Technical and Energy Performance of an Advanced, Aqueous

Jul 24, 2015 - The integrated capture system was then evaluated in terms of the mass balance and the energy consumption of each unit. The results ... ...
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Technical and Energy Performance of an Advanced, Aqueous Ammonia-Based CO2 Capture Technology for a 500 MW Coal-Fired Power Station Kangkang Li,*,†,‡ Hai Yu,*,† Paul Feron,† Moses Tade,‡ and Leigh Wardhaugh† †

CSIRO Energy Flagship, 10 Murray Dwyer Circuit, Mayfield West, NSW 2304, Australia Department of Chemical Engineering, Curtin University of Technology Australia, GPO Box U1987, Perth, Western Australia 6845, Australia

‡

S Supporting Information *

ABSTRACT: Using a rate-based model, we assessed the technical feasibility and energy performance of an advanced aqueous-ammoniabased postcombustion capture process integrated with a coal-fired power station. The capture process consists of three identical process trains in parallel, each containing a CO2 capture unit, an NH3 recycling unit, a water separation unit, and a CO2 compressor. A sensitivity study of important parameters, such as NH3 concentration, lean CO2 loading, and stripper pressure, was performed to minimize the energy consumption involved in the CO2 capture process. Process modifications of the rich-split process and the interheating process were investigated to further reduce the solvent regeneration energy. The integrated capture system was then evaluated in terms of the mass balance and the energy consumption of each unit. The results show that our advanced ammonia process is technically feasible and energy-competitive, with a low net power-plant efficiency penalty of 7.7%.

1. INTRODUCTION Postcombustion CO2 capture (PCC) based on aqueous ammonia (NH3) has received great attention as a promising technology for the reduction of CO2 emission from fossil fuel power stations because it provides many technical and economic advantages over the conventional amine solvent, such as low cost, low regeneration energy, no sorbent degradation, and the simultaneous capture of multiple acid pollutants.1−3 Intensive research has been carried out to understand and evaluate the NH3-based CO2 capture process. Experimentally, pilot-plant and demonstration trials have been conducted by several industrial companies and research organizations such as Alstom,4,5 Powerspan,6 and CSIRO.7,8 The results from these tests confirmed the technical feasibility of NH3-based technologies and demonstrated many of the expected benefits, such as high CO2 removal efficiency, highpurity CO2 production, and low regeneration energies. Specifically, Alstom’s chilled ammonia process (CAP) can achieve a 90% CO2 capture efficiency, >99.5% CO2 product purity, and 2.2 MJ/kg CO2 regeneration energy.9,10 In collaboration with Delta Electricity, CSIRO evaluated the NH3 process in pilot-plant trials at Munmorah power station. An 80−90% CO2 removal efficiency was obtained with a purity of CO2 product >99%.7,8 However, the pilot-plant trials revealed some technical and economic challenges, with the most critical one being the high volatility of NH3. The CAP © 2015 American Chemical Society

process is designed to operate at low absorption temperatures (0−10 °C) for the significant reduction of NH3 slip, but this comes at the expense of a high chilling duty and the introduction of problems in slurry handling. The CO 2 absorption at relatively high temperatures (10−30 °C) in the CSIRO process led to high NH3 loss and high capital and running costs for recovery of the vaporised NH3. In addition to the NH3 loss, the trials by CSIRO process also identified a practical issue of solid precipitation in the stripper condenser and reflux line that causes the shutdown of the plant.8 Parallel to the pilot and demonstration trials, modeling studies were conducted to evaluate and improve the NH3-based capture processes.11−13 Darde et al.14 developed an extended UNIQUAC model to thermodynamically simulate the CAP process that estimated the total heat requirement for CO2 desorber and NH3 stripper at 2700 kJ/kg CO2; Niu et al.15 proposed a novel aqueous-ammonia-based PCC process, and their thermodynamic analysis showed that the energy consumptions for the CO2 stripper and the NH3 regenerator were 1285 kJ/kg CO2 and 1703 kJ/kg CO2, respectively. Recently, the focus of modeling work has been shifted to the Received: Revised: Accepted: Published: 10243

May 6, 2015 July 2, 2015 July 24, 2015 July 24, 2015 DOI: 10.1021/acs.est.5b02258 Environ. Sci. Technol. 2015, 49, 10243−10252

Article

Environmental Science & Technology

Figure 1. Flow sheet of an integrated aqueous-ammonia-based postcombustion capture process.

development of the rigorous, rate-based models for CO2 absorption and desorption using aqueous ammonia, which is considered to more realistically and accurately evaluate the NH3-based CO2 capture process, guide the process optimization, and scale up. Qi et al.,12 Li et al.,16 and Yu et al.17 have developed rate-based models for CO2 absorption and regeneration and validated the models using pilot-plant data. These models help obtain a detailed understanding of the interrelationship between the operating conditions and the CO2 capture performance. The rate-based models were also used to evaluate the new process configurations that could improve the ammonia process. Jilvero et al.18 found that the staged absorption can significantly reduce the ammonia vaporization. The work by Yu et al.19 showed that the richsplit process can help eliminate the solid precipitation in the overhead condenser and reduce the energy consumption. Despite these investigations, the rigorous rate-based model has not yet been used to assess the detailed technical feasibility of NH3-based PCC for a commercial-scale plant. None of the studies estimated the size of the columns and the amounts of packing needed using a rigorous rate-based model. Although Zhang et al.20 estimated the reboiler duty (CO2 regeneration energy) in an NH3-based capture plant designed to capture 50% of the CO2 emitted from a 500 MW coal-fired power station, the calculated regeneration energy was surprisingly high at 5750 kJ/kg CO2. This study aims to further advance the NH3-based PCC process by addressing the NH3 slip issue and applying new approaches to reduce the solvent regeneration duty. It also aims to evaluate the technical and energetic performance of the advanced NH3 process integrated with a 500 MW coal-fired power station. We have previously reported a novel system in which flue gas cooling, NH3 recycling, and SO2 removal were achieved simultaneously with a very low energy requirement by utilizing the latent heat in the high-temperature flue gas.16,21

Apart from this configuration, the advanced process also incorporated several novel approaches including high-temperature absorption, two-stage CO2 absorption, rich-split process, and stripper interheating to improve its technical and economic performance. In the current study, we employed the rate-based model to evaluate the potential benefits of these new configurations, and calculate the size of the columns, the amount of packing, and both the thermal and the electric energy consumption. To our knowledge, this is the first time that a systematic study of process optimization and intensification of an NH3-based PCC process for a commercial scale power station has been presented and analyzed in detail.

2. MATERIALS AND METHODS 2.1. Rate-Based Model. A rigorous, rate-based model developed in Aspen Plus V7.3 was employed to simulate the NH3-based CO2 capture process. The model of NH3−CO2− SO2−H2O system has been thermodynamically and kinetically validated against the experimental results including those from open literature and pilot-plant trials at Munmorah Power Station, New South Wales, Australia. The established rate-based absorber−stripper model enables the reliable and practical evaluation and better calculations of the energy requirements during the capture process. The information for the model (including the thermodynamic model, reaction model, kinetic mode, and model validation), has been described in detail in our recent publications.16,21 2.2. CO2 Capture System. Figure 1 shows the entire NH3based PCC process, which integrates the NH3 recycling unit, CO2 capture unit, and CO2 compression section with Australian black-coal-fired power station (500 MW gross power output, 35.6% generation efficiency).22 The power plant island is assumed to be either equipped with a natural draft cooling tower, which can normally supply plenty of cooling water between 16 and 20 °C,23 or near a cold location 10244

DOI: 10.1021/acs.est.5b02258 Environ. Sci. Technol. 2015, 49, 10243−10252

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Environmental Science & Technology

Figure 2. Effect of (a) NH3 concentration with a fixed lean CO2 loading of 0.25 and (b) lean CO2 loading with a fixed NH3 concentration of 6.8 wt % on the energy requirement for the CO2 capture unit and for the NH3 recycling units, at conditions of 25 °C for the inlet NH3 solution temperature, 6 bar for the stripper pressure, 85% CO2 capture efficiency, and a 10 °C temperature difference of solvent in and out of stripper.

interheating process (which reduce reboiler duty by the utilization of the heat in the hot lean solvent leaving the stripper).28−30 2.2.3. CO2 Compression. The compression process was modeled in six stages with fixed discharge conditions (110 bar, 40 °C) using a pressure-changer modular MCompr simulator. We used three intercoolers at stages 1, 3, and 5 in the CO2 compressor to remove gaseous NH3 and moisture from the pressurized CO2 product to meet the requirement for subsequent geological sequestration. 2.2.4. Auxiliary Equipment. A water separation unit was introduced to maintain the H2O balance in the entire system; details are in the Supporting Information. Auxiliary equipment such as heat exchangers, blowers, pipelines, and pumps is also integrated in the entire PCC system. The isentropic efficiency and mechanical efficiency of both pumps and blowers were set at 80 and 95%, respectively. 2.3. Energy Evaluation. To complete the assessment of this advanced NH3-based PCC system integrated with the power station, we performed an overall energy evaluation. The cooling and chilling duty was converted into electricity using a coefficient of performance of 5. The electrical duties of the CO2 compressor, pumps, and blowers were directly derived from the Aspen simulation results based on the rate-based model. The most important energy duty for solvent regeneration was provided by intermediate and low-pressure steam extraction from the water steam cycle of the power station. This led to a subsequent net efficiency penalty to the power plant. To quantify the power loss, we introduced an adequate correlation based on the Carnot cycle, where the heat duty from steam extraction was transformed into an equivalent electric power loss. The following equations have been proven to accurately calculate the power loss for extraction temperatures from 131 to 155 °C:31

where low-temperature water is available to cool the NH3 solvent to 25 °C. The flue gas from the power plant is typically at 120 °C and 2344.8 tons/hour (t/h) with 10.7% CO2 (418.5 t/h), 6.0% H2O, 7.8% O2, and 75.5% N2, as well as 200 ppmv SO2 (volume basis). Owing to the massive flue gas flow rate, a single PCC train results in an inner absorber column diameter of 20 m using Mellapak 250Y packing material. Modern construction practices such as ceramic-lined concrete towers24 may allow columns of such a large size to be built in the future, but for this study, the recommendation of Chapel25 was applied, setting the maximum absorber column diameter to 12.8 m. Thus, three parallel-process trains of CO2 capture with 12 m diameter absorbers were proposed to eliminate construction uncertainties, with each designed to deal with one-third of the total flue gas of 139.5 t/h CO2 (approximately 1 million tons per year). This practical design is intended to eliminate plant construction uncertainties, and the scale is close to the word’s first commercial PCC plant in Boundary Dam, which deals with the flue gas from 110 MW unit (1 million tons per year).26 A detailed column size estimation is found in Supporting Information. 2.2.1. NH3 Recycling Unit. The advanced NH3 recycling unit integrates the functions of flue gas cooling and NH3 recovery. It includes a wash column, in which vaporized NH3 is recovered by wash water, and a pretreatment column, in which the heat contained in the high-temperature flue gas is used to regenerate NH3 in the wash water and recycle it to the CO2 absorber. This advanced NH3 recovery process was proven to be technically effective, with >99% NH3 recycling efficiency and >99% SO2 removal efficiency with a very low energy penalty.21 2.2.2. CO2 Capture Unit. An advanced CO2 capture process with 85% capture efficiency was designed to address the identified issues including high cooling duty, NH3 vaporization, and energy penalty. These advancements include: (1) elevating the temperature of the CO2 lean solvent to 25 °C and using a relatively low aqueous NH3 concentration, intended to avoid both solid precipitation and the substantial energy input for solvent chilling; (2) applying a two-stage absorption with intermediate cooling to significantly reduce the vaporized NH3 levels;18 and (3) implementing process modifications of the rich-split process (which have been proven to effectively reduce solvent regeneration duty in process modeling19 and pilot-plant trials at the Australia Tarong power station)27 and the stripper

ΔPloss = θeff × θCarnot × MCO2 × Hreg

(1)

where ΔPloss is the electric power loss in MWe, MCO2 is the CO2 capture rate in kg/h, and Hreg is the mass specific heat duty of regeneration in MJ/kg CO2. The additional power loss in the power cycle due to steam extraction, θeff, is calculated by θeff = 0.6102 + 0.00165(Tsat,stream − 273.15)

(2)

The Carnot efficiency θCarnot is defined as: 10245

DOI: 10.1021/acs.est.5b02258 Environ. Sci. Technol. 2015, 49, 10243−10252

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Environmental Science & Technology

Figure 3. (a) Heat requirement; (b) energy duty of the CO2 compressor and the reboiler temperature; and (c) net efficiency penalty as a function of the stripper pressure at 6.8 wt % NH3 concentration and 0.225 CO2 loading, with a 10 °C temperature difference of the solvent in and out of stripper.

θCarnot = 1 −

Tcond Tsat,stream

even if the staged absorption process was applied. This raised the energy burden of the NH3 recycling unit, including an increase in heat duty from 0.126 to 0.309 MJ/kg CO2 and an increase in the chilling duty from 0.128 to 0.323 MJ/kg CO2. It is worth mentioning that the energy duties in the NH3 recycling unit were significantly lower than that for solvent regeneration. This is mainly attributed to the proposed NH3 recycling process using waste heat in the flue gas and the staged absorption configuration significantly reducing NH3 slip by more than 50%. A minimum heat requirement was observed at 6.8 wt % NH3, at which the total heat duty was 3.91 MJ/kg CO2 and the chilling duty 0.259 MJ/kgCO2. 3.1.2. Effect of Lean CO2 Loading. Similar to NH3 concentration, lean CO2 loading also had a conflicting effect on the energy performance. Low CO2 loading caused a high NH3 loss and raised the reboiler duty, while high CO2 loading decreased the CO2 absorption capability and increased the solvent circulation rate, thereby raising the total heat requirement. As shown in Figure 2b, at a balanced lean CO2 loading of 0.225, the total heat duty reached a weak minimum of 3.86 MJ/ kg CO2, accompanied by a chilling duty of 0.257 MJ/kg CO2. At this CO2 loading, the solvent circulation rate was 31.7 ton per ton CO2 captured, with cyclic CO2 loading values from 0.225 to 0.41. It is worth mentioning that although the lean loading increased by a large increment from 0.175 to 0.30, the CO2 loading of rich solvent increased only slightly, from 0.40 to 0.43. In our simulation, the rich CO2 loading was limited to the range of 0.4−0.45. When absorption begins, the lean solvent

(3)

where Tcond is the condenser temperature in the water steam cycle, assumed to be 313 K in this study, and Tsat,stream is the saturation temperature of the extracted steam at the extraction pressure, which is assumed to be 10 K higher than that of the reboiler temperature in K.

3. RESULTS AND DISCUSSION 3.1. Parameter Sensitivity Study. 3.1.1. Effect of NH3 Concentration. As shown in panel a of Figure 2, NH3 concentration has conflicting influences on the energy requirement of the CO2 capture unit and the NH3 recycling unit. Increasing the NH3 concentration reduced the heat requirement for solvent regeneration because high NH3 concentrations increased the CO2 absorption rate and the CO2 absorption capacity per kilogram of solvent. The increase of NH3 concentration from 4 to 10% led to a decrease of the solution circulation rate from 5637 to 3337 t/h, which decreased the sensible heat and, consequently, the regeneration duty from 4.18 to 3.82 MJ/kg CO2. Moreover, increasing the NH3 concentration decreased the regeneration temperature from 132.7 to 126.2 °C. This means that lower-quality steam would be required from the power station, which is likely to decrease the steam consumption for solvent regeneration. However, increasing the NH3 concentration from 4 to 10% significantly increased NH3 slip from 7240 to 17721 ppmv, 10246

DOI: 10.1021/acs.est.5b02258 Environ. Sci. Technol. 2015, 49, 10243−10252

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Environmental Science & Technology

Figure 4. Stripping process modifications of (a) the rich-split process with the split stream fed to stage 2 and the unsplit stream 2 to stage 5; (b) the stripper interheating process with interheated solvent in and out at stage 5; and (c) the combined process with the split stream fed to stage 2, the unsplit stream to stage 5, and interheating at stage 8. The total stage number of the stripper is 15, with a condenser at stage 1 and a reboiler at stage 15. There is a 10 K temperature approach between the rich solvent entering the stripper and the lean solvent leaving the stripper.

the net efficiency penalty reached a minimum at a stripper pressure of 10−16 bar. When analyzing energy consumption, the determination of the stripper pressure should take into account the construction cost of a high-pressure stripper, which generally increases with pressure. Considering the small penalty difference between 10 and 16 bar, the lower pressure was selected. At a 10 bar stripper pressure, the regeneration energy was 3.27 MJ/kg CO2 with a reboiler temperature of 145.5 °C and condenser duty of 1.45 MJ/kg CO2. 3.2. Process Modification. Process modifications of richsplit and stripper interheating were proposed to further reduce the energy consumption of solvent regeneration. Simplified flow sheets are shown in Figure 4, while Table 1 summarizes the simulation conditions and results of process modifications. 3.2.1. Rich-Split Process. The rich-split process modification shown in Figure 4a is an efficient method to reduce the reboiler duty via the recovery of the steam generated in the stripper. The cold rich stream was split to recover the energy contained in the upcoming high-temperature water vapor, while the rich solvent was heated to release part of the CO2. To figure out how the energy was saved by the rich-split process, we determined the distribution of the three heat requirements as shown in panel a of Figure 5. It can be seen that the rich-split process significantly decreased the heat of vaporization with an increasing split fraction, which was favorable for lowering the condenser duty and the subsequent regeneration duty. However, at higher split fractions, the cold split stream started to cool the stripper, and more sensible heat was required to heat the split solvent to the required temperature, which increased the reboiler duty. Effectively, the heat of vaporization and sensible heat were competing with each other, resulting in an appropriate split fraction that maximized the saving of the energy duty. As shown in Figure 5b, when a 0.05 split fraction was applied, the total energy duty reached a minimum of 3.28 MJ/kg CO2 with a reboiler duty of 2.89 MJ/kg CO2 and a condenser duty of 0.39 MJ/kg CO2. 3.2.2. Interheating Process. The interheating process shown in Figure 4b exchanges heat between the hot lean stream leaving the bottom of the stripper and the semilean solvent extracted from the middle of the stripper before the hot lean stream goes to the main cross-exchanger, thus making better

quickly absorbs CO2 due to the relatively fast carbamate formation rate. When CO2 loading increases close to or greater than 0.5, the very slow reaction of bicarbonate formation will dominate CO2 absorption. This is consistent with the modeling results of Jilvero et al.,18 in which CO2-rich loading exceeding 0.5 is not recommend. Hence, we suggest that the rich loading should not target more than 0.5 in the NH3 process to ensure a relatively fast absorption rate and, therefore, a smaller absorber. 3.1.3. Effect of Stripper Pressure. Generally, the heat requirement of solvent regeneration consists of three components: heat of CO2 desorption, sensible heat, and heat of H2O and NH3 vaporization (details are given in Supporting Information). As shown in Figure 3a, the heat of desorption changed very little with an increase in stripper pressure. This is understandable because the heat of desorption is an inherent property of the solvent and is predominately dependent on the NH3 and CO2 concentration in the solvent (the composition of rich solvent entering the stripper and the CO2 desorption rate are the same at different pressures). The sensible heat also changed very little due to the same temperature difference (10 °C) of solvent in and out of the stripper, and the same solvent flow rate is used at different pressures. The reduction of regeneration duty by elevating the stripper pressure was primarily attributed to suppressing the H 2 O and NH 3 vaporization, which reduced the reboiler duty from 4.12 MJ/ kg CO2 at 4 bar to 2.87 MJ/kg CO2 at 20 bar. Moreover, a rise of stripper pressure significantly reduced the energy penalty of CO2 compression due to the elevating inlet pressure to the CO2 compressor. The power duty and chilling duty of CO2 compression decreased from 289 to 131 kJ/kg CO2 and from 454 to 271 kJ/kg CO2, respectively (Figure 3b). However, high stripper pressure also has some drawbacks. As indicated in eqs 1−3 in section 2.3, a rise in the reboiler temperature will result in an increase the steam extraction (power loss) and a subsequent increase in the net efficiency penalty to the power station. Moreover, high stripper pressure will consume more energy for the solvent pump. To determine the optimal stripper pressure, we converted the energy consumption of three important parts (the stripper reboiler, CO2 compressor, and pump) into the electrical power and corresponding net efficiency penalty. As shown in Figure 3c, 10247

DOI: 10.1021/acs.est.5b02258 Environ. Sci. Technol. 2015, 49, 10243−10252

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Environmental Science & Technology

accordingly reduced the energy consumption of sensible heat and heat of vaporization (Figure 6b). As a consequence of interheating, the reboiler duty was reduced to 3.00 MJ/kg CO2. 3.2.3. Combined Rich-Split and Interheating Process. As discussed before, the role of the rich-split process was primarily to reduce the heat of vaporization while the interheating process reduced the sensible heat during solvent regeneration. The combined process as shown in Figure 4c is meant to enable better use of these advantages to further reduce the total energy consumption. As show in Table 1, when a 0.05 split fraction and interheating process was applied in combination, the regeneration duty was reduced to 2.46 MJ/kg CO2 while the cooling duty of the condenser was only 0.24 MJ/kg CO2. This is a great reduction of 24.8% in reboiler duty and 83.4% in condenser duty compared with the values for the reference case, which is very competitive and promising in terms of energy saving. 3.3. Process Assessment. 3.3.1. Mass Balance Analysis. An analysis of the system material balance was necessary to determine the material consumption, H2O and NH3 makeup, and intermediate and final product yield of each section and the entire process. Figure 7 shows the mass flow rates of the three most important components (CO2, NH3, and H2O) at various locations of the CO2 capture system at a CO2 capture rate of 118.1 t/h and a capture efficiency of 84.7%, demonstrating that the material balance of CO2, NH3, and H2O in each unit of the entire CO2 capture system was maintained (see detailed discussions in the Supporting Information). 3.3.2. Energy Penalty. As shown in Table 2, the PCC plant involved an energy-intensive process. Its large net efficiency penalty of 7.7% led to a 21.6% reduction of net electricity output compared to that of the power plant without PCC. The overall net efficiency penalty (7.7%) included all of the necessary energy consumption involved in the CO2 capture process but did not consider the cooling duty for producing cooling water. This is primarily attributed to the proposed advanced process elevating the inlet CO2 lean solvent at 25 °C, which avoided the heavy reliance on the chilled water (