Natural Gas Buses: Separating Myth from Fact
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U.S. DEPARTMENT of ENERGY, May 2000 OFFICE of ENERGY EFFICIENCY and RENEWABLE ENERGY Natural Gas Buses: Separating Myth from Fact Introduction Myth 2: CNG buses emit more ultra-fine particulates than diesel buses. Increasing numbers of transit agencies across North Fact 2: Although CNG buses emit over 90% less America are making the choice to convert their bus particulate mass than conventional diesels, the fleets to compressed natural gas (CNG), and even particulates that are emitted may be smaller in more are seriously considering it. Natural gas buses size. However, early studies that have compared now account for at least 20% of all new bus orders. CNG and diesel particulate matter (PM) size have However, it becomes difficult for fleet operators to reached contradictory conclusions. Techniques are fairly evaluate the potential benefits of an alternative being developed to better measure PM size distrib- fuel program if they are confronted with misinforma- ution and understand how that distribution is tion or poor comparisons based on false assumptions. affected by test procedures. This fact sheet addresses some of the most common Myth 3: CNG buses create more greenhouse gases than misconceptions that seem to work their way into diesel buses because they emit more methane, anecdotal stories, media reports, and even some which is a greenhouse gas that is 20 times poorly researched white papers and feasibility stud- stronger than carbon dioxide. ies. It is an expanded version of information that was presented on behalf of the U.S. Department of Fact 3: Natural gas has inherently lower carbon diox- Energy at the South Coast Air Basin Alternative Fuel ide emissions compared to diesel. Considering the & Electric Transit Bus Workshop in Diamond Bar, total fuel cycle of both diesel and CNG including California, on March 15, 2000. the emissions created during fuel production, CNG buses appear to have total greenhouse gas emis- sions that are very similar to, if not slightly better CNG Bus Myths and Facts than, diesel buses, despite emitting higher levels of methane. Myth 1: CNG buses emit the same amount of particulate matter (PM) as diesel buses. Fact 1: CNG buses emit virtually no visible PM or black soot at the tailpipe. In chassis dynamometer testing conducted by West Virginia University, CNG buses consistently emit dramatically less PM than diesel buses. For example, testing of commer- NREL/PIX 06651 Norton, Paul cial buses in Boulder, Colorado, on the central busi- ness district (CBD) driving cycle demonstrated a 97% PM reduction and a 58% reduction in oxides of nitrogen (NOx) with buses running on CNG rather than diesel. Note that the trace amount of PM associated with CNG is generally attributed to crankcase lubricating oil consumption, not the fuel. Commercial bus in Boulder, Colorado, being tested on West Virginia University’s portable chassis dynamometer U.S. DEPARTMENT of ENERGY Myth 4: CNG buses are much more expensive than because natural gas combustion inherently diesel buses. produces lower levels of these emissions. Fact 4: CNG buses cost about $25,000 to $50,000 more Myth 6: The availability of ultra-low sulfur diesel fuel and than a conventional diesel bus (depending on the diesel hybrid-electric bus systems will negate the model and any special equipment that might be emissions benefits of CNG buses. ordered), but CNG fuel usually costs less than diesel fuel. At 25 cents per gallon savings, the Fact 6: Testing of prototype hybrid buses using typical CNG bus could pay for itself in just a little advanced emission control technologies and more than 3 years. And greater savings in fuel cost experimental ultra-low sulfur diesel fuel has can result in even quicker paybacks. The incremen- shown particulate matter emissions levels close tal cost of diesel-hybrid buses is estimated to be to those of natural gas engines, which need only about twice that of CNG buses, but payback analy- a minimal amount of exhaust emission control. sis will be based on improved fuel economy alone However, NOx emissions from these buses were since there will be no per-gallon cost savings with higher than those from CNG buses. In addition, the the continued use of diesel fuel. special diesel fuel required if these emission control systems are to function properly on diesel engines Myth 5: Advanced emission control technology for diesel is not expected to be commercially available engines will make CNG buses unnecessary. nationwide for 5 years or more (and then it will cost more than conventional diesel fuel). Further, Fact 5: Diesel emissions from new buses have even with the use of ultra-low sulfur diesel fuel, improved dramatically in the past 10 years, with natural gas emission control devices will most like- most of the gains resulting from improved engine ly be less expensive and longer lived than those for design. In the future, it’s almost certain that diesel diesel engines simply because there is inherently engines will have to rely on emission control less NO and PM to control with natural gas and devices to further reduce emissions. However, x because natural gas has fewer contaminants that the durability of these emission control devices degrade these devices. in use on diesel and diesel-hybrid buses depends greatly on fuel quality and is still to be deter- Myth 7: CNG bus fuel tanks are prone to explode. mined. Should these devices degrade as the buses age and accumulate mileage, future emissions will Fact 7: The technology for making CNG tanks is well be much higher . similar to those of the high known and mature. In the event of a vehicle colli- emitters observed within the aging gasoline vehi- sion, CNG fuel tanks are much stronger and safer cle fleet on the road today. Also, diesel buses emit than either diesel or gasoline fuel tanks. The few toxics (such as benzene and 1,3-butadiene) that instances of CNG tank failures were studied care- CNG buses do not. Based on the testing available fully, and the problems, mostly involving support today, it appears that CNG buses will always have strap failure or tank abrasion during normal oper- PM and toxic emission benefits over diesel buses ation, have been remedied. These CNG cylinders power a transit bus in Tacoma, Washington. Pierce Transit/PIX 04035 Transit/PIX Pierce U.S. DEPARTMENT of ENERGY of lower incremental bus prices and the lower price of natural gas). CNG engines can be used in hybrid applications as well as diesel engines, with similar reductions in fuel consumption and even lower emissions. Warren Gretz, NREL/PIX 05813 Gretz, Warren Myth 12: CNG engines will be too expensive to use in hybrid buses. Fact 12: Although CNG tanks are more expensive than diesel fuel tanks, fewer of them are needed in a hybrid application. In addition, the emission controls necessary to meet future emission stan- dards are likely to cost less for CNG than for diesel “CNG Power Delivers Clean Rocky Mountain Air” engines. And CNG is less expensive than the ultra- in the Denver metro area. low sulfur diesel fuel required for advanced diesel engine emission control devices. If we examine Myth 8: CNG buses are unsafe. their costs on a life-cycle basis, it’s likely that CNG hybrid buses will be competitive in cost Fact 8: CNG buses have some different safety concerns with diesel-hybrid buses. than diesel fuel buses, but overall, there is no evi- dence that CNG buses pose any greater risk of fire Myth 13: Diesel buses are easier to maintain than CNG or explosion than diesel buses. Natural gas buses buses and require no special training for the have on-board gas detectors and other safety equip- maintenance staff. ment specially designed to ensure safe operation. Fact 13: Mechanics must be highly skilled to maintain Myth 9: CNG bus maintenance garages and refueling both types of buses. The mechanics must be famil- facilities are more dangerous than diesel facilities. iar with the properties of each fuel type and must be properly trained on each system (just as diesel Fact 9: There’s no evidence to support this claim. mechanics must receive different training than Both natural gas and diesel fuels are flammable— those who work only on gasoline vehicles). that's why they are useful as fuels. Each requires Advanced technology diesel propulsion systems— handlers to use safety and fire protection equip- such as hybrid-electric-diesel systems and the ment designed specifically for that particular type exhaust emission control systems required for of fuel. However, diesel bus facilities typically store achieving significant emissions reductions with much larger quantities of fuel on site than CNG diesel engines—are much more complex than facilities (usually 100,000 gallons or more, usually standard diesel power trains. These advanced in large underground tanks; typically only 500 gal- technology systems will require additional special lons or so of CNG fuel is stored on site). Ground training, tools, and test equipment beyond that soil contamination from leaking diesel tanks is required for regular diesel or CNG bus mechanics. another concern that CNG facilities do not face. Myth 10: Natural gas vapors are toxic to breathe. Fact 10: Natural gas vapors (which might escape in the event of a leak) are odorless and nontoxic to breathe. The familiar natural gas smell is actually an odorant that is added to the gas as a safety fea- 04036 Transit/PIX Pierce ture. Gasoline and diesel vapors, however, do con- tain toxics that are dangerous to ingest or breathe. And any fuel vapor that builds up in an enclosed space can cause asphyxiation. Myth 11: Hybrid technology will make CNG buses obsolete.