Alternate Fuel, Additional Revenue

There is a new fuel in town, compressed natural gas (CNG), that has advantages of efficiency and cost—advantages that will allow it to take control of a large segment of the waste hauling industry. New technological advances in the production and utilization of CNG will enhance these advantages further, leading to greater market penetration. This article will review the basics of a CNG fuel and engine system and examine these technological trends. How do these systems differ from traditional fuel systems? How or why are they better? What can these latest innovations do? What will innovation do to the market for CNG systems? Is this a growing market sector? Will more fleets be converting to CNG? And will this be a difficult or easy conversion?

Scissors, Rocks, Paper—Comparing CNG with Diesel and Gasoline
There are three hydrocarbon fuels available to power equipment and truck engines: CNG, gasoline, and diesel. What are their operational advantages? How can we make a proper apples-to-apples comparison? Their primary physical and chemical differences are summed up in table 1.

How to make an apples-to-apples comparison? First, we need a consistent unit of energy measurement. The weight of a gallon of gasoline is 6.3 pounds. Diesel fuel has a density of 7.1 pounds per gallon. The density difference is proportional to the energy content difference with diesel having both an energy content and a unit density that is 113% of gasoline. Converting to pounds, the higher heating value of gasoline becomes 19,109 to 19,737 BTUs per pound, and diesel’s heating value is 19,505 BTUs per pound.

A second obstacle to a direct comparison is the fact that fuel energy values can change with its composition, additives, oxygenating mixture (ethanol, MTBE), ambient air temperature, season, and so on. Some simplifying assumptions are therefore required to create standard values for each fuel type. These standard values are conservatively based on each fuel’s lower heating value. These standard values are given in “gasoline gallon equivalent” (GGE) of 114,000 BTUs. Since the lower heat value of CNG is 900 BTUs per standard cubic foot, 126.67 cubic feet of natural gas is required to equal one GGE. With a density at standard temperature and pressure of 0.044 pcf, 5.66 pounds of CNG are required to match the energy of 1 GGE (CNG is rated and sold in terms of GGEs). Given its energy density, and since gasoline engines tend to be less efficient than diesel engines, a GGE per hour can produce 15 horsepower, which is also the power output of 1 GGE (5.66 pounds) of CNG. Diesel is more expensive per gallon than gasoline. The efficiency, power, and cost factors make diesel more suitable for high-powered engines found in large, long-distance transport trucks and heavy earthmoving equipment, while gasoline dominates the market for personal cars and small utility trucks.

Note that both of the liquid fuels have a lower energy content by weight than does natural gas. So why not always use CNG? The problem has to do with the need to compress CNG under high pressure to make it dense enough to act as an effective fuel. Being in liquid form already, gasoline and diesel do not need this additional operating step. Unlike gasoline or diesel, CNG has to be kept in a pressurized tank. Safety concerns limit the maximum pressure in such tanks to 3,600 psi. Under this pressure, CNG has a density of 11.2 pcf, which equals 1 GGE per 0.51 cubic feet (equivalent to 3.87 gallons). For practical reasons, CNG tanks are usually no larger than 20 gallons. Therefore, a filled 20-gallon pressure storage tank of CNG would have the same energy as approximately 5 gallons of gasoline.

So, while it has a baseline energy value per unit weight higher than that of gasoline, practical application of CNG requires the use of a less efficient storage tank almost four times the size of a gasoline tank with equivalent energy. The unavoidable result is a shorter operating radius or runtime between refills for trucks and heavy equipment. Given these inherent operational limitations of CNG, the question then becomes: why use it at all?

The first factor in its favor is cost. CNG can be considerably cheaper than gasoline or diesel. During the first half of 2019, depending on the grade of gasoline and the month it is sold, the price of gasoline at the pump has varied from $2.393 per gallon to $3.513 per gallon (source: US Energy Information Administration, “Weekly Retail Gasoline and Diesel Prices”). Natural gas prices for the past year have hovered consistently around $12.50 per one thousand cubic feet for the past five years (source: Average Annual Residential Average, US Energy Information Administration, “Short term Energy Outlook,” August 2019). That would be $12.50 per 900,000 BTUs (at 900 BTUs per cubic foot, see above) or $12.50 per 7.9 GGE (at 126.67 cubic feet of natural gas per 1 GGE), equal to $1.58 per GGE. Not only are natural gas prices more resistant to price fluctuations, but the typical price is also approximately one half that of gasoline on an equivalent per-unit-of-energy basis.

Other operational advantages include a significant drop in greenhouse gas emissions (a major consideration in the ongoing switch from coal to natural-gas-fueled electrical power generation) as well as the fact that the use of CNG fuel prolongs engine life compared to gasoline and diesel. Yet the limited range between fill-ups remains a problem. And so the best application for CNG fuel would be for those truck and equipment operations that rack up a lot of mileage or operating hours, but operate within a limited radius. In other words, it is the perfect choice for MSW waste collection operations and heavy equipment operating in the relatively small area of a landfill. For industrial engines, motors, and turbines utilizing a continuous feed of pumped CNG where there are no operating range or refueling constraints, natural gas is even more attractive. Besides, for many large landfills, CNG is a fuel they naturally produce onsite.

Homegrown Sources of CNG Fuel
Landfill gas (LFG) is the result of the decomposition of the organic portion of deposited waste. The production of LFG is a four-stage process that begins the day waste is deposited on the landfill’s working face. During the initial, short-duration “Aerobic Decomposition” stage, the initial oxygen supply in the waste voids feeds aerobic bacteria. These bacteria then perform hydrolysis on the organic waste portion (chemical reactions with moisture and water presenting the waste mass that result in the breakdown of complex organic molecules such as carbohydrates into simpler ones such as sugar) and aerobic degradation while generating heat.

The second stage, “Acidogenesis,” starts when the initial oxygen supply is depleted. Aerobic microbes displace the anaerobic bacteria to perform a fermentation that produces organic acids, hydrogen, carbon dioxide, water vapor, ammonia, and nitrogen, with hydrogen and carbon dioxide produced as byproducts of the fermentation process. The fermentation takes the simpler organic material previously produced by the aerobic bacteria and creates volatile fatty acids. It is at this stage that sulfur-reducing bacteria produce hydrogen sulfide (giving LFG its “rotten egg” smell).

The third stage, “Acetogenesis,” is a preparatory phase that sets the stage for true LFG production. While still under anaerobic conditions, the volatile fatty acids produced by the previous stage’s activities are converted into acetic acid, carbon dioxide, and hydrogen.

Lastly, full-scale LFG and methane production begins during “Methanogenesis.” This is the longest-lasting stage and sees the previously produced acetate and the last of the hydrogen converted into methane and carbon dioxide. Its length can be greater than the previous three stages and can last far longer than the final closure of a landfill. Therefore, methane can be produced decades after final closure well into the landfill’s post-closure care period.

How much LFG does a landfill produce during this last stage? That is a function of both the age of the deposited waste and its organic composition. Table 2 provides a fairly typical breakdown of MSW by weight, showing an approximate 60/40 split between organic and inorganic waste.

What is the amount and composition of LFG produced in the last stage of the decomposition of the waste’s organic component? A lot depends on the age and organic component of the waste. The 60% cited above is a long-term average. Some waste loads can be purely organic, such as a load of organic sludge. Another load can be purely inorganic, such as a load of broken concrete construction debris. But assuming a long-term average, over time the annual production of LFG should average about 0.272 cubic feet for every pound of waste (equal to 544 cubic feet per ton of waste).

We further assume that methane makes up about 45–50% of the total LFG generated by volume with another approximate 45–50% consisting of carbon dioxide and the remain 0–5% consisting of trace gases such as volatile organic compounds. These are long-term averages and can vary widely over the lifetime of the landfill and from landfill to landfill depending on local factors such as temperature, moisture content, and the percent of disposal space taken up by cover soils. But using these planning standards and the assumption that half of the landfill gas consists of methane, its annual production rate would be 0.136 cubic feet for every pound of waste (or 272 cubic feet per ton of waste). Converting the annual methane production rate to cubic feet per minute results in 0.0005 cubic feet per minute per ton of waste, or approximately 1 cubic foot per minute per pound.

Nationwide, the USEPA estimates that each American on average throws out 4.5 pounds of MSW each day, or over 0.82 tons each year. With a population of over 300 million citizens, the US on average generates over 246 million tons of waste each year. With about 72 million tons of waste being recycled each year, the remaining 174 million tons end up in a landfill (Source: USEPA Solid Waste Fact Book). Methane has an intrinsic heat value of 900 BTUs per cubic foot. The 272 cubic feet of methane produced each year by a ton of disposed waste would have a heat value of almost 245,000 BTUs. Therefore, the methane content of the total amount of landfilled waste has an annual value of 42,630,000,000,000 (42.63 trillion) BTUs nationwide. At 114,000 BTUs per GGE, this is equivalent to 373.9 million GGE, or 330.9 million diesel gallon equivalents (DGE).

How much of this is recoverable depends on the configuration of the landfill (especially if it is constructed with an impermeable cap utilizing a geomembrane to prevent intrusion of oxygen and escape of LFG), the completeness and overall efficiency of its active LFG extraction system, and how effectively and completely the methane can be extracted from the overall LFG stream. This last point is critical since only pure methane can be effectively used as CNG to power truck and equipment engines.

One method of LFG separation utilizes a carbon-dioxide wash unit to strip out the carbon dioxide and other impurities, leaving behind nearly pure methane. In this process, LFG is pretreated by being compressed, having hydrogen sulfide removed using a filter and then dehydrated to remove water vapor. The pre-treated LFG then enters the wash stack, which is essentially a refrigerated exhaust stack. The freezing temperatures utilize the fact that carbon dioxide will transition from a gas to a liquid at a higher temperature than methane. As the landfill gas rises through the refrigerant stack, the carbon dioxide becomes a liquid mist and droplets of carbon dioxide fall back down in a counter-current direction through the stack. As it falls, the liquid carbon dioxide acts as a filter to absorb and remove additional trace impurities. The liquid carbon dioxide is collected at the bottom. What remains of the landfill gas is an almost pure stream of methane that can be compressed and reutilized as CNG fuel.

Between the landfill configuration, the effectiveness of the extraction system, and the efficiency of the methane separation process, the amount of usable methane can vary widely from the total. Assuming a nationwide average overall system efficiency of 75%, the amount of methane that could be recovered from all landfills would be equal to almost 250 million DGEs.

Waste collection trucks are not the most efficient vehicles on the road. Powered by diesel fuel, they average just 3 miles per gallon. There are several reasons for this: the need to stop and idle the engine at each pickup point, the need to divert engine power into the compaction of waste, the heavy load to horsepower ratios, etc. A discussion concerning how to improve waste collection truck efficiencies would be an article in itself. But for now, the amount of methane that could be recovered from existing landfills would supply enough fuel for 750 million waste collection truck miles annually. But first, existing diesel engines would have to be retrofitted to utilize CNG fuel.

The Differences Between Diesel, Gasoline, and CNG Engines
CNG engines can come factory-made or modified from existing gasoline, diesel, or hybrid engines. Of the three, gasoline engines are the easiest to modify since a spark plug is needed to ignite both types of fuel. Conversion of diesel engines is no longer a significant part of the market while flex fueled engines face problems concerning “near-zero” nitrous oxide emissions requirements.

Conversion or factory-built requires the installation of extra-pressurized tanks and modified fuel lines. CNG fuel is fed under pressure while gasoline and diesel rely on suction. Other designs and operational differences between CNG and gasoline or diesel engines take into account the fuels’ differing chemical and physical characteristics.

One difference concerns 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 air-fuel mix also differs significantly. Gasoline engines utilize fuel injectors that feed the fuel in precise bursts of fuel/air aerosols that ensure that the fuel is thoroughly mixed with air before being fed into the piston cylinder via an inlet port. Diesel engines, utilizing a more energy-dense fuel, utilize direct injection into a piston cylinder that has been filled with compressed air (sometimes a pre-combustion chamber called the cylinder head is used). Gasoline engines utilize spark from a plug to ignite the fuel. Diesel engines, however, use compression that causes the diesel fuel to self-ignite at a lower temperature than gasoline.

As mentioned above, CNG has to ignite at a much higher temperature than gasoline or diesel. As such, CNG ignition utilizes both compression and ignition provided by a spark plug. Being a gaseous fuel is both an advantage and a disadvantage. Being a gas, CNG more readily mixes with air before ignition. However, as a gas, it has a much lower energy density. Unless modified before ignition, a CNG fuel flow will result in lower power output for each piston stroke. Therefore, turbochargers are used to increase air and fuel density before ignition and increase its power output. This allows CNG engines to match the power output of gasoline and diesel engines as measured by torque and horsepower.

The piston stroke cycle reduces and then increases the available volume within the combustion chamber. The ratio between its largest volume and smallest capacity is referred to as the compression ratio. Since it utilizes compression to ignite its fuel, diesel engines have a large compression ratio between 16 and 18 to 1. Gasoline engines have compression ratios between 6 and 10 to 1. CNG engines operate in the midrange with compression ratios between 10 and 12 to 1, and also require changes to the cylinder shape to allow for more efficient air/fuel mixing.

CNG has another distinct if indirect advantage: it produces far fewer greenhouse gases than other fossil fuels. A study commissioned by the California Energy Commission concludes that CNG vehicles produce almost 30% fewer greenhouse gas emissions than gasoline vehicles and almost 22% fewer than diesel vehicles (source: “Full Fuel Cycle Assessment: Well-To-Wheels Energy Inputs, Emissions, and Water Impacts,” August 2007). In addition to lower greenhouse gas emissions, CNG also produces 45% fewer hydrocarbons than gasoline, making CNG a clean-burning fuel, leaving little in the way of emissions after combustion. These emission reductions include carbon monoxide (CO) by up to 75%, nitrogen oxide (NOx) by 50%, 95% of particle matter (PM) by 95%, and volatile organic compounds (VOCs) by 55%.

The Latest Technical Advances in CNG Engines and CNG Production
In addition to extracting methane from LFG, traditional domestic production of natural gas from well fields and fracking has increased significantly in the past 10 years. Fracking technology is a separate subject, but it has unleashed a cornucopia of clean-burning energy. As a result, methane production in the US has increased from almost 75 trillion cubic feet per day in mid-2009 to over 110 trillion cubic feet by mid-2019 (source: “Monthly Crude Oil and Natural Gas Production,” US EIA, August 2019). This staggering increase in production has allowed natural gas to enter into non-traditional markets like transportations fuels.

The Department of Energy’s Advanced Research Project Agency-Energy (ARPA-E) is financing research into over a dozen new technologies in the field of methane fuel use. This includes their MOVE Program (“Methane Opportunities for Vehicular Energy) finding new ways to cost-effectively power passenger cars and other light-duty vehicles with CNG. These cutting-edge technologies include the following:

• The trouble with high-pressure storage tanks is that they need to be simple (and bulky) in design. Complex shapes create force concentration points where the tank wall could fail. “Blackpak” intends to solve this problem with tanks made from high-strength, high-surface-area carbon. This will form a sorbent-based natural gas storage vessel in which the sorbent material itself is the container. With such material, a pressurized storage tank is not needed at all, and this material can be shaped to fit any car or truck design and configuration. This also makes home refueling of CNG cars easier and safer. All in all, it will provide greater occupant comfort, reduced cost, and ease of operation.
• Speaking of at-home refueling, this offers a significant advantage over traditional commercial fuel pumps (which can take hours to completely refuel a vehicle). Eaton’s “Highly Efficient, Near-Isothermal Liquid-Piston Compressor for Low-Cost At-Home Natural Gas Refueling” system does just that. By tapping into a home gas pipeline, this system utilizes a traditional compressor powered by an electric motor to rotate a crankshaft. This, in turn, is connected to metal pistons that pump and compress the gas. But this system goes one step further with the replacement of traditional piston seals with a liquid that comes into direct contact with the natural gas. This eliminates the traditional problems with sealed piston compressors (high cost of manufacture, maintenance headaches, and short operational lifetimes).
• Ford is developing a “Covalent and Metal-Organic Framework High-Capacity” storage system. The goal is the development of an optimized advanced porous material within a storage system. By utilizing the surface energy attraction of the natural gas, this porous material stores more gas inside of a containment tank and at lower pressure. The result will be more efficient gas storage and lower production costs.
• OnBoard Dynamics’ “Vehicle-Integrated Natural Gas Compressor” is a hybrid approach that lets an internal combustion engine compress natural gas for onboard storage. The car can therefore perform the compressing operation instead of the fuel pump system. It also avoids the expense of a standalone home refueling system. The car, in effect, becomes its own refueling station. For less than $400, the onboard compressor would pay for itself in less than six months.
• “Fully and Intricately Conformable, Single-Piece, Mass-Manufacturable High-Pressure Gas Storage Tanks” developed by REL are low-cost, conformable gas tanks. One of these tanks utilizes an internal structural cellular core that increases the tank’s strength, allowing it to be made in almost any shape. REL is focusing on manufacturing methods that produce the desired results while reducing production costs (up to 70% less cost). Small-scale prototypes that have passed the tests for strength, rugged durability, cost, and safety are being scaled up into full-scale storage units.

Current and Future Markets for CNG Vehicles
Ongoing increases in technological advances and reductions in costs will lead to an ever-growing market for CNG transportation fuel. As this is a very effective way to achieve sustainable transport, this trend should be applauded. Currently, more than 86 countries use CNG powered vehicles. The number of CNG vehicles has increased from 9.5 million in 2009 to 26.13 million in 2018. Projects are for more than 30 million CNG vehicles by 2021. By region, Asia-Pacific has the most CNG vehicles, followed (in order) by South America, North America, Europe, the Middle East, and Africa.

As a percent of the total natural gas market, CNG vehicles still lag behind residential, industrial, and power generation uses, with only 4% of natural gas consumed. Given the abundance of available methane and the need to minimize greenhouse gas emissions, this market can only expand further. To encourage the use of CNG for transportation, the European Union has instituted a tax-reduction incentive. The EU is also looking to minimize its dependence on foreign natural gas sources by allowing more and diversified suppliers of methane.

Though small compared to other natural gas uses, the CNG engine market has exploded in recent years. Grow projects show an increase from the current 2.4 million CNG vehicles to over 3.9 million a decade from now. Even projected falls in oil costs will not hamper this growth (source: “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). CNG bus and truck markets will also expand from 12.6–6.4% annually with 35 million vehicles (mostly light trucks) projected to be on the road by 2020. In most markets, the emphasis will be on those vehicles that meet local needs for short-range hauling (urban commuter buses, warehouse delivery and shipping, and waste collecti