Efficiency Impact Associated with a change of fuel on a steam boiler p 2

[flyingscot]

I am trying to get a handle on the typical impact of changing from HFO firing to gas firing for a steam boiler plant. I have heard arguements that thermal efficiency will drop (by between 2 and 4%) as a result of increased latent heat losses in the fuel gas due to the higher hydrogen content in gas. This argument seems a little simplistic to me as a multitude of other factors may act to minimise this impact or even result in a small efficiency improvement. Does anyone have any good insights/experience???? What is the likely impact of a fuel change on efficiency and why?

Thanks

 

[Jonkatz]

THere are a number of variables that DIRECTLY affect the FUEL efficiency of a steam boiler so its hard to compare simplisticly Oil vs Gas only, depending on the specific composition of an Oil vs the fuel gas, the efficiency could be lower/higher in one vs the other... But they shouldn't vary by more than +/- 1% even if there is alot of hydrogen in the fuel GAS.
FOR EXAMPLE: In one typical case -All other variables being equal:
#2 Fuel Oil is about 0.5% more efficient than 100% Methane gas

However: Fuel Oil firing is USUALLY less efficient due to other factors: specifically, atomizing medium decreases efficiency slightly, and also, higher levels of excess air are usually required for good combustion, which reduces efficiency.
Hope this help you
J. Katz

 

[jmw]

J Katz advises that Fuel oil firing is usually less efficient due to other factors.
When using heavy fuel oils for burners this is often the case because efficient combustion depends on optimising the atomisation of the fuel. This is dependent on the kinematic viscosity. It is usual to heat the oil in order to control the viscosity. Ideally this is done by measuring the kinematic viscosity and using this measurement to control the fuel oil heater. This is very successfully done with HFOs used for Large diesel engines where the grade of oil is usually comparatively light (typically from 250 to 750cst); and because engines don't respond well to catalytic finings etc, the oil is centrifuged prior to the final stage heater. In burner applications the fuel oil can be significantly more viscous and less clean (e.g. bunker C) nor, despite the lower cleanliness,is it cleaned before the burners. This means that few visometers have been successful and most burner operation uses temperature measurement for heater control. The assumption being that the temperature is a good indicator of viscosity. Unfortunately oil quality variation means that this is not a good assumption and it is usual to augment simple temperature control with a combination of strategies including flame inspection, laboratory analysis and excess oxygen control. In a petrochemical plant burning commercial grades of HFO, for example, excess oxugen might be around 4.5% and in plant burning tallow as much as 6 or 7%. The excess cooling this causes can add significantly to the costs.
Fortunately new viscosity measurement technologies are available which provide virtually maintenance free viscometers which measure a variety of parameters including the density, base density, dynamic and kinematic viscosity and Ignition index.
See press releases on

http://www.solartronmobrey.com

This means that efficiency can be improved and this may affect the choice to swicth from heavy fuel oil to natural gas, especially as supply and demand problems has seen some some consumers see significant rises in gas prices.

 

[way]

At several of our power plants, we burn #6 Fuel Oil and Natural Gas. I have seen our Heat Rate improve by 4% when we are burning #6 Fuel Oil. Unfortunately, I have no idea why.

That is about all of my experience on the issue.
Way

 

[jmw]

http://www.steamesteem.com/S

a good referance site for boilers and related questions is

http://www.steamesteem.com/

 

[op9]

Yes the theory about the higher hydrogen content in nat gas will produce more water and hence the heat needed to evaporate it reduces the available heat is true in practice,if you compare "apples with apples".

However as Jonkatz points out excess air is generally left higher with fuel oils which could negate this advantage. Heavy oil burners are capable of running with lower excess air, but because they are more likely to go rich and produce soot, an extra margin is generally left or their combustion must be monitored more closely.

Other factors to consider (for and against):
1) The nat gas flame is far less radiant which means that less heat transfer takes place in the furnace so the gas exiting the furnace is hotter than in the oil fired version. The convection passes then must be able to compensate, but often more heat is lost up the stack. This fact needs special attention on firetube boilers where the higher temps entering the first tube pass can result in metal overheating and eventual cracks in tube and plate, especially if there is scale on the waterside. On old boilers a derate is prudent when converting to NG.
2) The radiance of the oil flame is actually glowing carbon, which must be completely burnt. But there is always a tendency to deposit soot on the heat transfer surfaces (tubes)which reduces the heat transferred to the water and here again will raise flue temps. Obviously regular soot blowing will keep this in check. But you're using steam for this.
3) The burner on nat gas will usually have far better turndown than on heavy oil. This may be of advantage,depending on your type of operation, as the burner will fire longer instead of cycling ON/OFF if the cutout pressure is exceeded. Fuel efficiency on NG will be better in this case.
4) Assuming you are using steam atomising for the heavy oil, this is not needed for nat gas. So a steam saving.
5) Nat gas is actually quite slow burning and if furnace conditions with regard to dimensions are not suited complete burning at furnace exit may not be complete and excessive incomplete emissions (CO) will be produced. This is also true with too much combustion air chilling the flame. Generally however the production of excess CO is an enviromental consideration as the efficiency gained in reducing it will probably not be practically noticeable.

Hope this helps. Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au

http://home.iprimus.com.au/nissenr

 

What are the advantages and disadvantages of uaing marine gas oil as compared to marine diesel oil in a medium speed diesel engine?

 

[helphelp1]

the eff. firing gas has three varibles going for it which COULD make it more eff. These are:
1. Less excess air required, therefor less heat loss due to excess air.

2.Lower flue gas temperatures can be obtained as you do not have to worry about acid dew point in the stack gas.

3.Better heat transfer in the heating surfaces due to cleaning burning fuel.

You also, have to look beyond the boiler, whats the impact upon station aux. power and heat rate.

 

[sdduck]

I have done natural gas & oil boiler performance calculations for a utility sized unit when I was working for a major boiler vendor. They showed that the efficiency on natural gas was 85.6% vs an oil efficiency of 89.7%, even though the natural gas excess air was 5% vs 10% for oil. Identical boiler with the same heat output for both cases.

 

[athomas236]

Hello,

Even in the UK we get the same order of difference in efficiencies between gas and oil firing as mentioned by sdduck.

regards

athomas236