natural gas vs. fuel oil 2

[pabasolo]

I am trying to decide wheter I should use natural gas or fuel oil for our furnaces.
Here in Uruguay, the cost of equal heat in gas doubles that of fuel oil.
What inefficiencies, other costs, pros/cons should I also take into account?
thnks
pablo

 

[25362]

Pablo, pro and con factors are multiple, and at the end a reputable heater manufacturer may give you the basis for carrying out an economic and environmental analysis to decide on which fuel to base your selection. Probably on both.
Mixture must precede burning. Liquid fuels need good filtration before entering the heater and a viscosity lower than 20 cS for mehanical atomization. Steam atomization can handle oils of up to 60 cS. Oils must be atomized to very tiny droplets of 10-50 microns in size, with a huge increase in the surface-to-mass ratio, to enable their almost instantaneous (microseconds) vaporization. However, for burning to occur, there must be an ample supply of heat to the atomizer discharge.

For yet non-existing fired heaters, liquid fuels tend to need more xs air than gaseous fuels. This has to do with efficiency of contacting air and liquids for a chemical reaction (combustion) to take place over a short period of time, and the need to add reactants to push the reaction towards the desired products of combustion. This is only a tendency when using natural draft. Xs air acts as coolant or heat sink in the furnace. When there is too much xs air the flame-gas temperature falls, and more fuel sould be fired to compensate. However, the optimum amount of xs air may be finally determined by the heaters' design, and not by the type of fuel.

Liquid fuels also may contain water and mineral (ash-forming, fouling) matter that contribute less heat than the hydrocabons per unit weight, and may result in air pollution and in metallurgical and refractory attack, such as from V₂O₅. This oxide can even become a catalyst to convert SO₂ into SO₃ accelerating sulphuric acid corrosion.

Gas firing, on the other hand, needs careful PCV sizing, in particular when using variable-composition refinery fuel gas. What ash-content is to liquid fuel, the % of inerts and H₂S are to fuel gases. "Afterburning" effects due to "shortages" of air can lead to high heat fluxes in places where they are not wanted. Most start-up explosions have happened in furnaces heated with gas, when the heaters haven't been duely purged with air or steam.
Sometimes gas-fired heaters show low frequency vibrations and noise, because of resonance of the whole system: airbox, burner, radiant cell, flueducts.

Flame stability and impingement depend on many factors: type of burner, type of fuel, amount of fuel, type of atomization, flame position in the radiant zone, aerodynamic factors, size and number of burner throats relative to the volume of the radiation zone.

Pablo, como puede ver el asunto no es simple y requiere un estudio a fondo.

Good luck.

 

[pabasolo]

Thanks to all
You have been extremely helpful!
bye

 

[davefitz]

Usually the final decision on which fuel to burn is a financial decision; it is generally accepted that the present worth based on a life cycle cost analysis is representative of the value of all resources used over the life of the process. Modern methods of financial analysis may adjust the costs for "externalities", such as the cost to the environment for emmissions such as SO2, CO2, VOC's etc., and these are higher with oil than with natural gas.

The oil fired heater will require an oil storage tank, transfer pump system, atomizing air or steam system, and ignitor, flame scanner, etc., which may represetn higher initial costs than a natural gas burner system. The oil fired stack gas temperature must be higher ( about 125 C vs 93 C)than for a gas fired unit, which leads to a lower efficiency based on LHV.

The oil fired unit will have corrosion issues at the exhaust end, which may imply a more costly stack and exhaust duct and perhaps a stainless economizer in some cases.

Finally, in some areas of South America, oil supply may be more reliable than natural gas due to the risk that the gas line may suffer outages due to political issues, while a oil storage tank might be more secure in some cases.

Another possible option, at lower cost, is orimulsion, instead of oil.

 

[jmw]

Orimulsion is apparently no longer available to new contracts (a search for orimulsion in the Eng-Tips fora will bring you some good discussions here).

If you decide to go the oil route, the heavy fuel oil will require heating to reach the optimum viscosity and I would advise using viscometers to control the heater rather than temperature, as has been common before now due to a lack of suitable viscometers.

Today there are suitable viscometers. They have been used for all sorts of difficult fluids including heavy fuel oil in power stations and black liquor in paper mills (several good technologies for this).

There are some good bio-fuel alternatives emerging such as pitch fuel, derived from the fractions of crude tall oil. Crude tall oil is recovered in the production of sulphanate pulp. Though of lower effective thermal value than an equivalent mineral fuel oil, it is very low in sulphur.

As said above, in the end it comes down to the finances.
If the fuel prices are significant enough and the scale of operations supports it, then dual fuel becomes an option. Some power stations operate on coal or heavy fuel oil. Some engines run on gas or heavy fuel oil.

Some plant in Europe, and especialy in the Uk, was designed with gas as the exclusive fuel. As gas becomes depleted the costs have increased significantly i.e. within the planned life of the plant.

So there is no magic or universal answer.


JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[athomas236]

See thread 610-92660 for more infoemation on Orimulsion

athomas236