Scholarly article on topic 'Carbon capture effects on water use at pulverized coal power plants'

Carbon capture effects on water use at pulverized coal power plants Academic research paper on "Environmental engineering"

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Abstract of research paper on Environmental engineering, author of scientific article — Haibo Zhai, Edward S. Rubina

Abstract Significant quantities of water are utilized in thermoelectric power plants, mostly for the purpose of cooling. Water is becoming critically important for low carbon power generation. To reduce greenhouse gas emissions from pulverized coal (PC) power plants, post-combustion carbon capture systems are receiving considerable attention. However, current systems require a significant amount of cooling, which puts further pressure on cooling water resource. This paper quantifies cooling water use and evaluates technical and economic effects of carbon capture on cooling systems at PC power plants. Included are recirculating systems with wet cooling towers and air-cooled condensers (ACCs) for dry cooling. For a wet cooling tower, water has to be provided to make up losses due mainly to evaporation and blowdown. Adding an amine-based carbon capture system to the plant would approximately double water use. When air-cooled condensers (ACCs) are used as the dry cooling system, in spite of the advantage in reducing water usage, the dry cooling system has a much larger capital cost than the wet cooling system, depending strongly on specific site and system characteristics.

Academic research paper on topic "Carbon capture effects on water use at pulverized coal power plants"

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Energy Procedía 4 (2011) 2238-2244

Energy Procedía

www.elsevier.com/locate/procedia

GHGT-10

Carbon capture effects on water use at pulverized coal power plants

Haibo Zhaia1*, Edward S. Rubina

a Department of Engineering and Public Policy, Carnegie Mellon University Pittsburgh, PA 15213, USA

Abstract

Significant quantities of water are utilized in thermoelectric power plants, mostly for the purpose of cooling. Water is becoming critically important for low carbon power generation. To reduce greenhouse gas emissions from pulverized coal (PC) power plants, post-combustion carbon capture systems are receiving considerable attention. However, current systems require a significant amount of cooling, which puts further pressure on cooling water resource. This paper quantifies cooling water use and evaluates technical and economic effects of carbon capture on cooling systems at PC power plants. Included are recirculating systems with wet cooling towers and air-cooled condensers (ACCs) for dry cooling. For a wet cooling tower, water has to be provided to make up losses due mainly to evaporation and blowdown. Adding an amine-based carbon capture system to the plant would approximately double water use. When air-cooled condensers (ACCs) are used as the dry cooling system, in spite of the advantage in reducing water usage, the dry cooling system has a much larger capital cost than the wet cooling system, depending strongly on specific site and system characteristics.

© 2011 Published by Elsevier Ltd.

Keywords: carbon capture; water use; cooling systems; power plants

1. Introduction and Objective

Thermoelectric generation accounted for approximately 39% of freshwater withdrawals in the United States, ranking only slightly behind agricultural irrigation (the largest source of freshwater withdrawals) [1]. Significant quantities of water are utilized in thermoelectric power plants, mostly for the purpose of cooling. Water is becoming critically important for low carbon power generation. To reduce greenhouse gas emissions from pulverized coal (PC) power plants, post-combustion carbon capture systems are receiving considerable attention. However, current systems require a significant amount of cooling, which puts further pressure on cooling water resource. In some areas, the pressure on water resource availability has led to increasing use of dry cooling systems in thermoelectric

* Corresponding author. Tel.: +1 412 268 1088; fax: + 1 412 268 1089. E-mail address: hbzhai@cmu.edu.

doi:10.1016/j.egypro.2011.02.112

power plants as they can significantly reduce water use [2]. Thus, a more careful evaluation of water usage is needed to better understand the performance and cost impacts of CO2 capture systems at power plants. The major objective of this paper is to quantify water use and evaluate technical and economic effects of carbon capture and storage (CCS) on water use for cooling systems at pulverized coal (PC) power plants. Evaluated are wet cooling towers and air-cooled condensers (ACCs) for dry cooling.

2. Method and Tool

Makeup water usage of cooling systems is systematically quantified based on mass and energy balances for PC plants. The cooling technologies considered include wet towers and air-cooled condensers (ACCs) for dry cooling. The performance models are linked to engineering-economic models that calculate the capital cost, annual operating and maintenance costs and total annual levelized cost of the specified system and plant. The water systems modules are embedded in the Integrated Environmental Control Model (IECM) developed by Carnegie Mellon University for the U.S. Department of Energy's National Energy Technology Laboratory (USDOE/NETL). The IECM is a well -documented publicly available model that provides systematic estimates of performance, emissions, cost and uncertainties for preliminary design of fossil-fueled power plants with or without CO2 capture and storage [3-4]. Detailed technical documentation for each of the IECM cooling options and power plant systems discussed in this paper are available elsewhere [3].

3. Base Case Studies

Base cases were designed to characterize the performance and cost of wet and dry cooling system options for subcritical PC power plants without carbon capture, which are typical of most current plants. As shown in Figure 1, major environmental control systems included selective catalytic reduction (SCR), an electrostatic precipitator (ESP) and flue gas desulfurization (FGD).

Cooling

^ Tower

Boiler SCR ESP FGD

Figure 1 Schematic of an illustrative baseline power plant without carbon capture

Table 1 summarizes key technical and economic design assumptions for the base case plants. All plants in this paper are evaluated on a basis of 550 MW power net output in order to compare cases with different design parameters and configurations. The results regarding the performance and costs of the base plant with wet and dry and cooling systems are given in Table 2.

Table 1. Key assumptions for the baseline cases

Parameters Value Technical parameters

Net plant output (MW) 550.0

Boiler type subcritical

Environmental controls SCR+ESP+FGD

Coal type Illinois #6

Ambient air pressure (kPa) 101.4

Ambient air temperature (oC) 25

Ambient relative humidity (%) 50

Cooling water temperature drop across the wet tower (oC) 11

Cycle of concentration in the wet cooling system 4

Turbine backpressure for the dry cooling system (inches Hg) 4

Air-cooled condenser (ACC) plot area per cell (m2) 110

Initial temperature difference for ACCs (oC) 27 Economic/financial parameters

Cost year 2007

Plant capacity factor (%) 75

Fixed charge factor 0.148

Plant life time (years) 30

Water cost ($/m3) 0.26

Coal cost ($/tonne) 46.3

General facilities capital (% of process facilities capital, PFC) 10

Engineering & home office fees (% of PFC) 10

Project contingency cost (% of PFC) 15

Process contingency cost (% of PFC) 0

As shown in Table 2, the plant with a wet cooling tower has a smaller gross size and higher efficiency than the plant with dry cooling because the wet cooling system needs less auxiliary power to maintain the operation. Total makeup water for the wet cooling system is 2.5 liters per kWh (net), which is about 2.3% of the total recirculating cooling water volume. Tower evaporation accounts for 75% of water losses, while blowdown accounts for approximately 25%. Tower drift and other losses are small, less than 1% of the total. Both evaporation losses and the cycles of concentration (a design parameter related to cooling water quality) affect tower blowdown rate. For the dry cooling system there is no makeup water required.

The procedure and cost categories established by the Electric Power Research Institute are used to calculate the total capital requirement (TCR) for each power plant sub-system [5]. Cost elements take into account the process facilities capital (PFC), general facilities cost, engineering and home office fees, contingency costs and several categories of owner's costs (including interest during construction). All costs in IECM Version 6 were updated to 2007 U.S. dollars based on recent studies by the USDOE [6]. The resulting TCR for the base plant design is $90 per kW and $224 per kW for the wet and dry cooling systems, respectively. Thus, the dry cooling system has a much larger capital cost than the wet cooling system. The wet system contributes approximately 5% of the total plant capital cost, whereas the dry system accounts for about 12% of the total plant capital cost. Due to its less capital cost and higher efficiency, the total cost of electricity (COE) for the base case plant with wet cooling is $4.0 per MWh less than for the plant with the dry cooling.

H. Zhai, E.S. Rubina/Energy Procedia 4 (2011) 2238-2244 Table 2. Results for the baseline cases using the IECM

Performance and cost measures PC plant with a wet cooling system PC plant with a dry cooling system

Gross power output (MW) 593.3 600.7

Net plant efficiency, HHV (%) 36.1 34.6

Total cooling system makeup water (tonnes/MWh) 2.46 0

Number of air cooled condenser cells 63

Cooling system total capital requirement ($/kW) 90.4 224.4

Cooling system levelized annual cost ($/MWh) 3.9 7.2

Plant total capital requirement ($/kW) 1788 1940

Plant revenue requirement (COE) ($/MWh) 69.1 73.1

4. Effects of Carbon Capture Systems

We evaluate the effects of carbon capture systems on wet and dry cooling systems for three types of power plants including subcritical, supercritical and ultra-supercritical (USC) steam cycle designs.

4.1. Wet Cooling Systems

We investigate the effect of the addition of an amine-based post-combustion CO2 capture system on cooling water requirements. Table 3 summarizes major performance parameters of the capture system based on recent studies by USDOE [6]. A carbon capture system affects PC plant performance in two major areas: the steam cycle and the cooling system. Low-quality steam extracted from the steam turbine is used to separate captured CO2 from the rich amine solvent. CO2 regeneration has heat requirement of approximately 3500 kJ/kg CO2 product. As a result, the steam cycle heat rate increases by about 25% and the net plant efficiency decrease by roughly 11 to 12% across the three plant types.

Table 3. Major performance parameters of the amine-based carbon capture system

Parameters Values

Process sorbent FG Plus

CO2 removal efficiency (%) 90

Sorbent concentration (wt, %) 30

Temperature exiting direct contact cooler (oF) 113

Maximum CO2 train capacity (tonnes/hr) 209

CO2 compressor capacity (tonnes/hr) 299

Lean CO2 loading (mol CO2/mol sorbent) 0.19

Nominal sorbent loss (kg/ tonne CO2) 0.3

Liquid-to-gas ratio (mol MEA liquid/mol flue gas) 3.015

Gas phase pressure drop (psia) 1

Solvent pumping head (psia) 30

Pump efficiency (%) 75

The operations of the direct contact cooler, the CO2 absorption and stripping processes and CO2 product compression also need cooling water [7-9]. As a result, the total cooling duty for the carbon capture system requires 91.2 tonnes of cooling water per tonne of CO2 product for the base case design. This cooling water is from the plant

cooling system. Thus, the total makeup water required for the plant cooling increases proportionally with the added demand of the CO2 capture system.

Figure 2(a) shows that decreased thermal efficiency (steam cycle effect) and increased cooling demands for CO2 capture (cooling system effect) result in a substantial increase in the size and makeup water requirement of the plant cooling system. Adding CO2 capture leads to water use increases (for cooling) by 83 to 91% across the three plant types. Thus, the availability of cooling water is critically important for low-carbon power generation using CCS. In the meanwhile, as shown in Figures 2(b) and 2(c), the wet cooling system has capital cost increases by 63 to 74% relative to a plant without CO2 capture, and its total levelized cost including higher operating costs for energy and other items of cooling increases by more than 90%.

Ultra-supercritical

Supercritical

Subcritical

□ w/ CCS

□ w/o CCS

Makeup Water Needed (L/kWh)

Ultra-supercritical Supercritical

□ w/ CCS

□ w/o CCS

Subcritical

0.0 40.0 80.0 120.0 160.0 Total Capital Requirement ($/kW)

Ultra-supercritical Supercritical Subcritical

w/ CCS □ w/o CCS

Total Levelized Cost ($/MWh)

Figure 2. Effects of carbon capture and storage (CCS) on wet cooling systems

4.2. Dry Cooling Systems

A design challenge appears when a post-combustion CO2 capture system is added to the PC plant since there is no cooling water readily available to meet the cooling demands of the capture unit. Therefore, an auxiliary cooling system is required for the capture process. In this study a wet recirculating system of the type described earlier is assumed to be the auxiliary cooling system. We treated the auxiliary cooling system as an added operating cost for the capture system. That is estimated to be $0.035 per tonne of cooling water required. Figure 3 shows the size of the power plant dry cooling system for the three PC plant types. In addition, makeup water required for an auxiliary wet cooling system for carbon capture is 2.5, 2.3 and 2.0 L/kWh for subcritical, supercritical and ultra-supercritical plants, respectively.

For these cases, the cooling duty of the primary dry cooling system decreases as the steam is extracted from the steam cycle for use in CO2 sorbent regeneration. For example, adding CO2 capture leads to that the number of condenser cells for the subcritical plant drops by three relative to the base plant without capture. However, for a given plant type, the auxiliary cooling of the CO2 capture system requires the amount of makeup water that is comparable to use of the wet cooling system at a plant without carbon capture as discussed in the base case. Thus, a large amount of water is still needed if an amine-based CO2 capture system with conventional water cooling is added to a plant with primary dry cooling.

Ultra-supercritical

Supercritical

Subcritical

0 20 40 60 80

Numberof ACC Cells

Figure 3. Effects of carbon capture at power plants with primary cooling by air-cooled condensers

5. Conclusions

This paper systematically evaluates the performance and cost of dry and wet cooling systems for illustrative power plant configurations with and without carbon capture systems. Makeup water usage is highlighted in evaluating the wet cooling tower for PC plants. Makeup water is mainly used due to evaporation and blowdown losses. Improving the thermal efficiency of the plant steam cycle (via supercritical units) helps reduce wet cooling system size, makeup water usage and associated costs, using air cooled condensers.

In assessing the performance and cost of a dry cooling system, a crucial design parameter is the initial temperature difference (ITD) between the ambient air and steam from the turbine exhaust. In spite of the advantage in reducing water usage, the dry cooling system has a much larger capital cost than the wet cooling system, depending strongly on specific site and system characteristics.

Current post-combustion CCS systems have additional cooling demands that nearly double the consumptive water use at a PC plant with conventional wet cooling towers. For a plant with dry cooling, a large-scale auxiliary

cooling system would be required to support the C02 capture process. Thus, there is a need for lowering consumptive water in R&D programs focused on developing new carbon capture systems for power plants.

Acknowledgements

This work was supported by the U.S. Department of Energy's National Energy Technology Laboratory (DOE/NETL) (contract No. DE-AC26-04NT4187) . Michael B. Berkenpas provided invaluable assistance with the IECM computer code. All findings, conclusions, opinions or recommendations expressed in this paper are those of the authors alone and do not reflect the views of DOE or any other government agency.

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