September 2016

Special Focus: Refining Technology Developments

Reduce industrial energy bills through improved combustion control

The last few years have been an extremely challenging period for the oil industry due to the adverse global economic environment and the impact of unfavorable external factors.

The last few years have been an extremely challenging period for the oil industry due to the adverse global economic environment and the impact of unfavorable external factors. In response, the downstream division of MOL Group launched a new downstream program with the clear objective of returning to the top quartile in Europe in terms of both operational efficiency and profitability.

The new downstream program covers all elements of MOL’s integrated downstream business, from supply chain management through refining and petrochemicals, to wholesale and retail. The program encompasses hundreds of independent actions targeting an increase in earnings before interest, taxes, depreciation and amortization (EBITDA) of $500 MM–$550 MM in 2014 vs. 2011. The main focus of the program is cost reduction (60%) and increased revenue (40%) in several fields:

  • Value chain optimization
  • Market management
  • Asset management
  • Resource and process efficiency.

The program’s asset management initiatives focus, among other aspects, on the reduction of energy costs. This article presents one of the most successful initiatives within this energy management pillar—reducing the energy costs of fired heaters, which are the most significant energy consumers in the refining sector.

Program scope: Reducing heat transfer costs

MOL Hungary (a member of MOL Group) examined the biggest furnaces within its 165-Mbpd Danube refinery, with a view to improve the efficiency of these fired heaters. Using data from a third-party study prepared in 2010, MOL Hungary identified a large gap in heater efficiency at the Danube refinery. Most of the refinery’s furnaces are approximately 40 years old and have potential for efficiency improvements. The identified gap offered a huge savings potential that would significantly enhance the company’s position in the present competitive business environment. This savings potential provided the impetus for the initial examination of the furnaces.

Fig. 1. Ideal firing range, according to CO and O<sub>2</sub> content curve.
Fig. 1. Ideal firing range, according to CO and O2 content curve.

As a first step, the physical and mechanical conditions of the furnaces were assessed, followed by an analysis of operating parameters. The parameter with the most influence on a heater’s efficiency is the amount of excess air in the combustion air. The amount of excess air is determined by measuring the oxygen (O2) content. The ideal O2 level (the lowest possible that allows complete combustion) depends on several factors, such as fuel type, burner type, humidity changes in the air, moisture content changes in the fuel, varying furnace loads, fouling of the burner system and the mechanical behavior of combustion equipment (FIG. 1).

Keeping these factors in mind, new targets for O2 content in flue gas and standardized operational procedures were set. These actions brought small benefits, but they also prompted a change in mindset. Since many of the aforementioned factors are continuously changing, the ideal amount of O2 continuously changes, as well. A higher firing rate induces greater turbulence through the burners, providing better mixing of fuel and air and enabling operation with a lower excess O2 before unburned fuel (represented by carbon monoxide [CO]) appears.

Fig. 2. Location and measuring range of TDLS instruments.
Fig. 2. Location and measuring range of TDLS instruments.

CO is usually the first combustible gas component to appear when the combustion fuel/air ratio starts to become too rich (and burning becomes imperfect, inefficient and even dangerous). To keep the O2 content at the most desirable level in a safe way, the CO content must be measured reliably. It is also important that the measured O2 and CO values are detected not only at one dedicated point (e.g., one point in the flue gas stack), but are also representative of the entire combustion process and, moreover, as close to the flame as possible.

The most common technology for measuring O2 content in flue gases is the in-situ ZrO2 (zirconia) probe, which uses point measurement and, therefore, does not provide a good average reading. New tunable diode laser spectroscopy (TDLS) technology has the capability to measure the average content of flue gas components all the way through the firebox, from just above the flame (FIG. 2).

Moreover, the average O2, CO and methane (CH4) contents can be measured properly with TDLS, without interference. This technology allows a more precise fuel/air ratio to be achieved, in the interest of improving energy efficiency (FIG. 3). Consequently, the previously discussed technology holds the potential for achieving significant cost reductions while simultaneously alleviating environmental problems.

Fig. 3. Specific energy consumption and flue gas O<sub>2</sub> contents of Furnace No. O<sub>2</sub> in the aromatics unit.
Fig. 3. Specific energy consumption and flue gas O2 contents of Furnace No. O2 in the aromatics unit.

The use of TDLS increases measurement accuracy, thereby improving operational safety. While the upfront cost of a TDLS analyzer is relatively high, its reliability (no moving parts, which means minimal wear and tear) equates to very low maintenance costs, making the total cost of ownership competitive.

Heater pilot project

A pilot project was carried out on a fired heater at the Danube refinery in 2013. This first application of TDLS onsite proved relatively successful. Only a TDLS analyzer was installed, without alteration to any other part of the furnace; neither combustion control nor any other devices were changed. Despite the minimal modification, the flue gas O2 content was greatly reduced.

Based on the successful pilot test and on a subsequent technical and economic assessment, it was determined that combustion control on other furnaces with high energy consumption should be upgraded.

Energy efficiency program

Shortly after the pilot project was finalized, a multi-year energy efficiency improvement program was launched at the Danube refinery that involved most of the largest fired heaters. The efficiency of these furnaces was found to be much weaker than expected; however, their technical condition was sufficient, so revamp costs were not exorbitant. Moreover, as these heaters operate with a relatively small number of burners, their control systems are not complicated.

Several technical measures were incorporated into the furnace upgrades:

  • Switching traditional zirconia flue gas oxygen analyzers for more modern ones, based on TDLS technology
  • Changing existing “simple” fuel/air ratio controls to a more sophisticated solution involving additional field measurements, such as arch draft and fuel density measurement
  • Replacement and refurbishment of existing end-of-life field devices—e.g., fuel gas flowmeters and flue gas dampers
  • Replacement of old, obsolete burners where necessary.
Fig. 4. Result from Furnace No. O<sub>2</sub> in the aromatics unit, shown in the combustion efficiency curve.
Fig. 4. Result from Furnace No. O2 in the aromatics unit, shown in the combustion efficiency curve.

These changes allowed more precise and sophisticated combustion control to be achieved, since the O2 content could be kept much closer to the optimum combustion value.

FIG. 4 shows a 4% (from 7%–8% red curve to 2%–3% blue curve) reduction of excess oxygen in Furnace No. O2 of a xylene separation column at the refinery’s aromatics unit. This result indicates a fuel savings of approximately 42,000 GJ, worth €500,000/yr. Furthermore, carbon dioxide (CO2) emissions were reduced by decreasing the amount of fuel used.

Two phases of the program have already been completed. In the first phase, three fired heaters were upgraded, including Furnace No. O2 in the aromatics unit, which has the largest thermal input (36.8 MW) at the refinery. In the second phase, five more heaters were upgraded, including all three furnaces of the No. 3 crude distillation unit (CDU3), each with a thermal input of 30 MW.

The first two phases of the program have so far resulted in a total savings of €2.1 MM/yr across the eight fired heaters.

Beyond the mentioned reduction in fuel consumption, it is also noteworthy that greenhouse gas emissions have been significantly decreased. The CO2 emissions per GJ of fuel are approximately 50 kg, so CO2 emissions have been reduced by 2,100 tpy. Furthermore, this new technology brings a higher level of operational safety, especially during the most critical startup and shutdown phases of furnace operation.

The program has now moved into its third phase. During this phase, two heaters in the No. 2 CDU will be completely refurbished, including replacement of each burner, installation of air blowers and air preheaters, and complete replacement of the combustion control system. The anticipated savings from these two furnaces are €1.5 MM, which reflects the scale of this reconstruction.

In the future, similar changes are planned on three more fired heaters to close the previously mentioned efficiency gap. These heaters are relatively small, so the payback time will be a little longer; however, the expected savings will still be €100,000/furnace.


Improving the efficiency of fired heaters in an oil refinery or petrochemical plant carries significant economic potential. The best way to achieve this economic gain is to install advanced combustion fuel/air ratio controls, using modern measuring technologies like the TDLS analyzer, which is suitable for the precise measurement of O2, CO and CH4 in flue gas. Numerous benefits are available with advanced controls:

  • Improvement in energy efficiency
  • Improvement in operational safety through CO and CH4 measurement
  • Reduction of air pollution (better combustion and lower fuel consumption)
  • Information direct from the firebox.

Despite this success, there will always be room for improvement and better solutions. HP

The Authors

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