Coal Blending Optimization Under Uncertainty. Jhih-Shyang Shih, Center for Energy and Environmental Studies, Department of Engineering and Public Policy, Carnegie Mellon University; and H. Christopher Frey, Department of Civil Engineering, North Carolina State University
Coal blending is one of several options available for reducing sulfur emissions from coal-fired power plants. However, decisions about coal blending must deal with uncertainty and variability in coal properties, and with the effect of off-design coal characteristics on power plant performance and cost. Sulfur content, ash content and heating value are treated as normally distributed random variables. Since the properties of coal are random variables, there is a risk that the specified requirement of the mixture can not be met 100 percent of the time. Therefore, one must consider the risk (or, conversely, the reliability) associated with potential exceedance (or compliance) of coal quality specifications. Given a characterization of variability in the properties of coals that comprise a blend, we may ask the following questions: (1) How should we specify coal quality in terms of constraints and the probability of exceeding those constraints due to variability in coal properties? (2) Given an explicit probabilistic description of acceptable blended coal quality, how can one optimize coal blends to minimize emissions and/or cost? (3) How can the variance in coal properties be minimized by coal blending? (4) What are the trade-offs between different objectives for coal blending (e.g., minimizing expected cost, minimizing expected emissions, or minimizing variance in cost or emissions)? (5) What is the benefit of reducing coal property measurement error? This paper will answer these questions taking into account the probabilistic nature of coal properties and the need to consider downstream effects of changes in coal properties on plant performance, emissions, and cost. A mathematical programming model is developed and applied to generate insight into: (1) coal blending practice for a power plant; (2) implications of coal blending for regulatory compliance; and (3) investment decision making for power plant modifications required for coal blending and for improvement of coal measurements.