Daniel P. Duffy 2016-11-16 10:41:15
There is a quiet (and sometimes not so quiet) revolution taking place in the way America powers its motors, engines, and turbines. Industrial motors, traditionally powered by diesel and gasoline fuels, are transitioning to a new fuel source—natural gas. Compressed natural gas (CNG) and its cousin liquid propane gas (LPG) are becoming the fuel of choice for energy and transportation applications. This transition raises certain questions concerning the switch from traditional diesel and gasoline powered engines. How are the manufacturers of industrial motors handling the transition to running on natural gas? What are the challenges posed by integrating natural gas powered engines, and how are engineers and industrial managers overcoming them? What affect does the conversion to natural gas have on the operating efficiency of engines used for transportation or energy production? Welcome to the Golden Age of Gas Hydraulic fracking is a technological improvement that has quite literally changed the world. It is the biggest energy game-changer of recent times. This drilling technique has opened a treasure trove of energy for industrial and transportation uses. The combination of horizontal drilling equipment and application of high-pressure fluids has allowed drillers access to trapped oil and natural gas that was once inaccessible. Fracking is only one source of what is termed “unconventional” natural gas. These gas sources can be accessed by special means such as fracking, but don’t lend themselves to extraction with traditional methods given their location dispersed in rock formations instead of concentrated oil and gas fields. Another source of natural gas is coal bed methane. This is naturally occurring and has long been extracted if only as a safety measure for coal mining. Even so, deeper formations may also require fracking of the coal itself to release its methane. Recent developments involve the injection of generally modified bacteria that essentially “eat” the coal and convert it to methane for extraction by a standard wellhead. Other sources include shale gas extraction and gas extraction from tight sand formations, both of which are susceptible to fracking methods to release their entrained methane. Biogas from anaerobic digesters, landfills, and other organic sources remains a large segment of this industry. And lastly, Japanese teams are involved in advanced research and development to safely extract methane from frozen clathrate formations on the ocean floor. As promising as all of these developments are, methane from unconventional sources will have to earn its position in the energy marketplace. And as an energy source that produces half the greenhouse gas emissions per British Thermal Unit (BTU) generated as coal does, it has some significant environmental advantages as well. But, it will be in competition with advances in solar energy (photovoltaic cells, photosynthetic cells, and concentrated solar) along with associated advances in battery storage and smart grid connectivity, as well as biofuels and wind energy. The recent fall in the price of oil adds another competitive pressure. But having to choose between a cornucopia of multiple, efficient, and clean energy sources is not a bad problem to have. In the meantime, the flooding of the energy market with unconventional natural gas has been an economic game-changer as well as a technological breakthrough. As a result of fracking, the cost of American domestically produced natural gas has tumbled from $12 per million BTU equivalents to below $4 in less than six years. Comparisons to other energy sources are equally impressive. On a comparative BTU basis (1 barrel = 42 gallons of crude oil = 5,729,000 BTU for US-produced crude oil), natural gas was selling for the equivalent of less than $20 per barrel. As a result, by using CNG or LPG, the owner of a truck fleet or operator of a turbine generator can achieve considerable cost-savings by making the switch to methane. Original equipment manufacturers (OEMs) in the transportation and power generation industries have taken notice and offer new equipment and vehicle lines powered by natural gas. Scissors, Rock, Paper—or Gasoline Versus Diesel, Versus CNG Table 1 sums up the operational, chemical, and energy differences between diesel, gasoline, and CNG fuels. So, what are the operational advantages of CNG fuel? To determine this, an operator has to first make an apples-to-apples comparison between the three fuels. Since the energy value of a fuel can change with temperature, season, and its oxygenate mix (ethanol, MTBE, etc.), the values given above can be simplified into standard values. A standard gasoline gallon equivalent (GGE) is rated as having an energy content of 114,000 BTUs. A standard cubic foot (SCF, at standard temperature and pressure) of natural gas yields 900 BTUs of energy—the lower heat value is used for the standard rating. Therefore, 126.67 cf of natural gas is required to equal the energy equivalent of one gallon of gasoline. Natural gas has a density at standard pressure and temperature of 0.041 to 0.045 pcf. So approximately 5.66 pounds of natural gas has the same energy content as one GGE (conversely, approximately 6.5 lbs of natural gas equals the energy of one gallon of diesel, or “diesel gallon equivalent”). CNG is usually rated and sold in the US in terms of GGE. On average, it takes approximately one gallon of diesel fuel per hour to generate 20 horsepower of energy either for transportation or energy production. Since it is less energy-dense (and because gasoline engines tend to be less efficient than diesel engines) one gallon of gasoline per hour generates 15 horsepower. This then is the potential power output of 1 GGE of CNG. Engines that fully convert from diesel to CNG or are modified to operate as “flex fuel” CNG or diesel hybrid engines take these ratios into account. This conversion involves the addition of an extra-pressurized tank and dedicated fuel line. With this configuration, an operator or driver can change fuels by flipping a switch. The transmission fuel line changes the pressure for engine use and is regulated by a solenoid valve. Upon entering the piston chamber of the engine, it is ignited and burned in the same way as gasoline. Unlike liquid fuel, CNG is kept in a pressurized tank. These tanks have a typical service pressure of 3,600 psi. At this pressure, CNG has a density of 11.2 pcf. 1 GGE of CNG, therefore, requires 0.51 cubic feet (3.87 gallons) of space. So a 20-gallon tank filled with CNG would hold the same amount of energy as about five gallons of gasoline. So why use CNG if it has such a smaller energy density than gasoline and would result in shorter operating times and distances for engines operating equivalent sized machinery and vehicles? First, CNG can cost considerably less than gasoline or diesel. In recent years, gasoline at the pump has experienced wide price swings with highs of $3.75 per gallon in the third quarter 2014 (nationwide average as reported in the “Clean Cities Alternative Fuel Price Report,” US Department of Energy’s Alternate Fuels Data Center, April 2016) with CNG price per gallon of $2.20 during the same time period. While the price of gasoline has fallen since then, the price of CNG has shown no such erratic volatility, holding steady and also falling to approximately $2.00 in the second quarter 2016. These price fluctuations show the CNG market to be less susceptible to political impacts or market manipulations, with CNG prices holding steady for an extended duration. This steadiness has been noted by Sam Abuelsamid of Navigant Research: “A significant portion of the cost advantage of natural gas has evaporated in the past 12 months as a result of the collapse of world oil prices. However, various regional factors also affect the markets for NGVs [natural gas vehicles], including ongoing political tensions, the availability of refueling infrastructure, tightening tailpipe emissions requirements, and total cost of ownership.” Long term, the horse to bet on remains natural gas. In addition to a significant drop in greenhouse gas emissions (a major consideration in the ongoing switch from coal to natural-gas-fueled electrical power generation), there are other operational advantages to the use of CNG that result in additional cost savings. Use of CNG also prolongs engine life compared to standard diesel or gasoline fuels. So for the operators of vehicles and equipment that rack up a lot of mileage or operating hours, but operate within a limited radius to minimize the need for more frequent fill-ups, CNG is an obvious choice—a choice that should be made based on overall lifetime costs. For industrial engines, motors, and turbines utilizing a continuous feed of piped CNG there are no range or refuel constraints, making natural gas even more attractive. Differences Between CNG and Gasoline or Diesel Engines CNG is used as a fuel by internal combustion reciprocating engines used for both transportation and power generation. So how do CNG engines differ in design and operation from standard liquid fuel engines? The design and operation of an internal combustion engine, of any kind, has to take into account several physical and chemical characteristics. First are the temperatures and pressures at which the fuel auto-ignites. Gasoline and diesel auto-ignite at 482°F (250°C) and 410°F (210°C) respectively. By comparison, natural gas auto-ignites at 1,076°F (580°C). The next characteristic is the air-fuel mix required for clean, efficient burning in the cylinder. Liquid fuels do not mix as well with air as does a combustible gas like methane. In traditional engines, fuel and air are mixed in the carburetor before being fed into the combustion cylinder. Modern gasoline engines utilize fuel injection that feeds the fuel in an aerosol form in precise bursts into the inlet manifold or inlet port to ensure that the fuel is thoroughly mixed with air prior to being fed into the cylinder. Modern diesel engines, on the other hand, utilize direct injection into the piston cylinder filled with compressed air (or a pre-combustion chamber called the cylinder head). Gasoline engines ignite the fuel with a spark from a plug, while diesel engines use compression to make the diesel fuel (with its higher-energy content) self-ignite at a lower temperature than gasoline. CNG, being less energy-dense, has to auto-ignite at a much higher temperature than liquid fuels. Like gasoline, CNG is compressed and ignited with a spark plug. And, even though CNG (being a gas instead of a liquid) mixes faster and more efficiently than liquid fuel, its lower energy density can result in lower output power for cylinders of the same size. This hurdle can be overcome by the use of turbochargers to increase the air and fuel density and get a more powerful ignition. Even so, CNG engines are easily capable of matching the power needs of liquid fuel engines in terms of torque and horsepower. These differences in compression ratios (the ratio of the volume of its combustion chamber from its largest capacity at the end of the piton’s power stroke, to its smallest compressed capacity immediately prior to combustion) can require changes to the combustion chamber or modifications to the pistons. A diesel engine has a compression ratio between 16 and 18 to one. Gasoline engines utilizing spark plugs instead of compression for ignition have compression ratios six and 10 to one. CNG engines operate at compression ratios between 10 and 12 to one. Pure CNG engines will require changes to the cylinder shape to allow for proper air and fuel mixing. For hybrid vehicles, the compression ratio has to be adjusted to allow for the use of CNG and prevent engine knocking. Testing indicates that the optimum compression ratio to operate a hybrid CNG or diesel engine without knocking is between 16 and 17 to one. The easiest way to modify the compression ratio in a modified hybrid is by altering the piston stroke length to match the desired compression ratio. An equivalent change can be achieved by altering the air-fuel ratio. Furthermore, since diesel engines do not use a spark plug, a conversion to CNG requires a replacement of the injector with a spark plug installed through the valve cover. Making The Transition to CNG Engines The recent growth of CNG engine market has been nothing if not remarkable, with projections for annual sales of natural gas vehicles (NGVs) projected to grow from 2.4 million units last year, to 3.9 million a decade from now—a growth of 62.5%—despite drops in oil prices along with gains in technology driving down the costs of electrical storage batteries. According to Navigant research, this market increase for NGVs will continue despite headwinds from falling oil prices, which are also projected to continue falling over the next decade. (“Natural Gas Passenger Cars, Light Duty Trucks and Vans, Medium/Heavy Duty Trucks and Buses, and Commercial Vehicles: Global Market Analysis and Forecasts”, Navigant Research, 2015). Additional projections indicate that the market for NGV buses and trucks will increase annually at a rate of 12.6% and 6.4% between now and 2022. The number of NGVs worldwide is expected to reach 35 million by 2020, most of these being light duty trucks. NGV market growth will focus on fleet markets that meet local needs (in accordance with the physical characters of CNG described above) such as urban transit buses, warehouse delivery and shipping vehicles, and waste collection trucks. Slower growth is expected for passenger cars, light-duty trucks, heavy-duty trucks and heavy equipment, and long-haul commercial shipping. Overall, the growth in CNG powered engines and vehicles will be slower than during the days of the oil price spikes a few years ago, but can be counted on to remain steady. Further growth in CNG-related infrastructure is also anticipated, first and foremost being CNG refilling stations. Also of importance to increased use of CNG engines are the technological developments by natural gas engine manufacturers, fuel-storage tanks, and fuel lines, and the upgrades to convert diesel and gasoline engines to hybrid use of CNG. Economic benefits and environmental improvements are also key to further use of CNG engines. The payback period is critical. As a rule of thumb, if CNG is priced at half the cost of diesel, the payback period for the cost of a conversion can be between one to one and a half years. Keeping natural gas prices down will be key to further engine and vehicle development as the lower price of natural gas is typically necessary to offset higher manufacturing costs for NGVs. The cost of NGV automobiles can be $2,000 more than standard gasoline cars, though post-manufacturing hybrid conversions can cost as little as $800. Larger utility trucks and commuter buses can cost over $30,000 to retrofit to use CNG. Natural Gas Turbines—Growth and Potential Natural gas not only powers industrial, vehicle, and equipment engines, it fuels electrical power-generating turbines as well. Long a significant part of the electrical power generator mix, natural gas turbines have been expanding their market share in response to lower natural gas costs and release much lower greenhouse gas emissions in comparison to coal. Though in part driven by regulatory concerns regarding climate change and greenhouse gas emissions, natural gas is displacing coal largely due to fuel price differentials, as well as the inherent durability and superior efficiency of gas turbines. Market projections see the global gas turbine market growing at an annual rate of approximately 3.5% until 2020. Gas turbines work in a manner not that different from jet engines. There are three main components to a gas turbine: the compressor, the combustion chamber, and the turbine itself with its spinning rotator blades. The compressor draws in outside air and force-feeds it into the compression chamber at high pressure and speeds in excess of hundreds of miles per hour. The combustion chamber is equipped with rings of fuel injection ports that inject natural gas into the chamber at a constant inflow rate. Inside the chamber, the natural gas and air are thoroughly mixed to create a high-efficiency burn, which is ignited at temperatures greater than 2,000°F (1,093°C). The exploding gas expands into the turbine at high pressures. Inside the turbine is an array of airfoil blades that spin and rapidly rotate the turbine as the expanding hot gasses pass through. This spins the turbines rotating blades, which generate electricity and draw in more pressurized air to feed the combustion chamber, making the combustion process more efficient. But what really makes natural gas efficient for energy generation is its use in combined-cycle power plants. This type of plant utilizes the still hot exhaust from the turbine to provide energy for a second stage of energy production, such as a steam power plant. This configuration can increase the overall efficiency of the system by 50 to 60%, and at significant cost savings over simple coal fired boilers. Still, further use can be made from the gas turbine exhaust in the form of cogeneration to provide heating and cooling for adjacent buildings. Natural gas turbines are easily scalable and also readily lend themselves to decentralized and distributive power systems less than 50 MW in size, providing local and regional flexibility in power generation. Taking their place next to smaller gas turbines are diesel and natural gas powered reciprocating engines used to generate electricity. They operate as described above, but at a much higher power output. Given its flexibility, low and volatile fuel costs, operational efficacy, and superior overall life cycle costs, natural gas deserves its place in any distributed energy mix alongside renewable energy from solar panels, concentrated solar power, and wind. Major CNG Engine Suppliers Cummins Westport is a major manufacturer of natural gas engines for heavy duty on highway applications that meet and exceed US EPA emissions standards. Their ISX12 G is a larger displacement natural gas engine that is used in a variety of heavy-duty vehicles, such as regional haul trucks, vocational and commuter transit, and refuse collection and transport applications. Operating on natural gas (either CNG or LNG), it has a displacement of 11.9 liters, operates at up to 400 hp, and produces up to 1,450 lb-ft of torque for vehicles with operating weights up to 80,00 lbs. Their ISL G Near Zero (NZ) NOx natural gas engine takes emission standards to a new level. With ratings up to 320 hp and 1,000 lb-ft of torque, the ISL G Near Zero is a mid-range engine for vehicles up to an operating weight of 66,000 lbs, that is certified by the EPA and Air Resources Board (ARB) in California to meet the 0.02 g/bhp-hr optional Near Zero NOx Emissions standards for medium-duty truck, urban bus, school bus, and refuse applications. The ISL G Near Zero NOx emissions are 90% lower than the current EPA NOx limit of 0.2 g/bhp-hr and also meet the 2017 EPA greenhouse gas emission requirements. CWI natural gas engines have met the 2010 EPA standard for particulate matter (0.01 g/bhp-hr) since 2001. Cummins Westport Inc. designs, engineers, and markets six to 12 liter spark-ignited natural gas engines for North American commercial transportation applications such as trucks and buses. Cummins Westport is a joint venture of Cummins Inc. (NYSE:CMI), and Westport Innovations Inc. In addition to natural gas engines for transport, Cummins NPower also provides natural gas generators for distributed energy systems and backup emergency power applications. Their natural gas and dual-fuel reciprocating engine generator sets provide power in the 55 kW to 815 kW ranges. They typically operate with Cummins’ PowerCommand control system. These generators can be configured for both the 50 Hz and 60 Hz markets. Caterpillar natural gas generator sets are designed to run both on natural gas and with the option to use flexible fuel configurations. Their wide variety of generators ranges from 20 to 9,700 kW, and are customizable to particular markets and needs. Their generators include tailored designs for natural gas, biogas, coal gas, and alternative fuels. They can come equipped with heat recovery models for cogeneration applications, further improving operational efficiency. They can operate independently, be tied into the local power grid, or serve as backup power generators. DE Daniel P. Duffy, P.E., writes on topics of energy and the environment.
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