In our previous blog, 'What's the Best Air Pollution Control Technology for My Process?', we reviewed the many factors that impact the choice of air pollution control equipment. In situations where a facility is considering their air pollution control needs, it is important for the plant's engineering staff to discuss the application data with potential suppliers. Application data are the details about the process operation such as uptime, maximum VOC loading, maximum air flow, energy usage concerns, permitted emission rates, and other process operation data that may be critical to equipment design. This will help determine the available options that will allow them to meet or exceed their environmental goals or outcomes. The air pollution control equipment supplier will then be able to direct the customer to the appropriate Best Available Control Technology (BACT) or Maximum Available Control Technology (MACT).
In order to provide more understanding of how we help our customers determine their BACT or MACT, we'll be looking at a few example scenarios here on the CPI blog.
Scenario 1: Thermal Recuperative Oxidizer
Your process has an airflow in the range of 500 to 50,000 Standard Cubic Feet per Minute (SCFM) at extremely high temperatures and higher than average (>25% LEL) volatile organic compound (VOC) loading, or have inorganics with the process gas VOC. You may want to consider thermal oxidation as your preferred technology. A thermal oxidizerwill typically utilize a primary heat exchanger, which is used to pre-heat the incoming process air, classifying it as a thermal ‘recuperative’ oxidizer. Not only is this oxidizer able to adequately handle the process characteristics mentioned, it can also accept both particulate-laden and mildly corrosive exhaust streams, all while achieving destruction rate efficiencies in excess of 99% as a standard.
There are a couple of drawbacks to these systems:
- Capital cost - the materials of construction (stainless steel) make this control device relatively costly compared to other technologies.
- Cost to operate - because the primary heat exchange is accomplished via surface area, the size is limited, and thus the limit thermal efficiency is typically less than 70%. This can make the oxidizer expensive to operate unless the process gas can become a form of supplemental fuel through the heat release of the combustion of the process BTU content.
If an exhaust stream only meets a couple of the characteristics (e.g. particulate-laden but low VOC loading), and a thermal recuperative oxidizer is still the BACT, there are opportunities to lessen the impact of the operating costs. Secondary heat recovery from the tail end exhaust of the oxidizer is a great way to recuperate some of the operating expense of these types of systems. Examples of secondary heat recovery include pre-heating of process make-up air, steam generation, building heat, and hot water.
Thoughtful consideration needs to be taken when considering a secondary heat recovery project to compare the project cost versus the return on investment (ROI). Recent natural gas prices have made it difficult to justify heat recovery projects, as the payback time frame often slips past two years. When considering a thermal recuperative oxidizer, CPI will always provide an analysis of the oxidizer exhaust flow and temperatures to determine if secondary heat recovery is a viable option.
Please watch for the next blog in this series: “Identifying Your Ideal Air Pollution Control Technology, Scenario 2”