PTQ Q2 2022 Issue

pressure to secure the supply of air for combustion and to guaran - tee the confinement of hot gases inside the equipment. This lack of airtightness in the heater makes it impossible to automate the opera - tion fully. It imposes on operators (i.e., the human-furnace interface) the ultimate and definitive respon - sibility for fuel consumption, CO 2 emissions, and the preservation of mechanical integrity of their fired heaters. Various alternative control sys- tems have been proposed or attempted to automate the opera - tion of natural draft fired heaters. However, most of these systems have run into the same obstacle: the difficulty in isolating equipment as large and as hot as a fired heater. The lack of airtightness makes the use of automatic control to regulate the operation based on the oxygen content in flue gases an impossible challenge to overcome due to air in-leakage through joints between the casing metal sheets, crooked and/or worn frames of observation windows, or annular spaces through which the process coil tubes enter or exit the radiant section. Asset sustainability, inefficient operations, operator competen - cies, and compliance with refinery safety standards are also major chal - lenges to this sector’s viability and its transition to a lower carbon operation mode. Put in plain terms, while top- down initiatives such as regulatory requirements and strategic plans are important, at a practical level the thermal performance and oper - ational reliability of process heaters depends much more on the knowl - edge, skills, and care of the oper - ators to control air intake. In turn, this is an important enabler for refinery margins and viability. Any rational approach to manag - ing an oil refinery to meet the above challenges and reduce Scope 1 emis -

Day-to-day process heater control: the operational milieu As illustrated in Figure 1 , in order to heat the process fluid (A), the com - bustion reaction in a fired heater requires ambient combustion air (C) and a fossil fuel (D) that enter through the burners. The air intake is a function of heater draft and excess oxygen, while the fuel intake is controlled automatically by the temperature set for the hot pro - cess fluid (B). Both draft and excess oxygen are measured at the top of the radiant section (radiant arch). The logical and sequentially organ- ised operating procedure lies in the draft control of the furnace so that it coincides with the values recom - mended by the designer. In natu - ral draft heaters, console and field operators should closely coordinate their efforts to control both the stack damper opening (usually remotely via the heater distributed control system, DCS) and the burner regis - ter openings (always manually) in order to maintain draft and excess air levels (i.e., the air-to-fuel ratio, A/F) within previously established optimum operational goals. Figure 2 illustrates the operational scenarios faced by process heater front-line operators and the options they have to control them. The de facto definition of draft and excess air: ‘wasteful’ operation In many instances, operators and supervisors, trying to ‘protect’ the furnace from a deficient air con - dition, define de facto operational conditions for draft and excess air, perhaps due to a lack of a thor - ough understanding of technical fundamentals or even an absence of guidelines. These artificial defi - nitions imply operating the furnace with high levels of draft (<-1.5in H 2 O) and excess oxygen (>4.0% wet basis). However, this operating mode can be described as ‘energy wasteful’. Additionally, operating condi- tions outside the window recom - mended by the designers can not only result in higher emissions of CO 2 , they also affect the mechanical integrity of the equipment. For example, the high level of excess air caused by higher draft

E. Flue gases

Damper

Stack

A. Cold process uid

Collective coil

Radiant arch Draft and Ex. O

Radiant coil

B. Hot process uid

Burners

C. Combustion air

D. Fuel

Figure 1 Draft and excess O 2 measurement in a fired heater

sions, as defined by GHG protocols, 4 requires, as a prerequisite, refinery engineers and front-line operators to have a clear understanding of basic combustion engineering and heat transfer principles. Increasing the thermal efficiency of a direct fired furnace reduces the operating costs and the carbon footprint. A one per - centage point increase in efficiency in a process furnace equates to an identical percentage decrease in fuel consumption. Operating a process heater in a technically, economically, and environmentally desirable condi - tion involves a few basic actions that generally do not entail unu- sual expenses. In fact, most of these measures, mainly related to mainte - nance work, would be aligned with typical capital expenditures cuts at refineries.

Excess O (%)

7.0

6.0

5.0

4.0

3.0

<1.5

Radiant arch

High

Correct

Low

-0.20

-0.10

0.0

Draft (in. HO)

-0.40

-0.30

Figure 2 Draft and excess O 2 control in fired heaters

58 PTQQ 2 2022