Topic last reviewed: 10 April 2013
Sectors: Downstream, Upstream
Gas turbines, boilers and other fuel-fired equipment (e.g. furnaces and kilns) combust fuel with the purpose of releasing chemical energy as heat. For industrial applications, they generate power, heat or steam, for direct use in processes or for operating other equipment such as steam turbines to produce shaft power, or as catalysers for chemical reactions.
The combustion efficiency of any combustion process is dependent on the amount of oxidant (air) that is used in relation to the amount of fuel, and on how well they are mixed. Controlling this air-fuel mixture, while still maintaining adequate mixing and minimizing waste heat, are key factors for maintaining high efficiencies.
Nearly 80% of all the improvements in energy performance in the industry derive from fuel combustion management. Improvements in the original equipment design, increased reliability, waste heat management, proper monitoring, and use of key parameters during operation are examples of these fuel management options.
Some commonly used techniques to decrease fuel usage and improve energy efficiency and combustion equipment performance are as follows:
- Combustion air, load or feedwater pre-heating
- Fuel heating using waste heat
- Effective monitoring (fuel flow rate, flue gas temperature, excess air, etc.)
- Flue gas energy recovery — boiler economizer, thermal oil heating, additional steam generation, fuel or air preheat
- Soot blower use to keep heat transfer surfaces clean, depending on the fuel
Figure 1: schematic of a combustion air pre-heating system
|Years experience in the industry:||21+|
Range of application:
|These technologies can be used in a variety of applications within oil and gas operations including, but not limited to, heating systems, power generation and flue gas recovery.
|Efficiency:||30–40% for gas turbines in open cycle and above 50% in combined cycle, and 70–95% in boilers and process heaters — efficiency is usually higher for large capacities and even loading.
|Guideline operational costs:||Depends mainly on fuel costs, air excess (efficiency), soot blower operation (depending on the fuel), campaign duration, and maintenance frequency.
|Typical scope of work description:||Monitor the combustion equipment’s performance characteristics, fuel usage, and exhaust/flue gas profile. Identify potential alternatives for efficiency tune-ups and techniques to improve system performance and reliability. Perform a cost feasibility analysis (direct and indirect costs, and length of payback period) for each identified technique/system to improve energy efficiency. Implement the selected energy efficiency improvement systems and techniques.
|Technical:||Stack gas temperatures and flue gas oxygen (or carbon dioxide) concentrations (too high or too low) are indicative of potential for improving combustion efficiency by optimizing input air-fuel ratio (Reference 5).
|Operational:||To achieve higher efficiency it is best to operate continuously, avoiding repeated start-ups, and securing stable heat distribution in the equipment.
|Commercial:||Resource efficiency and cost savings due to less fuel consumption.
Reduce emissions of VOCs, including hazardous air pollutants, NOX, SOX and other air pollutants.
|Environmental:||Reduce GHG footprint
- Increased cost savings
- Improved equipment performance and reliability
- Reduction in environmental impact
Industry case studies
The following two case studies are industry examples of improving the performance of combustion equipment. The first relates to the reliability of the process heater operation (during the time interval between two successive shut downs for maintenance)—increasing an operation’s efficiency depends largely on increasing the stability and reliability of the equipment. The second case study is related to the combustion process and the importance to be given to secondary systems as a means of maintaining high performance standards.
Case study 1: Expanding the operating limits of a fired heater for a specific crude oil composition with higher efficiency and extended campaign
Increasing the operational periods of crude oil processing equipment (to reduce costs) can result in potentially damaging conditions to atmospheric distillation units with loss of efficiency, requiring even more rigorous operational procedures to assure best performance over the duration of the campaign. The fired heaters used in crude processing are the critical equipment in this context because they are the most susceptible to failure whenever operational limits are stretched.
This case study originated from the tube failure analysis of a fired heater exposed to an extended campaign duration and increased production capacity, resulting in internal coking. The crude processing fired heater exhibited deformation (the ‘oranges’ in Figure 3) in eleven tubes of the radiation section, leading one of them to collapse (see images below). Because of this catastrophic tube failure an emergency shut-down was necessary to replace the deformed tubes.
Figure 2: Tubes with deformation
Figure 3: process heater schematic
The goal of this study was to evaluate the causes for the collapse of the heater tube and establish a procedure to determine new extended operating limits under optimal operational conditions, avoiding its recurrence.
Process simulations were conducted for different load conditions for two products: condensate (API = 64) and atmospheric residue (AR), with specific gravity equal to 0.921. Based on the simulations, the maximum temperature of film and wall in the heaters was determined. The analysis indicated a consistency between maximum temperature and the region where the coking of the tubes occurred.
The study found that the heater capacity is limited when there are high levels of condensate (API = 64), as film and wall temperatures above the recommended limits are observed for these situations. This is due to the combination of the following factors:
1) Higher than required heat transfer in the region of the flame.
2) Reduction of the internal heat transfer film coefficient by the change in flow regime (vaporization of the crude inside the pipe).
It is therefore recommended that the operational limit of a fired heater should not be determined only by its design and thermal duty; the influence of the crude composition and the operational limits for each type of crude must also be taken into account.
It is advisable to determine the maximum allowable load, skin and film temperatures expected for each unit campaign, depending on the crude oil processed. This action will avoid operational problems and allow for extended campaigns with different types of products while maximizing the efficiency through the campaign.
The analysis also considered the operational conditions for the fired heater:
- Gas flow per pass
- Release of heat load per burner
- Air distribution per burner
The operational conditions may lead to an increase in temperature beyond the limits calculated for each type of crude. Therefore, optimizing combustion operational conditions to keep the fired heater as homogeneous as possible, enhancing the thermal load distribution (balancing heat distribution between the radiation and convection zones), and keeping optimal air/fuel ratio for each type of load will contribute to a win-win situation with higher and more stable operational efficiency and an extended campaign duration, as well as a higher operational capacity.
The operational limit of a fired heater cannot be determined only by its original thermal design duty, but should also consider the real operational conditions including crude (load) composition. This is important to correctly assess the risk of crude vaporization that could result in greater skin temperatures. It is difficult to estimate the efficiency gains related to the extension of the campaign, since this is very case-specific. However, it can be seen that the savings are potentially significant, especially when the duration of the campaign is determined by the quantity of product requested by the client; interruptions can cause delivery deadlines to be missed, which may be costly.
Case study 2: Analysis of fouling formation in a fuel oil-fired boiler
The burning of fuel oil in boilers requires careful control in order to reduce coking and the formation of particulate matter, and thus achieve greater efficiency, stability and reliability.
The goal of this study was to evaluate problems of fouling in the flue gas circuit observed in boilers burning ultra-viscous fuel oil (see images below).
Figure 4: Fouling formation in a heavy fuel oil-fired boiler
The evaluation was conducted through interviews with experts, gathering information about the operating conditions, sampling fuel oil in storage and pumping systems, and laboratory analysis of samples to determine viscosity and solid content.
The results showed that viscosity plays a key role in fouling, and solids present in the fuel play only a minor role in the underlying case. Other parameters such as nebulization quality, fluid dynamics of flue gases (speed, impact stagnation, vortex, etc.) and adhesion (surface roughness, presence of organic metals, etc.), can lead to different deposition rates for the same oil burning in different equipment.
The outcome of the study resulted in the following actions and recommendations to improve the performance of the boiler:
- Control of operational parameters, especially the temperature of the stack, is essential to prevent fouling. While a gradual increase in temperature is normal within limits, higher temperatures indicate that fouling is isolating the heat exchanger pipes, resulting in capacity and efficiency loss.
- Avoid emptying the oil tank — always try to keep the oil tank full.
- Atomization steam and viscosity must be kept under strict control to allow good nebulization. It is recommended to keep steam superheated, at about 15°C above the saturation temperature.
- Monitor and maintain the atomizing nozzles to avoid wear of the orifice. Variations in orifice diameter of 10% above the average diameter, or 10% between two crossed diameters, indicate the need for replacement of the nozzle. Erratic atomization greatly increases the presence of organic particulate material, favouring the adhesion of inorganic compounds on the walls of the tubes.
- Periodically sample the bottom of the oil tank and analyse for ash content. High values above 1000 ppm may require removal of the ballast to prevent it from being passed to the boilers. Another solution is to recycle the tank bottoms, increasing the dilution.
- In scheduled shut-downs, it is recommended that the tank(s) are cleaned and inspected.
- It is recommended filters are used (which allow cleaning during operation by switching two filters in parallel) in at least three points of the oil supply system to the boilers/burners, e.g.:
- in the suction of the feed oil pump to the burners (exit of receiving tank);
- in the main pipeline to the burners;
- before the entry to the burner.
Compliance with these recommendations has great potential to increase operational stability and reliability at higher efficiency rates. Actual performance of boilers/burners operating with highly viscous fuel oil may improve by up to 5 percentage points, and maintenance intervals could increase by 2–4 months with no significant reduction in capacity until the end of the campaign. As an illustration, a large boiler operating at its MCR of 450 t/h steam production could save US$ 300,000.00 (US$250.00/tonne oil) per year if a 2.5% improvement could be achieved for 20% of the operational time (the duration of the campaign) for 8000 hours of operation per year (disregarding any extension of campaign).
- Kreith, F. and Goswani, Y. (2007). ‘Handbook of Energy Efficiency and Renewable Energy’. Taylor & Francis Group, USA.
- Baukal, C.E. (ed), (2001). ‘The John Zink Combustion Handbook’. CRC Press, USA.
- Petrobras’ technical archives
- Gilman, G.F. (2010). ‘Boiler Control System Engineering’ (Second edition). International Society of Automation, Research Triangle Park, North Carolina, USA.
- U.S. Department of Energy (2012). ‘Improve Your Boiler’s Combustion Efficiency’. Steam Tip Sheet #4, Advanced Manufacturing Office, U.S. DOE, Washington DC.