CNG VERSUS GASOLINE
S. K. ANSARI (CHEMICAL ENGINEER)
SEEMA ANSARI (TELECOM ENGINEER)
Feb 8 - 14, 2010
Compressed natural gas (CNG) is one of the fuels that have undergone several researches and studies before being proposed as an ideal replacement of conventional fuels. The use of CNG fuel has proven potential to reduce emissions. CNG has a Research Octane Number (RON) is approximately 130, which allows an engine to operate at a high compression ratio.
CNG also has an excellent lean flammability limit allowing for lean burning operation, which reduces carbon monoxide, nitrogen oxide emissions.
However, the main problem with CNG engine is the lack of acceleration. The CNG engine or dual-fueled engine has low performance as compared to gasoline engine.
One significant factor that reduces the CNG engine performance is its low volumetric efficiency due to low density of the CNG fuel which is 8 lb/ft3 or 1.07 lb/gallon.
PROPERTIES OF CNG AND OTHER FUELS
Properties Gasoline Diesel LPG CNG Physical State Liquid Liquid Gas Gas Boiling Range (∞F @ 1 atm) 80 to 420 320 to 720 -44 to 31 -256 Density (lb/ft3) 43 to 49 49 to 55 31 8 (lb/gal) 5.8 to 6.5 6.5 to 7.3 4.1 1.07 Net Energy Content Btu/lb 18,700 - 19,100 18,900 19,800 21,300 Btu/Gallon 112,000 - 121,000 123,000 - 128,000 82,000 22,800 Auto-ignition Temperature (∞F) 450 - 900 400 - 500 920 - 1,020 1,350 Flash Point (∞F) -45 125(min) 100 - 150 -300 Octane Number Range (R+M)/2 87 to 93 N/A 104 120 Flammability Limits L = 1.4 L = 0.7 L = 2.4 L = 5.3 (Volume % in air) H = 7.6 H = 5.0 H = 9.6 H = 14 Human Exposure Limit for Fuel (ppm) 500 N/A N/A Non-Toxic
EQUIVALENCY OF CNG VERSUS GASOLINE
- 1 liter of gasoline contains 30,115.70 Btu/liter.
- 1 liter of gasoline is equivalent to 33.46 ft3/liter of CNG at 200 bar.
- Heating value of NG is equivalent to 900 Btu/ft3.
- Therefore 33.46 ft3/liter x 900 Btu/ft3 of NG = 30,114 Btu/liter.
- This mean that 33.46 ft3 of NG is equivalent to 1 liter of gasoline.
- Density of CNG is 8 lb/ft3.
Density of gasoline is (43 to 49) lb/ft3, depending upon which types of additives are added. In Pakistan PSO adds various additives as compared to Caltex.
Boyle's Law mathematical calculation and experimental results show that 1 US gallon gasoline (0.833 imperial gallon, 3.79 liter) is equivalent to 2.42 kg of CNG. A normal vehicle usually covers 8 ñ 10 Km/liter of gasoline.
CNG cylinder weight and its capacities:
- 40 kg cylinder 6 kg CNG.
- 50 kg cylinder 8 kg CNG.
- 55 kg cylinder 10 kg CNG.
- 60 kg cylinder 12 kg CNG.
The price of CNG (current) is Rs. 53.63/kg. The price was increased 20% from Rs.48.54/kg last month. The average mileage from 1 kg of CNG is approximately 9 km with A/C. Cost of 1 liter of gasoline is Rs.66. The difference between CNG and gasoline prices is 18%.
Cost of CNG kit is maximum Rs.50,000. Depreciation of tenure for the kit is 5 years. The cylinder has to be replaced according to the guidelines of Hydrocarbon Development Institute of Pakistan. Depreciation per year is Rs.10,000 or Rs 28/per day. Usually, tank is filled once in a day. Cost of the kit per day is approximately Rs.140.
The overall cost of CNG fuel becomes Rs536.30 (CNG fuel cost when filled once) plus Rs.2.80 (Dep./day cost) and Rs.140.0 (kit cost per day). Rs. 679 cost is for 10 kg of fuel.
For 1 kg of CNG fuel, cost will be Rs.67.9 or approximately Rs.68/kg. (Wear and tear for the engine is not included otherwise the cost will further increase). CNG is expensive than gasoline in the current scenario by 14%.
CNG is only beneficial if the overall beneficial difference is 50% as compared to gasoline.
BENEFIT OF CNG
Compressed Natural Gas is a substitute of gasoline (petrol) or diesel. It is considered an environmentally clean alternative. It is made by compressing methane (CH4) extracted from natural gas. Argentina and Brazil are the two countries with the largest fleets of CNG vehicles. Pakistan is the largest user of CNG in Asia and the third largest in the world.
The CNG sector of Pakistan attracted over Rs 70 billion investments during the last few years as a result of liberal and encouraging policies of the government. Presently, more than 2,700 CNG stations are operating in the country in 85 cities and towns, and 1000 more would be setup in the next three years. The sector provides employment to above 30,000 people in Pakistan.
After the recent increase in the CNG price most of the CNG users are thinking of gasoline as right choice.
When it comes to transportation, one thing has become increasingly clear over the past decade: The world can no longer depend on oil as single energy source.
Oil reserves are simply running out. Demand for oil in both developed and developing countries is rising significantly, and by 2020 the world could face an energy crisis. Even more disturbing is the well-publicized environmental damage caused by emissions of diesel- and gasoline-run vehicles.
NATURAL GAS RESERVES IN PAKISTAN
Pakistan has limited natural gas reserves. At present, recoverable natural gas reserves in Pakistan are 32 Trillion Cubic Feet (TCF) and reserve to production (R/P) ratio based on current production of 3.7 Billion Cubic Feet per Day (BCFD) is 23 years. One of the significant developments in local gas market is the increase of the natural gas share in primary energy supply mix from about 40% in 1990-2000 to over 52% in 2004-05 in about five years. If the demand /supply trend of past five years is maintained, the R/P (reserve to production) ratio reduces significantly necessitating urgent need to increase natural gas supply. The government has accordingly placed the highest priority to (a) enhance indigenous natural gas supplies through intensifying exploration efforts and (b) tap other types of energy like DME from methanol (c) expedite its efforts to gasify coal deposits.