Application of waste energy recovery schemes in natural gas processing plants

  • Adesola Oluwasijibomi Olufade

Student thesis: Master's Thesis

Abstract

Waste energy recovery (WER) has gained increasing adoption in the industry to reduce energy consumption and CO2 emissions. This Thesis investigates WER in the natural gas (NG) industry through a thermo-economic and exergy analysis of three solid oxide fuel cell (SOFC) schemes built with WER capacity for reverse osmosis (RO) seawater desalination, power generation and cooling applications in natural gas processing plants located in the Arabian Gulf. In the first scheme (denoted as SOFC-HX-ORC-RO), SOFC exhaust gas waste heat is recuperated using a thermal oil loop to drive a single-stage organic Rankine cycle (ORC) which supplies mechanical power to the high pressure pump of a RO unit. In the second scheme (denoted as SOFC-HX-CORC-RO), the single ORC is replaced by a cascaded (i.e., two-stage) ORC to increase ORC mechanical power output. In the third scheme (denoted as SOFC-HX-ARS-ORC-RO), part of SOFC exhaust heat is used to drive a single-effect lithium bromide – water (LiBr-H2O) absorption chiller to provide process cooling. In addition, for each scheme the option of preheating the RO feed water through the ORC condenser is considered. The use of an energy recovery turbine in the RO unit is also considered. Waste heat recovery scheme thermodynamic performance is evaluated using standard mass and energy balance equations in Aspen HYSYS software, while the RO system performance is evaluated with Reverse Osmosis System Analysis (ROSA) software. A simple thermodynamic modeling methodology is adopted which provides a preliminary assessment of scheme feasibility. The predictions indicate that the SOFC-HX-CORC-RO scheme is the best performing scheme in terms of annual fresh water production and economic feasibility. This scheme produces an annual average of 429 m3/h of fresh water. RO feed seawater preheating yields minimal increase in fresh water output. The exergy efficiency of the scheme is 49%. The scheme has an estimated payback period of 3 to 17 years and primary energy savings of 40 to 115 MMSCF of NG per year depending on utility prices, SOFC CAPEX and environmental emissions tax rates. The scheme net present value is of approximately US$ 250 million, and internal rate of return of approximately 28.5%. The scheme specific water cost and benefit-to-cost ratio are of US$ 2.42/m 3 and 2.9, respectively, which signifies that the investment is profitable. Retrofitting an existing operational GT unit having same net power output as the SOFC and separate RO unit with the proposed SOFC WER scheme would lead to CO2 savings of approximately 28 million ton/year. The analyses presented indicate the techno-economic feasibility of the proposed WER scheme. Model refinement areas are proposed that would need to be addressed to yield more accurate thermodynamic and economic predictions.
Date of Award2014
Original languageAmerican English
SupervisorValerie Eveloy (Supervisor)

Keywords

  • Applied sciences
  • Mechanical engineering
  • 0548:Mechanical engineering

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