Comparing Non-Steady State Emissions under Start-Up and Shut-Down Operating Conditions with Steady State Emissions

Abstract

Although often neglected, the non-steady state operations of industrial facilities are more likely to result in increased emissions and process safety incidents compared to steady state operations. Regulatory authorities such as the United States Environmental Protection Agency and the Ontario Ministry of the Environment and Climate Change do not require industrial facilities to assess and report emissions under non-steady state operating conditions such as start-up and shut-down events.

It is demonstrated that emissions under non-steady state operation can be higher than those under steady state operation and that non-steady state emissions have the potential to exceed applicable regulatory emission limits.

A literature review has been conducted that compares non-steady state emissions under start-up and shut-down operating conditions with steady state emissions for several industrial sectors. Where available, trends have been developed to identify the circumstances, i.e. the industrial sector and contaminant, under which the assessment and consideration of emissions from start-up and shut-down events is necessary for each industry. The thesis also compares the two most commonly used air dispersion models: AERMOD and CALPUFF using a case study approach and recommends the use of CALPUFF as the more conservative approach. CALPUFF is then used to model the greenhouse gas emissions from the full load operation (steady state) and start-up conditions (non-steady state) of a combined cycle power plant to identify the worst-case emissions scenario.

The studies conclude that emissions under both, steady state and non-steady state operating conditions, must be modelled and assessed to ensure that the impacts of released emissions are modelled and studied in a conservative manner that takes into account all scenarios to determine the impacts of the worst-case scenario. The studies demonstrate that the worst-case operating condition may be different for each contaminant. Some contaminants have higher emissions during steady-state operating conditions, while others have higher emissions during non-steady state operating conditions. This was observed to depend on the nature of the industrial process and the type of contaminant. Considering these different operating scenarios is particularly important when emissions associated with non-steady state operation have the potential to exceed applicable regulatory emission limits, and to possibly cause an adverse impact on public health and the environment. Therefore, emissions under both, steady state and non-steady state, operating conditions must be assessed, controlled and reported to the regulatory authorities to ensure that emissions under the worst-case scenario are addressed, consequently preventing the emissions from adversely impacting public health and the environment.

The study recommends that regulatory authorities require industrial facilities to assess their emissions under non-steady state operating conditions as well as under steady state operating conditions to ensure that the emissions under both conditions are controlled below the applicable regulatory emission limits.