(181a) Multiscale Modeling and Optimization of Wet Flue Gas Desulfurization

Sulfur dioxide (SO₂) emissions are known to not only cause health issues, but also have an adverse effect on the environment in various ways. SO₂ is known to be a precursor of acid rain and plays a major role in the surface reactions that lead to the depletion of the ozone layer. The primary source of SO₂ emissions is flue gas from fossil fuels-based power plants. Several Flue Gas Desulfurization (FGD) technologies have been incorporated in power plants. These include Limestone Scrubbing, Lime Scrubbing Sodium Carbonate Scrubbing, Magnesia Scrubbing and a few others. Of these, the most popular technology is the limestone slurry system called Wet Limestone FGD (WFGD), primarily because of its high SO₂ removal capacity and low capital and operating costs. Several types of scrubbers such as spray scrubbers, venturi scrubbers and packed beds are used for limestone scrubbing. The most widely used system is the counter-current spray scrubber. In this system, flue gas enters from the bottom and the limestone slurry in the form of droplets is introduced from the top through nozzles distributed over several spray headers. SO₂ from the flue gas reacts with limestone to form calcium sulfite, which is oxidized at the bottom of the scrubber to form the byproduct gypsum.

Due to fast cycling of power plants and variabilities in the sulfur content in the coal, it can be very difficult to satisfy SO₂ emission standards at the outlet of FGD units. Furthermore, specifications of gypsum must also be satisfied during load following operation. In addition, efficiency of the FGD units should be maximized. A rigorous dynamic model of the FGD unit can be helpful in satisfying these challenging performance conditions. A number of rate-limiting mechanisms occur in the limestone droplets while the mechanisms at the scrubber bulk also play a critical role. For instance, the instantaneous reactions comprise of dissociation of species (e.g. SO₂, sulfites, bisulfites, bicarbonates etc.) and finite rate processes involve limestone dissolution, sulfite oxidation and crystallization. These processes have to be coupled with the scrubber bulk hydrodynamics and considered simultaneously. Therefore, a multi-scale dynamic model is required to capture the large number of ions and complex thermodynamic and chemistry. Due to the fast ionic reactions in the limestone droplet and presence of many ionic and molecular species, the system of equations is stiff, highly nonlinear, and not well posed. Therefore, they can be challenging to solve reliably during dynamic simulation.

In this work, we present an implementation of FGD simulation for the limestone scrubbing unit. It is a multiscale model. At the slurry droplet scale, we solve a two-point boundary value problem (BVP) to characterize the absorption of SO₂ in a slurry droplet. This is a Differential-Algebraic Equations (DAE) model based on penetration theory and, as such, requires rigorous analysis and reformulation to obtain and solve a so-called index-1 DAE model. At the scrubber bulk scale, the physical processes include complex factors such as velocity, size distribution, collision and coalescence between the slurry droplets which play a significant role in the SO₂ absorption at the droplet scale.

We use Pyomo, an open source framework for modeling and optimization, to implement the spray scrubber model. We demonstrate the performance of this multiscale model for the limestone WFGD system. First, we show that the BVP associated with SO₂ absorption is an ill-posed, high-index DAE model and present a systematic reformulation procedure to obtain a well-posed, index-1 DAE model that lends to a tractable numerical solution. Second, we simulate SO₂ scrubbing operations for the base line loadings of a coal-fired power plant. Using multi-level spray flowrates as decision variables we also demonstrate the optimization of operating costs and efficiency. Finally, using the Pyomo framework with IPOPT, we demonstrate the dynamic optimization of the FGD operation, such as maximizing efficiency, under transient plant conditions such as ramping up and down in response to varying SO₂ loadings.