Apr-2023

Digital platform for intelligent operation of fired heaters

An innovative web-based tool that optimises fired heater operation and reduces carbon footprint while doing so.

Grandhi Srivardhan, Rupam Mukherjee and Shilpa Singh
Engineers India Limited

Article Summary

Fired heaters account for more than 60% of the total energy consumed in refining crude oil. With energy prices rising continuously over the years, refiners are increasingly pressured to reduce this critical operating cost through better and more efficient management of fired heaters. And interestingly, fired heaters do present opportunities if looked at critically.

Any increase in furnace efficiency, minor or major, has a strong rate of return on operating expenditure and benefits the global commitment towards sustainable development by reducing the carbon intensity of refining operations.

Digitisation in fired heater operation can lead to a multitude of advantages. Benefits are well defined. For example, it enables refiners to fine-tune their fired heaters in real-time. Such digitised operation addresses, monitors, and evaluates fired heaters in refineries while adapting to prevalent operating and ambient conditions, and it also directs necessary actions to maximise operating efficiency. This optimisation exercise on periodic intervals can lead to tons of fuel savings annually, translating into lucrative monetary benefits for refining operations and helping to reduce carbon emissions. In fact, a 1% increase in fuel efficiency of ~80 MMkCal/hr heat duty furnace in a 200,000 bpsd refinery can lead to 900 tons of fuel saved in an operating year.

Refinery furnaces are often operated throughout the year with a fixed set of operating parameters or with only minimal adjustments resulting from existing control loops. However, many other parameters often go unnoticed from a refiner’s perspective, which, if optimised, might lead to higher earnings. The intent of digitised operation is to focus on these seemingly benign parameters on a real-time basis and identify actions for more efficient operation against the backdrop of strong computational algorithms and established engineering practices.

Why real-time optimisation?
Fired heaters are well known to be the energy guzzlers in hydrocarbon processing industries, amassing as much as a two-thirds share of the energy consumption in the crude fractionation process. Interestingly, the progress of technology or available options for increasing the efficiency of fired heaters has plateaued over the years. This is primarily because of the limitation posed by the sulphur dew point in fuel, which still looms large as one of the major determining factors in heater efficiency.

Balance apprehension lies in ensuring that the industry is not playing too safe on this aspect and intends to utilise the asset to its full potential. However, the question at this point is whether the fired heater engineer in charge is fully aware of the best achievable potential for the current scenario or case of operation. Or are they relying on set points and standard operating procedures, which primarily apply to some other operating condition? AI-based digital tools can play a pivotal role in such situations, bridging the gap between the current operation and set operating scenarios. While optimising the efficiency, safety and reliability of the equipment is accorded prime importance.

Quite often, the calorific value of fuel takes centre stage while talking about fired heater efficiency. While calorific value is indeed the prime character, fuel sulphur content is the dark horse of efficiency calculation. It can overturn figures on paper; more so, it can damage equipment irreversibly at site. But what has fuel sulphur got to do with furnace efficiency?

The answer is acid corrosion due to sulphur in flue gases. Then, the focus shifts towards protecting furnace auxiliaries from acid corrosion, which is inevitable every time the flue gas temperature in the furnace circuit goes below the sulphur dew point. To ensure equipment safety, furnace designers maintain the flue gas temperature at the system exit or, in other terms, ‘the stack temperature’ above the dew point, knowing full well that high-temperature gases going into the atmosphere are a necessary evil associated with fuel combustion in fired heaters.

Fired heaters are built to operate over decades, and governmental legislature often changes over this long period of time. A classic example is the case of fuel sulphur content. Nowadays, in India, the pollution control authority mandates a maximum SOx emission of 850 mg/Nm3 from new fired heaters, which operate with 100% fuel oil. This can only be achieved when fuel oil sulphur content is limited to nearly ~0.5 wt%. Interestingly, a number of existing refinery fired heaters and auxiliaries have been built for a sulphur content of 1-1.2 wt%.

Directionally, flue gases from 1.2 wt% sulphur fuel oil will have a significantly higher dew point than the 0.5 wt% sulphur-containing fuel. Thus, an inherent potential for increasing the flue gas heat recovery exists within such fired heaters, which can be tapped to increase efficiency effectively (see Figure 1). In fact, actual sulphur content in fuel oil has come down to 0.10-0.25 wt% sulphur in the majority of cases wherever oil firing is still practised. In contrast, original operating instructions or limitations of maintaining a high stack temperature consistent with 1 wt% sulphur fuel oil still hold well in the standard operating procedures of the plant.

To bridge this gap, the proprietary EngRT-Htr digital platform has been created. Considering the worldwide focus on net zero, it may not be prudent to put furnace ‘optimisation’ activity on standby until a fully-fledged revamp or makeover is undertaken. Optimisation of operation and increase in efficiency is the first step ‘towards’ the desired goal.

Many other parameters often go unnoticed from a refiner’s perspective, which, if optimised, might result in lucrative returns (such as the re-setting air requirement for the exact mix of fuel firing). Digital interventions can focus and identify actions for more efficient operation (see Figure 2).

Inertia effect in fired heater operation
In any fired heater designer’s tenure, they encounter many styles of furnace operation depending on the user’s choice of comfort and confidence. But considering the criticality of this equipment, operators seldom choose to deviate from the furnace’s instructions. Although most installations face significant changes in fuel specifications, limitations in identifying or quantifying efficiency improvement potential arising from these changes bring some inertia to being too flexible.

Thus, bringing digital interventions to such robust equipment takes time. Numerous interactions, feedback, and data evaluations warrant a gradual transition. A conversation with one of the plant managers went as follows:

“Hello, Sir; indeed, the hot oil heater is operated very efficiently. However, we feel there is room for improvement.”

“Why do you say that? The heater is being operated as per the designer’s documents only. The datasheet states that the flue gas temperature at the APH exit should be 155°C, and we are maintaining exactly that through bypassing of air. Designer parameters are supposed to recommend the best efficiency and safety, right?”

“That is true, sir. However, the parameter stated in the datasheet is for fuel oil sulphur content of 1 wt%, which is the worst-case scenario. However, for the past year, your fuel oil sulphur has never breached 0.3 wt% sulphur. You could have lowered the flue gas exit temperature without compromising safety. That would have enhanced your efficiency by 0.7-0.9%.”