Analyze Flue Gas for Proper Air/Fuel Ratio

A proper air/fuel gas ratio will result an optimal performance out of the boiler. Also, a proper ratio can prevent the boiler from producing excessive sood and emission gases that can cause a serious accient. Monitor the air/fuel ratio and adjust to the proper portion to achieve the best performance out of the boiler. The recommended percentage of oxygen is at 3.0% with a corresponding of 15% of excess air. It is also recommended to initiate maintenance program that use the flue gas analyzer once a week for inspection and adjustment of the boiler.

Case study:
It is assume that the boiler in this plant is CB Boiler with 85 HP. It can produce steam at a rate of 2,316 lbs/hr at 15 psig with 213 °F. It is estimated that the input gas for this boiler is at 3.5 MMBtu/hr. The flue gas analyzer is used to moitor the current ratio.From the flue gas analysis at the boiler, the oxygen is at 10.1%, the temperature of 270 °F, the excess air of 82.7% and efficiency at 81.3%. The reading reveals a potential to reduce excess air that reflected unnecessary energy consumption. The optimum amount of O2 in the flue gas fired boiler is 2.0% with 10% of excess air. But to maintain the optimum level is difficult for most of the boiler. The recommended excess air is at 15%.From the Boiler efficiency table below, 15% of excess air is corresponding to the percentage of oxygen at 3.0%. The efficiency with the proposed recommendation is at 83.5%. For the Mohawk boiler with 85 HP, steam output at 2,316 lbs/hr and pressure at 15 psig, the fuel consumption is approximately 3.5 MMBtu. Therefore, the energy savings, ES, can be calculated by:

ES = IG x H x ( 1 – (E1/E2))

IG = Input gas of the boiler MMBtu/year
H = Hours of operation per year
E1 = Current boiler combustion efficiency
E2 = Propose boiler combustion efficiency

ES = 3.5 x (24 x 6 x 52) x ( 1 – (0.813/0.835)) = 690.51 MMBtu/year

Boiler Efficiency Table:

Energy Cost Savings:
Average utility cost of gas, Ug, in this plant is at $7.44/MMBtu. The energy cost savings of this boiler are:

ECS = ES x Ug

ECS = 690.51 x 7.44 = $5,137.39/year

Implementation Cost and Payback:
The flue gas analyzer is recommended for the plant to have for monitoring the oxygen, excess air, temperature and efficiency of the boiler in order to adjust the fuel/air ratio with the right portion. It is also recommended to perform the analysis once a week. The cost of the flue gas analyzer is $1750. The recommended model is Testo 330-1 combustion gas analyzer kit with printer. The labor cost associated with this monitoring is estimated at $520 where it requires 24 hours/year, where each session will take 30 minutes of inspection and adjustment, and $20/hour for the wage. The total cost of this implementation is $2,270. The energy cost savings will pay back the cost within 5.3 months.

Adjust Air/Fuel Ratio Calculator

To properly calculating the air/fuel ratio, a flue gas analyzer is needed to read some of the required items below:

Size of your boiler: /MMBtu

Stack temperature from the flue gas analyzer: °F

Current boiler efficiency from the flue gas analyzer: %

Hours of operation of the boiler per year: Hours

Natural gas cost: $ /MMBtu

Labor cost of adjusting the fuel/gas ratio in-house: $ /Hour

Energy Savings Summary

It is recommended that the optimum level of setting this boiler is at 15% of excess air and 3% of Oxygen. The flue gas analyzer can be purchased and in-house adjustment can be conducted at lower cost comparing to contracting the maintenace agents to do it each month. The propose efficiency of your boiler is %. Therefore, the savings are:

Natural gas savings: $ /Year

Implementation Cost included with a cost of flue gas analyzer and a labor cost to inspect and adjust the boiler every week: $

Simple Payback Period: Years