Natural Gas Chemical Looping
CHEM. LOOPING INTERNATIONAL
Final Report
CHE-4080
Dr. Holles
Spring 2014
Nasser Aiyd Alhajri
Daniel Debroy
Esjae E. Eiden
Valeriya Litvinova
Moises Vazquez
Management Summary
The combustion of fossil fuels for energy is a human activity that leads to a large amount of carbon dioxide emission. The carbon dioxide levels in the atmosphere increased due to the use of fossil fuels. According to the Environmental Protection Agency, before the industrial revolution the atmospheric concentration of carbon dioxide was 35% less than today’s levels [Yeh, 2009].
Energy is significant factor for a modern society. Electricity is a primary source of energy in the United States [Carbon Dioxide Emissions, 2013]. However, the combustion of fuel for the electricity generation is the main cause and attribute of CO2 emissions. In the United States, statistical data shows that electricity generation radiates massive amounts of carbon dioxide annually.
Many advanced technological processes and practices have been developed to capture CO2, Chemical Looping Combustion is a significant one. Chemical looping combustion is a significant process in which one has to separate the combustion fuels into oxidation and reduction reactions. Proceeding with the process the reactions are swept out into two independent reactors (air and fuel reactors). The required oxygen is provided to the fuel by a suitable metal oxide in this process known as oxygen carrier. The fuel reacts with the metal oxide in the fuel reactor and reduces it. The reduced metal oxide circulates to the air reactor where it is oxidized with air. The metal oxide keeps circulation between the fuel and air reactors to make a chemical reaction going. Therefore, this process is named as Chemical Looping Combustion. The product of the fuel reactor consists of H2O and pure CO2 in which both products can be separated and CO2 can be captured, moreover the air reactor produces N2 and O2.
A fixed capital investment of $1.8 billion and a total capital investment of $2.2 billion will be required for the total project. The variable costs are $5.2 million. The NPV0 is $11.9 billion and after 10 years the NPV10 is $4.3 billion. The IRR is 36.0% and the payback period is about 2.1 years. This process is using a new technology, so having an IRR of 36.0% is reasonable but has a high risk [Peters, 2006]. As a conclusion, this project can be profitable even if the cost of the chemical looping combustion is high.
Contents
1.Project Definition
1.1 Business Opportunities
1.2 Key assumptions
1.3 Key issues
1.4 Project Goals
2.Process Information
2.1 Chemical Looping Combustion
2.2 Carbon Dioxide Separation
2.3 Electricity Generation System
3.Reactions and Properties
3.1 Oxygen Carrier
3.2 Reactions
3.3 Properties
4.Material Data
4.1 Text Flowsheet
4.2 Overall Material Balance
4.3Key Recycles
5. CO2 sequestration
5.1 Storage and transportation of CO2
5.2 Economics for CO2 storage and transportation
5.3 CO2 storage regulations...... 20
6. Nitrogen Sales
7. Economics
7.1 Overall Economics
8. Government Regulations
8.1 Permits
8.2 OSHA Law & Regulations
9. Conclusion
Appendix
A1. Flowsheets (detailed)
A2. Detailed Material Balance
A3. Physical Properties details
A4. Economics/Production Cost Statement
A5. Equipment List w/sizes, Sizing Bases, Costing Bases
A6. Piping and Instrumentation Diagram
Symposium Presentation
Database\Progress Reports\Symposium\CHE4070_Final Presentation_Chemical Looping.pptx
References
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1.Project Definition
A 500 MW power plant is designed and fueled by natural gas. Chemical Looping technology will be used to produce CO2.This project consists of three parts: a Chemical Looping Combustion, Carbon Dioxide capturing and electricity generation. Electricity and CO2 will be sold as products and H2O will be re-engaged in the process. Electricity is generated from steam turbines. To be a beneficial supplier to Wyoming, Colorado and Utah this plant will be located between Green River and Rock Springs because it is near by several natural gas pipelines and the Green River as a source of waterwhile CO2 is captured and stored.
1.1 Business Opportunities
To have a sustainable economy that is environmentally friendly, it is necessary to find a clean, cheap, and abundant energy supply. Chemical looping is an environmentally friendly process, and if the efficiency is high enough, it can be a competitor for other electricity production processes. The projected energy supply through year 2030 will be drawn from oil, coal, natural gas; renewable forms of energy; and nuclear energy, in that order. Competition for the chemical looping process includes traditional combustion processes. Among these, fossil fuels account for more than 86% of the world’s energy supply. For primarily economic reasons, fossil fuels will continue to play a dominant role in the world’s energy supply for the foreseeable future. Natural gas has been a common energy source for heating, electricity generation, and hydrogen production. The price of natural gas varies significantly with locations and source. It is projected that the natural gas price will decrease from $4.44/MMBtu in 2014 to $4.11/MMBtu in 2015 [U.S. Energy Information Administration, 2014]. Despite the projected price decrease, the share of natural gas in world energy consumption is expected to remain at 24% from year 2005 up to 2030 [ Fan,NJ,2010].
1.2 Key assumptions
The effective performance of the chemical looping particles or oxygen carrier particles is important to have a successful operation of chemical looping process. Metal oxide, support, and agent are effective particles of the chemical looping. These particles have to have desired properties in order to be effective ones [Fan, 2010].
To have a successful operation process, the list below contains the assumptions that were made:
- The oxygen carrier has a good oxygen carrying capacity, long-term recyclability, and good mechanical strength [Fan, 2010].
- Both in the Air reactor (oxidation) and the fuel reactor (reduction) have a good gas conversion with the oxygen carrier. That is, a fully conversion of the fuel into CO2 and H2O and is achieved when reacting with the oxygen carrier. Also, the reduced oxygen carrier that is circulating to the air reactor is assumed to be able to fully oxidize through reaction with air [Fan, 2010].
- The oxygen carrier remains environmental friendly and not harmfully [Fan, 2010].
- Pure CO2 will be captured and to be sold, CO2 will be sent directly to pipeline [Fan, 2010].
1.3 Key issues
Compared to other normal power plants, this project is costly because it is using a new technology and capturing CO2. Currently, a lot of reaches is studying how to improve the oxygen carrier circulation between the two reactors. A catalyst is mostly needed to improve some properties of the oxygen carrier. To be mentioned, the chemical looping process is not existed in any commercial power plants. So, the scaling of this project was based on pilot power plant references.
1.4 Project Goals
Selecting the appropriate oxygen carrier and reactors type was achieved. An Aspen+ simulation file was developed and the plant location was decided. A process optimization was considered including a final economic analysis, and also reliability and safety analysis.
2.Process Information
The process designed has two main parts: chemical looping combustion (with carbon dioxide separation), and electricity generation (or boiler). These two steps can be seen in Figure 1.
Figure 1. Flowsheet of the overall Chemical Looping Combustion System: a) Combustion section, b) Boiler section
2.1 Chemical Looping Combustion
The Combustion system consists of two interconnected fluidized bed reactors, in which a metal oxide circulates in order to achieve a reversible reaction that produces two separate product streams. Chemical Looping Process can be seen in Figure 2.Because of the amount of electricity produced, there will be three identical lines, each with the same design specifications. This part is subdivided in three sections.
Figure 2. Chemical Looping Process
2.1.1 Oxidizing Section
A detailed scheme of the oxidizing section is detailed in Figure 4 in the appendix. The inlet air is at room temperature and atmospheric pressure (77⁰F, 14.7 psia). The total air flow required is 23.21 MMft3/hr, equivalent to 1.710 Mlb/hr. The air is only slightly compressed to compensate for the pressure drop in the process, due to heat exchangers and the reactor itself. The pressure drop in the process is calculated to be around 1.6 psi, so the compressor (AIRCOMP) has to elevate the pressure of the air to 16.3 psia, resulting in a slight temperature increase. The total power used by this compression is 1.76 MW. This air has a composition of 79% N2 and 21% O2 by volume. Traces of other components are ignored for the calculations.
The air is then preheated, using a tube and shell heat exchanger and the product from the oxidizing reactor (HE01). The total area of heat exchange for this step is 40,500 ft2, equivalent to 13,500 ft2 for each heat exchanger. The flow of preheated air is at 705⁰F. This air then enters the oxidizer. This process will be detailed in section 2.1.3. The product of this reactor has a composition of 96% N2 and 4% O2 by volume [Sharma, 2011]. Due to the nature of the reaction, this gaseous mixture is at a high temperature (1832⁰F), so heat can be further extracted from it to produce steam. Using a heat exchanger (HE02), which is part of the boiler system detailed in 2.3, about 111 MW are extracted from this stream, reducing the its temperature to around 842⁰F. The product stream is then used to preheat the incoming air as mentioned before. The final product is at 98⁰F and 15 psia, and has the same composition as stated previously. The amount leaving the system is around 1.376 Mlb/hr, which is a considerable amount. It can be sold to a Nitrogen consuming plant; however, because of the rather large flow, using a compressor to send it to a pipeline, which would require a compression to about 400 psia (compression ratio of 27), would be extremely expensive as well as energy consuming, so this alternative is not recommended.
2.1.2 Reducing Section
A detailed scheme of the oxidizing section is detailed in Figure 5. The fuel we are using is natural gas, which we assumed to be pure methane for this process. The inlet flow is at 77⁰F and 350 psig, and we use a flow of 83,591 lb/hr of methane, equivalent to 78.06 Mft3/hr. This flow is passed through a turbine to reduce the pressure. For this reason, we use the turbine (FUETURB), which reduces the pressure to 16 psia. The power produced by this expansion is 2.42 MW. The fuel then proceeds to enter the Reduction reactor, further explained in 2.1.3. The product flow of this reactor is 66.7% H2O and 33.3% CO2. Due to the nature of the reactor operation, the flow is also at high temperature, 1787⁰F. Part of this heat is used to produce steam in the boiler section, detailed in 2.3. We can extract 67 MW from this process, resulting in a stream of Carbon Dioxide and Water at 392⁰F. This stream is taken to the Carbon Dioxide Separation Unit detailed in section 2.2.
2.1.3 Reactor Section (Chemical Looping)
This is the main section of this process, and it is where most of the heat is extracted for steam generation, and the basis of the process itself. Due to its nature, it produces two separate streams: one of Nitrogen and Oxygen, and another one of Carbon Dioxide and Water. This allows easy separation of the Carbon Dioxide produced, and it is why it is considered a clean technology when compared to traditional combustion processes. The details of the reaction used are in section 4.1.
The detailed scheme of the Looping process can be seen in figure 6 in the Appendix. The information used to build this cycle was extracted from the article by Sharma, R [Sharma,2011]. Some of the parameters used for the design were the fluidizing velocities in the reactors, temperatures, conversion rates and densities. The following table details the values used.
The system used has a circulating metal oxide, with two interconnected fluidized bed reactors. The air reactor has a higher speed (9.6 m/s), compared to the fuel reactor (0.16 m/s), due to the reduction/oxidation rates found in literature. This influences the design of the reactors, further detailed in Appendix A5, because it makes the air reactor have a smaller diameter (almost half), compared to the fuel reactor. The residence time for the fuel reactor (30 s) is also higher than that of the air reactor (which is around 9.2 s), because the endothermic reaction taking place in the fuel reactor is slower. However, because of size limitations, the reactors used will be smaller than those specified in the article. Because of this, for each air reactor, there must be five fuel reactors to ensure the fuel speed and residence time in the reactors’ stays the same.
This system operates at a very high temperature (1832⁰F for the air reactor, 1787⁰F for the fuel reactor). The fuel reactor is at a lower temperature because it is endothermic, so it uses some of the sensible heat (around 33 MW) of the hot metal oxide to complete the reaction. The reaction taking place in the air reactor is highly exothermic, so heat can be extracted from it while maintaining an extremely high temperature. A piping system to produce steam is installed in the reactor itself, and is used to extract 333 MW. This is the part of the whole process that produces the most energy.
The flow is a type of recycle, in which the iron oxide (Fe2O3/Fe3O4) circulates between the reactors, being oxidized and then reduced, and then repeating the process. The metal oxide’s reactivity and durability is enhanced by doping with Al2O3, in a 60:40 mass ratio of iron oxide and aluminum oxide. The stream ME is the reduced stream, and it has a mass flow of 31,233 Mlb/hr Fe2O3, 9,648 Mlb/hr Fe3O4, and 27,010 lb/hr Al2O3. As it enters the oxidizer, the Fe3O4 is oxidized, and produces a stream with 41,214 Mlb/hr Fe2O3 and 27,010 Mlb/hr Al2O3. The amount of oxygen carried is then 342 Klb/hr O. The reaction rates account for the relatively low amount of Oxygen transferred.
2.2 Carbon Dioxide Separation
The detailed scheme for the carbon dioxide separation unit can be seen in Figure 7 in the Appendix. The flow coming from the reducing section, which contains 66.7% H2O and 33.3% CO2, has to go to a separation unit. This is one of the most important steps of the process, because it differentiates it from conventional combustion processes. These generally have a product flow of Nitrogen, Water, and Carbon Dioxide. The separation of Nitrogen and Carbon Dioxide is expensive and require specific equipment. The separation of Carbon Dioxide and Water, on the other hand, is pretty straight forward, and it can be done in a simple flash tower that takes advantage of the vapor-liquid equilibrium of the system. The system is still expensive, because it requires cooling down and compressing high amounts of flow gas.
The cooler lowers the gas temperature from 392⁰F to 72⁰F, using cooling water at 55⁰F that heats up to 95⁰F. This requires a very large cooler: 168,000 ft2. This is one of the constraints of the process, and requires multiple heat exchangers in parallel to accomplish. The temperature of the outlet flow of gas was specified to obtain a high purity carbon dioxide stream out of the flash tower. After the mixture is cooled, it passes through a flash tower, which produces two streams. The liquid phase, which has a flow of 185,579 lb/hr, is composed of mainly water (99.98% Water) and traces of Carbon Dioxide. Though slightly acidic (due to solubility of Carbon Dioxide in water), it can be used for other purposes in the process or discarded easily. The vapor phase, with a flow of 231,364 lb/hr, is mainly Carbon Dioxide (99%) and a small amount of water, carbon monoxide, and hydrogen. This stream is at atmospheric pressure, so it has to be compressed in order to send it to a pipeline. The specification used was 365 psia, so the compression ratio is high (about 25). However, the flow isn’t too high, so it can be compressed using compressors that use 10.4 MW.
2.3 Electricity Generation System
For the design of the electricity generation system, an article by Zhou, S. and Turnbull, A. was used. Figure 3 is a diagram of a steam cycle in a conventional fossil fuel electricity generation plant. The system consists of an acuotubular boiler, which heats the incoming water using pipes and a superheater to produce superheated steam. This steam passes through a high-pressure turbine, and the outlet steam is reheated. The steam then passes through two lower pressure turbines, which are connected to a generator. The system is closed so all the water is recycled.
Figure 3. Simplified steam cycle in a power plant
Also, conditions of inlets and outlets of turbines were found and used for the model of the chemical looping plant, as described in Table 2.
Table 2. Typical inlet and outlet temperatures and pressures of steam for a fossil-fired steam turbine with a rated output of 500 MW.
Turbine / Inlet / OutletTemperature (⁰F) / Pressure (psia) / Steam wetness / Temperature (⁰F) / Pressure (psia) / Steam wetness
HP / 1047 / 2199 / / / 685 / 609 / /
IP / 1047 / 545 / / / 446 / 45 / /
LP / 444 / 44 / / / 86 / 9 / 8%
Adapting the values and cycle found to the Chemical Looping Plant, a suitable electricity generation section was designed. A detailed scheme of the boiler section is detailed in Figure 7 in Appendix. Though the boiler appears to be a single block of operation, it is actually comprised of piping that passes through the Oxidizing Reactor (OXIDIZER) and the two heat exchangers found in the reducing and oxidizing sections of the plant (HE02 and HE03). So although we are using a model similar to the one in Figure 3, the actual plant would not have a boiler per se, but a complex piping system that goes through the equipment mentioned, through which water/steam will be transported to have heat transferred to it. The total energy that can be extracted from the reactor and heat exchangers is 510 MW. This energy will be distributed between the main heating section (which uses 325 MW) and the reheating section (which uses the remaining 185 MW).
The flow of water was determined to be 3,473,000 lb/hr. This amount was determined assuming the power loss of the heating system was 100% efficient, meaning that all the energy extracted from the reactor and heat exchangers would be effectively transferred to the water. The turbines used for the process have an isentropic efficiency of around 0.8, which is on the higher side of the steam turbines found for commercial processes. For the efficiency of the steam generation however, Aspen+ has included calculations regarding the use of energy extracted from the process and used to heat up the water. The efficiency is set around 0.6, which means the efficiency of the whole electricity generation process is around 48%.