Computational Combustion and Emission Analysis from a mcchp System Compared to a Conventional Power Plant

Transcription

1 Computational Combustion and Emission Analysis from a mcchp System Compared to a Conventional Power Plant SIMONA PARASCHIV 1, SPIRU PARASCHIV 1, ION V. ION 1, NICOLAE BADEA 2 1 Department of Thermal Systems and Environmental Engineering 2 Department of Electrical Engineering University Dunarea de Jos of Galati 47 Domneasca Str, , Galati ROMANIA sparaschiv@ugal.ro Abstract: - The paper presents a numerical simulation to analyze the pollutants generation in the combustion chamber of a Stirling engine integrated into a micro-combined cooling, heating and power system (mcchp). Fuel combustion is one of the important problems in thermofluid mechanics. In the last years, Computational Fluid Dynamics (CFD) techniques are used due to developing of numerical techniques and computer capabilities. This CFD analysis provides information on flame temperature, CO2 and NOx concentrations for the biogas combustion of in mcchp system. The pollutant emissions of biogas combustion in a Stirling engine combustion chamber are compared with the emissions of a conventional power plant. Key Words: biogas, coal, CFD, combustion, pollutant emissions, Stirling engine, mcchp. 1 Introduction The global demand for energy continues to increase while traditional energy resources are becoming scarcer. With the growth of industrialization and world population, fuel consumption will increase enormously in the foreseeable future so that, unless improvements are made in combustion process and air cleaning, the pollution level is bound to rise sharply. Adoption of environmentally benign and renewable energy conversion technologies is essential if our society is to retain its advanced lifestyle in the face of global development. The energy sector by converting primary energy resources, have significant effects, direct or indirect, local scale, regional or global on air, water, soil and subsoil. The strongest impact is found in the case of fossil fuels burning especially the coal combustion that attract many negative consequences in terms of pollutant emissions. Combustion plants use large quantities of fossil fuels and raw materials extracted from the earth's natural resources and turning them into useful energy. They generate a range of residues, wastes and emissions by large amounts throughout the environment. This paper presents a comparison study of the emissions generated from a coal-fired power plant versus a mcchp system [1]. Combined heating and power (CHP) was used in industrial applications in the late 19th century. Development and implementation of cogeneration systems in buildings has been made easier in recent years due to rapid development of technology [2]. Trigeneration is an effective use of the fuel energy both in terms of economical and environmental protection than that of the centralized production. [3], [4], [5]. Trigeneration systems have a big potential for economic and energy savings due to efficient use of fuel. Trigeneration involves the use of fuel for the combined production of electricity, heat and cooling [6], [7]. The cogeneration/trigeneration systems (CHP/CCHP) used in the residential sector should be developed and designed to be able to produce heat or cold and electricity simultaneously from a single source of energy [8]. The development of CHP/CCHP systems in the EU is characterized by a great diversity, both in terms of the scale and the nature of development [9]. 2 Estimation of emissions from coalfired power plants The most important air pollutant emissions from burning fossil fuels are SO2, NOX, CO, particulates and greenhouse gases such as CO2 and N2O. Other substances such as heavy metals, acid fluoride, halogenated compounds, unburned hydrocarbons, volatile organic compounds and dioxins, although ISBN:

2 are emitted in small amounts, they have a significant impact because of their toxicity and persistence. The most important pollutants emitted by power plants are presented below: - Particulate matter (PM) emissions The PM emissions from coal combustion are directly related to the ash content of the coal. The particles consist of: soot, high molecular condensed hydrocarbons, ash particles and unburned carbon particles. Particles are particularly damaging also because they absorb gases or other toxic substances and carry them to the lungs, producing an effect similar to ionizing radiation. - Sulphur oxides emissions The SOx emissions from coal combustion depend on the sulphur content of the coal. The sulphur content of coal and oil varies widely, but generally in the range of 0.3 to 5% by weight. Sulphur dioxide is emitted from similar sources as suspended particles, so they both are regarded as traditional pollutants of urban areas. The highest level of these pollutants occurs during the sulphurous smog. Of the total sulphur dioxide it has been estimated that more than half is deposited as acid solution on buildings and vegetation, on both of which it has a deleterious effect. - Nitrogen oxides emissions Main nitrogen oxides emitted during fossil fuel combustion are oxides of nitrogen (NO), nitrogen dioxides (NO2) and nitrous oxide (N2O). The first two of these gases form a mixture known as NOx, representing more than 90% of the NO emission of large combustion plants. NOx formation is dominated by three key mechanisms, characterized by the origin of nitrogen and the environment where the reaction takes place: thermal NOX, formed by the reaction of oxygen with nitrogen from the combustion air; fuel NOx, formed from the nitrogen content in fuel; prompt NOX, formed instantly into flame in the presence of molecular nitrogen compounds hydrocarbon feedstock. Fuel NOx formation depends on the fuel nitrogen content and oxygen concentration in the reaction medium. The amount of NOX produced from the fuel is higher in plants that use coal as the amount of nitrogen is greater in its structure than in other types of fuels. Nitrogen dioxide is a phytotoxic substance; that is, it can cause damage to vegetation. Nitric oxide can react with haemoglobin in the blood stream to form metheglobin which is not suitable for oxygen transport. Nitrogen dioxide is the most toxic of the nitrogen oxides. With increasing dosage the following sequence is observed: odour perception, nasal irritation, discomfort in breathing, respiratory distress, pulmonary oedema and finally death. - Carbon dioxide emissions Different fossil fuels produce different amounts of carbon dioxide per unit of energy released. The carbon from the coal composition is converted to CO2 during the combustion process. A rise in temperature due to the greenhouse effect would have numerous, fearsome consequences, such as the shifting of climatic zones and with them the shifting of the inhabitable regions of the earth, the expansion of the deserts, as well as the flooding of larger areas of land due to melting of polar ice masses. Emissions Determination Determination of the pollutant emissions is based on measurements made with specialized equipment. Where such equipment is not available, emissions calculation is done using "stock assessment methodology emissions of SO2, NOx, particulate matter (fly ash) and CO2 from thermal power plants" issued by the Protection Service Environmental Strategy and Economic Development Division - CONEL. This method of calculation is based on fuel consumption and emission factors. The amount of pollutant discharged into the atmosphere (emission) is given by: E= B LHV e [ kg / h] where e is the emission factor, the quantity of pollutants discharged into the atmosphere per unit of heat introduced with the fuel in the boiler. For various pollutants, emission factors are determined experimentally and their characteristics depend on the fuel used, type of construction of combustion plants and their thermal power. Romania is a country where the production of electricity is state controlled and own. Most of combustion facilities are running both for electric and thermal generation (co-generation). The major fuel is the low calorific Lignite (LHW = kcal/kg), that is supported by gas or oil (up to 15%). The coal s and the oil s content in sulphur and nitrogen are quite high, and the SO2 and NOx emissions are of considerable extend, that exceeds several times the maximum admitted value, not mentioning the great amount of ash [10]. Table 1 gives an inventory of calculated pollutant emissions generated by a power plant functioning with Pit coal. ISBN:

3 Table 1. Pollutant emissions of a power plant. Parameter of the Steam Boiler type C4 Steam mass flow rate: 420 t/h Value Steam pressure pn=138bar Steam temperature ts = 5400C Power plant capacity: 50MWt Fuel type Pit Coal Low Heating Value, LHV (kj/kg) Elemental composition of the used coal as received (% mass) Carbon, C Hydrogen, H Sulphur, S Oxygen, O Nitrogen, N Ash, A Total Humidity, W Fuel consumption for nominal 21.6 power, B (t/h) Thermal efficiency, ηt (%) 81.4 Emissions (kg/h) ECO ESO2 492 ENOx Eash 2618 results indicate that the NOx emissions produced by the mcchp are 67% less than NOx emission produced by the studied coal fired power plant. In Figure 1 are shown the temperature contours which are dependent on specific heat. Fig. 1 - The temperature contours within the combustion chamber. Figures 2-5 show the contours of mass fraction of CH4, O2, CO2 and H2O. 3. Modelling of the pollutant emissions generated from biogas-fired mcchp Table 2 shows the composition and physical properties of the biogas used by the mcchp combustion chamber. The biogas it is primarily a mixture of methane (CH4) and inert carbonic gas (CO2) and is characterized based on its chemical composition and the physical characteristics. Table 2 - Characteristics of biogas Types of fuel Biogas Composition 68% CH4 26 % CO2 1% N2 0% O2 5 % H2O Net Calorific kj/nm3 Value In this section it has been carried out a study to investigate the formation of pollutants in a mcchp combustion chamber. The study was performed using the FLUENT package. The results are compared with the emissions generated from coalfired power plant. The analysis of the simulation Fig. 2 - Contour for mass fraction of CH4. Fig. 3 - Contours for mass fraction of O2. ISBN:

4 Fig. 4 - Contours for CO2 mass fraction. Fig. 7 - Contours of NO mass fraction: thermal NOx generation. Fig. 5 - Contours for mass fraction of H2O. In Figure 6 is shown the formation of both thermal and prompt NOx. Analysing the results we can note that the peak concentration of NO is located in the region of high temperature where oxygen and nitrogen are available. Fig. 8 - Contours of NO mass fraction: prompt NOx generation In Figure 9 are shown the NOx (ppm) contours for NOx generation. Fig. 6 - Contours of NO Mass Fraction: Prompt and Thermal NOx. In figure 7 and respectively 8 are presented the formation of thermal and prompt NOx. Fig. 9 - Contours of NO (ppm). 4. Results and discussions The goal of this research is to compare the NOx emissions from coal combustion in electric power sector and from biogas combustion in a mcchp system. The biogas is a clean fuel, composed primarily of methane and the main products of the ISBN:

5 combustion of the biogas are carbon dioxide and water vapour. The biogas combustion produces small amounts of nitrogen oxides, carbon dioxide and carbon monoxide and does not produce particulate matter. The biogas is an important source for electricity generation because its exploitation is related not only to energy production but also to waste treatment, while being subject to regulations related to environmental protection. Approximately two thirds of the biogas produced is used for electricity production and one third for the production of heat. Using biogas as a renewable energy source could be a small step, but important in reducing dependence on fossil energy imports, increasing the local economy and reducing GHG emissions. The simulation results indicate that the NOx emissions produced by the mcchp are 67% less than NOx emission produced by the studied coal fired power plant. Nomenclature MCCHP - micro combined cooling heat and power MCHP - micro combined heat and power LHV - lower heating value [kj/kg] CHP - combined heat and power CFD -Computational Fluid Dynamics E - the mass flow rate of pollutant discharged into the atmosphere [kg/h] B - amount of fuel consumed during that period [kg/s] e - emission factor [kg/kg] [5] K. Brown, S. Minett, History of CHP developments and current trends, Applied Energy, Volume 53, Issues 1-2, 1996, Pages [6] P. Katsigiannis, D. Papadopoulos, A general technoeconomic and environmental procedure for assessment of small-scale cogeneration scheme installations: application to a local industry operating in thrace, greece, using microturbines, Energy Conversion and Management, Volume 46, Issue 20, December 2005, Pages [7] O. Balli, H. Aras, A. Hepbasli, Exergoeconomic analysis of a combined heat and power (chp) system, International Journal of Energy Research, Volume 32, Issue 4, pages , 25 March [8] M. Biezma, J. Cristóbal, Investment criteria for the selection of cogeneration plants a state of the art review, Applied Thermal Engineering, Volume 26, Issues 5-6, April 2006, Pages [9] N. Badea, E. Ceanga, S. Caraman, M. Barbu, Numerical Simulation of the Conceptual Model for mcchp- Stirling Engine based on Renewable Energy Sources, 9th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING (ICOSSSE '10), Iwate Prefectural University, Japan, October 4-6, 2010, Pages [10] I. Ionel, Impact on the air quality due to Romanian power plants, Nr. 233, VDI Verlang GmbH Düssedorf, References [1] E. Cardona, A. Piacentino, A methodology for sizing a trigeneration plant in mediterranean areas, Applied Thermal Engineering; Volume 23, Issue 13, September 2003, Pages [2] K.C. Kavvadias, A.P. Tosiosa, Z.B. Maroulisa, Design of a combined heating, cooling and power system: Sizing, operation strategy selection and parametric analysis, Energy Conversion and Management, Volume 51, Issue 4, April 2010, Pages [3] D. Wu, R. Wang, Combined. cooling heating and power: a review, Progress in Energy and Combustion Science, No. 32, 2006, pp [4] J. Hernández-Santoyo, A. Sánchez-Cifuentes, Trigeneration: an alternative for energy savings, Applied Energy, Volume 76, Issues 1-3, September-November 2003, Pages Acknowledgements The authors would like to acknowledge to EEA Financial Mechanism for financing the research on Integrated micro CCHP Stirling Engine based on renewable energy sources for the isolated residential consumers from South-East region of Romania (m- CCHP-SE), under the contract No. RO-0054/2009. ISBN: