University of Nairobi Water Distillation by Solar Power Report this is the subject Water distillation by solar power this is the required thing Page lim

University of Nairobi Water Distillation by Solar Power Report this is the subject

Water distillation by solar power

this is the required thing

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in the attachment is the example of the report Hybrid Solar Flameless Combustion System: Modeling
and Thermodynamic Analysis
Seyed Ehsan Hosseini
Email Address: seyed.ehsan.hosseini@gmail.com, shosseini@atu.edu
Abstract
In this paper, the idea of using flameless combustion in a hybrid solar combustion system is
investigated by modelling and thermodynamically analyzing a gas turbine system. In this
regard, a gas turbine system coupling with hybrid solar flameless combustion including
heliostat solar field, central receiver, flameless combustor and power generation system are
modeled. In conditions that sun power is not adequate to heat up the combustion air over autoignition temperature of the fuel, air is passed through the first stage combustor. To provide a
basic cycle for comparison, a common gas turbine with preheater is modeled as well. Energy
and exergitic-based analyses of various systems are evaluated and environmental footprints
reduction of proposed optimum cycle is assessed and compared to the basic case. The results
illustrate that Nitrogen Oxides formation in hybrid solar flameless combustion is significantly
lower than common gas turbine system. While the gas turbine with preheater generates 67
micro grams Nitrogen Oxides per kWh, hybrid solar combustion system produces less than 7
micro grams Nitrogen Oxides per kWh. In comparison to gas turbine system, fuel consumption
decreases in hybrid solar flameless combustion system from 0.1875 kg/s to 0.16 kg/s (about
14.7%) when solar share is considered just 40%. Since the inlet air of flameless combustor is
charged from solar heater outlet, increasing air temperature enhances the share of solar energy
in the system which results in overall exergy reduction in the system. The proposed system
shows significant environmental benefits and based on the available technologies, it is suitable
for high temperature solar towers.
Keywords: Flameless Combustion, Solar Energy, Pollutant, Gas Turbine
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1. Introduction
Renewable energy sources have great potential to reduce greenhouse gases (GHG) emission
from power generation systems [1]. Currently, the contribution of solar, wind and geothermal
energy is estimated about 0.4% of the global energy demand, while conventional fossil fuel
resources account for around 80% [2]. Statistics illustrate that more than 78% of CO2 emission
is generated by fossil fuel power plants [3]. In addition to the growing global concerns about
climate change due to increasing rate of energy consumption, arising of the exploitation of
finite fossil fuel resources menaces the energy security of the world [4]. It is projected that the
world energy consumption will increase by 70–100% before 2050 [5]. Among renewable and
sustainable energy sources, development of clean and abundant solar energy is perceived to be
a promising solution to mitigate the negative effects of climate change and ensuring energy
security [6]. However, solar power generation without thermal storage systems provides only
a diurnal power due to its intermittent nature [7]. Furthermore, the addition of electrical or
thermal energy storage increases the overall cost of the electricity. Nevertheless, more
investigations are still underway to decrease the investment costs and technical constraints of
adopting solar power generation. The main goal of these endeavors is to generate up to 11% of
the global electricity demand by solar energy by 2050 [8]. As a promising method of the
efficient and low cast solar power generation, the solar thermochemical process is developed
widely where the solar energy is upgraded to the high-quality chemical energy of the fuel. In
this method, the solar thermochemical process utilizes the concentrated solar radiation as a
source of heat to drive an endothermic chemical conversion [9]. Solar steam reforming of
natural gas (NG) was experimented and a steam injected gas turbine power plant were
integrated for solar syngas production and application [10]. The use of photocatalysts to split
water into stoichiometric amounts of Hydrogen and Oxygen (overall water splitting) without
the use of external bias is of particular interest due to its simplicity and potential low cost of
operation [11]. The steam-gasification of coal into syngas was developed using concentrated
solar energy as the source of high-temperature process heat [12]. Experimental investigation
of a 5kW solar chemical reactor for the steam-gasification of petcoke in a high-flux solar
furnace was performed and the effects of varying the particle size and slurry stoichiometry on
the degree of chemical conversion and energy conversion efficiency were examined [13] . Solar
hydrogen production was investigated as an endothermic process which is applied for efficient
and low cost solar power generation [14].
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1.1 Hybrid solar combustion power plants
Investigations confirm that hybrid plants have remarkable advantages over solar only plants,
especially for near term markets. These advantages include lower capital investment as well as
lower energy costs, the opportunity for higher energy conversion efficiency and higher valued
energy due to dispatchability [15]. The photovoltaic technology (PV) as the direct way of
power generation from solar energy is extending its focus from decentralized small-scale power
generation systems towards large-area bulk electricity generation [16].
Hybrid solar combustion (HSC) technology has been recently developed to achieve costcompetitive power generation, especially in the regions that direct sunlight is abundant [17].
As the major option to accelerate market introduction of solar power generation, HSC systems
has illustrated so many advantages compared to solar-only systems. Reduction of technical and
economic risks, higher efficiency of the system, fewer start-up and shutdown-losses and
reduction of part load operation are the main advantages of HSC systems. Moreover HSC
system guarantees the required power delivery to the grid and combustion system can be kept
stand-by to compensate the fluctuating power supply of solar system [18]. In the hybrid solar
gas turbine (HSGT) system, solar power is used to preheat the pressurized air before injection
to the combustion chamber. In HSGT system, since the temperature of the pressurized air is
about 800-1000oC after passing the solar tower, lower amount of fossil fuel is consumed in
combustion chamber to maintain the inlet temperature of the turbine in optimum temperatures
(950-1300oC) [19]. The high efficient and cost effective of HSGT implies a great potential for
cost reduction of environmentally friendly HSC power plants [20]. Thermoeconomic concept
was applied using genetic algorithm for optimization of an Integrated Solar Combined Cycle
(ISCC) system that produces 400 MW of electricity. It was found that the cost of electricity
generated by steam turbine and gas turbine in the optimum design of the ISCCS are about 7.1%
and 1.17% lower with respect to the base case [21].
In the ISCC systems, waste heat of the exhaust gases of gas turbine (GT) is applied to preheat
water and to superheating steam, while solar energy is employed to make additional steam.
Electricity generation by ISCC also results in significant reduction of carbon dioxide emissions
compared to the fossil fuel power plants and, therefore, less environmental impacts [22]. ISCC
system as the modern combined cycle power generation with GT and steam turbines (ST) and
additional thermal input of solar power was initially proposed in 1993 [23]. Since then, several
schemes have been suggested for integration of solar power to supply low temperature energy
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into top and bottoming sections of combined cycle power plant (CCPP). The performance of
an ISCC system and justified the preferences of this system in comparison with a Solar Electric
Generating System (SEGS) was evaluated [24]. In this research, thermal power from parabolic
troughs was integrated to the Rankine cycle (bottom cycle) of a combined cycle power plant
and it was found that the generation cost of electricity (kW h) is significantly low in ISCC
system. High temperature air provided by solar system which is fed into the Brayton cycle of
a CCPP could increase the efficiency of the system around 30% and decrease the cost of
electricity about 6 cent per kWh in such CCPP [18]. The thermodynamic characteristics of an
advanced oxy-fuel hybrid power system (AHPS) using the advanced process simulator Aspen
Plus and compared their results with a solar thermal hybrid steam (STHS) turbine power
generation system as the reference was analyzed [25]. It was concluded that the efficiency of
the AHPS is 95.90%, which is 21.61% higher than that of the STHS. Also, exergy efficiency
of the AHPS was recorded 55.88%, which is 2.13% higher than that of the STHS.
Although, valuable attempts have been made to develop power generation technology by
hybrid solar combustion systems, limited scientific sources can be found about combustion
method in HSC system. The performance of a hybrid solar receiver combustor operating with
Moderate and Intense Low oxygen Dilution (MILD) and conventional combustion was
compared [26]. The effects of the dominant geometrical parameters, burner configuration and
flux distribution of the incident solar radiation on the heat transfer mechanisms, heat losses,
thermal efficiency and heat flux distribution were investigated with a three-dimensional
computational fluid dynamics (CFD) model of the device. Two different burners were
considered; non-premixed and partially premixed flames; one with and the other without swirl.
It was stipulated that the device can obtain a similar thermal performance in each mode of
operations. Moreover, the thermal efficiency of the system increased with the length-todiameter cavity ratio and/or solar peak flux at the focal plane [27]. It was pointed out that the
rate of fuel consumption as well as cost of electricity can be reduced up to 41% and 6%
respectively if MILD combustion is employed in HSC [28].
Since conventional combustion or flame mode is usually used in the gas turbine cycle and solar
energy is used to preheat the combustion air, using preheated air in conventional combustion
increases pollutant formation, especially NOx, exponentially [29]. In this paper the effects of
combustion mode on the performance of a HSC system is developed. To find inefficiencies in
systems, exergy analysis is carried out by simulating the cycles using MATLAB program.
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2. Flameless Combustion
Flameless Oxidation (FLOX), also known as High Temperature Air Combustion (HiTAC), or
Colorless Distributed Combustion (CDC), or Low NOx Emission Injection is a new combustion
technology which is capable to accomplish low NOx emissions and high efficiency among
various techniques [30]. Excess Enthalpy Combustion (EEC) is the principle for the technology
of flameless combustion with preheated air [31]. Uniform heat flux distribution as well as low
NOx formation are the main characteristics of flameless combustion [32]. Furthermore,
flameless was found to be an effective combustion method that can decrease the rate of fuel
consumption and the size of equipment up to 30% and 25% respectively [33]. Noise production
is substantially higher with conventional flame modalities compared to flameless combustion.
Experimental studies have shown significant noise reduction (>7 dB) from flameless
combustion. It was reported that noise in conventional combustion is 98 dB while in flameless
combustion is 82 dB. This may be attributed to smaller volumetric expansion within the
flameless chamber and the uniform distribution of its features [34]. Highly preheated oxidizer
augments the flame temperature in conventional combustion which exponentially increases the
formation of NOx however, in flameless combustion higher temperatures of combustion air are
desirable [35]. Different methods were experimented to preheat the flameless combustion air.
Regenerative and recuperative burners were used in flameless combustion systems [29]. An
electrical heater was applied to increase combustion air temperature higher than the autoignition temperature of the fuel [36]. A CCPP using flameless burner was modeled as a
supplementary firing system located after GT [37]. The diluted and extremely high temperature
turbine exhaust is used to run flameless combustion in this model. In another attempt, using a
self-preheated reactor was experimented to preheat the oxidizer in biogas flameless combustion
[38]. A quartz cylinder was employed as a flameless combustor and quarts pellet in the entrance
of the oxidizer to maintain the temperature over self-ignition temperature of the fuel inside the
chamber [39]. Using a self-regenerative burner was proposed to experiment natural gas and
biogas flameless combustion mode. A new design of boiler operated in flameless mode was
proposed where this flameless boiler design was very similar to that of industrial chamber with
regenerative burners [40]. The thermal energy generated by flameless combustion phenomena
was transferred to the water in the boiler tubes mainly through radiative heat transfer. The 1200
°C exhaust gases were passed through a regenerative burner to decrease the temperature to 150
°C before being released into the atmosphere. The primary air enrichment and preheating (PRP)
5
burner is equipped with a preheating chamber, which one end is connected to the primary
combustion oxidizer and the other end opened to a chamber to preheat the oxidizer [41].
3. System Description and Modeling Formulation
In this paper, a GT system with HSC technology which includes heliostat solar field, central
receiver, combustion and power generation system is modeled. Fig1 illustrates HSC system in
which the solar system concentrates solar energy from solar field (heliostat field) to a receiver
located on the top of tower operating as a convective/radiative heat exchanger. The
concentrated solar energy is converted into heat absorbed by the air from compressor. Highly
temperature air enters combustor to mix with natural gas (NG) as the fuel. The temperature
difference between the solar receiver outlet temperature (around 1000∘C) and the turbine inlet
temperature (950–1300∘C) is closed by combustion chamber which runs in flameless
combustion mode. In this condition, the outlet temperature of solar heater is maintained over
auto-ignition temperature of NG. The constant turbine inlet conditions are provided by
combustion chamber despite temperature variations in solar system output. The detailed data
about the receiver were developed by [19].
Fig1. Solar flameless combustion system (Note: During the sunny hours, air passes
through the solar heater. In hours without sun power, air passes through the first
combustor)
During hours of the day that sun power is not adequate to heat up the combustion air over autoignition temperature of NG or during the night, air is passed through the first stage combustor.
In this combustor, air is heated up to auto-ignition temperature of NG and introduced to second
stage chamber (flameless combustor). To provide a baseline cycle for comparison purposes, a
common GT-preheater configuration (Fig 2) is also modeled.
Fig2. Gas turbine with preheater
To model the system, following assumptions are made to render the analyses in a traceable
way:
•
It is assumed that all processes work under steady state condition.
•
Air and combustion products are considered ideal gas.
•
Natural gas is used as the fuel in combustion system.
•
The kinetic and potential exergies were assumed negligible.
6
•
Reference state is T0= 293.15K and P0= 1.01bar.
Table 1. Sensitivity of solar coupled cycle’s performance parameters to SHR
3.1 Exergy Analyses of the system
Exergetic based analysis is an integral part of thermodynamic assessment of any power plants.
Exergy analysis was extended from economic analysis and life cycle assessment of power
plants and developed to figure out the structure in nature as a form of entropy generation
minimization (EGM) [42]. The scenario of future power generation systems and the role of
exergy analysis in their development was discussed and the author stipulated that exergy
analysis will help the designers to come up with a good decision on how to improve the
performance of a system. It was pointed out that exergy analysis is a powerful tool in evaluating
destructions in a power cycle while these wastes have not be seen through energy analysis [43].
Exergy analysis was employed to evaluate the performance of solar thermal power systems
[44].
Exergy is divided into physical, chemical, kinetic and potential exergy, which the two latter
components are neglected in this study as the elevation and speed have negligible changes [45].
Physical exergy is the maximum theoretical useful work achieved as a system interacts with an
equilibrium state. The chemical exergy is equal to highest amount of achievable work when
the corresponding substance is brought from the environmental state to the reference state by
heat transferring and heat exchanging of substance only with the environment [46].
4. Feasibility of the plan
The idea of the two-stage solar boosted combustion is developed to understand the performance
of the system and effectiveness of the proposed method to reduce emissions. However, the
employed method is a thermodynamics approach which provides components inlet-outlet state
points. A component design process is required to achieve this thermodynamics performance
obtained here especially for combustors. This process design needs detail heat, reaction and
flow analyzing to achieve their system level performance evaluated here. During the process
design, there is always limitations which may prevent the component to reach the
thermodynamics design goals and a new thermodynamics analysis might be designed iteration
process in most cases required. Despite of this nature of the design process, due to the maturity
of combustors and gas turbine components technology, design of this system is highly possible
to the authors’ point of view. Another important parameter which affect the feasibility of the
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plant is the cost. Although the cost analysis needs detail calculations to reach a conclusion, in
authors’ point of view, the proposed plan will be economically as attractive as normal solar GT
plants, or even more attractive if environmental costs reduces significantly. The reason is that,
cost of combustor is much lower than solar field and tower. Hence, even if the modification
proposed here leads to a new combustor design it is not affecting the cost of Solar-GT coupled
system hugely. In addition, the positive effects of NOx reduction in environmental cost and
taxes can cover the modification cost.
5. Results and Discussion
Solar flameless combustion (SFC) and two stage combustors (TSC) are proposed in this
research for their environmental benefits, because when the second combustor is operating in
flameless combustion mode, NOx formation is reduced significantly. To understand effects of
this approach on the performance and environmental footprints of the cycle, a comparison
between three different cycles is carried out and results are presented here. Fuel saving and
environmental benefits are reasons for implementing the solar flameless combustion. For
comparison, gas turbine with preheater (GTPRE) is used as base case for comparison since it
is a well-known configuration for gas turbine.
6. Conclusion
Since solar energy and flameless combustion technologies are known as environmentally
friendly energy conversion systems and to increase the rate of efficiency and decrease fossil
fuel consumption, hybrid solar flameless combustion system and two stage combustion system
were modeled in this paper. Fuel saving due to implementing coupling solar energy with
flameless mode was calculated. Results show that proposed configuration has significa…
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