Introduction

Climate change is one of the greatest challenges facing humankind today. Human induced emissions make up the majority of the greenhouse gases driving anthropogenic climate change, and fuel combustion processes make up the majority of greenhouse gas emissions in the process industry - such as fired heaters, power boilers, pulping chemicals processing, wood drying processes, etc…

Reliable measurementation of plant emissions is challenging, but also very important as wrong measurements result in losses - losses in terms of productivity or expensive fines as a result of violating environmental regulations.

Measurementation Issues

In reality, stack emissions from most processing plants are calculated based on first principle equations on the DCS, or on the supervisory systems using the BTU of the Fuel gas used in the fired heaters, which in most cases will be by laboratory analysis and sampled infrequently. Because of the infrequency in BTU results, the emissions from the plant are not calculated on a real-time basis, resulting in an unreliable measurement of stack emissions in the processing plant, indirectly leading to losses in $$$.

Traditional Approach for Emission Control

Emission controls are usually implemented on the DCS or on the computer based supervisory systems at process plants, and were used primarily for maintaining and complying with the environmental regulations. In most cases these controls are operated on a Feedback/Feedforward mechanism so that the SOx, NOx, CO, CO2, etc. emissions from the stack are within the specifications provided by the government.  

These control schemes are designed, developed, tested with Process Simulators to validate their performance under various operation scenarios, and then implemented in a real plant. However, the simulators only consider ideal conditions, using mathematical models that neglect the effect of unmeasured disturbances and the actual constraints and conditions in the actual plant.

A Real Example of Emissions Affecting Plant Profitability

An Olefins plant is a typical example of stack emissions directly affecting the profitability of a processing plant. The below figure represents the typical bottlenecks in Olefin units such as a Quench tower being unable to handle a higher load of feed, resulting in:

• Improper quenching
• Charge gas compressor being limited by performance at higher load
• Drier unable to remove water properly
• Cracking heaters unable to crack Ethane due to stack emission limitations
• Separation columns unable to handle large mass transfers
• etc…

The constraints on other sections of the Olefin plant are usually caused by improper design or mechanical limitations of equipment creating a real bottleneck for optimizing the plant and improving profitability, whereas the limitation on the cracking heaters are usually caused by stack emission limitations/constraints. Cracking heater constraints allow process engineers and operations engineers to de-bottleneck the limitation by properly identifying the root cause of the problem and taking corrective actions.

Typically model based multi-variable controls used in Olefin plants try to control and operate the fired heaters safely and optimize the complete Olefin plant operation by pushing towards the constraints, like emission constraints on fired heaters. Emission calculations being done based on measurements that are not made in realtime, the result is unreliable stack emission values thereby creating a wrong indication that the plant has already reached constraint, thus losing all opportunities to explore the plant and optimize the plant operation towards a profitable region. The emission constraint on cracking heaters directly results in reducing the load on the front end section of the Olefin plant, thereby causing an impact on the product yield and specific consumption of the plant.

Normally in Olefin plants, the fuel gas flow for cracking heaters will be used as raw flow without any compensation for pressure, temperature and specific gravity which also results in inaccurate consumption of fuel gas flow against the actual demand. Operating the heaters with uncompensated fuel flow over a period of time results in overconsumption of fuel gas, which results in unnecessary loss and affecting the specific consumption of the plant.

Countermeasures

Reliable online analyzer installation on the fuel gas line for measurement of BTU helps in the usage of right value at the right time to calculate the right emissions from the fired heaters in plant. In the event of maintenance of the analyzer, the last good value from the analyzer or the laboratory results can be fed by the operator for emissions calculation, which will help the multi variable control system to push the plant to the constraints realistically, helping the plant throughput to be maximized to the actual constraints rather than to a pseudo constraint condition.

Compensating the fuel gas flows based on the actual pressure, temperature, and density will provide a realistic and accurate compensated flow helping the multi-variable controllers to set a cost minimization objective on the fuel gas flow, without compromising on the safety and controllability of the cracking heaters.

Conclusion

In a nutshell, accurate real time measurements of the fuel gas system with proper corrections helps in maximizing the throughput and minimizing the cost until the actual constraints are hit, thereby indirectly increasing the profit of the plant.

Did you enjoy this article?? Read more: Combustion Optimization at MOL Group & Combustion Management in Fired Heaters