They can be opaque, translucent or invisible. They parade through the sky in Technicolor green, yellow, orange, brown and red, or basic shades of black and white. Varying in fragrance from odorless to offensive, they hug the ground, disperse rapidly into the atmosphere, or just smother the sky as an oppressive blanket of smog.
What are they? They are plant emissions. Like a many-headed dragon, they come back to life with each plant expansion. Your job is to perform a desktop emissions estimate and to do it well enough so you don’;t have to independently confirm its accuracy with an even more expensive stack test. Intended as a tool to help you estimate plant emissions, this article opens with a set of holistic principles and rounds out with an explanation of the fundamental process activities used to model emissions.
Emissions are best estimated by following these five holistic principles:
1. The process engineer knows his process; the modeler knows his model. Yet the two
2. The goal is to estimate emissions, not assess blame.
3. Iterations to improve the accuracy of the model are unavoidable.
4. Emission modeling and permitting should be assigned to different people.
5. Ensure paper and electronic trails exist so the estimate can be reproduced.
The modeler needs to establish at the outset a relationship with key personnel who understand the process, can communicate its nuances and are willing to ask the modeler any question they like to make sure the estimate is realistic and representative. Absent this relationship, pretension builds up, questions are avoided and simulation outputs will be contrary to what all parties desire.
Had I been working in an environment that was receptive to cries for help, I would have spoken up before the circumstances spoke for me. The lesson of this anecdote is that you must quickly establish an environment of trust on any emissions inventory team. The goal is to estimate emissions using practical, reproducible procedures, not to chase down errors so those responsible can be ostracized and embarrassed.
Despite these potentially stressful iterative revisions the meticulous modeler can take comfort in one irrefutable fact: He or she has been asked to quantify the emissions from a complex set of processes, the dynamics of which no two professionals are likely to agree upon entirely. Therefore, due to the high likelihood of necessary changes, allocate resources for iterations and clarify in your scope that iterations will be limited to two, three or some other single-digit value.
Master emissions estimates
There are several software programs on the market to help you perform an emissions inventory. One program I have used is Emission Master, which estimates emissions based on vapor liquid equilibrium. It produces sound estimates of emissions that conform to EPA-approved calculation methodologies. In fact, EPA agents requested information from the maker of Emission Master when developing its methodologies.
In simplest terms, using software to estimate process emissions is similar to a game you might have played as a child. Remember how you made a flip book by rippling a set of index cards with a figure on each that is drawn slightly different at the top? The rippling appears to set the figure in motion. It is the same with emissions estimation software; only the cards in our estimate are activity models. Prepare enough activities in the proper order and you will have accurately simulated process emissions. Below are fundamental emission activity models found in Emission Master that when properly used, can represent virtually any process imaginable. You will find that other software programs have similar models.
Beyond these fundamentals, most emissions estimating programs can simulate special-case emissions from vacuum operations, gas evolution, solids drying and empty vessel purges. With any of these activities, the modeler can set up process condensers and aftercondensers, along with scrubbers and other control devices, to more accurately estimate process emissions.
Estimate with conviction
Combining powerful emissions estimating software with the holistic principles can result in an emissions estimate of convincing accuracy. Unfortunately, a modeler who shorts himself on the application of software, principles or both will find his estimate second-guessed and perhaps rejected, or subject to nerve-wracking rework. With this article and the proper software, you are forewarned and forearmed, so don’;t let it happen to you.
Don Dickerson is a registered chemical engineer and senior environmental engineer for Lockwood Greene, a division of CH2M HILL. He works in Augusta, Ga. E-mail him at email@example.com.