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December 2010 Vol.53 No.12: 3284–3293 doi: 10.1007/s11431-010-4164-4

Technical-economic evaluation of O2/CO2 recycle combustion power plant based on life-cycle WANG Yun, ZHAO YongChun, ZHANG JunYing* & ZHENG ChuGuang State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China Received April 26, 2010; accepted October 13, 2010

In this study, a detailed technical-economic analysis on a O2/CO2 recycle combustion power plant (Oxy-combustion plant) retrofitted from the existing coal-fired plant (with a capacity of 2×300 MW) in China was carried out by using life cycle assessment (LCA) and life cycle cost (LCC) method. The CO2 emissions, investment cost, cost of electricity and CO2 avoidance cost within the life cycle were calculated respectively. The results showed that the CO2 emission avoidance rate of retrofitted Oxy-combustion plant in the life cycle was about 77.09% without taking account of the CO2 compression; the annual cost increased by 5.9% approximately, the net power decreased by 21.33%, the cost of electricity increased by 34.77%, and the CO2 avoidance cost was about 28.93 USD/t. Considering the compression process of CO2, the avoidance rate of CO2 emission was about 73.35% or so; the annual cost increased by 9.35% approximately, the net power decreased by about 26.70%, the cost of electricity increased by 49.13%, and the CO2 avoidance cost was about 45.46 USD/t. The carbon tax (the CO2 tax) should be more than about 24 USD/t and 34 USD/t under the condition of considering CO2 compression or not, respectively, which is beneficial to promote transformation of existing coal-fired plant for reducing the CO2 emissions. life cycle assessment, coal-fired power plant, O2/CO2 recycle combustion technology, cost of electricity, CO2 avoidance cost Citation:

Wang Y, Zhao Y C, Zhang J Y, et al. Technical-economic evaluation of O2/CO2 recycle combustion power plant based on life-cycle. Sci China Tech Sci, 2010, 53: 3284−3293, doi: 10.1007/s11431-010-4164-4

The Intergovernmental Panel on Climate Change (IPCC) believes that CO2 capture and storage (CCS) technology of power plant is the most effective and potential technical means to fight against global climate change and greenhouse gas emissions. The CO2 capture technologies of power plant mainly include pre-combustion capture, post-combustion absorption, O2/CO2 recycle combustion technology and so on. In particular, with rapid development and gradual maturity of oxygen preparation technology, O2/CO2 recycle combustion technology has become the most potential CO2 capture technology of power plant on a large-scale, since it can be applied to the existing power plant boiler without great changes. For a reasonable choice

*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010

in practical applications and decision-making assessment, it is necessary to conduct a comprehensive technical and economic analysis. The economy of various types of CO2 removal systems for different types of power plants have been analyzed and compared broadly by domestic and overseas scholars according to their own actual national or local conditions. Chalmers University [1] evaluated a retrofitted 865 MWh lignite-fired power plant in Germany, it was found that the cost of electricity was 64.3 USD/MWh, corresponding to a CO2 avoidance cost of 26 USD/t. Singh et al. [2] studied the techno-economic of CO2 capture from an existing coal-fired power plant, which was retrofitted with the approaches of MEA scrubbing vs. O2/CO2 recycle combustion, and believed that the CO2 capture cost for the MEA case was 55 USD/t while the O2/CO2 case was 34 USD/t. Xiong et tech.scichina.com

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al. [3] evaluated an economic feasibility study of O2/CO2 recycle combustion technology based on existing coal-fired power plants, and their results indicated that the cost of electricity ranged from 46.37 USD/MWh to 55.04 USD/MWh, and the CO2 avoidance cost ranged from 19 to 35 USD/t. These research results above have shown that the investment on the systems of power plant and the cost of electricity will rise sharply after using O2/CO2 recycle combustion technology, which makes the CO2 avoidance expensive. However, these results may change according to the difference of power plant scales, location, efficiency, coal kinds, fuel prices, operating factor and financing cost. All the above studies solely consider the direct CO2 emissions from fuel combustion and the pure production cost of power plant, but have no techno-economic evaluation and analysis based on full life cycle. By using LCA and LCC methods, this paper presents a full life-cycle techno-economic evaluation for the existing 2×300 MW coal-fired power plant before and after utilizing O2/CO2 recycle combustion technology.

1 1.1

Subjects, methodology and system boundary

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generator and 2×1065 t/h pulverized coal boiler) in China. The CO2 emissions, investment cost, cost of electricity and CO2 avoidance cost are quantified from a life-cycle perspective. The retrofitted Oxy-combustion plant is shown in Figure 1, and the main parameters of the existing 2×300 MW coal-fired power plant are shown in Table 1. The proximate analysis and ultimate analysis of the coal used in power plants are listed in Table 2. 1.2

Methodology

Life cycle assessment (LCA) is a methodology for evaluating the environmental load, raw materials and energy consumption of processes or products (goods and services) during their life cycle from cradle to grave. Compared with the traditional environmental impact assessment (EIA) and environmental audit (EA), the advantage of LCA is that it is not limited to evaluate the pollutants from the product phase, and also considers the entire life cycle of production and industrial activities, including raw material extraction, material processing, construction, transportation, operation,

Subjects

The combustion air is replaced by the mixture gas consisting of pure oxygen produced from ASS (air separation system) and partial boiler flue gas. O2/CO2 cycle combustion technology can make the concentration of CO2 in flue gas reach up to 95% or more, then CO2 is recycled directly to avoid greenhouse gas emissions effectively through the direct liquefaction of final flue gas, while the release of NOx, SO2 and other pollutants are also reduced effectively [4]. In this study, O2/CO2 cycle combustion technology is to retrofit an existing 2×300 MW homemade sub-critical coal-fired power plant (direct air-cooled condensing steam turbine

Table 1

Table 2

Figure 1

Schematic diagram of the retrofitted Oxy-combustion plant.

Main performance parameters for the existing coal-fired power plant Plant capacity

2×300 MW

Plant life

30 a

Annual net generation (full load)

5.256 billion kWh

Loading rate (the 1st year)

40%

Efficiency of power generation

39%

60%

Coal consumption (full load)

5632 t/d

80% 1536000 t

Efficiency of boiler

92%

Loading rate (the 30th year) Loading rate (2∼29 a) The construction period

Hours of operation

6000 h/a

Annual coal consumption

3a

Proximate analysis and ultimate analysis of the coal Proximate analysis Component

Var

FCar

Aar

Mar

wt%

10.06

59.68

22.3

7.96

Ultimate analysis Component

Car

Har

Oar

Nar

Sar

Qnet,ar (MJ/kg)

wt%

62.45

3.09

2.85

0.55

0.80

23.60

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maintenance, recycling, and final disposal [5, 6]. According to ISO standards, an LCA study is composed of four phases: goal and scope definition, inventory analysis, impact assessment and interpretation [7, 8]. The traditional techno-economic evaluation considers only the pure production cost of product, but the cost analysis of the each phrase in life cycle is not enough and it lacks the decision on life cycle cost of product. In the 1960s, the US Defense Department first proposed the concept of life cycle cost, and on this basis, developed the analysis method of LCC (life cycle cost), which has been applied to the fields of weapon equipment and power plant construction [9]. The life cycle cost is defined as “the sum of the discounted values of cost incurred over a period of time with operation, maintenance, repair and disposal of an item or the system” by the US National Institute of Standards and Technology (NIST) [10]. 1.3

System boundaries

In order to obtain environmentally friendly solution by fully considering the environmental and economic factors of decision-making, two methods of LCA and LCC are integrated to establish the corresponding model of calculation and evaluation (in the theoretical framework of LCA,LCC's the goal and scope, the system boundary, functional unit allocation and logistics data will be consistent with that of LCA) to improve decision efficiency of power plant's power generation, emissions reduction, etc. In accordance with the life-cycle analysis framework, the system boundary of Oxy-combustion plant retrofitted from existing coal-fired power plant includes two series: 1) Life cycle of power plant, including four stages of building materials mining, building materials transport, plant construction and decommissioning of power plant; 2) life cycle of fuel, including four stages of fuel extraction, fuel transportation, fuel consumption, and electricity use. To simplify the calculation process, the following assumptions are made in the evaluation model: First, the construction period, the service life, yearly load factor and demand of the main building materials of the Oxy-combustion power plant should be consistent with those of a traditional coal-fired power generation system; the amount of annual coal consumption, coal mining and power generation efficiency can be adjusted according to boiler efficiency and power consumption, and the phases of CO2 transportation and storage are not considered. Secondly, the distance between the mine and power plant is 500 km; the transportation distance among equipment, building materials and waste residue is 50 km, and all are transported by the same diesel train (a total of 85 wagons, the coal loading capacity of each wagon is 77 t, the service life is 30 a). Thirdly, the oxygen is produced by using the cryogenic air separation technology, oxygen purity from an existing air separation unit ASU (60000 Nm3O2/h) is above 95%, and 70% to 80% of the flue


gas is recycled, CO2 volume concentration (dry) of the exhaust gas stream from the boiler rear is more than 95%. Fourthly, all the energies consumed in the flue gas pretreatment, oxygen producing, CO2 compression and recovery are provided by this power generation system. Among them, the station service power consumption rate of power plant is 5%, the power consumption of de-SOx and de-NOx device are calculated by 2% of the generating capacity. Fifthly, the factors and processes that have little impact on the calculated results can be ignored. Sixthly, in order to facilitate the comparison and uniform evaluation, according to the average exchange rate in 2008 (1 USD = 6.94 RMB), the coal price is 70 USD/t. The life cycle analysis boundaries of the system are shown in Figure 2.

2 Life cycle CO2 emissions inventory 2.1 LCA–based evaluation model and parameter settings The traditional CO2 accounting methods only consider direct CO2 emissions from energy generation process, while the life cycle CO2 emissions also need to consider CO2 emissions from the upstream and downstream of the system, which is the new application of the LCA method in the field of carbon footprint calculation. In the evaluation model of life cycle CO2 emissions, the amounts of CO2 emissions from the production of materials are shown in Table 3, which is considered as the basic data to calculate the system’s life cycle CO2 emission inventories in all stages. The life cycle model to calculate CO2 emission of the system is as follows:

Figure 2

System boundaries of the life cycle analysis.

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Table 3

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CO2 emissions from construction materials and energy [5, 11–13] Items

Steel

Aluminum

Iron

Copper

Cement

Concrete

CO2 emissions (g/kg)

1160

1740

1160

1304

1167.18

30

Items

PVC

MEA

heavy oil

fuel combustion (g/km·kg)

electricity (g/kWh)

CO2 emissions (g/kg)

1497

26.5

3216

0.0162

406.8

n

n

T = T f + yfuel ∫ (1 − ηcap )η PG dt + yext ∫ PE dt + TD 0

(1)

0

where T is the total CO2 emissions in the full life-cycle (t), Tf is the CO2 emissions from system construction (t), yfuel is the emission from unit fuel combustion (g/kg), ηcap is the capture efficiency (%), η is the combustion efficiency (%), PG is coal consumption (t/d), yext is the emissions of unit electricity production (g/kWh), PE is the consumption of electrical energy (kWh), TD is the emissions from material recovery (t), and n is the operating life (a). 2.2

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The life cycle emissions of power plant

Oxy-combustion power plants use pure oxygen instead of ordinary air as the combustion-supporting fuel, therefore, it is necessary to retrofit auxiliary equipment, such as ASU on the existing facilities. Nowadays, the main method of separating high-capacity oxygen from air is cryogenic. The equipment has been localizized and the technologies are improving day by day. To control the combustion temperature and heat transfer, extra fans are needed to install to meet the requirement of flue gas recirculation. For the change of combustion-supporting medium, the characteristics of O2/CO2 cycle combustion (including ignition, combustion and flame stability), gas radiation heat transfer, desulfurization and denitrification will all change. This requires the comprehensive design and improvement to the combustion chamber, thermal heating surface, etc. The corresponding studies both in laboratory and pilot scale have been started by many researches in different countries [14, 15]. The results have shown that O2/CO2 cycle combustion technology has lower NOx emissions, and the thermal NOx has been reduced by about 75% or more; high SOx removal efficiency can be achieved through boiler limestone injection technology [16, 17], eliminating the investment on flue gas desulphurization and denitrification equipment of conventional coal-fired power plant. After transformation, in the life cycle of Oxy-combustion power plant, the CO2 emissions in the process of construction mainly include the emissions from the infrastructure construction, the manufacture and installation of power plant equipment, and the transportation of related building materials, which are stated as follows. 1) The emission inventory in the construction phase mainly comes from the production process of construction materials, and the main construction materials, such as steel, aluminum, iron, etc., are estimated

in accordance with plant engineering reports [5, 18, 19]. 2) The equipment manufacturing and installation emissions of power plant can be attributed to the production process emissions of raw materials for the equipment manufacturing. When it comes to the feasibility of counting, only the emissions in the process of manufacturing and installing large equipment will be calculated. 3) CO2 emissions in the phase of building materials transportation are mainly from fuel combustion emissions, manufacturing and decommissioning emissions of vehicles. The decommissioning of power plant occurred in the 30th year, and emissions from the decommissioning process are set as 10% of construction emissions according to the literature [20]. Taking account of the changes of equipment in the transformation process and the extent of the impact on LCA, this paper assumes that Oxycombustion power plant's demand for the main building materials is basically the same as that of the conventional coal-fired unit. The details amounts are shown in Table 4. 2.3

Life cycle emissions of fuel

In the fuel life cycle, CO2 emissions from coal mining and processing stages contain the emissions caused by its mining process and energy consumption. 1) CO2 emissions of coal mining process mainly come from the production process of mining tools, the limestone for mining and electricity production process, which is estimated by the sum of CO2 emissions from the production of steel, electricity and limestone. 2) The CO2 emissions of coal treating are mainly derived from the production process of power consumption in the coal washing process and the transportation process of the solid waste. For appropriate conversion according to per coal consumption, load rates of power plant and ref. [19], the amount of raw coal needed by the coal-fired units in the mining process is about 1973500 t/a; the quantity of Oxy-combustion power plant needed is about 1880000 t/a; Table 4 Amounts of construction materials for a 2×300 MW Oxy-combustion power plant Material/process

Unit

Amount 30432

Steel

t

Aluminium

t

251

Iron

t

371

Concrete

t

95254

Cement

t

30432

Building materials traffic

t

156742

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the amount of steel needed to manufacture mining tools is 5265 t/30a; the limestone used to mine 10000 t/a with long wall method is 180 t and the power required is 141000 kWh; annual electric quantity and the amount of solid waste generated in fuel processing stage are calculated by the amount of moisture and ash free (MAF) coal. The amount of MAF coal accounts for 60% of the yield and the electricity required in the processes of coal preparation and washing is 790 kJ/t·MAF coal. The waste output of solid waste is 0.35 t/t·MAF coal. The CO2 emissions during the operation of Oxy-combustion power plant mainly come from direct emissions of coal combustion, indirect emissions of equipment power consumption, and emissions caused by solid waste transportation. 1) Direct emissions of CO2 after coal combustion depend on coal type, coal consumption, boiler efficiency, the absorption of CO2 in the desulfurization process and so on. Nsakala et al. studied the U.S. 450 MW coal-fired power plants [21], and indicated that the boiler combustion efficiency under the condition of air combustion is 88.13%, while in the way of O2/CO2 combustion it can increase to 90.47%. Yan et al. calculated the boiler efficiency of a domestic 300 MW coal-fired power plant [22]; they found that the boiler efficiency could rise from 91.57% under air combustion to 96.76% under O2/CO2 combustion. In this paper, the increased range of the boiler efficiency in calculation is set as 2%–5%, so the corresponding coal consumption and coal mining required are also reduced. 2) The operation of the milling, the circulating water and heating system devices in the power plant, and new equipment, such as, producing of oxygen, CO2 compression and recovery, will consume a lot of electricity and cause indirect CO2 emissions. 3) The coal using in the power plant is transported by train. The amount of coal transportation is calculated by coal consumption and the load rate of power plants, about 11290 round trips in 30 a. CO2 emissions in the coal transportation process mainly come from train fuel combustion, train manufacturing and decommissioning. The amount of steel and aluminum required for manufacturing each carriage is respectively 6713 and 45 kg [19]. Plant operation will also produce large amount of solid ash, and the transportation of solid ash will also consume fuel and generate CO2 emissions. The annual output of solid ash is calculated by annual coal consumption and the amount of coal drybased ingredients. During the operation of Oxy-combustion power plant, the theoretical volume of oxygen v0 (Nm3/kg) needed for Table 5


1 kg as-received basis fuel burning and the actual amount of O2 V (NMm3/h) supplied for coal burning are respectively calculated by

ν 0 = (Car /12 + H ar / 4 + Sar / 32 − Oar / 32) × 22.4,

(2)

V = PGν 0α / 24,

(3)

where PG represents for coal consumption of power plant (t/d); α is the excess air coefficient (set to 1.15). The formulas can also be used to calculate the number of the air separation units. At present, the power consumption of large-scale air separation unit is about 0.38–0.40 kWh/Nm3·O2. Considering the losses during production and transportation, the value of 0.42 kWh/Nm3·O2 is taken. The power requirement of CO2 compression (derived from Damen) [23] is calculated by γ −1 ⎡ ⎤ ZRT Nγ ⎢⎛ p2 ⎞ Nγ ⎥, − 1 W= ⎜ ⎟ ⎥ M γ − 1 ⎢⎝ p1 ⎠ ⎥⎦ ⎣⎢

E=

(4)

W , ηisη m 3600

(5)

where W is specific work (kJ/kg·CO2); E is specific electricity requirement (kWh/kg·CO2); Z is compressibility factor (0.9942); R is universal gas constant (8.3145 J/(mol K)); T is suction temperature (313.15 K); γ is specific heat ratio (cp/cv) (1.293759); M is molar mass (44.01 g/mol); ηis is isentropic efficiency (80%); ηm is mechanical efficiency (99%); Nγ is number of compressor stages (4); p2 is discharge pressure (11 MPa); p1 is suction pressure (0.101325 MPa). The minor CO2 emissions from the building materials, manufacture and installation process and energy consumption during the construction and operation of Oxy-combustion power plant have been ignored in this study. More relevant material flow and the main parameters are shown in Table 5.

3 The life-cycle cost analysis From the perspective of the life cycle, LCC of Oxy-combustion power plant is composed of the costs of construction, fuel, operation and maintenance, and scrap. The life

Material flow and the main parameters in the case of O2/CO2 cycle combustion

Combustion technology O2/CO2 Combustion technology O2/CO2

Coal consumption (t/d)

Coal mining (104 t )

Boiler efficiency (%)

5364

188

97

Power consumption of oxygen production (kWh/Nm3·O2) 0.42

Capture efficiency (%) 90

CO2 capture (t/d) 6057

Oxygen (t/d) 6536 Power consumption of CO2 compression (kWh/t) 111

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cycle cost model [24, 25] is as follows: LCC = Cs ⋅ f + Os ,i + Es ,i + Ds ⋅ s, −1

⎡ q q −1 ⎤ −1 f =⎢ − ⎥ , ( p + n) ( q 1) q ( q − − 1)q p ⎦ ⎣ ( p+n)

s=

p

q −1 , q( p + n) − 1

(1 + i ) q= . (1 + r )

(6) (7) (8) (9)

In eq. (6), LCC represents the life-cycle cost (MUSD/a), Cs is the construction cost (MUSD), f is the annual capital recovery factor, Os, i is the operation and maintenance cost (MUSD/a), Es, i is fuel cost (MUSD/a), Ds is scrap cost (MUSD), s is the sinking fund factor. In eqs. (7)–(9), p is the construction period (a), n is the operating life of power plant (a), i is the interest rate (8%), r is the average rate of inflation, according to China’s inflation rate in the period of 1998–2008 (−0.8%, −1.4%, 0.4%, 0.7%, −0.8%, 1.2%, 3.9%, 1.8%, 1.5%, 4.8%, 5.9%, respectively), thus 1.13%. According to “Fisher effect theory”, eq. (6) is amended in this paper, with the comprehensive consideration on the time value of money (i), inflation rate (r) factors, the project construction period, lifespan and so on. Based on the above formula and data, f and s are obtained as about 12.93% and 0.53%, respectively. Construction cost is the total money involved in the design and construction period of the power plant, including preliminary fees, management and technical service fees, financing and equipment purchasing costs, construction and installation costs, commissioning and trial production. It is the cost paid before the commercial operation of power plants, but must be recovered from the cost in the effective economic service period. Compared with conventional coalfired unit, Oxy-combustion power plant need to upgrade the structure of coal-fired boilers and add a large ASU equipment. Although there are no specific investment and construction on desulfurization and denitrification equipment, it is still essential to make some treatment for flue gas [26]. The major parameters of investment required on construction of coal-fired unit and transformation of coal-fired power plant are summarized in Table 6. Operation and maintenance costs include the fixed operating costs, variable operating costs, overhaul and spare parts costs, etc. Fixed operating costs include staff salaries, maintenance cost, sewage charges, management fees and insurance; variable operating costs, including consumable materials (excluding fuel), such as water, various chemicals, oil materials and air filter media; overhaul and spare parts costs contain the expenses of labor, materials and thermal components. As experience and data are scarce, it is difficult to calculate the operation and maintenance cost exactly, and which are generally calculated by the 4% of the total

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Table 6 Investment costs of coal-fired power plant and the added investment costs of O2/CO2 recycle combustion retrofit Devices Base plant (no de-SOx and de-NOx devices) Among: Boiler De-SOx devices (Wet-FGD) De-NOx devices (SCR, pure NH3) ASU (60000 Nm3O2/h)a)

Unit

Investment cost

MUSD

410

MUSD

40

MUSD

21 [27]

MUSD

35 [28]

MUSD

24 7% of cost of Boiler upgrades MUSD base boiler [29] 2.5% of total investment b) MUSD Fuel gas treatment cost of base plant [26] a) The number of ASU is calculated by formula, power is 25 MW [3]. b) The energy consumption of fuel gas treatment is 8% of gross electrical output.

construction cost [2]. The fuel cost is related with not only coal price, coal consumption and the calorific value of coal, but also the hours of units under the state of the maximum load. Coal price is mainly decided by the cost of mining and transportation, and to some extent influenced by oil price and market price. The annual fuel cost of unit is calculated by coal price, coal calorific value, the coal consumption and the hours under maximum load. The equation is as follows [3]: Es , i =

CF HPG (Qnet,ar / Qn ), 24

(10)

where Es,i is the annual cost of fuel (MUSD/a), CF is the price of coal (USD/t), H is the annual hours of operation of the power plant (h), PG is the coal consumption amount (t/d), Qnet,ar is the net low calorific value of coal (MJ/kg), Qn is the low calorific value of standard coal (29.27 MJ/kg). Scrap cost is the cost paid for the cleanup and destruction after the ending of product's life cycle. The products of different types and uses have different scrap costs. Some scrap cost can produce a certain amount of residual income to offset the related costs, and at this time scrap cost should be negative, such as the normal retirement of equipment. However, some can not generate any residual income, but rather their cleanup and retirement need to spend a lot of money, then the scrap cost is positive, such as chemical products and nuclear products. In the process of product obsolescence, a certain human, material and financial resources are consumed, but meanwhile it may bring some income. Therefore, this cost is needed to be counted carefully. This cost is generally estimated by referring to historical data, and scrap cost is set as the 5% of the total investment in this paper [24].

4 The cost of generation and CO2 avoidance cost in life-cycle The effect of CO2 capture on the cost of generation is one of

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the most important indicators to measure the economic of CO2 avoidance emissions of power plant. The CO2 avoidance cost is also an economic indicator widely used in the field of CO2 emission control systems and used to measure the consumption cost for avoiding the production or emission of CO2. In order to fully measure these two key economic parameters, this paper estimates the cost of generation and CO2 avoidance cost of Oxy-combustion power plant from the life cycle perspective. The calculation models [25] are shown as follows: COE = ∑

LCC , Wnet H

(11)

where COE is the life-cycle cost of electricity (USD/MWh), LCC is the life cycle cost (MUSD/a), Wnet is the net electricity output (MW), and H is the annual hours of operating (h) . ⎛ Tair TO2 (CC )avoided = (COEO2 − COEair ) ⎜ − ⎜ Wnet-air H Wnet-O H ⎝ 2

⎞ ⎟ , (12) ⎟ ⎠

where (CC)avoided stands for the life cycle CO2 avoidance cost of Oxy-combustion power plant (USD/t); COEO2 is the life cycle cost of electricity of Oxy-combustion power plant (USD/MWh); COEair is the life cycle cost of electricity of coal-fired power generation unit (USD/MWh); TO2 is annual CO2 emissions of Oxy-combustion power plant in the life cycle (t); Tair is annual CO2 emissions of the coal-fired unit in the life cycle (t); Wnet-O2 is the net electricity output of Oxy-combustion power plant (MW); Wnet-air is the net electricity output of the coal-fired unit (MW).

5

Results analysis

The technological retrofit was carried out for a 2×300 MW coal-fired unit by using O2/CO2 cycle combustion technology. Based on the LCA and LCC analysis, the amount of CO2 emissions, the costs of investment, electricity and CO2 avoidance were calculated on the whole life cycle system. Life cycle embodies in three processes, i.e. construction and


renovation, operation and maintenance, disintegration and retirement of Oxy-combustion power plant. A variety of product ingredients (such as equipment, raw materials, processes, etc.) and various links involved in the process were calculated in detail and a calculation list of the entire life process was obtained. 5.1

The life cycle CO2 emissions

The calculation of CO2 by LCA shows that the life cycle CO2 emissions of coal-fired power plant are 7374.82 t/d, among which the proportions of power plant’s operation emissions are about 98.82%, the share of emissions from fuel extraction is approximately 0.64%, the proportion of emissions from fuel transportation is about 0.44%, and the share of emissions from plant construction and decommissioning is only 0.1%. On this basis, with the O2/CO2 cycle combustion technology to transform coal-fired power plant and a CO2 capture efficiency of 90%, the life cycle CO2 emissions after transformation are 1689.78 t/d without considering the CO2 compression stage. Among them the emissions shares in oxygen stage and power plant operation period respectively account for 44.90% and 50.31% of the total emissions; the shares of fuel extraction and transportation emissions are about 2.67% and 1.82%, respectively; CO2 capture within the life cycle is about 5685.04 t/d and the reduction rate reaches about 77.09%. Considering the case of CO2 compression phase, the life cycle CO2 emissions are 1965.64 t/d, among which the emissions of oxygen stage, power plant operation, and CO2 compression account for 38.65%, 57.63% and 13.93% of total emissions, respectively. The proportions of fuel extraction and transportation emissions are about 3.25% and 3.22%; CO2 capture of Oxy-combustion power plant within full life cycle is about 5409.18 t/d, while reduction rate is about 73.35%. It can be found that Oxy-combustion power plant can effectively control the emissions of greenhouse gases, but energy consumption in the oxygen phase can result in higher indirect CO2 emissions (Table 7).

Table 7 Life cycle inventory results for the coal-fired power plant and Oxy-combustion power plant CO2 in equal shares to 30 a (t/d) Stages and processes of life cycle

2×300 MW coal-fired power plant (including desulfurization and denitrification)

2×300 MW Oxy-combustion power plant (capture efficiency: 90%)

Power plant construction

6.82

6.82

Coal mining

47.29

45.04

Coal transportation Plant operation Power plant decommissioning Oxygen production

32.31

30.77

7287.72

850.07

0.68 -

0.68 758.76

CO2 compression

-

273.50

Total life cycle emissions

7374.82

1965.64

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The life cycle cost

The calculation results show the life cycle cost of coal-fired power plant is 158.51 MUSD/a, among which the construction cost is 60.27 MUSD/a, operating and maintenance cost and fuel cost are 98.11 MUSD/a, accounting for 38.02% and 61.90% of the total cost, respectively. If not considering CO2 compression, the life cycle cost of Oxy-combustion power plant is 167.88 MUSD/a, only 9.39 MUSD/a higher than that of conventional coal-fired power plant, with an increase of approximately 5.9%; net power has dropped by some 21.33%, from 558 to 439 MW. When considering CO2 compression, the life cycle cost of Oxy- combustion power plant is 173.33 MUSD/a, about 14.82 MUSD/a higher than that of conventional coal-fired power plant, with an increase about 9.35%. Among that the construction cost is about 75.67 MUSD/a, and the operation and maintenance costs and fuel cost are 97.49 MUSD/a, accounting for 43.66% and 56.25% of the total cost, respectively; the net power drops from 558 to 409 MW, by 26.70%. According to the life cycle cost analysis, the upkeep cost (including operation, maintenance and fuel costs) of Oxycombustion power plant after transformation is basically consistent with that of coal-fired unit. However, the upkeep cost of coal-fired unit is 23.87% higher than its construction cost, while the upkeep cost of Oxy-combustion power plant is only 12.58% higher than its construction cost. Therefore, the upkeep cost of transformed power plant is relatively lower, so it is necessary to have a deep trade-off analysis between them to improve the overall economy. The concrete calculation results are summarized in Tables 8 and 9. 5.3

The life cycle cost of electricity and CO2 avoidance

Based on the above results, it can be obtained that the life cycle cost of electricity of coal-fired power plant is about 47.34 USD/MWh by eq. (10). After transformation by O2/CO2 cycle combustion technology and if the CO2 compression stage is not considered, the cost of electricity is Table 8

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approximately 63.80 USD/MWh, which has increased by about 34.77%; otherwise, the cost of electricity is about 70.60 USD/MWh, increased by about 49.13%. IPCC announced changes in the scope of CO2 avoidance costs of power plants (without regard to transportation and storage phase) is about 15–75 USD/t [30]. This paper uses the life cycle avoidance cost of Oxy-combustion technology, and it is about 28.93 USD/t without considering the case of CO2 compression. It is about 45.46 USD/t with considering the case of CO2 compression, by eq. (11). Specific results are listed in Table 10. According to the neoclassical economic theory, a carbon tax is the most market efficient economic means to reduce CO2 emissions, and is the most important cost-effective emission reduction measure. Carbon tax is a Pigovian tax, which is in fact a product consumption tax based on the carbon content of fossil fuels or carbon emissions consumption, to increase the financial burden for business or individual not committed to reducing CO2 emissions. So far, the carbon tax is applied to more than 10 countries, including Austria, Czech Republic, Denmark, Estonia, Finland, Germany, Italy, Netherlands, Norway, Sweden, Switzerland and the United Kingdom. In addition, Japan, New Zealand and other countries are also considering the carbon tax. Because of the standards of carbon tax are still in a research stage, and there are many different standards, from 7 to 61 USD/t range [31, 32]. In this study, the carbon tax (the CO2 tax) was applied to the total cost of conventional coal-fired power plant and Oxy-combustion power plant to analyze the economic viability of Oxy-combustion power plant, which may provide a decision reference for tax collection policy of CO2 emissions (Figure 3). The cost of electricity of conventional coal-fired power plant and Oxy-combustion power plant are calculated by choosing different units of the tax value. The CO2 emissions from the coal-fired power plant are more than those of the Oxy-combustion power plant, and the influence of carbon tax on its cost of electricity is more than that of Oxy-combustion power plant. The increase rate of cost of

Life-cycle costs for coal-fired power plant Parameter Construction cost Operating and maintenance cost Fuel cost Scrap cost Net power

Unit MUSD MUSD/a MUSD/a MUSD MW

Value 410.00 16.40 79.47 20.50 569

Desulfurization

Construction cost Operating and maintenance cost Scrap cost Net power

MUSD MUSD/a MUSD MW

21.00 0.84 1.05 563

2.72 0.84 0.01

Denitrification

Construction cost Operating and maintenance cost Scrap cost Net power

MUSD MUSD/a MUSD MW

35.00 1.40 1.75 558

4.53 1.40 0.01 158.51

Base plant

Total

LCC (MUSD/a) 53.02 16.40 79.47 0.11 -

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Table 9

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Life-cycle costs for Oxy-combustion power plant

Power plant construction, operation and CO2 capture

CO2 compression

Parameter Construction cost of base plant Construction cost of flue gas treatment facilities Construction cost of boiler upgrades Construction cost of ASU Operating and maintenance cost Fuel cost Scrap cost Net power Purchase and construction cost Operating and maintenance cost Scrap cost Net power

Unit MUSD MUSD MUSD MUSD MUSD/a MUSD/a MUSD MW MUSD MUSD/a MUSD MW

Value 410.00 10.25 2.80 130.15 22.13 74.08 27.66 439 32.00 1.28 1.60 409

Total

LCC (MUSD/a) 53.02 1.33 0.36 16.83 22.13 74.08 0.15 4.14 1.28 0.01 173.33

Table 10 Life cycle cost of electricity and CO2 avoidance 2×300 MW Oxy-combustion power 2×300 MW coal-fired power plant plant (no compression) (including desulfurization and denitrification) Total life cycle CO2 emissions (t/a) Net power (MW) Annual hours of operating (h) CO2 emissions of unit electricity production (t/MWh) Life cycle cost of electricity (USD/MWh) Life cycle cost of CO2 avoidance (USD/t)

2×300 MW Oxy-combustion power plant (compression)

2691809.31

617632.81

717460.31

558

438.53

409.12

6000

6000

6000

0.80

0.23

0.29

47.34

63.80

70.60

-

28.93

45.46

Figure 3 The life cycle cost of electricity under different CO2 tax rates.

coal-fired power plant is significantly higher than that of Oxy-combustion power plant as the standard of carbon tax increases. These two costs are equal when the carbon tax reaches about 24 USD/t without considering CO2 compression stage; otherwise, the carbon tax will reach about 34 USD/t, in favor of the market and scale development of O2/CO2 abatement technologies.

6

Conclusions

Carbon abatement technologies are comprehensively measured and evaluated from the two aspects of environment and economics by integration of LCA and LCC methods, which is consistent with sustainable development and the basic

principles of modern economics, and thus it is beneficial for the selection and development of low-carbon technologies and make reasonable decisions. The following conclusions can be drawn from the theoretical analysis and the results. 1) In the whole life cycle, CO2 emission of the conventional coal-fired power plant life cycle is 7374.82 t/d. After the retrofit, CO2 emission of Oxy-combustion power plant life cycle is about 1689.78 t/d without considering the case of CO2 compression, the rate of CO2 avoidance is about 77.09%; the CO2 emissions amount is about 1965.64 t/d considering the case of compressed CO2, the rate of CO2 avoidance is about 73.35%. The retrofit is effective to control greenhouse gas CO2 emissions. 2) In view of the LCC, without considering CO2 compression, the life cycle cost of Oxy-combustion power plant

WANG Yun, et al.

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is 167.88 MUSD/a, about 9.39 MUSD/a higher than that of the existing coal-fired power plant, or an increase of approximately 5.9 %; the net power decreases by about 21.33%. Considering CO2 compression, the life cycle cost of Oxy-combustion power plant is 173.33 MUSD/a, higher than the existing coal-fired power plant of about 14.82 MUSD/a, an increase of about 9.35%, the net power decreases by about 26.70%. The upkeep costs of coal-fired unit and retrofitted Oxy-combustion power plant are the same, but the upkeep costs of coal-fired unit are 23.87% higher than its construction costs, and the upkeep costs of Oxy-combustion power plant are only 12.58% higher than its construction costs, and the upkeep costs are relatively lower. 3) The life cycle cost of electricity of the conventional coal-fired power plant is about 47.34 USD/MWh. After retrofit by O2/CO2 cycle combustion technology and without the CO2 compression stage, the full life cycle cost of electricity is about 63.80 USD/MWh, an increase of about 34.77%; abatement cost is about 28.93 USD/t. If considering CO2 compression stage, the life cycle cost of electricity is about 70.60 USD/MWh, an increase of about 49.13%, and the abatement cost is about 45.46 USD/t. 4) For different tax rates, the costs of electricity of the existing conventional coal-fired power plant and Oxy-combustion power plant were calculated and compared. The results indicated that the two cases are the same when the carbon tax is about 24 USD/t without considering the CO2 compression, and that the two cases are the same with consideration of the CO2 compression when the carbon tax is about 34 USD/t. The latter tax rate can be conducive to promoting an active carbon emissions reduction of conventional coal-fired power plants. This work was supported by the National Natural Science Foundation of China (Grant Nos. 40972102, 50936001, 50721005) and the National Basic Research Program of China (“973” Program) (Grant No. 2010CB227003). 1

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