Safer and efficient combustion for fired heaters and boilers

A technological approach to achieving efficiency improvements, CO2 emissions reduction, and parallel effects in NOx emissions control in a crude oil-fired heater.

Author Francisco Rodríguez, Enrique Tova, Miguel A. Portilla, Miguel A. Delgado, Sixto López and Alfonso García-Marcos Suanzes

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Article Summary

Combustion optimisation offers the greatest potential for economic savings in industrial-fired heaters and boilers. This article presents the fundamentals and results of applying ABACO (Advanced Boiler Automation for Combustion Optimisation) technology in a 480 MMBtu/h fired heater. The improved controls provided by this technology resulted in increased efficiency, minimum pollutant emissions (CO2, CO, NOX, SOX, particles), and safer operation.

This technology relies on the closed-loop control of combustion based on monitoring in-furnace conditions and enhanced regulation capabilities. The sampling and direct measurement of actual gas composition at each burner result in a balanced firing pattern and overall optimisation of the combustion process. While preserving the unit from detrimental combustion conditions, maximum safety, reliability and process profitability are achieved. These optimised scenarios are durable, and the process is continuously surveyed to maintain sustainable operating conditions.

The effectiveness of this approach has been demonstrated in more than 150 testing programmes developed in over 75 industrial facilities worldwide, including 20 combustion units firing gaseous fuels and/or liquid fuels. The main results in refinery-fired heaters and boilers are:
- Improved unit combustion efficiency, resulting in fuel consumption savings of up to 7%. Equivalent CO2 and SOX emission reductions
- Simultaneous reduction in NOX emissions (t/h) up to 50% (by excess Oâ‚‚ decrease and burner tuning)
- Increased heater capacity
- Controlled combustion performance leading to higher unit safety, capacity, and availability
- Control of unburnt fuel and CO emissions
- Reduced preventive maintenance and corrective costs
- Increased fuel flexibility
- Efficient operation for variable duty or load scenarios.

The reduction in fuel consumption typically leads to combined fuel and CO2 savings of over 1 million USD/yr for unit duties around 400 MMBtu/h. These savings give rise to extremely short payback periods that can even be below one year.

Combustion tuning
Combustion is a relatively opaque process offering great potential for economic savings in industrial boilers and fired heaters. Despite the economic and environmental importance of the combustion process, there is usually a low level of monitoring and control. The process is typically governed by a few global variables, such as excess oxygen or process stream results, with no direct control of the combustion conditions.

Fired heater or boiler operation is typically supported by standard procedures rather than effective online information and optimised flame control. In most multi-burner applications, the standard monitoring used for controlling global excess oxygen in the combustion unit does not represent the real average in-furnace excess O2, which introduces a critical restriction for combustion tuning.

In recent years, a considerable amount of attention has been given to the application of combustion tuning for efficiency optimisation, operational safety, and reduction of pollutant emissions. However, the cost-effectiveness of these tunings is greatly limited by the referred restrictions on combustion monitoring and control. This situation is even more relevant in scenarios of high variability in fuel properties, load profiles, and/or burner arrangement for multi-burner systems. In these cases, uncontrolled combustion conditions might force operators to apply ‘too conservative’ operating settings, which are far from the optimum tuning.

To solve these limitations, a methodology and technical approach known as ABACO has been developed for combustion process optimisation in fired heaters and boilers with different designs and fuels (oil, gas, coal, pet-coke, and biomass).

Technical approach
Efficiency and emissions in industrial fired-heaters and boilers depend largely on the correct distribution of fuel and air supplied to the combustion process. Improper fuel/air ratios at critical locations are severely detrimental to these important parameters and the operational safety of the unit (see Figure 1).
The ABACO approach relies on closed-loop control of combustion based on in-furnace monitoring. This combustion optimisation technology makes possible the individual optimisation of any single burner, which results in an overall optimisation of the combustion process.

This approach turns out to be a cost-effective alternative to the retrofit of the combustion system (for example, burner substitution) or a valuable complementary tool after the retrofit has been completed. Applying this technology to an existing combustion unit requires minimum modifications and a very limited outage period for the new equipment implementation.

As illustrated in Figure 2, the following elements are used in this integrated approach:
- Advanced monitoring technologies
- Regulation capabilities improvement
- Expert system for optimised combustion control.

Advanced monitoring technologies
Monitoring the local in-furnace combustion conditions makes surveillance possible, which is essential for implementing optimised operating scenarios. This monitoring guides the operator to obtain the best tuning of any individual burner and, therefore, overall optimisation of the combustion unit.
ABACO-Opticom technology characterises the flue gas concentration profiles (Oâ‚‚, CO, CO2, NOX, SO2) in the envelope of each burner while avoiding the application of interpolation software or averaged results.

Gas samples are individually taken and are not affected by the fuel typology or boiler configuration. Sampling probes can be fixed, non-cooled, or retractable water-cooled (see Figure 3); both are designed to withstand any temperatures within the furnace at the required sampling points.

The use of ABACO-Opticom results in the identification of operating issues, such as significant fuel and air imbalances between furnace areas that can give rise to:
- High CO levels
- Corrosion or fouling problems
- Produce tubes overheating and coking
- Jeopardising unit operational safety.

Acquiring the capability to identify these bespoke operating issues makes possible the supervision and optimisation of operating conditions for all load scenarios and changing conditions, such as co-firing and the use of different fuels while maintaining adequate control of safety limits.

Regulation capabilities improvement
Optimised operating scenarios are performed by the improvement of the fired heater regulation. These include:
- Automation of existing manual regulations from the control room, including both burners and draft dampers (see Figure 4)
- Revamping of existing equipment, such as burners or wind box, to increase the tuning potential.

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