Properties of Natural Gas

Transcription

1 Properties of Natural Gas Topics Covered: Physical Properties Physical State Color, Taste and Odour Composition Specific Gravity

2 Properties of Natural Gas Topics Covered: Chemical Properties Required Air/Gas Ratio Heating Value or Heat Content Flame Temperature and Flame Speed Ignition Temperature Limits of Flammability Combustion Process

3 Physical State of Natural Gas Natural gas has a boiling point of -258 F(-161 C) Therefore natural gas, in its application and transmission will always be in a gaseous state. In measuring gases volume is measured in ft3 or cf and gas flow rates in ft3/hr or cfh imperial units. Liquid volume is measured in gallons and liquid flow rates in gallons per hour (gph) imperial units.

4 Physical State of Natural Gas In the metric system gas volume is measured in cubic metres (m3) and gas flow rates in cubic metres per hour (m3/h) Liquid volume is measured in litres, and liquid flow rates in litres per hour (L/h)

5 Color, Taste and Odour Natural gas in its pure form is an odourless, colourless and tasteless gas. A safety precaution is that natural gas must be detectable by smell. To achieve this, the local gas company adds an odourant to the natural gas called Mercaptan. The amount added is small, one lbs per million cubic feet of gas. This gives it the rotten egg smell.

6 Composition The composition of natural gas can vary from region to region throughout North America. The amount of variance is very small since largest component of natural gas is methane. Methane composes 85% - 90% of the total substance of natural gas.

7 Composition Methane is a simple hydrocarbon, a substance consisting of carbon and hydrogen. Methane is composed of one carbon atom linked with four hydrogen atoms. Methane gas is a very light fuel has, a fuel gas is a gas used to supply heat energy. As the number of carbon and hydrogen atoms in fuel gas is increased, the gas becomes progressively heavier.

8 Composition Heavier fuel gas has an increased heating value as a heat fuel source. Natural gas as a fuel gas will contain methane and heavier hydrocarbons such as ethane, propane and butane. Due to natural gas being 85-90% methane, the properties of natural gas are usually regarded as the same properties as methane.

9 Typical Natural Composition Analysis Methane 89.68% Ethane 4.68% Propane 1.73% Butane 0.67% Pentane 0.15% Nitrogen 2.89% Carbon Dioxide 0.20%.34 grains sulfur per 100 cubic feet of natural gas

10 Specific Gravity The specific gravity of natural gas is approximately 0.60 where air is equal to 1.0 This property of natural gas is an advantage in the handling as escaping natural gas will tend to rise and vent itself since its lighter than air. A heavier fuel gas like propane tends to sink and remain in pockets at ground level, creating a possible hazard if points of ignition are present.

11 Specific Gravity Also the mixing of the natural gas with air in the burner system can be easily accomplished to establish the correct flame characteristics. Flame characteristics are the color, shape and intensity of the flame, which indicates the quality of the combustion process taking place. Standard conditions in the gas industry are psia and 60 F or kpa and 15 C

12 Toxicity Natural gas in nontoxic, there is nothing in the composition of natural gas that is injurious to health in any way. This is in contrast to manufactured or coal gas, which contains carbon monoxide, a very toxic and poisonous substance.

13 Required Air/Gas Ratio The required air/gas ratio is the cubic feet or cubic metres of air required to burn one cubic foot or cubic metre of natural gas completely without having any excess air in the combustion products. The required air/gas ratio is sometimes called the theoretical air. The required air/gas ratio for natural gas is 10/1

14 The Yellow Flame A yellow flame is a flame where there is no premixing of the natural gas with the required air for combustion. All the required air for the combustion process is supplied around the flame as it burns at the burner port. The flame will have a slightly blue color zone at the burner port followed by a larger bright area.

15 The Yellow Flame The brighter portion of the flame is the luminous and yellow in color. The blue portion of the flame is where the hydrogen is burning at a greater speed than the carbon that burns at a lower speed in the yellow portion of the flame. If this flame makes contact with a cold surface, sooting will take place, this is called impingement.

16 The Blue Flame The blue flame is quite different from the yellow flame. It is blue in color, small, non-luminous and has a higher temperature than the yellow flame. The blue flame is a result of some premixing of the fuel with the required air for combustion. The air required for the combustion process can be broken down into two air supplies, primary air and secondary air.

17 The Blue Flame Primary air, is air mixed with the fuel prior to its ignition. Secondary air, is air mixed with fuel around the flame as in the yellow flame. 1 cubit foot of air will release 100 Btu of heat from any fuel source. 1 cubic meter of air will release MJ of heat.

18 The Blue Flame The initial combustion takes place on the surface of the inner flame cone as the oxygen from the primary air combines with the gas. The body of the inner cone is composed of unburned fuel and primary air. Partial combustion takes place in the bright blue area resulting in the release of intermediate products of combustion.

19 The Blue Flame In the outer cone, oxygen in the secondary air is mixed with the intermediate products of combustion. This gives the final and complete combustion that takes place on the rim of the outer cone. Complete combustion is achieved by the presence of secondary air.

20 The Blue Flame For the blue flame the outer cone can strike or impinge a cold surface without affecting the combustion as in the yellow flame. However the inner cone cannot make contact with any cold surface, since incomplete combustion will occur, giving carbon monoxide and aldehydes.

21 The Blue Flame Carbon monoxide is a colorless, odourless extremely toxic gas. Aldehydes have a sharp and penetrating odour, it is also toxic and irritates the eyes, nose and throat. Carbon Monoxide poisoning is a common cause of death in gas appliance related accidents.

22 Excess Air The required air supply is the sum of all primary and secondary air needed to perfectly complete the combustion of the fuel. In practice, additional air must be supplied to ensure the complete combustion of the fuel. This air supply above the ideal or theoretical amount is called Excess Air.

23 Composition of Air Oxygen in air supports the combustion process of the gas, since air consists of oxygen and nitrogen, not all air is involved in the combustion process. Nitrogen is an inert gas and does not contribute in the combustion process, air is composed of 20% oxygen and 80% nitrogen.

24 Heating Value or Heat Content The heating value of a fuel is the amount of energy released by a measured amount of the fuel during the combustion process. The heat energy released is measured in BTU s. 1 Btu raises 1 lb. of water 1 degree Fahrenheit Heating value for natural gas is approximately 1000 Btu/ft3 or MJ/m3

25 Heating Value or Heat Content The gas consumption of an appliance is rated on an hourly basis, in terms of flow rate and heat rate or input rate. The input rate is the amount of heat energy generated by the appliance as it burns the fuel during each hour of its operation. Input Rate = Heating Value of Gas Gas Flow Rate

26 Flame Temperature The flame temperature for natural gas is approximately 3500 F or 1954 C The maximum obtainable flame temperature can only be achieved at perfect combustion where excess air or fuel will not absorb combustion heat. The highest temperature occurs at a point just above the outer flame cone.

27 Flame Speed The flame speed is the speed at which the flame front moves towards the air/gas mixture issuing from burner port. The flame front is the dividing line between the air/gas mixture and the combustion products. In a stable flame the velocity of the air/gas mixture exiting from the burner port is equal to the burning velocity of the flame.

28 Flame Speed Flame speed depends on the type of gas and the amount of air in the air/gas mixture. Increasing the primary air with constant burner input will increase the flame speed, sharpening the flame and the inner cone height decreases. The flame speed for natural gas is approximately 1.0 ft/sec or m/s

29 Flame Speed Fuel gases that contain high amounts of hydrogen, have a relatively high flame speed. The higher flame speed allows the burner to achieve a high degree of flame stability. Excess flame speed due to excessive primary air usually causes the burner flame to lift off the burner head face.

30 Ignition Temperature Ignition temperature is the temperature at which the combustible mixture of natural gas and air will initiate and maintain the combustion reaction. The ignition temperature for natural gas is approximately 1200 F or 649 C

31 Limits of Flammability The limits of flammability are the upper and lower percentage of gas in the air/gas mixture that will support combustion. The air/gas mixture can be too fuel lean or too fuel rich to support combustion. Lower Limit for Natural Gas = 4% Upper Limit for Natural Gas = 14%

32 Limits of Flammability The upper and lower limits of flammability is the fuel s range of combustibility, in relationship to the percentage of fuel in the air/gas mixture that will support combustion. When the combustion of the fuel is not controlled within the confines of a burner system, the limits of flammability can be called the Explosive Limits. There are the Lower and Higher Explosive Limits.

33 Limits of Flammability L.E.L. = Lower Explosive Limits U.E.L. = Upper Explosive Limits A portable explosive meter detects the lower explosive limits of a fuel gas. The principle operation is based on the Catalytic Combustion. The filament consists of a catalytic material that will initiate the combustion of the mixture at a low temperature.

34 The amount of current flow across the bridge is a function of the change in filament resistance that is affected by the heat released in catalytic combustion. Limits of Flammability The filament, upon the heat of combustion, will change its electrical resistance. As the filament resistance varies, the balanced wheatstone bridge will now be out of balance, and a current will flow across the bridge.

35 Limits of Flammability The meter is calibrated in % of gas in the mixture. Pointer rises slowly and stops between 0 and 100, the sample is below the L.E.L. Pointer moves to the extreme right and stays there the sample is between 4% and 14% Pointer moves rapidly and returns within scale the sample is above the H.E.L.

36 Combustion Combustion is the rapid combination of a fuel with oxygen resulting in the release of heat. To achieve the combustion or burning of a fuel, three components of the combustion process must be present as seen in the combustion triangle. Remove any one part of the combustion triangle and combustion can not be sustained.

37 Combustion Triangle Heat

38 Combustion Natural gas is a hydrocarbon consisting of carbon and hydrogen in some combination. The combustion of natural gas can be considered as the combination of carbon and hydrogen. C + O 2 = CC 2 + HHHH The combustion of the carbon produces carbon dioxide and heat.

39 Combustion OO = 22 2 O + HHHH The combustion of the hydrogen produces water in vapor form and heat. Combining both carbon and hydrogen as in natural gas, the combustion reaction would be. CC OO = CC H 2 O + HHHH

40 Combustion The products of Combustion natural gas and oxygen are carbon dioxide, water vapor and heat. The heat generated in the combustion process is called the Heat of Combustion and is determined by. Heat of Combustion = Heating value of gas fuel X gas flow rate.

41 Combustion However, it is only under certain applications that natural gas is burned using pure oxygen. The source of oxygen for combustion in domestic and commercial appliances is air. The composition of air is approximately 20% oxygen and 80% nitrogen, a ratio of 1 oxygen to 4 nitrogen. The combustion of natural gas in which air is the source of oxygen would be.

42 Combustion CC O N 2 = CC H 2 O + 8 N 2 + HHHH The products of natural gas and air are carbon dioxide, water vapor, nitrogen and heat. In the combustion of natural gas, where air is used the nitrogen does not enter the combustion reaction. Nitrogen is an inert gas and passes through absorbing heat, from the combustion reaction.

43 Properties of Gas Table Properties of Gas Natural Gas Propane Butane Formula Designation CH4 C3H8 C4H10 Toxicity Non Toxic Non Toxic Non Toxic Physical Properties Colorless, Tasteless, Odourless Colorless, Tasteless, Odourless Colorless, Tasteless, Odourless Odorant Mercaptan Mercaptan Mercaptan Specific Gravity Gas Specific Gravity Liquid Liquid to Vapor Ratio 600:1 270:1 235:1 Boiling Point -258 F -44 F 32 F Heat Content per ft3 1,000 Btu 2,520 Btu 3,260 Btu Limits of Flammability 4% L.E.L. 14% U.E.L. 2.4% L.E.L. 9.5% U.E.L. 1.9% L.E.L. 8.5% U.E.L. Max Flame Temp 3,400-3,550 F 3,600 F 3,625 F Max Flame Speed 25 per second 32 per second 33 per second Req d Air/Gas Ratio 10 ft3 air :1 ft3 gas 25 ft3 air : 1 ft3 gas 32 ft3 air : 1 ft3 gas