Optimizing Combustion Control with the TDLS8000

Introduction

Industrial Combustion sources such as thermal cracking furnaces and, process heaters play a critical role in the process industry. Because the large amount of fuel such as gas or fuel oil which is consumed in these processes, their combustion efficiency directly affects the performance and operational costs of the production facilities.
Incomplete combustion and the use of too much excess air can lead to higher level of toxic emissions such as CO (carbon monoxide), CO₂(carbon dioxide), NOx (nitrogen dioxide), and SO₂ (sulfur dioxide).

 

Challenges

Combustion requires fuel and air (oxygen), and insufficient air causes fuel residue, resulting in incomplete combustion with soot and smoke. On the other hand, excessive air caused problems, such as a larger amount of exhaust gas and heating of excessive air, resulting in lower fuel efficiency. Figure 1 shows the principle of the air-fuel ratio and state of combustion. The air-fuel ratio plotted on the horizontal axis shows the ratio of actual supply air to the theoretical amount of air required for fuel combustion (theoretical air amount).

For combustion furnaces such as heating furnaces and boilers in plants and factories, small-scale controllers such as single loop controllers are employed to optimize the air-fuel control ratio for improving the combustion efficiency. In large combustion furnaces, distributed control systems (DCS) and advanced control (multivariable predictive control, etc.) are used. These mainly control the air-fuel ratio and internal pressure of the furnace to prevent CO, CO₂ and NOx (nitrogen oxide) from being emitted and apply a cross limit circuit to prevent incomplete combustion while controlling combustion to maximize efficiency.

Figure 1 : Relationship Between Air-Fuel Ration and Heat Efficiency (Combustion)

 

Solution

To measure spectra, we employ our originally developed peak area method for accurate measurement even in process environments where the composition, pressure and temperature fluctuate simultaneously. The TDLS8000 suffers no component interference owing to its high spectrum measurement resolution and is directly installed in a process enabling the laser beam to travel through the process windows in a non-contact measurement as shown in Figure 3.

 

Measurement of O₂ and CO Concentrations in Combustion Gas by the TDLS8000

Figure 4 shows an example of using the TDLS8000 analyzer to measure O₂ and CO concentrations in a combustion furnace. This example shows that concurrent measurement of O₂ and CO concentration while gradually and manually reducing the amount of air supplied to the burner. This experimental data describes the situation; CO generation begun nearly at the O₂ concentrations of 2% and sharply increased at the concentration of almost 1.5%, resulting in incomplete combustion, then by increasing the amount of air supply to avoid incomplete combustion, the O₂ concentration rapidly increased and the CO concentration decreased again, resulting back to complete combustion. This data also indicated that the CO concentration increased from 100 ppm to 4,000 ppm in just a few minutes.

Figure 4 : O₂ and CO Concentration Changes in a Combustion Furnace at Low Oxygen Concentration Operation

 

Air-Fuel Ratio Control Utilizing CO and O₂ Concentrations

According to Lyman F. Gilbert, the CO concentration in the optimum combustion zone (having the highest heat efficiency per unit amount of fuel) is around 200 ppm irrespective of fuel types and devices. However, CO concentration increases rapidly once it has broken through as shown in Figure 4. Thus, either a stable combustion must be kept with sufficient supply of air or a control system must monitor the CO concentration in real-time and keep it constant at a relatively low level.

 

Estimation of Economic Effects by Air-Fuel Ratio Improvement

Because the fuel required for generating the same amount of energy can be saved by lowering the set value of O₂ concentration in exhaust gas when performing air-fuel ratio control, the direct economic effect by the decrease of O₂ concentration in exhaust gas can be estimated. As the economic effects by the improvement, reduction in fuel cost which can be achieved by reducing the excessive O₂ while keeping the furnace temperature constant is calculated. Table 1 summarizes a calculation example.¹

Table 1 : Example of Economic Effect Calculation by Reducing Excessive Oxygen

*1: Assume to be constant, though it varies depending on the temperature, CO₂ concentrations, etc.
*2: (m-l)*A0 is air volume not used for combustion

This example shows a trial calculation for 0.5% reduction of excessive oxygen in the heating furnace of a petroleum normal pressure distillation apparatus(topper) which process oil of 100,000 barrels per day. The result indicates a fuel saving of about 240 kiloliters, worth $113,000 per year assuming a price of $470 per one kiloliter fuel oil.

Furthermore, by reducing the amount of fuel used, CO₂ emissions can be reduced. The carbon emission per one kiloliter of fuel is about three tons, so CO₂ emissions are reduced by 720 tons per year.

 

Safety Control by the TDLS8000

A burner management system (BMS) safely controls the burner of the combustion furnace and includes an interlock mechanism and a safety shut-off mechanism to prevent explosion. The BMS must comply with safety standards based on risk assessment such as the international standard (ISO12100) and the EU, USA and Japanese standards (EU standard: EN746, USQ standard: NFPA86, Japanese standard: JIS B9700).

In addition to the measurement of CO the TDLS8000 can accurately measure CH₄ (Methane) levels providing an extra level of safety during start up where a flame out or a failed burner ignition can cause an explosion.

 

Conclusion

The TDLS8000 is attracting considerable attention from many refinery and petrochemical plants because it requires little maintenance and can measure the concentrations of O₂/CO/ CH₄ in the furnace in near real-time.

The technology for combustion efficiency optimization, by measuring O₂ can CO concentrations in this application note was established more than twenty years ago.

The TDLS8000 provides a near real time measurement of O₂/CO/CH₄ in the radiant section of a large scale combustion furnaces and process heaters allowing process owners an unprecedented opportunity to optimize combustion.

 

Product Recommendations

TDLS 200 (O₂/CO/CH₄ analyzer)
Note: Various options are available please consult your local Yokogawa Sales Office for more information.

Notes

• Response time: 2-5 seconds
• Process pressure up to 20 bar
• Interference Free
• Process temperature up to 1500ºC
• TruePeak Measurement
• Optical Measurement- No sensor contact with process

References:

1. Tsuneo, Hiraoka, “Reduction of Fuel Costs and CO₂ Control for Packaged Boilers,” Yokogawa Technical Report, Vol. 44, No. 2, 2000, pp. 85-86 in Japanese
2. American Petroleum Institute, “Instrumentation, Control & Protective Systems for Gas Fired Heaters”, API 556 2nd edition, April, 2011.