Power Engineering

Comparing Natural Gas and Diesel Generator Sets

For small power producers such as those operating mini-grids, a new generation of generator enclosures is increasing choice and reducing or eliminating some of the challenges with diesel. Photo courtesy: HIPOWER Systems
For small power producers such as those operating mini-grids, a new generation of generator enclosures is increasing choice and reducing or eliminating some of the challenges with diesel. Photo courtesy: HIPOWER Systems

 

Do the Old Rules Still Apply?

By Rafael R. Acosta, HIPOWER SYSTEMS

Only a few years ago, traditional logic for generator sets (gensets) held that diesel meant reliable and inexpensive but also noisy, loud and messy. Natural gas meant expensive and temperamental but also quieter and cleaner. Today, thanks to the development of new technologies in engines, enclosures and other components, the differences between diesel and natural gas generators is no longer so clearly defined.

Additionally, many firms follow conventional logic in their comparisons of these two fuel types and fail to give sufficient weight to the operating realities of their particular applications. In this article, we'll take a look at both new developments and long-standing considerations that impact generator selection by fuel type in the power generation industry.

The Engine Equation

Traditionally, the viewpoint has been that diesel engines provide response, power and longevity, while natural gas engines are more environmentally friendly. Although the environmental argument for natural gas still holds true, diesel is no longer the clear winner in terms of power and response. Spark-ignited (natural gas) industrial engine manufacturers can now optimize the RPM of these engines to make their transient response similar to that of diesel. Manufacturers are also producing natural gas units that can meet the 10-second startup requirement for backup systems that is traditionally associated with diesel engines, alone.

Use of gear-on-gear powertrains or two-pole alternators (as opposed to traditional four-pole alternators) has increased the overall performance and power of natural gas engines, as well. Manufacturers have also incorporated stronger, more resilient engine parts, such as hardened valves and seats, to boost performance and increase reliability.

In the area of energy density, there is no doubt that diesel has greater peak energy density than natural gas-by a factor of more than of three (generally 129btu versus 37btu). Even here, there are mitigating factors that may tip the scales in favor of natural gas.

Density is impacted by both engine and fuel conditions. A poorly maintained diesel engine, or one running sludgy fuel from a fuel tank filled with particulates, likely will outperform a natural gas engine, but it will not perform at its peak.

Even more significantly, diesel engines have a sweet spot of 50-70% of load, with 80% being the recommended maximum for long-term prime operation. Running them under a lighter load for long periods of time results in wet stacking, a condition that sends unburned fuel and soot into the exhaust system.

Operators that run engines under light loads often employ load banks to consume the excess energy. This approach reduces wet stacking but can waste a considerable amount of fuel. As an alternative, those operators could instead choose natural-gas-powered generators, which burn hotter than diesel engines. These engines are less likely to experience problems with unburned fuel, even if they are run at a lighter-than-optimal load.

 

Built to Last

 

Regarding longevity, diesel engines still tend to have longer lives, on average, than natural gas engines. However, many of the new technologies mentioned above not only enhance performance in natural gas engines; they also increase engine resilience and longevity.

In short, companies that plan to keep their engines operating at peak condition will still likely enjoy the greatest longevity from a diesel engine. If they allow it to fall prey to the operating challenges discussed in the previous section, longevity will suffer-in extreme cases, potentially cutting engine life in half.

For firms that use generators only for backup power, a natural gas engine may provide nearly the same effective life, given how infrequently it is used. Other considerations such as continuity of fuel source also come into play, perhaps outweighing diesel's benefits of performance and engine life.

Cost Comparison

It's a common misconception that industrial diesel engines are considerably less expensive than comparable natural gas models. Below 150kW, natural gas engines are actually more cost effective, even without factoring in the fuel differential.

For applications where more kW are required, power producers can create parallel configurations of smaller engines to provide them with the cumulative kW needed for the operation. Parallel systems have the advantage of supporting load sharing and management, making them one of the most cost-effective and environmentally friendly options, in terms of fuel use, for variable load applications such as mini-grids. Add to this savings the reliability and scalability of parallel systems where they replace a single, larger diesel generator (such as for backup power) and the benefits of such a solution are considerable.

For the past two years, natural gas genset suppliers have also been touting fuel prices as cost benefit. While the fall in natural gas prices is certainly making them a more attractive option, plummeting oil prices are having a similar effect on diesel generator fuel. We won't make a prediction here, because the outcome of fuel prices is anyone's guess.

Saving the Planet

One of the big advantages of natural gas, of course, is that it burns more cleanly than diesel. This comparison is exacerbated for any of the operating conditions mentioned in the previous section, where wasted fuel increases soot and dangerous emissions.

In addition, shortened engine life from wet stacking, light loads, inadequate maintenance and other common diesel generator issues can negatively impact emissions even more. This can be an problem, not only for the environment, but also for regulatory compliance with the EPA and other agencies that protect it.

The new rules require MACT (maximum achievable control technology) emissions controls and GACT (generally achievable control technology) management practices for both major sources and area sources of HAPs (hazardous air pollutants).

Engine maintenance and condition monitoring must be verified through reporting, and if operators allow engines to fall outside acceptable ranges with poor maintenance or fuel conditioning, steep fines can result. The fines for not keeping an engine in optimal running condition can quickly eclipse the added cost of a natural gas model. Consequently, firms should consider all of these factors before they make an engine choice.

Bi-Fuel add-ons give diesel engines the ability to burn natural gas, but there are no solutions that enable natural gas engines to burn diesel. Photo courtesy: HIPOWER Systems
Bi-Fuel add-ons give diesel engines the ability to burn natural gas, but there are no solutions that enable natural gas engines to burn diesel. Photo courtesy: HIPOWER Systems

Bi-Fuel Is not Bi-Directional

One of new technologies beginning to really make headlines is bi-fuel. With this technology, diesel engines can run up to 75 percent natural gas, with the gas being introduced through the air intake.

Functionality is usually provided by an add-on system, including a highly sensitive, intelligent controller that adjusts and optimizes the ratio of gas to diesel during startup, to address transient response needs or performance issues, or during ongoing operation.

Bi-fuel systems can save operators up to 50 percent on fuel costs, based upon the cost of diesel relative to natural gas, and they make diesel engines more environmentally friendly.

Going into the details of bi-fuel solutions is beyond the scope of this article. but HIPOWER SYSTEMS posted a detailed article on bi-fuel in June if you would like more information.

For the purposes of this article, the point is that bi-fuel solutions do not work both ways, and due to differences between the fuel types and the respective ignition systems (compression versus spark-ignited), we do not anticipate they ever will. As a result, if you want the option of burning diesel in any percentage, along with the flexibility to burn a large amount of natural gas, you must purchase a diesel generator.

They can come equipped with bi-fuel add-ons, or operators can add them at a later date.

Fuel Supply and Storage

Fuel handling is another area where diesel gensets and natural gas generators are not converging.

As with other criteria, the benefit of one over another often depend upon the situation and need.

For example, it is widely promoted that natural gas delivered from a pipeline can run a generator indefinitely in the case of a disaster. This is true in most situations, but there are caveats. It's fairly evident that any natural disaster that disrupts the earth, such as an earthquake, can cause supply disruption.

Of greater surprise to many enterprises, weather-related events such as hurricanes and tornadoes can cause damage to buildings that disrupts their natural gas lines.

This in turn can impact gas pressure in the surrounding area or even force a utility to shut the gas supply off.

Any power provider considering a natural gas genset as a backup power supply to keep utility offices running in the event of storm or for other reasons should be aware of these possibilities.

For diesel, fuel availability is limited only by delivery frequency. If diesel trucks can service a site, then fuel supply is no problem.

However, in remote locations that use generators for backup or prime power, weather can factor into fuel delivery, as well.

Additionally, diesel fuel can become contaminated with water, particulates and algae over time. (Ultra-low-sulfur diesel fuel is especially prone to degradation.)

For remote locations that cannot easily undertake routine fuel sampling and cleaning, automatic fuel polishers, which consist of a pump and filtration system, can be incorporated into a diesel genset.

Protection from the Elements; Protection for People

Another traditional factor in comparing natural gas to diesel has been the cold-sensitivity of diesel fuel and engines.

This has become less of an issue in the past decade or so, as fuel additives and special fuel types were developed to reduce diesel gelling in temperatures to as low as -20oF.

More recently, the rise of enclosed generators with winter add-ons has made gelling even less of a concern for diesel engine performance.

These developments are especially welcome in the remote locations common to mini-grids, wind and solar farms, and other small energy producers.

Gelling is primarily a concern when it enters the engine (specifically the fuel filter, where gelled fuel clogs up the filter, preventing a freely flowing fuel supply to the engine).

Heated enclosures powered by the generator itself, not to mention fuel filter heaters and other localized heaters, do an outstanding job of keeping the engine and its liquids at the proper operating temperature.

Modern enclosures for diesel generators also make it easier to dampen sound and vibrations.

In areas where noise or vibration is a sensitive issue, enclosures with rock wool insulation can close the gap that traditionally separates quieter natural gas generators from diesel models.

The Final Analysis

There are other minor factors to consider when comparing diesel generators to natural gas gensets, such as fire safety (advantage, diesel) and spill risk (advantage, natural gas).

Even here, companies on both sides are taking steps to level the playing field.

Diesel engine makers are incorporating 110 percent spill containment catch basins into their enclosed generators, and natural gas generator manufacturers are making their enclosures more fire proof.

They both want as much generator business as possible, so both will continue striving to erase any perceived drawbacks.

With 81.6 percent of units shipped, globally, in 2013 per research firm IHS, the diesel genset market far outpaces that of natural gas.

However, natural gas generator sales are running nearly 12 percent per year, and sales of alternative energy options, such as generators that run biodiesel, are increasing, as well.

The impact will be enough, IHS predicts, that diesel's market share will have dropped 10 points by 2018, to 71.1 percent.

This battle, propelled with manufacturers that continue to innovate on a near-monthly basis, ensures the contest between diesel and natural gas generators will be running for quite some time.

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