专业英语论文脱碳方法总结CO2 removal process

Research and development of CO2 Removal Processes Abstract：The problem of global warming due to increasing atmospheric CO2 concentration is arguably the most important environmental issue that the world faces today. There are several technologies for decarbonation, such as physical method, chemical absorption, membrane separation, et. ASPEN PLUS simulation software has been used to simulate and optimize the process of decarbonation. According to the result of simulation, energy and material consumption are reduced through rational optimization, which then cut down the cost of decarbonation.

Key words: decarbonation ; ASPEN PLUS ; simulation ;

1 introduction

The Inter-governmental Panel on Climate Change _IPCC.( IPCC, 1990).has published a report concluding that human activities result in the production of four gases (carbon dioxide, methane, chlorofluorocarbon and nitrous oxide) which significantly contribute to global warming, and that the global warming caused by increasing emissions of these gases is one of the most serious environmental problems[1-3]. Among these gases, CO2 has the greatest adverse impact on the observed greenhouse effect causing approximately 55% of the observed global warming.

It is imperative to reduce carbon emission by using new energysaving energy, improving energy efficiency and carbon capture and storage (CCS). In the coming few decades, new energy will not play a major role in the energy supply. The contribution to the carbon emission reduction through saving energy and improving energy efficiency is after all limited [4]. It has huge potential for the contribution to carbon emission reduction by CCS, which will cut down the consumption of climate change mitigation [5-6]. Hence, the reduction of CO2 emissions from the fossil fuel energy systems is considered to be the most urgent to slow down the global warming trend.

2 CO2 removal methods

Several alternative strategies have been proposed to reduce the emission of CO2 into the atmosphere. These strategies include fuel alternative, energy conservation and improving power generation efficiencies [7-9]. However, their implementation may have a limited impact on the emission reduction of CO2. Therefore, various end-of-pipe technologies have also been tested to remove and recover CO2 from flue gas stream.

These include physical adsorption, chemical solvent absorption, membrane separation, cryogenic separation, biological fixation as well as the O2/CO2combustion process.

2.1 Physical absorption

Physical absorption, it relies on CO2 solubility in some absorption change with temperature to absorb CO2. In this work, the Pressure & Temperature Swing Adsorption (called “PTSA”), CO2 adsorption by pressure and temperature swing, has been employed. In the PTSA method, CO2 is adsorbed on an adsorbent at near the normal pressure, and then the adsorbent is heated and CO2 is regenerated under depressurization. Addition of a moderate temperature swing can increase regenerative capacity and reduce power consumption (mainly for the vacuum pump). This system can be expected to be an economically effective system by providing maximized utilization of yet unused energy within the thermal power plant as the heat source for this system.

The pilot plant was designed [10], based on the mass balance data obtained from the bench scale test and on the temperature swing test results. For the PTSA process, it is considered that addition of temperature swing introducing gas to the adsorption column using a steam gas heater should be most effective if added to the first stage of low concentration CO2. In the case of physical adsorption, it is estimated that concentration of the process gas can greatly affect the plant performance, and in this test about a 1.5% increase from 10% of the CO2 concentration in flue gases led to about a 25% drop in the electric power consumption, and thus, it was found that the

CO2 removal efficiency greatly depended on the CD2 concentration in the flue gases

2.2 Chemical absorption

Because of CO2 low concentration of coal-fired power plant flue gas (10%~15%), as well as which contains SO2, NOX and some other acidic gas, CO2 in the flue gas of coal-fired power plant is generally removed with SO2, NOX simultaneously by chemic absorption [11]. Normally absorbent are monoethanolamine (MEA), aqueous ammonia (NH3·H2O), aqueous 2-amino-2-methy-1-propanol (AMP), diethanolamine (DEA) and aqueous sodium hydroxide (NaOH) [12].

Chemic absorption, it relies chemist reaction between CO2and absorption to separate CO2 from flue gas. The general absorption is organic-ammine [13]. Heat the absorbed liquid to about 100°C, it will release high concentration CO2 to reuse. This method belongs to wet absorption; it can combinative with existing wet desulphurization device [14]. It based on a reaction of a weak base (amine) with a weak acid (CO2) to produce salinities that are easy to dissolve in water. The equation is:

23222CO NH R CO O H NH R -?++-

Ammonia seems to be an alternative solvent for removing CO2 from flue gas. This is partly due to that ammonia reagent which has already been used in the DeNOx process [15].such as selective catalytic reduction and selective non-catalytic reduction. In the flue gas systems, The ammonia gas or solvent is also an excellent reagent for removing SO 2 and HCl from waste gas streams. Therefore, it may be possible using the ammonia reagent to scrub all acid pollutants including the CO 2 greenhouse gas. However, NH3 scrubbing is a new process for removing CO2 emissions from flue gas in the environmental engineering field and limited information on its technical as well as economical effectiveness is available [16-17]. A review of the NH3/CO2reactions is referred to Bai and Yeh, who conducted a preliminary study of CO2removal by NH3 scrubbing. The results showed that possible reactions between CO2 and NH3：

)

(3)()()()(3)

()()()()(2422324223s HCO NH l O H g CO l NH s CO NH l O H g CO l NH ?++?++

2.3 Low-temperature separation

Low-temperature separation makes CO2 condense by low temperature, then separate CO2 from the others. In this way, sometime flue gas should be dried before separation to prevent the pipeline being blocked by freezing water vapor.

2.4 membrane separation

Packed columns have been used successfully in gas absorption applications for several decades. However, industrial-scale packed columns are very large, expensive to build, and suffer from a variety of operational problems including liquid channeling, flooding, entrainment and foaming [18]. The gas absorption membrane(GAM) systems have been tested in a variety of applications including the capture of carbon monoxide, sulfur dioxide, Hydrogen sulfide, and mercury[19–22].Qiand Cussler[23,24]were the first to adapted this membrane application for the removal of CO2 from gas streams. Their initial work has led to a number of studies on the use of GAM systems for CO2 removal applications.

In general, a GAM system contains a microporous membrane that separates the gas and liquid phases. Mass Transfer occurs as the solute gas diffuses through the membrane and absorbs into the liquid solvent, as seen in Fig. 1 for CO2 absorption. This arrangement offers several advantages including independent liquid and gas flow, a very high surface area to volume ratio, and linear scale-up [25]. On the negative side,The membrane itself adds an additional level of resistance to the mass transfer process. This resistance can be significant if the membranes are wetted by the absorption solution. Several authors have confirmed that a non-wetted mode of operation is preferable in GAM systems [26-28].Membrane wettability is one of the main obstacles facing GAM technology if large-scale CO2 absorption plants are going to be realized.

Fig.1.Mass transfer in a microporous GAM system

3Process simulation with Aspen plus

3.1 process modeling of CO2 absorption with aqueous ammonia

The monoethanolamine (MEA) scrubbing is widely used in the gas purification and carbon capture processes. However, the operating cost of the MEA process is high due to huge energy consumption in regenerating and operation problems such as 13corrosion and solvent degradation. Aqueous ammonia is considered as the promising substituted solvent. The advantages of aqueous ammonia are high CO2 loading capacity, no absorbent degradation, potential of capturing multiple flue gas components (SO2, NO2, and CO2), low energy requirement, and resistance to oxidation. Many researchers have studied the CO2 capture using aqueous ammonia, including reaction mechanism, kinetics, removal efficiency, and mass transfer coefficient.

Process simulation provides the means to interpret laboratory experiments and quantify the process performance among various CO2-capture processes using aqueous ammonia. Darde et al.[29]have presented thermodynamic analysis of the chilled-ammonia process. It calculates the activity coefficient for the liquid phase using the extended UNIQUAC model and the gas phase fugacity using the Soave-Redlich-Kwong (SRK) equation for the volatile compounds. Mathias et al[30] have quantitatively evaluated the chilled-ammonia process for CO2 capture using thermodynamic analysis and process simulation. The Electrolyte-NRTL model for the

NH3-CO2-H2O system has been used in the Aspen Plus simulator. It should be noted that the process simulation model used by Darde et al. and Mathias et al. assumes physical and chemical equilibrium and does not consider reaction kinetics and vapor-to-liquid mass transfer rate in the absorber. It is known that the rate-based modeling approach is rigorous and offers higher model fidelity over the traditional equilibrium-stage modeling approaches [31]. Therefore, developing the rate-based process modeling of CO2 absorption with aqueous ammonia is necessary.

The design of the pilot plant is based on a standard absorption and desorption flow sheet and partial or complete separation of gas mixtures. This is an integrated pilot plant operating with absorber, stripper, separation system, and connected unit operations.

3.2. Model simulations

The RateFrac model used the rigorous Electrolyte-NRTL thermodynamics to predict enthalpies [32-35], entropy, equilibrium vapor pressures of NH3, CO2, and H2O, and solution speciation. Thermodynamic models for electrolyte systems require a chemistry model. The chemistry model of NH3-CO2-H2O system is presented in Table 1.chemical equilibrium is assumed in the chemistry model.

Table 1 Chemistry Model for the NH3-CO2-H2O System

Process optimization can be divided into two main procedures: absorption and desorption. In this study, only the absorption process optimization is investigated in order to attain the control mechanism and optimization approach. Moreover, Process optimization with rate-based model simulations will provide theoretical guidance for energy-saving technology. For the CO2absorption, main optimization target

parameters are the CO2removal efficiency, CO2loading of CO2-rich solution, and NH3concentration of outlet gas. The optimization parameters and their setting operational parameters for process optimization with the rate-based model are shown in Supporting Information.

4conclusion

The methods of decarburization are introduced, which are physical adsorption, chemical solvent absorption, membrane separation, cryogenic separation, biological fixation as well as the O2/CO2combustion process. Importantly, the Aspen plus software is used to simulate and optimize the CO2removal process of chemical solvent absorption. The Thermodynamic Model including Electrolyte-NRTL in RateFrac model are performed to predict enthalpies, entropy, equilibrium vapor pressures of NH3, CO2, and H2O, and solution speciation. At last the optional temperature, operating pressure, CO2efficency can be obtained through Aspen simulation.

In all CO2emissions, CO2emissions from coal-fired power plant is in a large proportion in China, so the study of the CO2 emission control of the coal-fired power plant is of the great significance. A promising outlook to future reduction of power consumption for CO2 removal could be obtained.

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