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This paper considers comparative assessment of combined-heat-and-power (CHP) performance of three small-scale aero-derivative industrial gas turbine cycles in the petrochemical industry. The bulk of supposedly waste exhaust heat associated with gas turbine operation has necessitated the need for CHP application for greater fuel efficiency. This would render gas turbine cycles environ-mentally-friendly, and more economical. However, choosing a particular engine cycle option for small-scale CHP requires information about performances of CHP engine cycle options. The investigation encompasses comparative assessment of simple cycle (SC), recuperated (RC), and intercooled-recuperated (ICR) small-scale aero-derivative industrial gas turbines combined-heat-and-power (SS-ADIGT-CHP). Small-scale ADIGT engines of 1.567 MW derived from helicopter gas turbines are herein analysed in combined-heat-and-power (CHP) application. It was found that in this category of ADIGT engines, better CHP efficiency is exhibited by RC and ICR cycles than SC engine. The CHP efficiencies of RC, ICR, and SC small-scale ADIGT-CHP cycles were found to be 71%, 60%, and 56% respectively. Also, RC engine produces the highest heat recovery steam generator (HRSG) duty. The HRSG duties were found to be 3171.3 kW for RC, 2621.6 kW for ICR, and 3063.1 kW for SC. These outcomes would actually meet the objective of aiding informed preliminary choice of small-scale ADIGT engine cycle options for CHP application.

Gas turbine is a very satisfactory means of producing mechanical power. It is designed to be highly effective in producing aligned high thrust and power [

Some processes in the petrochemical industry occur at relatively moderate temperatures (below 600˚C), and steam is generally the source of their heat energy supply. Such processes include the likes of refining and transformation of crude oil by separation, conversion, and purification carried out in refineries [

The benefit of CHP is illustrated in

The decision to use aero-derivative gas turbines is mainly based on economical and operational advantages. Gas turbine manufacturers have found that to reduce cost of designing and developing new gas turbines, a more effective approach is to develop high performance industrial gas turbines by modifying aircraft gas turbine engines [

turn lead to reducing maintenance operation and enhancing gas turbine availability in industrial applications [_{x} control requirements because they are suitable for power augmentation by steam injection. For instance, the GE LM series industrial aero-derivative gas turbines are meeting NO_{x} requirements as low as 25 parts per million (ppm) using steam injection. Other merits of aero-derivative gas turbines include low weight-to-power ratio, compactness, and hence, lesser erection and startup time [

However, deciding on choice of small-scale ADIGT cycle option for CHP application poses some difficulty for engineers and decision-makers. Hence, the objective of this paper is to carry out performance comparison of simple cycle (SC), recuperated (RC), and intercooled-recuperated (ICR) small-scale ADIGT cycles in CHP, that would aid good and informed choice of turbine cycle option for the purpose of use in small-scale CHP application. The novelty of this research work is in the area of comparing the performances of SC, RC, and ICR aero- derivative gas turbine cycles in small-scale CHP. Previous works only considered CHP analysis of the simple engine cycle, and as such, considering CHP performance of advanced cycles (RC and ICR) actually presents a wider range of options for small-scale CHP engine cycle choices.

CHP systems are either developed as “topping cycles” or “bottoming cycles” as illustrated in

process with subsequent utilisation for power generation [

A set of heat exchangers that utilises the exhaust heat of a gas turbine to produce steam is referred to as heat recovery steam generator (HRSG). Three types of HRSG are identified, namely, unfired, supplementary fired, and exhaust fired. The most common and widely used HRSG is the unfired type because it is simple in design and cheap [

Approach point is the difference between the temperature of saturated steam and the temperature of water entering the evaporator, whereas pinch point is the difference between the gas temperature leaving the evaporator and the temperature of saturated steam [

Using the notations in

To model the design point performance of a CHP plant is to match the parameters of HRSG with the design point of the gas turbine given particular consideration to desired steam flow or temperature and saturation pressure. In doing so, pinch and approach points are selected by the engineering judgement; and from gas turbine exhaust gas flow, the HRSG temperature profile, duty, and steam flow are established. Using pinch technology and thermodynamic properties of steam, the computation of CHP HRSG gas/steam temperature profile and steam flow is done as follows: Gas turbine exhaust gas temperature and mass flow are imported from gas turbine performance simulation while the HRSG pinch and steam saturation pressure (which fixes the steam saturation temperature―T_{c}) are selected by the engineering judgement. In this design the steam saturation pressure is 10bar. With the notations of _{x}) is given by Equation (1)

where

The superheated steam temperature (T_{e}) is chosen as required by the industrial process heat demand. The steam flow (w_{s}) is computed from total heat transfer in super-heater and evaporator using heat balance above pinch as defined by Equation (2)

where 0.99 = heat loss factor,

0.02 = blow down factor,

Equation (3) defines the super-heater duty (Q_{super})

Gas temperature drop in the super-heater (ΔT_{4y}) is given by Equation (4)

This implies that exhaust gas temperature to evaporator (T_{y}) is calculated using Equation (5)

Evaporator duty (Q_{evap}) is determined with the aid of Equation (6)

Similarly Equation (7) defines Economiser duty (Q_{econ})

Gas temperature drop in the economiser (ΔT_{x}_{1}) is given by Equation (8)

This implies that exhaust gas exit temperature from the economiser (T_{1}) is calculated using Equation (9)

Total HRSG duty (Q_{HRSG}) is computed by Equation (10)

The electrical efficiency could be assumed, such that

where

Heat to power ratio of the CHP is given by Equation (11)

Equations (1) to (11) are referred from [

Equation (12) is used to compute First Law CHP efficiency (η_{1})

where

Equation (13) is used to compute Second Law CHP efficiency (η_{2})

The denominator of Equation (13) is the availability rate of the fuel consumed,

where

The CHP design point simulation for the SS-ADIGT was done using TURBOMATCH (a gas turbine engine performance simulation code) [

The technical performance of the simple, recuperated, and intercooled-recuperated SS-ADIGT cycles derived from helicopter engines have been analysed by the author in [

The HRSG would normally not operate at the design point due to variations in the inlet gas conditions and steam parameters. The inlet gas conditions in turn would depend on gas turbine off-design variation in ambient conditions, firing temperature, altitude, power setting, etc. This makes the CHP plant exhibits varying outputs. The CHP off-design performance was simulated with TURBOMATCH engine off-design. The off-design performances of the SS-ADIGT-CHP with changing conditions of the engines are shown in Figures 6-10.

At design and off-design conditions the RC and ICR ADIGT engines exhibit better CHP efficiency than the

Parameter | Values for the SS-ADIGT engines | ||
---|---|---|---|

Simple cycle | Recuperated | ICR | |

Steam saturation temperature (K) Pinch point Approach point Superheated steam temperature (K) Steam mass flow (kg/s) Economiser feed water temperature (K) Super-heater duty (kW) Evaporator duty (kW) Economiser duty (kW) HRSG duty (kW) Gas turbine exhaust mass flow (kg/s) Gas turbine exhaust temperature (K) Gas temperature at evaporator exit (K) Gas exit (stack) temperature (K) Gas turbine power (kW) GT Thermal efficiency Heat: power ratio CHP efficiency | 457 15 8 673 1.10 388 530.07 2198.23 334.74 3063.05 5.65 885 806 422 1567 0.296 2.09 0.56 | 457 15 8 673 1.14 388 548.80 2275.89 346.57 3171.26 5.64 901 819 420 1567 0.336 2.16 0.71 | 457 15 8 673 0.94 388 453.69 1881.45 286.50 2621.63 5.64 826 758 429 1567 0.339 1.79 0.60 |

SC engine as shown in

in CHP efficiencies of RC and ICR over SC at design-point (DP) are 16.5% and 3.8% respectively. This superior performance is due to the lower heat input from burning less fuel in the advanced cycle engines. Looking at

On the other hand, the SC engine produces more HRSG duty than the ICR cycle as shown in

Besides, as plotted in

The foregoing analysis of technical performances of small-scale aero-derivative industrial gas turbine-CHP cycles has led to the conclusions that for small-scale ADIGT-CHP, better CHP efficiency is exhibited by RC and ICR cycles than Simple engine cycle. Also, it was found that the RC engine produces the highest HRSG duty. Therefore, it could be said that performance comparison of simple, recuperated, and intercooled-recupe- rated small-scale ADIGT cycles in CHP has been achieved. This sort of analysis would actually aid concerned engineers, product developers, and other key decision-makers to logically make good and informed choice of small-scale ADIGT engine cycle option for the purpose of use in CHP application.

Very essentially, the authors would want to thank Professor Pericles Pilidis and Dr. Theoklis Nikolaidis of the Department of Power and Propulsion of Cranfield University, United Kingdom, for their invariable contributions to this research.

BarinyimaNkoi,Barinaadaa ThaddeusLebele-Alawa, (2015) Comparative Assessment of Combined-Heat-and-Power Performance of Small-Scale Aero-Derivative Gas Turbine Cycles. Journal of Power and Energy Engineering,03,20-32. doi: 10.4236/jpee.2015.39002