By Lester W. Davis, Jr.
A facility's carbon footprint is related to the CARBON emitted to the atmosphere. There has been pressure in recent years to lower the amount of carbon emitted from industrial facilities. Expectations are that this pressure will continue via increasingly stringent legislative mandates that facilities must meet.
The energy needed to drive most refining and petrochemical plant
processes is provided in a Fired Heater through the combustion process.
This energy is known as HEAT. Heat is transferred to the process by
two heat transfer mechanisms: radiation and convection. The
carbon emitted during the combustion process is in the form of carbon
monoxide (CO) and carbon dioxide (CO2). CO is not a
factor when the combustion process is performed adequately (i.e., with
sufficient O2 and air/fuel mixing). The stoichiometric
combustion process for the fuel methane, CH4, which is common
in natural gas, is defined as
CH4 + 2O2 --> CO2 + 2H2O
This simple correlation shows that when additional CH4 (fuel) is required, additional carbon (CO2) will be emitted, thereby increasing the facility's carbon footprint.
To satisfy a given process condition, the process is required to absorb X MBTU/per unit time. The amount of heat resulting from the combustion of methane (known as Heat of Combustion), is about 913 BTU/ft3 or 21,520 BTU/lb. Therefore, a process requiring an absorption rate of 100 MBTU/hr will require about 1,095.3 ft3/hr of methane. However, the amount of methane actually consumed during the combustion process depends on the thermal efficiency of the fired heater.
Thermal efficiency is impacted by the following variables:
For an oil refinery with a throughput of about 150 kB/D, a saving of about 100 to 200 k$/yr can be achieved with a 1% decrease in excess O2. This is based upon a fuel cost of about 2$/MBTU. The recognized wet excess O2 levels for a natural draft fired heater with manual control of the stack damper and burner air registers are in the range of 3.0 to 3.5%. A natural draft fired heater equipped with automatic draft and O2 control can operate at 1.5 to 2.0% excess O2. It is recommended that an O2/combustibles analyzer be installed when targeting an O2 operation of 2.0% or lower. Also, the fired heater should be tested to determine its minimum O2 capability.
Fired heaters typically operate under less than atmospheric pressure or under negative pressure. During unit start up when a burner is initially placed into operation, the hot flue gases which are lighter than air will naturally rise throug the stack, creating a draft. Thus, the term NATURAL DRAFT OPERATION arises. The natural draft pressure is at its lowest where the flue gas is transitioning from the radiant section to the convection section. If there are any openings in the fired heater casing, ambient air will be infiltrated or leaked into the fired heater. The air infiltrated must be heated up to the stack temperature or the process will be cooled. Therefore, additional fuel will be required to prevent this from occurring (which of course increases both the cost of fuel required and the carbon emitted).
At this point, we have the same situation as discussed above: a high O2 operation resulting in a low efficiency. Air typically enters the fired heater casing through tube penetrations such as convection section return bends, crossover tubes leaving the convection section and entering the radiant section, and of course radiant tube outlets. The excess O2 levels should be measured using the same portable O2 analyzer below and above the convection section. Use of the same portable analyzer will provide consistent readings whether on a wet or dry basis and the built-in analyzer tuning.
The draft at the Radiant Section to Convection Section flue gas transition should be minimized to minimize air leakage. The draft at this location should be automatically controlled by the stack damper positioner or the Induced Draft (ID) Fan outlet damper positioner. The draft at this point should be controlled to -0.1 to -0.25 in. H2O.
The utilization of an APH is germane to lowering your Carbon Footprint. The primary function of an APH is the reduction of fuel usage for the same heat absorption rate. An APH raises the radiant heat transfer rate while lowering the stack temperature. This is accomplished by transferring heat from the existing flue gas to the combustion air. This heat exchange raises the combustion flame temperature, which increases the overall Radiant Section heat transfer rate. However, there are a number of factors that should be evaluated when designing an APH:
Each of the factors metnioned above can be addressed by considering the following:
A fired heater is a major factor contributing to the carbon footprint of an industrial facility such as an oil refinery or petrochemical plant. Reducing the carbon footprint is both necessary to meet increasingly stringent legislative mandates, and has the potential to save a facility money by possibly decreasing fuel usage.
There are multiple ways to potentially reduce the carbon footprint of a fired heater, and this article briefly discussed several of them. In many situations, the carbon footprint can be reduced with either no or minimum capital investment. A focused audit done by an experienced fired equipment engineer can quickly identify the potential for reducing the carbon footprint of a fired heater, the various options that might be considered, and their relative impacts and potential costs. Carmagen Engineering has significant expertise that can be applied in this area and has done this at many locations worldwide.