Volume 22, number 1, march 2008

Natural gas: more efficient and cleaner than heavy fuel oil

Gaz Métro, with the assistance of the Natural Gas Technologies Centre (NGTC), recently produced a study showing that boiler rooms using natural gas are 2.91% more efficient on the average than when they burn heavy fuel oil. This study was per-formed in pulp and paper mills and
aluminium smelters. The results obtained are also valid for all industries using heavy fuel oil.

Often the market’s perception is that heavy fuel oil is more efficient than natural gas. However, to determine the boiler room overall efficiency, the losses related to the use of each fuel must be subtracted. Thorough analyses were produced of 15 boilers with six industrial users. All factors were scrutinized in depth.

For each boiler, the rated efficiency was calculated or measured first. This is the
useful energy produced over a representative reference period compared to the energy entering the boiler. In some boiler rooms, combustion tests at different steam production regimes served to
establish the boiler rated efficiency.

Results of the boiler efficiency study

    Fuel oil Natural gas
1 Stack 14.56% 17.44%
2 Radiation 1.76% 1.76%
3 Purge 1.03% 1.03%
4 Tank heating 0.17% 0.00%
5 Preheating before the burner 0.04% 0.00%
6 Atomization 1.57% 0.32%
7 Soot blowing 0.12% 0.00%
8 Average clogging 1.79% 0.00%
9 Pumping 0.20% 0.00%
10 Additional make-up water + chemicals 0.37% 0.00%
11 Additives 0.15% 0.00%
12 Corrosion 1.69% 0.00%
  Total losses 23.46% 20.56%
  Overall efficiency 76.54% 79.44%
  Overall gain in favour of natural gas   2.91%

Common losses

Certain losses are common to both natural gas and heavy fuel oil. For both fuels, these are stack, radiation and boiler water purge losses.

1. Stack

Combustion products involve a large amount of energy and these losses are the greatest for both energy sources (heavy fuel oil and natural gas).

2. Radiation

Radiation losses are caused by the heat released from the boiler walls. To establish this value, graphs available from the American Boiler Manufacturers Association (ABMA) show that the losses depend on the boiler capacity, the average production and the number of walls cooled with
boiler water.

3. Purges

Although boiler water usually requires softening and treatment, purges are necessary to prevent an increase in the concentration of products dissolved in water. The purges avoid the accumulation of deposits on the tubes and guarantee a good heat transfer. They also help avoid water overflows and ensure a good quantity of steam.

Additional losses attributable to heavy fuel oil

Heavy fuel oil must be treated, heated and pumped before its use in the boilers and they must also be cleaned regularly. Despite cleaning, deposits accumulate on the inner surfaces and the equipment is subject to corrosion. Losses due to these factors most be considered in the overall evaluation of boiler rooms efficiency. Here are the details for each of the above-mentioned items.

4. Tank heating

Heavy fuel oil is viscous and usually requires heating to 60°C, bringing it to a temperature that facilitates pumping. Heating coils located inside the tanks require a large quantity of steam to rise and maintain the required temperature.

5. Preheating before the burner

Heavy oil is preheated to 104°C with steam. However, 80% of the energy used in this step is returned to the boiler by the fuel oil. Nonetheless, at least 20% of the energy is lost in the transfer system.

6. Atomization

For more complete combustion, heavy fuel oil must be broken into small particles by steam injection in the burner. Although this step is not at all required when natural gas is used in the boiler, one site injected steam to lower the nitrogen oxide emissions. This was not mandatory to meet Quebec’s requirements.

7. Soot blowing

Despite steps 4, 5 and 6 described above, soot is deposited on the boiler tubes. To remove it, the cleaning process uses steam, which creates a blowing effect on the tubes. The boilers are equipped with several blowers, which are activated during the day. Losses ensue from this action.

8. Average annual smoke side clogging

The step described above, even if it is performed regularly, cannot ensure a thorough cleaning of all of the boiler inner surfaces. The accumulated soot reduces the heat transfer to the fluid to be heated and affects overall efficiency.

9. Heavy fuel oil pumping

The use of pumps is required to make the heavy fuel oil flow to the burner.

The ratio of the costs of using electric motors to the supply cost of heavy fuel oil allows a loss to be established in terms of efficiency related to this item.

10. Additional make-up water and chemicals

An additional volume of water is necessary to operate with heavy fuel oil. The steam consumed for soot blowing, atomization, heating and preheating must be replaced with make-up water. This water is pretreated with chemicals, which end up in the sewer. Additional costs are associated with this step, which translate into a loss of efficiency calculated according to the supply cost of heavy fuel oil.

11. Additives

Additives may be added to heavy fuel oil to reduce clogging in the tank.

12. Corrosion

The sulphur contained in heavy oil causes corrosion on the cold walls and reduces
the boiler’s useful life.

An annualized replacement cost is calcu-lated and divided by the annual heavy fuel oil purchasing budget. This becomes the loss due to premature corrosion.

In conclusion, this study clearly shows that natural gas is more efficient and cleaner than heavy fuel oil, representing a 2.91% difference for the 15 boilers analyzed recently. The environmental point of view1 should not be neglected, because, based on the operation of these 15 boilers, the reduction of atmospheric emissions is very significant; 25,5240 tonnes/year, or 32.7%, of greenhouse gases and almost complete elimination of sulphuric oxide.

Guy Desrosiers, Eng., CEM
Technical Advisor

1. For more details on the environmental aspects of the use of natural gas compared to heavy fuel oil, see the article Natural gas-fired industrial boilers published in informa-TECH Volume 20, Number 2 of September 2006.