Considering more stringent NOx reduction requirements in the refining and petrochemical industry, what are the optimal strategies for reducing NOx emissions from process heaters and SMR/PSA heaters?Jul-2022
Dirk Jan Treur, Becht, firstname.lastname@example.org
In case the current NOx emissions are above the proposed limits, a few different NOx reduction measures are available. The first and preferred option is always to improve operational performance, which usually results in lower emissions, but in many cases these operational measures are either impossible or insufficient. A few operational measures are:
• In case the burner design has primary and secondary air (air staging), decrease primary air. This will typically result in reduced flame temperature, which reduces NOx formation
• Decrease air preheat temperature; however, this may decrease thermal efficiency
• Reduce hydrogen content in the fuel mix, which is sometimes an option for refinery fuel gas. Less hydrogen results in lower flame temperature, which reduces NOx formation
• Water or steam injection may be used to cool down the flame, which reduces NOx formation. However, this primary measure also reduces energy efficiency but can make sense when NOx emissions are just above the emission limit value.
The second main option would be to install or replace the existing burners with new (ultra) low NOx burners, but the viability of this option depends on the firing duty, heat flux pattern, size of the heater, and firebox temperature. Low/ultra-low NOx burners typically have larger flame volume. Firebox dimensions should be large enough to allow for lower NOx burners, which can be challenging in case of process heater revamping. Also, the firebox exit temperature can increase with burner revamp projects, which may limit this option.
The third and often most expensive option would be to install DeNOx flue gas cleaning equipment. This will typically require additional space in the flue gas area and can reduce efficiency. DeNOx flue gas cleaning is a secondary NOx control measure and is a post-combustion NOx control technology, also known as an ‘end of pipe’ solution. Two widely used are the selective catalytic reduction (SCR) process and the selective noncatalytic reduction (SNCR) process.
SNCR and SCR technology should be considered only when the available low-NOx combustion technology provides insufficient NOx reduction to comply with the emission requirements. Furthermore, DeNOx technology cannot be installed easily in any system and relies on sufficient space, proper mixing with injectant, flue gas temperature, and residence time.
Both SNCR and SCR processes reduce NOx to N₂ and H₂O using ammonia or ammonia-based reagents like urea. The residual unreacted ammonia leaves the reactor as ammonia slip. The emission of ammonia slip is another pollutant, which typically also has legislative limits.
Hervé Lavieu, Heurtey Petrochem Solutions, email@example.com
There are several aspects to be considered:
• At the pre-combustion level, the solution of ‘fuel switch’, mostly meaning fuel oil (FO) to fuel gas (FG), appears to be beneficial. The switch from FO firing to FG firing will reduce the flame temperature and, as a result, reduce the NOx thermally generated. This switch will also be beneficial since, unlike the FO, the FG combustion will not contain organo-nitrogen compounds and chemically formed NOx will therefore be eradicated.
• At the combustion level, accurate control of excess air will also reduce NOx formation. At the post-combustion level, the installation of a DeNOx system (SCR) remains the more efficient solution for NOx reduction. SCR is a post-combustion method dedicated to converting NOx into N₂ and H₂O by means of reaction with ammonia (NH3) in the presence of catalyst. SCR systems can achieve a reduction of up to 95+% of NOx in the flue gas stream.
Ron Beck, AspenTech, firstname.lastname@example.org
There are several key aspects of reducing emissions:
• Accurate measurement and monitoring to understand levels and trends and relate those to process conditions in a unit
• Most effective removal of that pollutant from the flue gas or other emissions stream
• Efficient process operation, which can minimise emissions from a unit or a process overall.
Also, a pollutant such as NOx needs to be considered together with other plant parameters (carbon, SOx, energy, water use) because it becomes a multi-faceted optimisation problem to meet sustainability goals but still be profitable.
Digital solutions are increasingly critical in making the right abatement decisions for emissions such as NOx, mainly because changing process operating parameters is faster and more cost-effective than adding more control systems.
Improving efficiency by changing combustion conditions is essential to reduce emissions. Digital twin solutions and advanced process control (APC) apply open- and closed-loop modelling and optimisation technologies to stabilise operations, improve efficiency, and reduce and report emissions. AI technology can enhance rigorous models based on engineering first principles, using an approach called hybrid models to obtain highly accurate models to support engineering decisions. For example, Nissan Chemicals used AI to predict temperature profiles in an SMR for improving heat transfer, increasing efficiency, and reducing steam consumption. This would also reduce NOx.
Advanced multivariate analytics can help understand which process conditions in a complex process may be the biggest culprits in conditions amenable to NOx creation (and reduction). Applying this methodology (for example, using AspenTech’s PRO MV multivariate technology) can identify strategies to minimise NOx (and other pollutants).
Then, of course, a catalytic control unit, such as a selective catalytic reduction (SCR) unit, is needed to remove the remainder to reach permitted limits. Again, a hybrid model, as previously described, can be instrumental in operating the pollution control systems to maximise effectiveness and catalyst lifetime.