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Jan-2007

Controlling fired process heater emissions to reduce fuel costs and improve air quality

Historically, refineries have relied on relatively clean fuels for fired process heaters. With these clean fuels, there has been little need for controlling atmospheric emissions.

Scott T Eagleson and Nick Confuorto, Belco Technologies
Sudhanshu Singhania and Neeraj Singhania, Singhania Technical Services Pvt.
Ramesh B John, Lisha Engineering Co
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Article Summary
With today’s rising energy costs, many refiners are now looking for lower cost fuel options. A number of refiners have already found that there are economic advantages with firing lower cost fuels despite the fact that they are dirtier and require investment in emissions control systems. This paper reports on what technologies refiners are using to control flue gas emissions from fired process heaters and how they reduce costs. A case study is presented. The control of particulate, SOx and NOx emissions is considered, as is the importance of having a highly reliable emission control system.

Use of Fired Process Heaters in Refineries
Fired process heaters are integral to nearly every refinery and are in use to transfer heat to a fluid other than water. They are used for pre-heating crude oil and other feed stocks for a number of refinery processes where the use of steam from boilers may not be practical. Common refinery applications include distillation, hydrotreating, catalytic cracking, alkylation, reforming and coking.

With a fired process heater, fuel and air are combusted in a firebox to produce heat that is transferred to liquid (process liquid or a transfer liquid) contained within tubes. There are many suppliers that provide fired process heater technologies and designs. Systems differ with specific process applications, thermal efficiencies, draft types (forced, induced and balanced), waste heat recovery/steam generation, air preheating, designers/suppliers, and fuels. The differences resulting from these many variables will not be addressed herein. A simplified fired heater system with an emission controls systems is depicted in figure 1. This reflects only a single example for a single application.

The impact of the fuels being fired, the resulting flue gas emissions, and how/why to control these emissions is considered. Before proceeding with this discussion, it is important to note that reliability and uninterrupted operation are extremely important with fired process heaters as they support the operation of key refinery processes. Any systems or technologies controlling flue gas emissions must be extremely reliable. It is unacceptable to think of shutting down a key refinery process unit because of problems with an emission control system on a fired process heater. For this reason, many refiners have chosen to use emission control technologies that were first developed and proven for refinery fluid catalytic cracking units (FCCUs), as opposed to making use of less robust technologies used in general industry and power generation.

Various liquid and gas fuels are fired. These include natural gas, refinery fuel gas, propane, fuel oils, residual oils and refinery waste gases. Each has advantages and disadvantages. Cleaner burning fuels are those that produce very little particulate and sulphur oxide emissions. Unless nitrogen oxide emissions are a concern, emission control systems are not typically required. Natural gas is generally considered to be the cleanest of fuels. The downside with using clean fuels can be fuel costs.

Dirtier fuels are generally lower in cost and produce particulate and sulphur oxide emissions. The general trend is that the dirtier the fuel, the lower the fuel cost and the higher the particulate and sulphur oxide emissions. At what levels these emission need to be controlled, or if at all, depends greatly on local emissions standards and a refining company’s practices.

Air at ambient conditions or preheated air are most commonly used for combustion. It is somewhat common to use air vented from refinery process units that may contain ammonia (NH3) or other contaminants. With this approach the fired process heater also provides a waste gas incineration function, although the impact on flue gas emissions needs to be considered. Some have also considered the use of oxygen enrichment as a way to improve combustion and, in particular, reduce nitrogen oxide emissions.

Flue Gas Emissions
Like other combustion processes, fired process heaters produce a waste flue gas stream that is discharged/exhausted to the atmosphere. The flue gas is primarily nitrogen (N2), carbon dioxide (CO2), water (H2O) and oxygen (O2). Depending on the fuel, combustion air and system design, the flue gas may also contain smaller concentrations of pollutants. These include carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), particulate and metals.

Fired process heaters have long been designed to minimise the formation of CO in the combustion process. NOx (NO and NO2, typically expressed as NOx) formation is a result of system design, operation and the presence of nitrogen compounds in the fuel. A number techniques and process are used to minimise NOx formation during combustion. Other technologies are used reduce NOx emissions after NOx formation. These include selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR) and LoTOx5 using ozone oxidation. These have been the subject of many papers1,2,3 and will not be discussed herein.

SOx, particulate and metals in the flue gas are primarily a result of the fuels being fired. Sulphur and sulphur compounds in the fuel form SOx compounds (SO2 and SO3, typically expressed as SO2) during combustion. One can typically expect that all the sulphur in the fuel will end up as SOx in the flue gas. With firing dirtier fuels, SOx emissions from 100 to 3,000 parts per million dry volume basis (ppmdv) are not uncommon
Particulate is formed during combustion and is generally composed of ash (non-combustible compounds in the fuel) and unburned carbon. The ash content of the fuel is good indictor as to how much particulate will end up in the flue gas. Metals, typically nickel (Ni) and vanadium (V) are present in many heavy oils. These will end up as particulate in the flue gas and are controlled by the same technologies as particulate emissions. With firing dirtier fuels, particulate emissions (including metals) from 25 to 300 mg/Nm3 (dry basis) are not uncommon.

Controlling Emissions
The most common reasons for controlling flue gas emissions are regulatory and/or legal pressure to do so. How to best address meeting these requirements is going to be different for every refinery. It will depend greatly on how stringent emission regulations are and the cost/availability of cleaner fuels. An economical choice is to burn a lower cost dirtier fuel that requires an emission control system.
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