Thermodynamic Analysis of a Steam Injected and Recuperated Gas Turbine Air Compressor

Abstract

Compressed air and steam are perhaps the most significant industrial utilities after electricity, gas and water, and are responsible for a significant proportion of global energy consumption. Microturbine technology, in the form of a Gas Turbine Air Compressor (GTAC), offers a promising alternative to traditional, electrically driven air compressors providing low vibration, a compact size, reduced electrical consumption and potentially reduced greenhouse gas emissions. With high exhaust temperatures, gas turbines are well suited to the cogeneration of steam. The compressed air performance can be further increased by injecting some of that cogenerated steam or by conventional recuperation.This paper presents a thermodynamic analysis of various forms of the GTAC cycle incorporating steam cogeneration, steam injection (STIGTAC) and recuperation. The addition of cogeneration leads to improved energy utilisation, while steam injection leads to a significant boost in both the compressed air delivery and efficiency. As expected, for a low pressure ratio device, recuperating the GTAC leads to a significant increase in efficiency. The combination of steam injection and recuperation forms a recuperated STIGTAC with increased compressed air performance over the unrecuperated STIGTAC at the expense of reduced steam production. Finally, an analysis using a simpli- * Address all correspondence to this author fied model of the STIGTAC demonstrates a significant reduction in CO 2 emissions, when compared to an equivalent air compressor driven by primarily coal-based electricity and a natural gas fired boiler. NOMENCLATUREa, c, d, e Number of mol CMF Corrected mass flow rate EGT Evaporative gas turbine GTAC Gas turbine air compressor H Enthalpy h Specific enthalpy HAT Humid Air Turbine HRSG Heat recovery steam generator ∆h f o Enthalpy of formation M Mach numbeṙ m Mass flow rate Mo Molar mass NIST National Institute of Standards and Technology q Heat release/transfer r p Pressure ratio S Steam ratio STIG Steam injected gas turbine STIGTAC Steam injected gas turbine air compressor 1

Demonstration of Model-Based, Off-Line Performance Analysis on a Gas Turbine Air Compressor

“…It is emphasized that these efficiencies are low because this rig is a prototype. The companion work [6] demonstrates how a GTAC might be made significantly more efficient, through recuperation, steam injection, and cogeneration, with predicted reductions in greenhouse gas emissions of up to 50% compared with existing technologies.…”

“…This study showed that it was possible to bleed up to one-third of the cycle air flow for compressed air supply, limited by thermal considerations in the turbine. Although the efficiency of this prototype, unrecuperated configuration was low, significant improvements to the efficiency and greenhouse gas emissions of the device can be achieved through exhaust heat recovery, as detailed by a companion work [6].…”

“…The prototype GTAC, shown in Fig. 2, was designed as the first step in a larger development program [6]. The rig is built around a Garrett GT3076R automotive…”

“…where the tunable parameter c pd is constant across all operating conditions. This leads to a combustor pressure ratio r p;cc ¼ 1 À DP t;cc P t;cci (6) For the pressure loss due to nonadiabatic flow in a perfect gas, Oates [11] derives the result…”