Author: Hussam Zebian

Publisher:

Published: 2014

Total Pages: 222

ISBN-13:

Efficient and clean electricity generation is a major challenge for today's world. Multivariable optimization is shown to be essential in unveiling the true potential and the high efficiency of pressurized oxy-coal combustion with carbon capture and sequestration for a zero emissions power plant (Zebian and Mitsos 2011). Besides the increase in efficiency, optimization with realistic operating conditions and specifications also shows a decrease in the capital cost. Elaborating on the concept of increasing the performance of the process and the power generation efficiency, as part of this Ph.D. thesis, new criteria for the optimum operation of regenerative Rankine cycles, are presented; these criteria govern the operation of closed and open feedwater heaters, and are proven (partly analytically and partly numerically) to result in more efficient cycle than the conventional rules of thumb currently practiced in designing and operating Rankine cycles. Simply said, the pressure and mass-flowrate of the bleed streams must be selected in a way to have equal pinch temperatures in the feedwater heaters. The criteria are readily applicable to existing and new power plants, with no associated costs or retrofitting requirements, contributing in significant efficiency increase and major economical and environmental advantages. A case study shows an efficiency increase of 0.4 percentage points without capital cost increase compared to a standard design; such an efficiency increase corresponds to an order of $40 billion in annual savings if applied to all Rankine cycles worldwide. The developed criteria allow for more reliable and trustworthy optimization, thus, four additional aspects of clean power generation from coal are investigated. First, design and optimization of pressurized oxy-coal combustion at the systems-level is performed while utilizing a direct contact separation column (DCSC) instead of a surface heat exchanger for more reliable and durable thermal recovery. Despite the lower effectiveness compared to a surface heat exchanger, optimization employing newly developed optimal operating criteria that govern the DCSC allow for an efficient operation, 3.8 percentage points higher than the basecase operation; the efficiency of the process utilizing a DCSC is smaller than that utilizing a surface heat exchanger but only by 0.32 percentage points after optimization. Optimization also shows a reduction in capital costs by process intensification and by not requiring the first flue gas compressor in the carbon sequestration unit. Second, in order to eliminate performance and economical risks that arise due to uncertainties in the conditions that a power generation process may be subjected to, the designs and operations that allow maximum overall performance of the process while facing all possible changes in operating condition are investigated. Therefore, optimization under uncertainty in coal type, ranging from Venezuelan and Indonesian coals to a lower grade south African Douglas Premium and Kleinkopje coal, and in ambient conditions, up to 10°C difference in the temperature of the cooling water, of the pressurized oxy-coal combustion are performed. Using hierarchic optimization and stochastic programing, the latter shown to be unnecessary, an ideally flexible design is attained, whereby the maximum possible performance of the process with any set of input parameters is attained by a single design. While in general a process designed for a specific coal has a low performance when the utilized coal is changed, for the pressurized oxy-coal combustion process presented herein, it is demonstrated that designing (and optimizing) while taking into consideration the different coal types utilized, results for each coal in performance that is equal to the maximum performance obtained by a design dedicated to that coal. The third aspect considered is flexibility with respect to load variation. Particularly with the increase of the power generation from intermittent renewable energy sources, coal power plants should operate at loads far from nominal, down to 35%. In general this results in efficiency significantly lower than the optimum. Therefore, while keeping the turbine expansion line design fixed to that of the nominal load in order to allow for a full range of thermal load operations, an elaborate study of the variations in thermal load for pressurized oxy-coal combustion is performed. Here too optimization of design and operation taking into consideration that load is not fixed results in a process that is flexible to the thermal load; the range of thermal load considered is 30..100%. The fourth aspect considered is a novel design for heat recovery steam generator (HRSG), which is an essential part of coal power plants, particularly oxy-coal combustion. It is the site of high temperature thermal energy transfer, and is shown to have potential for significant improvements in its design and operation. A new design and operation of the HRSG that allow for simultaneous reduction in the area and the flow losses is proposed: the hot combustion gas is splitted prior to entering the HRSG and prior to dilution with the recycling flue gas to control its temperature as dictated by the HRSG maximum allowed temperature. The main combustion gas flow proceeds to the HRSG inlet and requires smaller amounts of dilution and recycling power requirements compared to the conventional no splitting operation. The splitted fraction is introduced downstream at an intermediate location in the HRSG; the introduction of the splitted gas results in increasing the temperature of the flue gas and the temperature difference between the hot and the cold streams of the HRSG, particularly avoiding small temperature differences which require the most heat transfer area. Results include area reduction by 37% without change in the compensation power requirements, or a decrease in the compensation power requirements by 18% (corresponding to 0.15 percent points of the cycle efficiency) while simultaneously reducing the area by 12%.