared with digestion reactors], is that you can use more lignin-based materials [like] corn husks or rice,” continues Watts. “It’s not digestion”—as is the case with the organics yielding gases in any bioreactor.

“You’re not just limited to carbohydrates and fats [and digestible organics] … of that nature,” again, typical of many municipal waste streams,” he adds. “You can use more cellulosic types of sources,” such as are found at granaries, farms, poultry coups, in forestry, at lumber mills, and at scrap wood or other plant vegetation sites.

These woody sources are typically less amenable to anaerobic digestion at a treatment plant relying on microbes, which also need controlled conditions.

By contrast, the material for pyrolysis can be gathered, dried in the air or sun as needed, and simply fed into a pyrolizer for rapid rendering into gaseous fuel. 

To begin the process, heat is first applied in the absence of oxygen or other reagent; soon, gases are emitted, effectively “kick-starting” its own pyrolyzing fuel source, so that the cycle becomes both self-sustaining and a net fuel and energy gainer.

At the end of the sequence, gases emerge for collection “as a nice mix, that we’ve been able to get to work successfully with our microturbines,” name-plated at 250 kW, says Watts.

Besides the woody materials noted above, other potential feed stocks might include dedicated produce like palm oil, rapeseed, or sunflower oil, for example, or mixed animal and vegetable biomasses, green cuts, straw, vegetable waste, and livestock manure, even having high-moisture content, according to product literature from a pyrolysis firm. 

For preparatory drying, exhaust heat from the engine can be applied.

Pyrolysis process speeds can also be hastened, and raw ingredients varied, to suit whatever stock is available and to alter the gaseous proportional outcomes—H2, carbon monoxide (CO), carbon dioxide, methane, and others—as desired.

Gaseous outputs are then dried and prepared to run engines. Even the ash can be recycled into high-quality fertilizer.

The system which IR mated to its microturbine is one patented by Solenia SA of Camorino, Switzerland; Solenia is the pyrolysis subsidiary of CO-VER Energy Holding, which support virtually the full spectrum of renewable technologies and alternative fuels.

As noted earlier, the small pilot plant is now powering IR’s first application, in northern Italy. Although, at present, only one turbine is being fueled with the pyrolysis yield, Watts points out that the plant is scalable, and it could easily run multiple generators.

How about the tiny plant’s commercial prospects as an energy generator?

Watts mentions that, in Europe, favorable feed-in tariffs strongly support renewables and, thereby, assure an eventual return on investment.

Solenia’s sales manager Stefano Bianchi also provides the following operational specifics:

With the pyrolysis chamber working virtually round-the-clock (as designed) or for an average of 7,500 hours per year (i.e., 20-plus hours per day), and stocked with wooden chips at 40% relative humidity at the dryer inlet, this volume of feedstock would enable production of “about one megawatt electricity,” using four of IR’s 250-kW turbines.

 “The gas produced by our pyrolysis reactor in this case is about 720 cubic meters per hour … and is mainly composed of methane, hydrogen, carbon monoxide, and carbon dioxide,” he adds.

Pyrogasification system from Swiss renewable-energy specialty firm, Solenia SA, can extract gases from a range of biomass sources.

The total turnkey cost: from 2,500 to 6,000 Euro per kilowatt electrical (kWe) installed, depending on plant size.

Bianchi adds that, using the other kinds of potential biomasses, “You need different quantities of material to reach the desired electricity production, as every biomass has a different heating value.”

 As for running continuity: Once the temperature reaches its critical point, “the reaction is stable and the gas production is continuous—provided you properly feed the reactor.”

Reactors are equipped for remote monitoring and control. And, when evaluating a site for possible development or for other testing purposes, a mobile portable pyrogasifier, capable of yielding 50-kilogram-per-hour biomass may be sited initially, prior to installing a pre-assembled modularized plants (250–1,000 kWe) for automated, continuous operation. Output of electricity and heat is currently available in the 500-kW–10-MW range. 

Burners, Adapting to Change
Running turbines on “boutique” fuels poses a few technical challenges to ignition systems, which, either slightly or significantly, must be reengineered.

 Hydrogen, for one (again, that’s the gas made by both the just-described pyrolysis and goal gasification), displays radically different combustion characteristics from methane: specifically, notes Acharya, “higher flame speeds … and a tendency for flashback,” when put through the standard premix injectors that are designed for natural gas. “You can’t just replace natural gas with hydrogen,” he says, as the flash will destroy the burner. 

Adding to the challenge is the fact that burners seek air-fuel ratio consistencies, for NOx control. For that matter, turbines can burn the same emerging renewable ethanol and butanol touted as the alternative fuel for transportation—along with more conventional energy liquids, ranging from diesels to kerosene and naphtha. At any rate, all kinds of ignition refinements are going to be the order of the day.

It was, in part, to devise these that LSU established its Turbine Innovation and Energy Research center in 2003, over which Acharya was named as the head. 

In 2008, his patent on “Efficient Premixing Fuel-Air Nozzle System” was filed, he notes; and a local firm is now interested in making them commercially.

But perhaps the real “hurdle” barring the way to a widespread refueling of turbines, Acharya explains, is not the hardware so much as the cost of H2 or any other exotic fuel production. These processes simply cost-out too expensively on a per-kilowatt-hour basis, to make much sense.

So far, anyway. Bio- or synfueled turbines, thus, do not seem likely to surpass natural gas ones, at least in the immediate future. Even so, explains Acharya, there’s obviously strong justification for exploring fuel efficiency and power improvement in general—as TIER is doing. Concerning both biofuels and the more conventional ones like gasoline or kerosene, the shortcoming in most of them, he explains, “is that the energy density of the fuels is
not high.”

Resulting fuels are burned for heat and power to sustain gasification and run a generator.

Nanotech Fuel Boosters
Specifically, on this score, one intriguing experimental approach now in progress at TIER is the addition of ultra-fine catalyst particles, such as cerium oxide, as an accelerant. Not too long ago, Acharya notes, fuel catalysts were stuck in the micron range. He says that now, however, “We can actually buy nanosized catalyst particles in the marketplace,” then mix them into gasoline, or kerosene, or whatever, “to enhance the combustion process.”

As a further gain, “Instead of having, let’s say, a two-foot-long engine, we can complete the combustion in half the volume, and we can make a smaller footprint.…” states Acharya. “So, that’s good for compactness.”

In addition: “You’re increasing the energy density of the fuel by mixing in a small amount of metal nanoparticles,” he adds.

Catalyzed higher energy also raises combustion temperatures; this enables most metal nanoparticles to burn well. However, some unburned particles “may be coming out of the tailpipe,” too, concedes Acharya. This poses yet another research challenge to determine the consequences and seek possible remedies.  

Full-scale commercialization of supercharged fuels is thus probably still a few years away, he suggests. 

Besides turbine fueling issues like these, other research conducted at TIER includes turbine aerodynamics; cooling effectiveness; fuel mixing; thermal barrier coatings; combustion; NOx reduction; and enhanced reliability, efficiency, and maintainability. TIER research partners include DOE, GE, Pratt & Whitney, Ishikawajima-Harima Industries Co., Ltd., Siemens, and others.

One Manufacturer’s Fuel Development
Solar Turbines Inc. has been steadily working on ways of burning an assortment alternative or atypical fuels in its turbines, the company reports.

A recent white paper, provided for use in this article, states that, “New applications of current technology are being actively pursued, and the development of new technology is continuing in response to customer identification of new, alternative fuel supplies and compositions.”

Solar’s turbines have long been capable of running on various fuels, and reports that “work is in progress to expand fuel flexibility to allow use of associated and raw natural gases, and landfill and digester gases,” he says.

Currently, dry low NOx emissions (DLE) turbines can burn landfill gas (LFG) with methane content in the range of 1,000 to 1,350 Btu, notes a company engineer; other models are expanding Solar’s capability from 1,350–1,600 Btu, and from 1,000–800 Btu. The Mercury 50 (DLE only) can handle 400–1,350 Btu.

Solar’s DLE natural gas burner can now also handle number-two diesel and kerosene, he adds.

Hydrogen can be used with conventional combustion gas turbines, but pose special challenges for Solar’s DLE gas turbine, similar to those that Acharya earlier described. Hydrogen—the lightest known gas—diffuses quite rapidly; it passes through gaskets, which are otherwise impervious to methane; it seeps through some metal flanges; and it is highly flammable.

Despite these challenges, Solar can use high H2 fuels with conventional combustion systems, but, to control NOx emissions, water injection is required. Solar has recently initiated a project with DOE to begin developing a DLE turbine capable of using similar higher H2 “synfuels” and gasified biomass, a company engineer states. 

Moreover, “Use of highly energetic fuels containing a higher percentage of hydrogen is currently restricted to conventional applications and requires special fuel and control systems for successful operation,” adds Acharya.

Biodiesel fuels are also widely touted these days, and—having a higher H2 content, density, and flashpoint than number-two distillate oil, but with a 10%-lower heating value—biodiesels can indeed be used in conventional combustion turbines. However, in a Solar DLE turbine, further development and testing is required, due to concerns with smoke signature and storage life, a company spokesman notes. Biodiesels may be used either in pure form or blended with petroleum-based diesels. Fuel speciation for B100 (100%) to B20 (20%) has been developed.

Heating value of some of these renewable biofuels is
approximately one-half to one-third the value of the pipeline gas; so, to get the energy required to run a turbine, “a large volume of these fuels, approximately two to three times the volume of natural gas fuel, must be injected into the combustor primary zone,” the white paper states.

For example, landfill gas carries only one-third the value of pipeline gas; so, the injector flow area must be expanded about three times larger to compensate. This large increase can then alter the mixing and combustion process within the engine.

Solar Turbines engineer Luke Cowell notes that, “Such effects are determined (modeled and tested) before these fuels can be used in a particular gas turbine engine model.” 

Fuels of medium heating value “may require a change in the injector flow area,” and usability of the low-NOx technology is limited, adds Cowell.

Fuels of low heating value require “even more extensive changes to the injector and fuel system,” he concludes. And these weak fuels currently cannot be used  with the company’s low-NOx system.

All in all, fueling with atypical or exotic prime-movers will likely demand adhering to special procedures for handling; controls; achieving or maintaining critical operating and supply temperatures and pressures; using higher Wobbe Index value fuels for startup; and recognizing part-load safety issues.

Biofuels for Street Racers? Not Likely 
A decade ago, Marine Turbine Technologies (MMT), of Franklin, LA, won some fame, and made the Guinness Book of World Records, with a production-model 320-horsepower (hp) turbine “Superbike.” Powered by a Rolls-Royce 250 turboshaft, it screamed along at a top speed of 227 mph.

MMT has since sought to nurture its image as an offbeat turbine innovator, though not necessarily related to fueling or even exclusively to transportation. 

A more conventional product-line for boats was complemented recently by a turbine harness as an exceptionally compact oil rig “fracing” pump. Typical units are skid-mounted, and use number 2 diesel kerosene self-contained in the platform.

But: Using any biofuels?

Jeff Stary, MMT vice president, notes that this “is nothing new or exotic,” but then again, not particularly in demand, either and, “not practical because there’s not enough Btu content to light it off.” 

As noted previously, ignition requires conventional higher-
WI fuel.

The first time MMT ran one of its turbines on any biofuel, he recalls, “We were all there listening to it,” waiting for the dramatic turning of the valve, the moment when the starter fuel cuts off, and rotors run on pure vegetable oil.

“If there was a cough or hesitation, it was imperceptible to me,” he says. “And, once you get it running, you can sustain [combustion] very easily.”

Even though a renewable fuel works just fine, though, Stary’s “not so sure that biofuels are the ultimate answer anyway,” because they’re currently “not that readily available.”

Thus, “In some respects, it’s a ‘feel good’ thing,” he says. “People want to see fuels that will protect the environment. But, biofuels “are still a long way from being mainstream,” he adds. 

At one point during the company’s tryout of biofuels, measurements were done to see if comparative emission came out cleaner than those of conventional diesels. If indeed this turned out to be so, it would mean that engines running on biofuels would likely be allowed to run longer, not violating air-quality permit limits.

The result? 

Surprisingly, classic petroleum distillate versus the bio kind came out a dead heat: around 57 part per million (ppm) NOx and 38 ppm CO output, on a 3,800-hp turbine, regardless of whether running “cooking oil” or number two diesel. 

As for MMT’s latest crop of exotic innovations: In 2008, the turbine engine firm introduced (if you can believe) a next-generation jet super-bike: a pumped-up 420-hp “Streetfighter.”

And more down to earth, perhaps as early as late 2009, the company may introduce a portable microturbine-sized
generator set.

The fuel?

Most likely, any conventional ones.

At least for now.