Combined cycle versus one thousand diesel power plants: pollutant emissions,ecological efficiency and economic analysis

The increase in the use of natural gas in Brazil has stimulated public and private sectors to analyse the possibility of using combined cycle systems for generation of electrical energy. Gas turbine combined cycle power plants are becoming increasingly common due to their high efficiency, short lead times, and ability to meet environmental standards. Power is produced in a generator linked directly to the gas turbine. The gas turbine exhaust gases are sent to a heat recovery steam generator to produce superheated steam that can be used in a steam turbine to produce additional power. In this paper a comparative study between a 1000 MW combined cycle power plant and 1000 kW diesel power plant is presented. In first step, the energetic situation in Brazil, the needs of the electric sector modification and the needs of demand management and integrated means planning are clarified. In another step the characteristics of large and small thermoelectric power plants that use natural gas and diesel fuel, respectively, are presented. The ecological efficiency levels of each type of power plant is considered in the discussion, presenting the emissions of particulate material, sulphur dioxide (SO₂), carbon dioxide (CO₂) and nitrogen oxides (NO_x).     

Ecological efficiency in CHP: Biodiesel case

This paper evaluates and quantifies the environmental impact resulting from the combination of biodiesel fuel (pure or blended with diesel), and diesel combustion in thermoelectric power plants that utilize combined cycle technology (CC). In regions without natural gas, the option was to utilize diesel fuel; the consequence would be a greater emission of pollutants. Biodiesel is a renewable fuel which has been considerably interesting in Brazil power matrix in recent years. The concept of ecological efficiency, largely evaluates the environmental impact caused by CO₂, SO₂, NO_x and particle matter (PM) emissions. The pollution resulting from biodiesel and diesel combustion is analyzed, separately considering CO₂, SO₂, NO_x and particulate matter gas emissions, and comparing them international standards currently used regarding air quality. It can be concluded that it is possible to calculate the qualitative environmental factor, and the ecological effect, from a thermoelectric power plant utilizing central heat power (CHP) of combined cycle. The ecological efficiency for pure biodiesel fuel (B100) is 98.16%; for biodiesel blended with conventional diesel fuel, B20 (20% biodiesel and 80% diesel) is 93.19%. Finally, ecological efficiency for conventional diesel is 92.18%, as long as a thermal efficiency of 55% for thermoelectric power plants occurs.     

Fuel cell operation on landfill gas at Penrose Power Station

This demonstration test successfully demonstrated the operation of a commercial phosphoric acid fuel cell (FC) on landfill gas (LG) at the Penrose Power Station in Sun Valley, CA. Demonstration output included operation up to 137 kW; 37.1% efficiency at 120 kW; exceptionally low secondary emissions (dry gas, 15% O₂) of 0.77 ppmV CO, 0.12 ppmV NO_x, and undetectable SO₂; no forced outages with an adjusted availability of 98.5%; and a total of 707 h of operation on LG. The LG pretreatment unit (GPU) operated for a total of 2297 h, including the 707 h with the FC, and documented total sulfur and halide removal to much lower than the specified <3 ppmV for the FC. The GPU flare safely disposed of the removed LG contaminants by achieving destruction efficiencies greater than 99%.     

Experimental studying of a small combined cold and power system driven by a micro gas turbine

A small combined cold and power (SCCP) system is presented. An experimental study of the performance of the SCCP system is described. The gas fuelled SCCP system uses a micro gas turbine generator set and an absorption chiller. The test facility designed and built is also described. The rated electricity power of the micro gas turbine generator is about 24.5 kW at the experimental conditions. When exhaust gas from the micro gas turbine is used to drive the absorption chiller, the rated cooling capacity is 52.7 kW without supplying fuel to burn in the absorption chiller and 136.2 kW with supplying about 78.9 kW LPG fuel to burn in the absorption chiller, respectively. Primary energy rate (PER) and comparative saving of primary energy demand are used to evaluate the performance of the SCCP system. PER of the SCCP system decreases rapidly with the decrease of electric power output when the electric power output is less than 10 kW. The calculated results also show that comparative saving of primary energy demand of the SCCP system decreases with the decrease of electric power output and the SCCP system do not save primary energy comparing to conventional energy system when the electric power output is less than 10 kW.     

Experimental evaluation of a low-power direct air-cooled double-effect LiBr–H2O absorption prototype

This paper describes a new small air-cooled double-effect LiBr–H₂O absorption prototype directly powered by fuel and discusses the experimental findings for some tests carried out in Madrid in 2007, with natural gas as energy source. The prototype, which has been designed to supply 7 kW of cooling power, was able to chill water up to 7–18 °C under extreme outdoor temperatures. A new flat-sheet adiabatic absorber was used allowing it to operate at outdoor temperatures about 45 °C without any sign of crystallization. A mean daily coefficient of performance (COP) of about 1.05 was obtained. Since this absorption machine does not need cooling tower, there is neither water consumption nor Legionella pollution. Moreover, it is a quite compact unit. The ratio of cooling power over volume is about 6.0 kW/m³, while for the only air-cooled absorption chiller, Rotartica 045v, in the marked until 2009 this ratio is 4 kW/m³. When comparing with electric chillers presently on the market, this prototype was found to have a cooling cost approximately 15.9% higher and an environmental impact 16.7% lower. The absorption prototype is a more environmentally friendly solution as it does not emit fluorinated refrigerants.     

Energy policy tools for agricultural residues utilization for heat and power generation: A case study of sugarcane trash in Thailand

Cane trash could viably substitute fossil fuels in heat and power generation projects to avoid air pollution from open burning and reduce greenhouse gas (GHG) emission. It is competitive with bituminous and other agro-industrial biomass. Using cane trash for heat generation project could provide a higher reliability and return on investment than power generation project. The heat generation project could be viable (Financial Internal Rate of Return, FIRR = 36–81%) without feedstock subsidy. With current investment and support conditions, the capacity of 5 MW option of power generation project is the most viable (FIRR = 13.6–15.3%); but 30 MW, 1 MW and 10 MW options require feedstock subsidy 450–1100 Baht/t-cane trash to strengthen financial viability. Furthermore, the revenue from carbon credit sales could compensate the revenue from current energy price adder and increases 0.5–1.0% FIRR of power generation project. Using cane trash for 1 MW power generation could reduce GHG emission 637–861 t CO₂eq and avoid air pollutant emissions of 3.35 kg nitrogen oxides (NO_x), 0.41 kg sulfur oxides (SO_x) and 2.05 kg volatile organic compounds (VOC). Also, 1 t steam generation from cane trash could avoid pollutant emissions of 0.6 kg NO_x, 0.07 kg SO_x, and 0.37 kg VOC. The potential of cane trash to cause fouling/slagging as well as erosion are not significantly different from other biomass, but chlorinated organic compounds and NO_x could be higher than bituminous and current biomass feedstock at sugar mill (bagasse and rice husk).     

Development of a thermoelectric refrigerator with two-phase thermosyphons and capillary lift

A thermoelectric domestic refrigerator has been developed, with a single compartment of 0.225 m³, for food preservation at 5 °C. The cooling system is made up of two equal thermoelectric devices, each composed of a Peltier module (50 W) with its hot side in contact with a two-phase and natural convection thermosyphon (TSV) and a two-phase and capillary lift thermosyphon (TPM), in contact with the cold side.The entire refrigerator has been simulated and designed using a computational model, based on the finite difference method. Subsequently an experimental optimization phase of the thermosyphons was carried out, until thermal resistance values of R_TSV = 0.256 K/W and R_TPM = 0.323 K/W were obtained. These values were lower than those obtained with finned heat sinks.Finally, a functional prototype of a thermoelectric refrigerator was built, and the results which were obtained demonstrate that it is able to maintain a thermal drop (Ambient Temperature–Inside Temperature) of 19 °C. The electric power consumption at nominal conditions was 45 W, reaching a COP value of 0.45. The study demonstrated that by incorporating these two-phase devices into thermoelectric refrigeration increases the COP by 66%, compared with those which use finned heat sinks.     

Life cycle assessment of a Brassica carinata bioenergy cropping system in southern Europe

The energetic and environmental performance of production and distribution of the Brassica carinata biomass crop in Soria (Spain) is analysed using life cycle assessment (LCA) methodology in order to demonstrate the major potential that the crop has in southern Europe as a lignocellulosic fuel for use as a renewable energy source.The Life Cycle Impact Assessment (LCIA) including midpoint impact analysis that was performed shows that the use of fertilizers is the action with the highest impact in six of the 10 environmental categories considered, representing between 51% and 68% of the impact in these categories.The second most important impact is produced when the diesel is used in tractors and transport vehicles which represents between 48% and 77%. The contribution of the B. carinata cropping system to the global warming category is 12.7 g CO₂ eq. MJ⁻¹ biomass produced. Assuming a preliminary estimation of the B. carinata capacity of translocated CO₂ (631 kg CO₂ ha⁻¹) from below-ground biomass into the soil, the emissions are reduced by up to 5.2 g CO₂ eq. MJ⁻¹.The production and transport are as far as a thermoelectric plant of the B. carinata biomass used as a solid fuel consumes 0.12 MJ of primary energy per 1 MJ of biomass energy stored. In comparison with other fossil fuels such as natural gas, it reduces primary energy consumption by 33.2% and greenhouse gas emission from 33.1% to 71.2% depending on whether the capacity of translocated CO₂ is considered or not.The results of the analysis support the assertion that B. carinata crops are viable from an energy balance and environmental perspective for producing lignocellulosic solid fuel destined for the production of energy in southern Europe. Furthermore, the performance of the crop could be improved, thus increasing the energy and environmental benefits.     

Combustion of spent mushroom compost and coal tailing pellets in a fluidised-bed

Previous investigations found that fluidised-bed combustion of spent mushroom compost–coal tailing pellets was preferred for these high ash content fuels. This paper considers the combustion tests carried out on these wastes in a laboratory-scale fluidised-bed, where parameters, including the pellet feedrate, primary/fluidising air flowrate and bed depth, were investigated. Based on the minimum air ratio of 2.5 required to achieve high combustion efficiencies of around 97%, the optimum operating conditions for the combustor employed were a pellet feedrate of 3.25 kg/h (180 kg/m²h) and a total air flowrate of 650 kg/m²h. A lower sand bed depth of 0.22 m was also deemed beneficial, as deeper beds resulted in slugging and noticeable reductions in combustion efficiency. Acid gas emissions (NO_x, SO_x and HCl) were found in limited concentrations, as species remained primarily as inorganic compounds in the flyash. Some N₂O is thought to have formed, as fluidised-bed combustors are particularly prone to this. The alkali index of the ash suggests probable fouling/slagging in the system. For industrial-scale combustion of these wastes, the combustion efficiency could be improved by the presence of secondary air jets to aid turbulent mixing.     

Experimentation on a cogenerative system based on a microturbine

The paper reports on experimental results of an energetic characterization of a cogenerative plant. The generator is a microturbine Turbec T100-CHP integrated in a heat recovery system. For operation in standard conditions the maximum electrical and thermal power generated are, respectively, 105 and 167 kW. Experimental tests were run by varying the electrical power produced between 50 and 110 kW with 10 kW stepping. For each step the set-point of the water temperature at the outlet of the recuperator was varied in the range 60–80 °C with 5 °C stepping. In every operating condition the measurement system allows the real-time calculation of the quantities needed for the energetic characterization of the plant, such as efficiency indices and PES (primary energy saving index). It is seen that performances remain essentially constant in the range 80–110 kW. A moderate decrease is then observed until about 60 kW, while a further reduction of the electrical power implies a clear worsening (PES decreases from about 30% to 16% in the tested range). Furthermore, environmental impact has been investigated with respect to gaseous and acoustic emissions. In particular the concentration of pollutants in exhaust gases, except for NO_x and CO₂, strongly increases by reducing the electrical power output.     

Negative pressure dependence of mass burning rates of H2/CO/O2/diluent flames at low flame temperatures

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H₂/CO/O₂/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO₂ up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO₂ was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N₂ and H₂O.Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO_x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO₂ + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O₂(+M) = HO₂(+M) in both pure and mixed bath gases, in rate constants for HO₂ reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H₂ or H₂/CO flames.     

Numerical calculation of local entropy generation in a methane–air burner

This study considers numerical simulation of the combustion of methane with air including 21% oxygen and 79% nitrogen in a burner and the numerical solution of the local entropy generation rate due to the high temperature and velocity gradients in the combustion chamber for various fuel flow rates (from 5 to 10 lpm). Swirling air flow is also used to burn the methane more efficiently. The effects of equivalence ratio (ϕ) and swirl number (S) on the combustion and entropy generation rate are investigated for different (consecutive) equivalence ratios (from 0.5 to 1.0) and swirl numbers (from 0 to 0.3). The numerical calculation of combustion is performed individually for these cases with the help of the Fluent CFD code. The volumetric entropy generation rate distributions and the other thermodynamic parameters are calculated numerically by using the results of the combustion calculations. The maximum values of the rates of reaction-1 and -2 decrease with the increase of ϕ. In the case of ϕ < 1, complete combustion occurs, and the combustion in the case of ϕ = 1 is very close to the complete combustion state. In the case of no swirl, the entropy generation rate decreases exponentially with the increase of ϕ in the cases of high Q_f, whereas they have quadratic profiles having their minimum values in cases of low Q_f. In terms of the entropy generation rates, the optimum equivalence ratios for Q_f = 5, 6, 7, >7 lpm in the case of S = 0 and Q_f = 10 lpm in the case of S = 0.3 are obtained as ϕ = 0.66, 0.8, 0.86, 1.0 and 0.92, respectively.     

Parametric study of recuperative VOC oxidation reactor with porous media

Numerical study of volatile organic compounds (VOC) oxidation reactor consisting of two coaxial tubes, filled with inert porous media is performed. Influence of incoming gas flux, adiabatic temperature of gas combustion, reaction rate constant, diameter of porous body particles, reactor size and heat losses on maximal temperature of reactor, recuperation efficiency, combustion front position is investigated. It is shown that maximum temperature and recuperation efficiency of reactor has extremum in the field of incoming gas flow rate and porous body particle size parameters (for simulated configuration of reactor maximum corresponds to U_G ∼ 2 m/s and d₀ ∼ 6 mm). Numerical simulation shows non-monotonous character of maximal temperature and recuperation efficiency dependence from side heat losses of reactor. The obtained results can be used for construction optimization of practical VOC oxidation reactors.     

Wind power utilization for water pumping using small wind turbines in Saudi Arabia: A techno-economical review

An attempt has been made, may be first time in Saudi Arabia, to utilize power of the wind for pumping the water for remotely located inhabitants not connected with national power grid. Small turbines of 1–10 kW have been chosen in conjunction with Goulds 45 J model water pumps to produce energy from wind and pump water using the produced energy at Arar, Rawdat Ben Habbas and Juaymah localities in Saudi Arabia. Wind speed measurements made at different heights using 40 m tall towers have been utilized in the present work. Higher wind speeds were noticed during summer time compared to winter time at all the locations. Both energy yield and cost of energy point of view, 2.5 kW wind turbine from Proven was found to be most suitable for wind power generation at all sites. It is shown that annual total water pumping capacity of 30,000 m³ is possible from a depth of total dynamic head of 50 m when using 2.5 kW Proven wind turbine with hub heights 15–40 m at all three sites with cost of water pumping as low as 1.28 US¢/m³.     

Research and development of a low-BTU gas-driven engine for waste gasification and power generation

Combustion characteristics of low-BTU gases (about 1000 kcal/N m³) were experimentally investigated in order to develop engine generators for waste gasification and power generation systems. Two simulated low-BTU gases, obtained from one-step high temperature gasification (hydrogen rich) and two-step pyrolysis/reforming gasification (methane rich), as well as natural gas, were tested in a small-scale spark ignition engine. Compared to the natural gas driven engine, the hydrogen rich low-BTU gas driven engine showed similar thermal efficiency but with significantly lower NO_x and hydrocarbon emissions and wider equivalence ratio range for stable engine operation. On the other hand, the methane rich low-BTU gas engine showed narrower equivalence ratio range for stable operation. The test results show engine performance more depends on combustion characteristics than on the heating value of the fuel gas. For better engine performance, hydrogen rich fuel gas is desirable.     

LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario

The poplar bioenergy system has been analysed applying life cycle assessment (LCA) to compare its environmental performance to: Ethiopian mustard bioenergy system and natural gas. The life cycle impact assessment (LCIA) shows that the use of fertilizers is the highest impact in four of the 10 environmental categories, representing between 39% and 67% of the impact in them. The diesel used in transport vehicles and agricultural tractors also has a significant impact in another five of the 10 analysed categories 40–85%. The poplar bioenergy system contributes to global warming with 1.90–1.98 g CO₂ eq MJ^?1 biomass produced. The production and transport as far as the thermoelectric plant of the poplar biomass consumes 0.02 MJ of primary energy per 1 MJ of biomass stored. In comparison with Ethiopian mustard and natural gas, it reduces primary energy consumption by 83% and 89% and the greenhouse gas emission by 84% and 89%, respectively. The results of the analysis support that the poplar bioenergy system is viable from an energy balance and environmental perspective for producing energy in southern Europe, as long as it is cultivated in areas where water is available. This latter point and the better environmental performance of both crops in comparison to natural gas allows us to affirm that the combination of several crops adapted to the local agro-climatic conditions of the territory will be the most suitable strategy in Mediterranean areas that wish to reach the global energy production targets in terms of biomass established by the European Union (EU).     

Combustion characteristics of rice-husk in a short-combustion-chamber fluidized-bed combustor (SFBC)

A short-combustion-chamber fluidized-bed combustor (SFBC), of 250 kW_th capacity, was developed and tested for combustion characteristics of rice-husk, i.e. combustion efficiency (E_c), heat rate intensity (I_c), temperature distribution, and gaseous pollutant emissions. The effects of fluidizing velocity, excess air, and combustor loading were analyzed. The results indicated that the system could operate without any secondary solid as bed material, and could achieve high combustion efficiency and high heat rate intensity. Solid recirculation within the bed, created by a solid recirculating ring and an air vortex, played an important role in efficient combustion, even in a relatively short-combustion-chamber. A maximum E_c of 99.8% and a maximum I_c of 1.54 MW_th m^?2 were realized. Increasing fluidizing velocity and excess air caused decreases in E_c. CO and NO_x emissions increased with increased excess air, and were in the range 50–550 ppm and 230–350 ppm, respectively.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

Optimal design of geometric parameters of double-layered microchannel heat sinks

This work uses an optimization procedure consisting of a simplified conjugate-gradient method and a three-dimensional fluid flow and heat transfer model to investigate the optimal geometric parameters of a double-layered microchannel heat sink (DL-MCHS). The overall thermal resistance R_T is the objective function to be minimized, and the number of channels N, channel width ratio β, lower channel aspect ratio α_l, and upper channel aspect ratio α_u are the search variables. For a given bottom area (10 × 10 mm) and heat flux (100 W/cm²), the optimal (minimum) thermal resistance of the double-layered microchannel heat sink is about R_T = 0.12 °C/m²W. The corresponding optimal geometric parameters are N = 73, β = 0.50, α_l = 3.52, and, α_u = 7.21 under a total pumping power of 0.1 W. These parameters reduce the overall thermal resistance by 52.8% compared to that yielded by an initial guess (N = 112, β = 0.37, α_l = 10.32, and α_u = 10.93). Furthermore, the optimal thermal resistance decreases rapidly with the pumping power and then tends to approach an constant value. As the pumping power increases, the optimal values of N, α_l, and α_u increase, whereas the optimal β value decreases. However, increasing the pumping power further is not always cost-effective for practical heat sink designs.     

Combustion characteristics of a 4-stroke CI engine operated on Honge oil,Neem and Rice Bran oils when directly injected and dual fuelled with producer gas induction

Energy is an essential requirement for economic and social development of any country. Sky rocketing of petroleum fuel costs in present day has led to growing interest in alternative fuels like vegetable oils, alcoholic fuels, CNG, LPG, Producer gas, biogas in order to provide a suitable substitute to diesel for a compression ignition (CI) engine. The vegetable oils present a very promising alternative fuel to diesel oil since they are renewable, biodegradable and clean burning fuel having similar properties as that of diesel. They offer almost same power output with slightly lower thermal efficiency due to their lower energy content compared to diesel. Utilization of producer gas in CI engine on dual fuel mode provides an effective approach towards conservation of diesel fuel. Gasification involves conversion of solid biomass into combustible gases which completes combustion in a CI engines. Hence the producer gas can act as promising alternative fuel and it has high octane number (100–105) and calorific value (5–6 MJ/Nm³). Because of its simpler structure with low carbon content results in substantial reduction of exhaust emission. Downdraft moving bed gasifier coupled with compression ignition engine are a good choice for moderate quantities of available mass up to 500 kW of electrical power. Hence bio-derived gas and vegetable liquids appear more attractive in view of their friendly environmental nature. Experiments have been conducted on a single cylinder, four-stroke, direct injection, water-cooled CI engine operated in single fuel mode using Honge, Neem and Rice Bran oils. In dual fuel mode combinations of Producer gas and three oils were used at different injection timings and injection pressures.Dual fuel mode of operation resulted in poor performance at all the loads when compared with single fuel mode at all injection timings tested. However, the brake thermal efficiency is improved marginally when the injection timing was advanced. Decreased smoke, NO_x emissions and increased CO emissions were observed for dual fuel mode for all the fuel combinations compared to single fuel operation.