a Science and Evaluation Branch of Alberta Energy Regulator , Alberta Energy Regulator , Alberta , Canada.
b Principal of Zelt Professional Services , Zelt Professional Services Inc , Alberta , Canada.
J Air Waste Manag Assoc. 2019 Apr;69(4):450-458. doi: 10.1080/10962247.2018.1547802. Epub 2019 Jan 4.
The United States Environmental Protection Agency (US EPA) flare pseudo-source parameters are over 30 years old and few dispersion modellers understand their basis and underlying assumptions. The calculation of plume rise from the user inputs of pseudo-stack diameter, temperature and velocity have the most influence on air dispersion model predictions of ground-level concentrations. Regulatory jurisdictions across Canada, the United States and around the world have adopted their own approach to pseudo-source parameters for flares; all relate buoyancy flux to the heat release rate, none consider momentum flux and flare tip downwash as adopted by the Alberta Energy Regulator (AER). This paper derives the plume buoyancy flux for flares burning a gas in terms of combustion variables readily known or calculated without simplifying assumptions. Dispersion model prediction sensitivity to flared gas composition, temperature and velocity, and ambient conditions are now correctly handled by the AER approach. The AER flare pseudo-source parameters are based on both the buoyancy and momentum flux, thus conserving energy and momentum. The AER approach to calculate the effective source height for flares during varying wind speeds is compared to the US EPA approach. Instead of a constant source for all meteorological conditions, multiple co-located sources with varying effective stack height and diameter are used. AERMOD is run with the no stack tip downwash option as flare stack tip downwash is accounted for in the effective stack height rather than the AERMOD model calculating the downwash incorrectly using the pseudo-source parameters. The modelling approaches are compared for an example flare. Maximum ground level predictions change, generally increasing near the source and decreasing further away, with the AER flare pseudo-source parameters. It's time to update how we model flares. Implications: What are the implications of continuing to model flare source parameters using the overly simplified US EPA approach? First, the regulators perpetuate the myths that the flare source height, temperature, diameter and velocity are constant for all wind speeds and ambient temperatures. Second, that it is acceptable to make simplifying assumptions that violate the conservation of momentum and energy principles for the sake of convenience. Finally, regulatory decisions based on simplified source modelling result in predictions that are not conservative (or realistic). The AER regulatory approach for flare source parameters overcomes all of these shortcomings. AERflare is a publicly available spreadsheet that provides the "correct" inputs to AERMOD.
美国环保署（US EPA）火炬伪源参数已有 30 多年的历史，很少有扩散模型能够理解其基础和基本假设。从用户输入的伪烟囱直径、温度和速度中计算羽流抬升对地面浓度空气扩散模型预测的影响最大。加拿大、美国和世界各地的监管管辖区都采用了自己的火炬伪源参数方法；所有方法都将浮力通量与热释放率相关联，而没有考虑动量通量和火炬尖端下洗，这是艾伯塔省能源监管机构（AER）采用的方法。本文从燃烧变量的角度推导出燃烧气体时火炬的羽流浮力通量，这些变量是已知的或无需简化假设即可计算的。现在，AER 方法正确处理了扩散模型对火炬燃烧气体组成、温度和速度以及环境条件的预测敏感性。AER 火炬伪源参数基于浮力和动量通量，从而实现了能量和动量的守恒。AER 计算变风速下火炬有效源高的方法与美国环保署的方法进行了比较。对于所有气象条件，不再使用一个恒定源，而是使用多个共置源，其有效烟囱高度和直径随风速变化。由于火炬尖端下洗已计入有效烟囱高度，而不是 AERMOD 模型使用伪源参数错误地计算下洗，因此 AERMOD 运行时不使用火炬烟囱尖端下洗选项。为了进行比较，对一个示例火炬进行了建模。使用 AER 火炬伪源参数，地面最大预测值发生变化，通常在源附近增加，在更远的地方减少。是时候更新我们的火炬建模方法了。影响：继续使用过于简化的美国环保署方法来模拟火炬源参数会产生什么影响？首先，监管机构延续了这样的神话，即火炬源的高度、温度、直径和速度在所有风速和环境温度下都是恒定的。其次，为了方便起见，违反动量和能量守恒原理做出简化假设是可以接受的。最后，基于简化源建模的监管决策导致的预测结果不保守（或不现实）。AER 火炬源参数监管方法克服了所有这些缺点。AERflare 是一个公开的电子表格，它提供了 AERMOD 的“正确”输入。
