CNG VS GASOLINE

Using compressed natural gas (CNG) vehicles instead of gasoline

Using compressed natural gas (CNG) vehicles instead of gasoline

Introduction

Fuel cycle analysis is requisite when comparing different vehicle technologies powered by different fuels. Figure 1 shows the significantly different greenhouse gas (GHG) emission results obtained by considering only vehicle tailpipe emissions (pump to wheels) versus considering both fuel production (well to pump) and tailpipe emissions (fuel cycle). A number of studies have been conducted to estimate fuel cycle emissions and energy use associated with various transportation fuels and vehicle technologies. The results of those studies were influenced by the assumptions made by individual researchers regarding technology development, emission controls, primary fuel sources, fuel production processes, and many other factors. Because different methodologies and parametric assumptions were used by different researchers, it is difficult to compare and reconcile the results of different studies and to conduct a comprehensive evaluation of fuel cycle emissions and energy use. Computer models for calculating emissions and energy use are needed to allow analysts and researchers to test their own methodologies and assumptions and make accurate comparisons of different technologies (Mondt, 2000).

Discussion and Analysis

At present, in the United States and worldwide, motor vehicles are fueled almost exclusively by petroleum-based gasoline and diesel fuels. Since the first oil price shock in 1973, efforts have been made to seek alternative fuels to displace gasoline and diesel fuels and achieve energy and environmental benefits. Some of the alternative fuels that have been researched and used are liquefied petroleum gas (LPG), compressed natural gas (CNG), liquefied natural gas (LNG), methanol (MeOH), dimethyl ether (DME), Fischer-Tropsch diesel (FTD), hydrogen (H2), ethanol (EtOH), biodiesel, and electricity. Production processes associated with gasoline, diesel, and each of these alternative fuels differ.

For fuel cycle analyses, gasoline often serves as the baseline fuel to which other fuels are compared. Because of emission concerns, requirements for gasoline quality are in place in many countries to help reduce motor vehicle emissions. In the United States, so-called reformulated gasoline, compared to the gasoline used in the 1970s, has much better quality in terms of emission reduction potential. Oxygen additives such as methyl tert-butyl ether (MTBE) or ethanol are often mixed with gasoline. To control emissions of nitrogen oxides (NOx) and particulate matter of size smaller than 10µm (PM10) with after-combustion treatments for diesel vehicles, regulations are in place or are being developed to maintain the sulfur content of diesel fuels at a very low level. The demand for both low-sulfur gasoline and low-sulfur diesel requires significant desulfurization efforts in petroleum refineries (Romm, 2004).

Liquefied petroleum gas, primarily propane, is produced in either petroleum refineries or natural gas (NG) processing plants. In the United States, about 40-50% of LPG is produced in petroleum refineries and the remaining 50-60% is produced in NG processing plants. Natural gas can be delivered to vehicle refueling stations, where it is compressed to above 3600 psi for storage on CNG vehicles, or it can be liquefied in central liquefaction plants and transported to refueling stations for LNG ...