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Publication Title | A THERMODYNAMIC COMPARISON OF THE OXY-FUEL POWER CYCLES WATER-CYCLE, GRAZ-CYCLE AND MATIANT-CYCLE

Organic Rankine Cycle

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A THERMODYNAMIC COMPARISON OF THE OXY-FUEL POWER CYCLES WATER-CYCLE, GRAZ-CYCLE AND MATIANT-CYCLE

Olav Bolland, Norwegian University of Science and Technology, N-7491 Trondheim, Norway, Tel: +47 73591604 - Fax: +47 73598390 – E-mail: Olav.Bolland@tev.ntnu.no

Hanne M. Kvamsdal, SINTEF Energy Research, N-7465 Trondheim, Norway,

Tel: +47 73550465 - Fax: +47 73593580 – E-mail: Hanne.Kvamsdal@energy.sintef.no

John C. Boden, (CO2 Capture Project), BP Oil International, Sunbury-on-Thames, TW16 7LN, UK, Tel: +44 1932 771929 – Fax: +44 1932 763439– E-mail: bodenjc@bp.com

INTRODUCTION

With the aim of developing technologies to reduce the cost of capture and geological storage of carbon dioxide from fossil fuel combustion, an international programme, The CO2 Capture Project, funded by nine leading energy companies, has been established. One of the technology areas targeted is oxy-fuel combustion, since this generates a flue gas consisting largely of carbon dioxide and water from which carbon dioxide is easily separated. The use of oxy-fuel combustion in gas turbine based power generation will require new equipment, but also provides an opportunity to develop new cycles which may offer higher efficiencies than current air-based combined cycle systems, thus partially offsetting the additional cost of oxygen production. Several such cycles have been proposed in the literature and this theoretical study compares the thermodynamic performance of three of them, referred to here as the Water-cycle, the Graz-cycle and the Matiant- cycle.

An evaluation of the three concepts is given in the following sections. The evaluation of the Water cycle concept and the Graz-cycle concept is based on simulations performed by the simulation tool PRO/II (SIMSCI Inc.) while the evaluation of the Matiant-cycle is based on literature statements only.

Computational assumptions. The following assumptions are common for the two computational parts of the present study:

♦ The SRK (Soave-Redlich-Kwong) thermodynamic system including use of steam tables in PRO/II was used for calculation of thermodynamic properties.

♦ The fuel is natural gas with the following composition: C1; 81.2 vol%, C2; 8.97 vol%, C3; 4.26 vol%, C4; 2.35 vol%, C5+; 1.0 vol%. Fuel pressure; 50 bar.

♦ The oxygen for the combustion is produced at atmospheric pressure with an energy requirement of 906 kJ/kg oxygen (0.25 kWh/kg).

♦ The heat exchangers were calculated with a pressure drop of 3%. The combustor(s) were calculated with 4% pressure drop. The adiabatic efficiency of 88% was assumed for all compressors while the turbines were calculated with an adiabatic efficiency of 85% (turbine cooling losses included).

♦ In the present work the compression of CO2 from the condenser pressure to 1 bar is calculated, using a

polytropic efficiency of 75%-85%. For intercooled compression of CO2 from 1 bar to the specified end pressure of 100 bar, a specific value of 390 kJ/kg (about 0.11 kWh/kg) was used ([1]).

WATER-CYCLE

The evaluation of the water cycle concept is to a large extent focused on publications given by [2]-[4]. The Water- cycle can be categorised as a Rankine type power cycle. The working fluid (approximately 90% water) is compressed in the liquid phase, and hot gases are expanded to provide work. In the publications ([2] to [4]), there are various schemes for the cycle configuration with respect to the reheat arrangement. Both single and double reheat is applied.

A flowsheet diagram of the process applied in the present study is shown in Figure 1. Production of oxygen and the compression up to the specific pressure levels are not shown here, but the energy requirement is included in the total energy efficiency calculations. The fuel is compressed (FUEL_COMP) and preheated (E4) before the high-pressure combustion takes place (HP_COMBUSTOR). Oxygen (O2_COMB), from a cryogenic air separation unit, is fed in a stoichiometric ratio with the fuel in the combustor. The combustor exit temperature is controlled by adding water (H2O_MIXED). The combustor exit flow (HP_TI) is expanded in a turbine (HP_TURBINE). The turbine exit stream flows to a secondary, or reheat, combustor (REHEATER). By adding fuel (MP_FUEL) and oxygen (O2_REHEATER) to the reheater, the exit temperature of this unit is controlled. The HP_TURBINE inlet temperature is 871oC which represent a very advanced steam turbine technology based on an uncooled, high pressure turbine while the REHEATER, in which the inlet temperature is 1427oC, represents a standard gas turbine technology based on a cooled, medium pressure turbine. The reheater exit stream is expanded in a turbine (LP_TURBINE). The temperature of this stream (LP_EXHAUST1) is rather high, 730-960°C depending upon combustion pressure and temperature. The turbine exhaust consists of typically 90% steam and 10% CO2. The exhaust is cooled down by fuel preheating (E1) and water heating (EX_COOLER). The water (H2O_MIXED) to the high-pressure combustor is preheated in the EX_COOLER. The exhaust starts to condense in the EX_COOLER and is further cooled by cooling water in the heat exchanger E3. Water and CO2 is split in the CONDENSER. The

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