October 2021

Special Focus: Plant Safety and Environment

Are furnace emissions proving to be your Achilles heel?

Furnaces, or fired heaters, provide the source of heat required for major industrial processes.

Singh, S., Mukherjee, R., Engineers India Ltd.

Furnaces, or fired heaters, provide the source of heat required for major industrial processes. As an energy-intensive industry, oil refining employs furnaces in most modern refinery process units. The prime source of heat in furnaces is fuel oil or fuel gas that is normally generated within the refinery; these are essentially anthropogenic sources of carbon. The carbon in these fuel sources ultimately ends up in the atmosphere in the form of carbon dioxide (CO2), a major pollutant and recognized as a prime greenhouse gas (GHG).

The planet has been reeling with increasing global temperatures that are poised to create havoc if the present rate of temperature rise continues unabated. As per COP21 (Conference of Parties, Paris Agreement 2015), a concerted effort is being adopted to reduce carbon emissions to limit the global temperature rise within 2°C above pre-industrial levels—more specifically, targeting all efforts to limit the temperature rise within 1.5°C.

Without a doubt, the task is difficult considering the climate’s antagonistic relationship with economic and financial parameters. However, carbon emissions reduction and carbon capture and sequestration (CCS) initiatives, as detailed in the Paris Agreement, are progressive steps toward sustainable and green development.

As part of this larger initiative, refinery furnaces have their own role to play. Furnaces are one of the largest polluters in terms of CO2 emissions, and the rate of carbon emissions is directly proportional to the type of fuel being fired. Therefore, any reduction in rate of fuel fired is amplified multiple times into the reduction in CO2 emissions. Considering that thousands of metric tons of CO2 are emitted per year from a large-scale refinery, the emissions issue can become any refinery’s Achilles heel in the decade to come as climate initiatives invariably gain momentum.

As the voices against carbon emissions grow louder, excellent progress is being made on this front with some valuable literature.1,2,3,4 However, most of these documented studies focus on the reduction in carbon emissions on a pan-refinery level. Literature with respect to the impact of optimization at the grassroots level or equipment level is scarce. This article has been framed considering this, as well as the impact of carbon reduction strategies in quantitative terms on a furnace of appreciable heat duty. This work also explores the magnitude of carbon emissions reduction that can be achieved by common yet easy-to-implement strategies.

Furnaces and their tryst with emissions

Both basic sources of fossil fuels in fired heaters—fuel oil or fuel gas that is generated internally within the refinery—are rich sources of carbon and generate approximately 9,500 Kcal/kg–12,000 Kcal/kg (kilocalories/kilogram) of energy of the fuel burnt. However, each kg of fuel combusted generates approximately 18 kg–20 kg of flue gas: nearly 15%–20% of flue gas mass is CO2. Effectively, each kg of fuel oil generates approximately 3 kg–3.5 kg of CO2, whereas common refinery fuel gas generates approximately 2.5 kg–2.7 kg of CO2.

While evaluating carbon reduction strategies for furnaces, it is imperative to review the strategies adopted on a pan-refinery level and then extend them to furnaces. It is unanimously agreed that the path to carbon reduction is defined by on the following three pillars:

  • Efficiency improvements and process intensfication
  • Fuel substitution and feedstock management
  • Carbon capture, utilization and storage (CCUS), end-of-pipe solution.

Extending these three pillars to a refinery furnace, proven and time-tested techniques exist that can be covered under the first two categories (i.e., efficiency improvements and fuel substitution). However, the end-of-pipe solution of CCUS remains to be proven on a commercial scale.

While it is well-accepted that CCUS provides a significant reduction in carbon emissions, multiple factors stand as roadblocks between easy implementation on common refinery furnaces. For example, a standard crude refining facility consists of some 30–35 furnaces scattered across various points in the refinery—no “single point source” of emissions exists where CCUS can be planned, unlike power plant or fertilizer plant furnaces where CCUS is easier to implement. Moreover, the sulfurous content and the oxygen content of the flue gas are captured to act as a hindrance for the amine-based reagent that forms the heart of CCUS processes.

Even if these challenges are overcome, the utilization component in CCUS is a major chink in the armor for a refinery. Common utilization strategies include injecting CO2 for enhanced oil recovery or for methanol generation, both of which are dependent on geographical proximity to the end user, as well as on prevailing markets for the end products.

It was decided to focus this study on the first two well-proven options: energy efficiency improvement and fuel substitution. CCUS has been kept beyond the scope of this article; however, it is fervently hoped that the technology is soon demonstratively proven on commercial scale refinery furnaces, alleviating the inherent apprehensions.

The impact of the first two strategies has been examined for this case study on a moderate duty furnace commonly found in oil refineries. As it is proven that a reduction in emissions is intricately linked to fuel consumption rate—or, in other words, an increase in operational efficiency—effort has been made to enhance the furnace efficiency to its best achievable figures. The most common strategies include:

  • Optimizing the current operation and plugging areas of inefficiency
  • Retrofitting the furnace with an air preheat system to effectively utilize residual heat from outgoing flue gas
  • Switching to fuel gas firing from existing fuel oil firing.

The first two steps exemplify efficiency improvement. Fuel substitution is demonstrated by the third step of fuel switchover, as well as an additional check case where fuel gas containing 60% hydrogen by volume has been investigated.

OPERATION ANALYSIS AND BASELINE DEFINITION

A Southeast Asian refiner was operating a furnace with the conditions detailed in TABLE 1. It was evident from the operating parameters that ample scope existed for improving performance. The refinery was facing an uphill task of curbing carbon emissions. Among many other furnaces in the refinery, the subject furnace was further investigated with respect to CO2 emissions.

A deeper analysis indicated that the furnace was running at far from ideal operating conditions. For example, the furnace’s fuel efficiency was a mere 75%, primarily due to the absence of air preheating or other heat recovery. In fact, the arch oxygen level of 6 vol% was quite high, which was a consequence of excess air being maintained at 38%. Therefore, it was evident the furnace presented ample opportunity to improve on operating parameters, which will also help in lowering the carbon emissions from its current level of 242 metric tpd.

The following steps emerged in pursuit of these strategies.

Step 1: Working on current operational lacuna—Excess air levels

Excess air levels impact the process from multiple directions. The higher excess air dilutes the flame zone with products that do not contribute to heat release, resulting in a lower heat source temperature and reducing heat transfer by radiation.

Secondly, higher excess air levels increase the mass of flue gas, increasing the sensible heat loss through flue gases. With more excess air, more heat is wasted in heating it from ambient temperature to the combustion temperature. Excess air leads to the burning of additional fuel, which is not desirable from an operational cost perspective nor from an emissions viewpoint. Accordingly, the excess air level was adjusted to 25% from the existing level of 38%; this 25% figure was in accordance with standard API guidelines, as well as common operating procedures for natural draft systems.

The results of this optimization exercise are shown in TABLE 2. It can be seen that excess air optimization—an essentially zero-investment solution—can lead to a carbon emissions reduction of ∼8 metric tpd.

Step 2: Opting for an air preheat system

It was evident that continuing with the current natural draft system was insufficient for substantial cuts in emissions. Reducing fuel consumption was expected to impact emissions, so the installation of an air preheat system was evaluated in detail. Sometimes, fuel may be so inexpensive that the installation of an air preheat system may not be economically justified; however, emissions reduction will always justify higher heat recovery investments. Accordingly, an air preheat system was envisaged for the case studied here and appreciable carbon reductions were achieved. Note: The excess air level could be trimmed further to 20% with the use of air preheat systems in lieu of better control over the combustion medium.

Step 3: Exploring avenues for further reduction—Exercising the fuel gas firing option

After the evaluation of an air preheat system, it was decided to utilize a lower-carbon fuel source. Accordingly, refinery fuel gas was evaluated for furnace firing. It is vital to judge the overall refinery fuel balance before proceeding with this fuel shift. A sudden shift to fuel gas would have created a dearth in the fuel gas network, leaving the refinery with excess fuel oil. It is important to consider the use of this left-over fuel oil on a pan-refinery level for effective utilization. Common residue processing units, such as a delayed coker unit (DCU), can process this leftover fuel oil and generate valuable products. A study on fuel gas consumption was conducted and results are tabulated in TABLE 2.

Step 4: Tightening the noose—Fine-tuning fuel gas firing parameters

Having established a substantial reduction in CO2 levels, it was time for the final fine-tuning. The inherently clean character of refinery fuel gas presented an opportunity to further enhance heat recovery by cooling the outgoing flue gas within 25°C of the acidic dew point. For refinery fuel gas with amine-treating facilities installed, the acid dew point is generally within 110°C–115°C with a level of 100 ppm hydrogen sulfide (H2S). Further, better burning characteristics and ease of combustion also presented an opportunity to further optimize the excess air level to within 15%. The results of this exercise are shown in TABLE 2.

Check case: Hydrogen-rich fuel gas firing

Hydrogen (H2) has been touted as the “energy of the future” and a promising solution to multiple persisting environmental and energy security issues. It was determined that the study would be incomplete without evaluating the H2-rich fuel gas firing case. For the sake of this study, fuel gas with a H2 content of 60 vol% (and residual as methane content) was evaluated, considering that 60 vol% H2 can be accommodated in an existing furnace with minor changes in hardware, although it is always recommended to thermally evaluate the furnace to match this situation. Results are shown in TABLE 3.

As evident, H2-rich fuel gas firing shows remarkable results when evaluated for carbon emissions reductions. Firing 60 vol% H2-rich fuel gas will reduce CO2 emissions to < 40% of the baseline figure. However, practical limitations of refinery and commercial fuel pricing must be evaluated before this strategy can be adopted. Enriching the fuel gas composition by H2 produced from fossil sources will not help the cause, as the overall CO2 generated in a hydrogen generation unit (HGU) may be higher than the reduction achieved in this step.

Critical checkpoint: Evaluation of furnace performance parameters

As shown in TABLES 1, 2 and 3, the conversion of a natural draft furnace to a balanced draft furnace—and further shifting to fuel gas firing—appreciably reduces flue gas quantity and causes a major shift in the heat profile within the furnace. A lower flue gas quantity results in a lower convective heat transfer and, therefore, a higher radiant heat flux. A higher radiant heat flux leads to a higher metal temperature and film temperature in the radiant section. This must be evaluated well before such a conversion is attempted. A comparison of critical parameters is provided in TABLE 4. An analysis of these tabulated parameters was critical to ascertain the following points:

  • Although the radiant flux increases as one moves to Step 4, the impact of these critical parameters can be absorbed within the case study furnace as sufficient design margins were already available.
  • Tube metal temperature and film temperature are critical parameters that showed an increase. Although these can be managed within the existing design (of the studied furnace), in many cases, the increase in these parameters may force internal hardware modifications to operate the furnace within a safe envelope.
  • An appreciable increase exists in the bridgewall or arch temperature, which can be managed within the existing design in lieu of inherent design margins and superior tube support metallurgy.

Further, the shift to fuel gas firing augmented with an air preheat system required the following modifications and cross-checking:

  • New burners suitable for forced-draft conditions
  • New air preheater with ductwork—adequate plot space is required for such heat recovery systems
  • New forced-draft fans and induced-draft fan must be installed
  • Dampers and other blinding provisions required
  • New control philosophy and complex control loop of the air preheat circuit.

Mission accomplished: Summarizing the study

With a primary focus on reducing carbon emissions from the furnace, the operating parameters and hardware change strategies were put to work. Results presented a strong sense of optimism with carbon emissions being reduced from 242 metric tpd to 140 metric tpd, which is a reduction of almost 43%. Refer to FIG. 1 for the stepwise reduction in carbon footprint of the furnace.

FIG. 1. Stepwise reduction in carbon footprint.

Another positive and promising result is visible in the appreciable reduction in fuel consumption. Fossil fuels are depleting quickly and are costly sources of energy. A savings of the magnitude realized in this case study will result in significant monetary savings.

The examination of the case study furnace establishes, once again, the cardinal rule of carbon emissions reduction: enhancing operating efficiency. Although all furnace geometries are unique, some commonalities are applicable.

The study executed here also upholds that strategic decisions aimed at reducing the carbon footprint of a 23-MMKcal/hr furnace can curb CO2 levels by approximately 100 metric tpd. Considering an average of 333 operating d/yr accounts for a carbon footprint reduction of 33,300 metric tpy. Considering carbon pricing—a common and well-debated buzzword—and assuming an average carbon price of $20/t of CO2, the revenue equivalent of more than $6 MM/yr is generated. This alone outweighs the cost of an air preheat system, even in cases where the applicability of the air preheat system is not commercially justifiable by fuel savings, such as for small-duty furnaces.

Environmental pledge

Our planet has seen radical changes since the industrial revolution. Fossil fuels have dramatically improved the global quality of life; however, the positives are accompanied by an associated battery of negatives, the worst being industry’s effects on the environment. With significant releases of GHGs, the planet’s natural cycles have been altered (e.g., the depletion of polar permafrost, rising sea levels and increasingly frequent coastal storms, tornados, forest fires, floods and droughts). An organized, concerted effort is imperative throughout the entire value chain to reduce GHG emissions.

As detailed in this article, well-established solutions—efficiency improvements, waste heat integration, fuel substitution and H2-rich fuel firing—were applied on a common refinery fired heater or furnace. The results are encouraging: significant carbon emissions can be reduced by existing, proven techniques that will pave the way for further advanced technologies. HP

LITERATURE CITED

  1. Ferguson, S. and M. Stockle, “Carbon capture options for refiners,” PTQ, 2Q 2012.
  2. Creek, T., “Role of carbon capture in CO2 management,” PTQ, 2Q 2004.
  3. Mukherjee, R. and S. Singh, “Evaluating hydrogen-rich fuel gas firing,” PTQ, 1Q 2021.
  4.  Mukherjee, R. and S. Singh, “Savings through subtle operational adjustments,” Hydrocarbon Engineering, April 2021.

The Authors

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