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Energy Efficient Carbon Fiber Production with Concentrated Solar Power: Process-Design and Techno-Economic Analysis Uwe Arnold, Andreas De Palmenaer, Thomas Bartholomaeus Brück, and Kolja Kuse Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04841 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Industrial & Engineering Chemistry Research

Energy Efficient Carbon Fiber Production with Concentrated Solar Power: Process-Design and Techno-Economic Analysis Uwe Arnold a,*, Andreas De Palmenaer b,**, Thomas Brück c, Kolja Kuse d a

b

AHP GmbH & Co. KG, Karl-Heinrich-Ulrichs-Str. 11, D-10787 Berlin, Germany

Inst. Textile Technology, RWTH Aachen University, Otto-Blumenthal-Straße 1, 52074 Aachen, Germany

c

Werner Siemens Chair of Synthetic Biotechnology & Director TUM AlgaeTec Center, Dept. of Chemistry, Technical University of Munich (TUM), Lichtenberg Str. 4, 85748 Garching, Germany d

TechnoCarbonTechnologies GbR, Oberföhringer Strasse 175 a, D-81925 München, Germany

1

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ABSTRACT

3

Carbon fibers if generated from

4

anthropogenic CO2 via algae based

5

carbon

fixation

and

subsequent

6

energy

and

cost

efficient

7

carbonization would be a sustainable

8

CO2 sink. They could replace materials, which currently are major GHG-sources, hence

9

contributing significantly to accomplish targets of 2015’s Paris Agreement. For the first time our

10

study presents an integration of concentrated solar power (CSP) technology into a carbonization

11

reactor (CR) for carbon fiber production combined with extensive energy recovery. Based upon a

12

mass and energy flow model of the corresponding CSP-CR concept techno-economic analysis

13

(TEA) was carried out, first with static base case values for broad variation analysis, and in a

14

second step with a dynamic economic model, embedded in a Monte-Carlo simulation, so as to

15

quantify risks from market and modeling uncertainties. First results indicate significant reduction

16

potential of energy consumption and cost. Economic viability perspectives are promising and

17

justify extended research and developments.

18

INTRODUCTION

19

Compliance with 2015’s Paris Agreement requires mitigating further increase of atmospheric

20

greenhouse gas (GHG) concentrations fast and effectively. Sole substitution of fossil energy

21

supplies by renewables, however, will not suffice to meet the Paris goals in time1. Hence, CO2-

22

sinks with lasting GHG-extraction and demobilization effects are to be installed fast and in a

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significant scale. Solution concepts a) of massive afforestation and reforestation2, b) of

24

combining biomass power plants (BE) with carbon capture and storage (CCS)3,4 equipment ( 2 ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research

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BECCS)2, and c) of carbon capture and (sustainable) utilization (CCU)3,4 are rapidly gaining

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attention worldwide.

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To date, essential construction materials, such as steel, aluminum, and cement, seem to be

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irreplaceable in our societies. However, conventional production processes of these materials are

29

highly energy intensive and therefore responsible for up to 21% of the global anthropogenic

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GHG emissions (see Supporting Information, SI, section 2.1). A substitution by a material

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derived from CO2 with equivalent performance and cost but reduced energy input would reverse

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the trend of CO2 emission and generate a value adding, permanent CO2 sink. Carbon fiber (CF)

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could be an answer to this challenge.

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Due to their reduced weight and high tensile strength carbon fibers and their composites are

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rapidly gaining importance in the automotive and aerospace industries5,6,7. Currently, the

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prevailing CF-precursor polyacrylonitrile fiber (PAN) is generated from petroleum based

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resources5. Moreover, the conversion of PAN into carbon fiber is extremely energy and cost

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intensive causing significant CO2 emissions. Cost of CF is currently prohibitive to deploy this

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advanced material into cost sensitive mass markets, such as the building construction industry.

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The development of new production processes that aim at a conversion of anthropogenic CO2 to

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carbon fiber with minimal energy and cost input would offer a significant CCU solution and may

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enable a disruptive transition in the construction, vehicle, and other producing industries. To that

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end processes that convert CO2 into carbon fiber by electrolysis8 and by algae based biochemical

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methods9 are evaluated technologically. In our previous manuscript9 we have delineated and

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analyzed a process route for the conversion of anthropogenic CO2 to PAN fiber via generation of

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algae biomass for the first time. In this study, we complete the process path by disclosing an

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energy-efficient method for the thermal conversion of PAN into carbon fiber using concentrated 3 ACS Paragon Plus Environment

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solar power (CSP) technologies in combination with extensive internal energy recovery for the

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first time.

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After wet spinning into PAN fiber in the conventional CF production process, PAN precursor

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is thermo-chemically converted into CF. Specifically, the PAN precursor is first stabilized

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(oxidized) under air atmosphere at temperatures up to 300 °C and then carbonized

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(denitrogenated) under nitrogen inter gas atmosphere and temperatures in excess of 1,500 °C.

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These processes including downstream exhaust gas treatment are highly energy intensive.

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Depending on the local energy prices, these processes comprise up to 33 % of the CF production

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cost (SI, section 6.1). In that regard, the typical energy demand amounts to about 340 MJ/kg

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without precursor production10. Hence, the high energy consumption is a main cost driver and

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environmental burden of CF production. Industry requires cost reductions of about 50% for a

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diversified application of suitable high quality carbon fiber11. Consequently, lowering cost and

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specific energy demand of CF production is a major target of numerous CF related recent

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investigations6,7,11,12,13,14.

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To achieve both, reduced cost and energy demand, without losing material quality, the present

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study suggests integrating concentrated solar power (CSP) technology into a carbonization

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reactor (CR) in combination with extensive internal energy recovery. This new technology

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platform is termed CSP-CR. The principle feasibility of carbonizing PAN fiber by means of

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concentrated light was demonstrated by Fraunhofer-Institute ILT at RWTH Aachen University,

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which treated oxidized PAN precursors with a laser beam as a concentrated light source15. These

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experiments enabled quantitative PAN carbonization. The second prerequisite of high quality

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carbon fibers are sufficiently high temperatures in the focal zone of the CSP setup. The 1 MW

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solar furnace at Odeillo in Southern France is reported to create temperatures up to 3,200°C16. 4 ACS Paragon Plus Environment

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Hence, a temperature of 1,200-1,400 °C should be achievable given sufficient collector aperture

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area and focusing quality. While multiple research on optimizing CSP for power production has

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been reported e.g. 17,18, an investigation of viability perspectives of CSP for CF production has not

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been disclosed to the authors’ knowledge.

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The data reported in this study present a primary feasibility analysis of the CSP-CR-concept,

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The system boundaries of our study encompass process design, static techno-economic analysis

77

(TEA) with average values for broad sensitivity analysis, and dynamic TEA embedded in a

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Monte-Carlo simulation for assessment of risk from market, design, and modeling uncertainties.

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The related work is reported as an early stage contribution to mandatory CCU technology

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developments. Our study applies a reverse engineering approach by initially analyzing the

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required conditions for economic viability of the investigated technology prior to proceeding

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towards more detailed and resource consuming developments.

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MATERIALS AND METHODS

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CSP-CR Process, Mass and Energy Flow Model. For analysis of the CSP-CR technology, a

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concise process concept (see figure 1) and a simplified mass and energy balancing model was

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developed. The predominantly integral CSP-CR-model was configured to enable sufficient

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differentiation for comparison with the conventional CF production process. Moreover, the model

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had to be appropriately simplified for early stage TEA and the reverse engineering approach.

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Some comprehensive system characteristics were covered by means of empirical mass or energy

90

specific coefficients and calibrated by means of the “MegaCarbon”-model of the Institute of

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Textile Technology (ITA) at RWTH Aachen University, which is a granular physics-based

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simulation model and process cost accounting tool of the comprehensive conventional carbon

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fiber production process19,20,21,22,23. 5 ACS Paragon Plus Environment

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The MegaCarbon model was developed 2010-2013 in a joint R&D project of ITA Institute of

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Textile Technology at RWTH Aachen University and industrial partners22,23. The model consists

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of 46 interlinked modules (see figure S13 in SI) which model mass and energy flows and cost of

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the different production steps of PAN-precursor production, carbon fiber production, and

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preparation and supply of operation media such as nitrogen, steam and other. Included are

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separate model components of cost accounting, factor input balancing and scenario analyses.

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Differentiated for the single elements of the process-chain including steam and nitrogen

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generation and waste gas abatement, the model calculates annual capital and operation cost,

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energy and media consumptions depending on plant capacity and carbon fiber quality input

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values. The specifics of investment and operation expenses were derived from data provided by

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industrial producers and plant operators. Additional details of the ITA MegaCarbon model

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including the model structure are presented in SI, section 4.2.

106 107

The model was modified to serve as a reference case (exclusion of the precursor production section, confinement to comparable system boundaries and related boundary conditions).

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Industrial & Engineering Chemistry Research

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Figure 1. Simplified process flow chart of the CSP-CR process for PAN precursor carbonization

110

to carbon fiber. Roving flow connection through the interface between oxidation and

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carbonization zone can be both continuous and decoupled. The power block scheme (here for

112

simplification 1-stage process) is symbolic and can represent a multi-stage process with

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intermediate storage capacities as well.

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The sequence of basic treatment steps of the CSP-CR is identical with that of the conventional

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process of PAN fiber oxidation and carbonization: pre-treatment, oxidation, carbonization, post-

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treatment, (see figure 1). Instead of electric furnaces and in analogy to a solar field of a CSP

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plant, the carbonization step takes place in a field of parallel solar heated carbonization reactors

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with parabolic trough reflectors, which focus sunlight upon transparent carbonization tubes in

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their focal axis. Each carbonization reactor tube comprises a heating zone (collector) and a heat7 ACS Paragon Plus Environment

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insulated extension zone (non-transparent, internally coated with reflecting surface) necessary to

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ensure sufficient reaction time of the moving carbon roving inside. With the reported heat

122

demand for carbonization, the determined average area specific normal irradiation (DNI) of the

123

sun, and an assumed efficiency ηSF of the energy conversion form irradiation to heat a required

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minimum collector aperture area ASF is calculated. With this area and a design-specific (pre-

125

selected) collector height hSC, a number of collector elements nSC, which is a function of the

126

required production capacity, a necessary minimum collector length lSC can be determined. The

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total length of the carbonization reactor tube lCR needs to exceed the minimum transfer path

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length of the roving in order to match with the minimum carbonization time of the roving (due to

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reaction dynamics). This path length is a function of the average longitudinal roving velocity.

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Figure 1 illustrates the related geometric parameters. The related formulae and additional details

131

are presented in section 4.1.4 of SI. The length of the zones and the geometry of the collectors are

132

results of the mass and energy flow model. Effective building ground demand of the plant is

133

derived by multiplying the total aperture area of the solar field with an empirical factor of 3.924,

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whose delineation is described in the SI (SI, section 4.3, equation number 30).

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Nitrogen serves as protective gas to enable the pyrolysis process inside the carbonization tubes.

136

The “exhaust gas” flow at the reactor’s exit has to be purified from gaseous products of the

137

carbonization reaction, i.e. from HCN, CH4, CO, NH3, C2H4, C2H6 and other by means of a state-

138

of-the-art RTO-component (regenerative thermal oxidation)25, which is a purpose-specific

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combustion-chamber fueled with natural gas. The major energy recovery unit of the CSP-CR-

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system is the power block, which may include an indirect Rankine or Brayton power cycle or a

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classic small CHP-plant if the combustion unit is integrated. Due to the high temperature level at

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the entrance of the power block’s primary side, a multi-stepped process may be required. The 8 ACS Paragon Plus Environment

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final selection of the power block’s energy conversion process is depending on the results of a

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future techno-economic system comparison on a higher level of detail. At present, a Rankine-

145

process was assumed. The electric power generated by the power block contributes significantly

146

to covering the electric power consumption of the CSP-CR-process.

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Behind the power block, treated exhaust gas of the carbonization reactor flows through heat

148

exchangers (e.g. double wall, shell exchanger or other) of the oxidation ovens. In closed N2-cycle

149

mode, the N2-stream is mixed with refill N2 behind the oxidation ovens to compensate for gas

150

losses and reduce the concentration of residual products of the combustion processes.

151

Subsequently, the gas stream is fed into the carbonization tube. Switch S in figure 1 denotes the

152

possibility of an open or semi-open N2 cycle where between zero and 100% of the N2-inflow to

153

the CR-tube are fed from an N2- supply (tank or attached air-splitting facility). The exiting part of

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the N2-effluent from the oxidation ovens encompasses heat recovery and cleaning units to be

155

released to the atmosphere. This simplified model covers the gross effects of the different

156

operation modes for N2 mass flows and energy balances by means of related parameters. At

157

present, details of the stabilization gas cleaning process and of heat recovery for pre-heating the

158

N2-feed are not modelled in-detail.

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Due to the exothermal reaction of the precursor oxidation, the temperature inside the oxidation

160

ovens has to be controlled very precisely. This is achieved by control of the hot N2-flow through

161

the oven system (by means of a bypass valve) and by control of the cool air inflow and heat

162

exchange between air in- and outflow. Behind the oxidation ovens, the gas flow exiting from

163

inside has to be cleaned from gaseous products of the stabilization reaction i.e. most importantly

164

from HCN and CO. Natural gas is required for this treatment by combustion and electric power is

165

consumed by the air fans and auxiliary devices of the oxidation/stabilization steps. 9 ACS Paragon Plus Environment

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In the process model, sophisticated heat recovery is generally assumed. The total demands of

167

electrical power (12.07 MJ/kg CF) and natural gas (118.30 MJ/kg CF) are determined empirically

168

by means of mass specific factors which were calibrated by the ITA MegaCarbon-model.

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The CSP-CR-model allows for both a continuous 24h-operation of the plant and an intermittent

170

“Sun-day”-mode with decoupled operation of the oxidation ovens and the carbonization tubes.

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The intermittent mode is governed by the varying daily sunshine period and requires extra thread

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spools at the oxidation/carbonization-interface and additional electric heating equipment for pre-

173

heating the oxidation ovens. Intermittent operation is associated with significant additional

174

control and organization efforts because of the daily variation and the seasonal sunshine

175

differences. The model requires selection of a geographic plant location to determine local DNI

176

(direct normal irradiance) and average hours of sunshine from underlying meteorological data.

177

The CSP-CR process model is a simplified integral balance of mass and energy flows of the

178

following CSP-CR main components: stabilization & oxidation ovens, carbonization reactors,

179

waste gas abatement and power-block, auxiliary elements including roving pre- and post-

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processing. The basic quantity is the carbon fiber mass flow, which is identical with the plant

181

production capacity. Precursor mass flow, nitrogen demand and number of required parallel

182

carbonization reactors are derived from the carbon fiber mass flow in combination with average

183

roving velocity, roving strength, material properties, and empirical specific precursor and

184

nitrogen demands per mass unit of carbon fiber. Material properties of carbon fiber and nitrogen

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are computed by means of empirical formulae from literature. The required heat input to the

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carbonization reactor is the sum of heating the roving from reactor entrance to exit temperature,

187

heating of nitrogen flow, and coverage of the pyrolysis-enthalpy of the endothermal

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carbonization reaction. 10 ACS Paragon Plus Environment

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The mass specific pyrolysis enthalpy was derived from literature data under the assumption of

190

approaching a practical optimum as elaborated by Liddell et al.10 and U.S. Dept. Energy12 leading

191

to an estimate of 65 MJ/kg CF. The carbon fiber specific demand of natural gas required for

192

waste gas abatement was calibrated by means of the MegaCarbon model at 2.99 m3N/kg CF. The

193

calculation of electric power produced from heat recovery and a Rankine cycle applies moderate

194

assumptions of 90% and 31% for the energy conversion coefficients of heat recovery and of the

195

heat-power-cycle. On the power demand side the process related consumption of electricity per

196

carbon fiber mass unit (without heating) was calibrated by means of the MegaCarbon model

197

leading to a specific value of 12.07 kWhel/kg CF.

198

The required aperture area of the solar field of the carbonization reactors is a function of the

199

required heat input to cover the energy demand of heating and pyrolysis of the carbonization

200

reactors, the collector geometry (height, length), the location specific solar irradiation (DNI), the

201

number of required parallel carbonization reactors and an average energy conversion efficiency

202

coefficient of the solar field. Based upon operating CSP-plants this coefficient was approximated

203

by 50%, which is a target value and needs validation in follow-up studies.

204

Supporting Information SI contains the complete equation set of the CSP-CR-process model

205

applied by the authors including the simplifying model assumptions, and the derivation of

206

empirical design factors (SI, section 4.1). Details of the model calibration by the MegaCarbon-

207

model are presented in SI-section 4.2.2.

208

Static Techno-Economic Analysis Method. The economic model applied to static TEA uses

209

time averaged values for the calculation of product sales revenues, capital and operation cost in

210

order to determine a break-even-price of the carbon fiber product which is defined by line 36 of

211

table S4 in SI (sum of direct and indirect annual cost plus venture & profit surcharge divided by 11 ACS Paragon Plus Environment

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212

annual production capacity). The simple model was applied, which compares the CSP-CR with

213

conventional CF production plants for selected case scenarios and in a broad variation analysis.

214

Primarily, fixed capital investment, usually consisting of ISBL (inside battery limits) plant cost,

215

OSBL (outside battery limits) plant cost, engineering cost and contingency surcharges is

216

determined so as to calculate capital cost. In the static cost-model ISBL plant cost consists of four

217

components: 1 carbonization reactors and solar field, 2 power block, 3 oxidation ovens, 4 sum of

218

all other plant components and aggregates. The following scale functions for ISBL plant cost

219

components were derived by means of regression analysis from anchor values of real plant

220

investments or were calibrated by means of the ITA MegaCarbon-model (for derivation of the

221

scale functions see SI, section 4.3):

222

CSP-CR-carbonization reactor:

223

, = 353.92 ∗   

224

with

225



KInv,CR:

.

∗ 1 + κ!"#!$ % € (1)

ISBL investment demand of the carbonization reactor field in €, cost year: 2017

226 227

'()

aperture area of solar collector field in m2

228

κ(*

empirical investment cost coefficient for CSP-CR specific additional

229

technical effort and cost, initial estimate: + 100%

230

'() is provided by the CSP-CR process model. The coefficients and the exponent of the scale

231

function (1) were derived from known data of two CSP-plants, which is a relatively poor data

232

base and needs additional empirical input. Compared with the established cost structure of a

233

standard CSP-power plant solar field, the empirical cost-coefficient κCSPCR represents the

234

additional cost in % which is caused by transparent absorber tubes instead of simple metallic 12 ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research

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ones, additional sensors and control devices, roving entry and exit gas locks, roving transport and

236

positioning elements and sturdier mechanical construction of the reflector, its framework and

237

foundation and other additional technical equipment. As an initial estimate and cost target for the

238

CSP-CR design a value of 100% was selected, assuming that the CSP-CR-specific features of the

239

collector/reactor field lead to a duplication of investment demand. This “duplication-assumption”

240

is a working-hypothesis and was the object of broad variation analysis within the study presented

241

here.

242

Power block:

243

-.,// ,** = 9,949.57 ∗  01 

244

with

*

.234

€ (2)

245

KInv,PP:

ISBL investment demand of the power block in €, cost year: 2017

246

5 67,**

installed power generation capacity of power block in kW

247

567,** is provided by the energy balance of the CSP-CR process model. The coefficients and

248

the exponent of the scale function (2) were derived by means of regression analysis with 23

249

anchor points (square of the Pearson correlation coefficient r2 = 0.993).

250

Oxidation ovens:

251

,89 = 77,360 ∗ 0>=  ⁄ 

252

with