Elsevier

Energy

Volume 91, November 2015, Pages 852-865
Energy

Integration of oxygen membranes for oxygen production in cement plants

https://doi.org/10.1016/j.energy.2015.08.109Get rights and content

Highlights

  • Energy, exergy and economic analysis of oxygen membranes in cement plants.

  • Oxygen enrichment of the tertiary air and full oxy-fuel cases.

  • Comparison of figures of merit of oxygen membranes with standard cryogenic plant.

  • CO2 and clinker selling price are the most sensitive parameters for the viability.

Abstract

The present paper describes the integration of oxygen membranes in cement plants both from an energy, exergy and economic point of view. Different configurations for oxygen enrichment of the tertiary air for combustion in the pre-calciner and full oxy-fuel combustion in both pre-calciner and kiln are examined. The economic figures of merit are compared with those from a standard cryogenic plant. Both oxygen enriched air and full oxy-fuel cases allow for an increase in clinker production, use of alternative fuels as well as on-site electricity production. In addition, the full oxy-fuel cases generate a concentrated CO2 source that can be used for enhanced oil recovery, in combination with biomass gasification and electrolysis for synthesis gas production, or possibly sequestered. The cases with oxygen enriched air provide very promising economic figures of merit with discounted payback periods slightly higher than one year. The full oxy-fuel cases have a discounted payback period of approximately 2.3 years assuming a CO2 selling price of 35 US$/ton. The sensitivity analysis of full oxy-fuel cases clearly shows that for the discounted payback period, the most sensitive parameters are the CO2 price and the clinker selling price.

Introduction

The cement industry is one of the main carbon dioxide emitting industries in the world, constituting approximately 5–7% of the global anthropogenic carbon dioxide emission [1]. The process emits, per ton of cement produced, around 900 kg CO2 [2], of which approximately 50% come from the conversion of limestone (CaCO3) into CaO (calcium oxide); another 40% are due to the fuel combustion in the kiln, 5% from transportation and the remaining 5% is from the electricity consumption [3], [4]. Modern cement plants have drastically reduced the fuel consumption by heat integration of all subsystems, and in many cases, replaced fossil fuels by biomass or waste [5], [6]. Thus the only option to significantly reduce greenhouse gas emissions in the cement sector is CCS (CO2 capture and sequestration) [7]. Different studies [3], [7] have pointed out that fossil fueled cement production is a very important process to consider in CO2 emission reduction plans because each mole of pure O2 generates 2–3 times the amount of CO2 generated in power production, due to the calcination of limestone [8].

Post-combustion capture and oxy-fuel combustion technologies have been proposed as possibilities for CO2 separation in cement plants [9], [10], [11]. Pre-combustion is not suitable because it is unable to capture the emissions from the conversion of limestone to lime. Post-combustion capture technology is considered a suitable short-term option for CCS in the cement sector because it doesn't require fundamental changes in the clinker-burning process, and could be retrofitted to existing plants. It relies on the downstream separation of CO2 using different chemical or physical measures like chemical absorption (amine scrubbing, chilled ammonia), membrane technologies, adsorption technologies, mineralization and calcium looping [12]. The most mature and proven post-combustion technology to separate the CO2 is chemical absorption using amine-based solvents. The CO2-rich solvent is then regenerated in a stripping process by addition of low-temperature heat, typically low-pressure steam. The energy requirements for the regeneration of the amines are high and additional installations such as a cogeneration plant are usually needed. In addition to the heat requirements for the solvent process, other challenges for post-combustion technologies are related to managing the dust and SOx emissions. Other post-combustion capture techniques, like membranes and solid sorption processes are in a research state and may lead to less energy-intensive capture options. A relatively new and upcoming carbon capture process is the carbonate looping process [13], [14], [15], [16], also known as the regenerative carbonate cycle. Unlike the amine and ammonia processes, a dry limestone-based absorbent is used to capture CO2. This technology uses DFB (dual fluidized bed) reactors (i.e. the carbonator and the calciner) with a continuous looping of Ca-bearing solids. The capture principle is based on the reaction of lime (CaO) with CO2, forming CaCO3 (calcium carbonate), at a temperature range between 600 °C and 700 °C, in the carbonator and the reverse reaction in the calciner (900 °C), producing a rich-CO2 gas stream and a quantity of calcined lime, which is active for further CO2 absorption. The calciner has to be operated under oxy-fuel conditions to prevent the dilution of the captured CO2 with nitrogen; the oxygen demand is about 40% in relation to the oxygen demand of an oxy-fuel kiln [17].

Oxy-fuel technology seems to be the most promising of the CCS technologies in the long term perspective [10] because of the low oxygen consumption (compared with power plants) and the high concentration of CO2 due to calcination. It can give efficiency gains in cement production and improve the energy demand but still needs further research and requires changes in the core cement production process that make it more suitable for new cement plants.

In full oxy-fuel combustion, nitrogen is eliminated and the resulting flue gas stream is highly concentrated in carbon dioxide, which can be compressed and liquefied for underground sequestration or used in EOR (enhanced oil recovery) applications [18]. Even if the valorization of CO2 in EOR is highly debatable due to elevated investment costs, CO2 transportation, energy consumption, plant operability and environmental and long-term consequences; it should also be noticed that in a future energy system, dominated by renewables, concentrated CO2 sources can actually become valuable and of high interest. CO2 can be used in combination with biomass gasification and electrolysis of steam to produce a synthesis gas that can be upgraded to liquid fuels.

Oxygen enrichment of air is less potent, compared with full oxy-fuel combustion, but an economically interesting method for reducing the carbon footprint of cement production. The method is well known in the industry to increase the production capacity [9], [10], [11] and the use of alternative fuels [12]. Many cement plants are limited by their induced draft fans that reach their limits before other equipment or systems tied to the kiln. However, increasing the production requires additional fuel and air for the combustion. In these cases, oxygen enrichment provides the necessary additional oxygen for combustion without the volume penalty of air. In addition to the immediate higher clinker production, oxygen promotes a more stable operation by creating a hotter, shorter flame and improving burning zone control. Over time, this added stability contributes to additional production. Some studies [19], [20] have quantified the benefits of oxygen enrichment of air in the kiln burner. An increase of 25%–50% of kiln capacity (short term experiments) was reported for an air oxygen enrichment of 30–35 vol.% [19]. The same report indicates that oxygen enrichment can reduce fuel consumption between 100 and 200 MJ/tclinker but, at the same time, the electricity consumption increases between 10 and 35 kWh/tclinker due to oxygen production. The modeling results of Duan et al. [20] estimate that when the O2 concentration is increased from 21% to 30%, the coal consumption decreases by 18%, the flue gas volume reduces by 13% and the CO2 concentration in the flue gas increases to 77%. Regarding air oxygen enrichment in the pre-calciner, potential capacity improvements of 3.5 and 4 tclinker/textraO2 have been reported in Refs. [21] and [22], respectively.

The costs of oxygen production are a significant barrier to the application of full oxy-fuel combustion and oxygen enrichment. Large scale oxygen separation from air is generally achieved by cryogenic distillation, which is a mature technology that allows for high purities (>99%). However, it is a very energy intensive process operating at very low temperatures and elevated pressures. The energy input to generate one ton of oxygen (>99%) has been reported to be, for large scale production plants, 220kWh/tO2 [23], 240kWh/tO2 [24] and 245kWh/tO2 [25]. A promising alternative to cryogenic distillation is MIEC (mixed ionic and electronic conducting) oxygen membranes (sometimes referred to as OTMs), operating at high temperatures (usually >700 °C). The driving force for the separation is an oxygen pressure difference across the membrane. High purity oxygen can be obtained because only oxide ions can be transported through the membrane. The working principle of the membrane is as follows: oxygen is adsorbed on the feed side, subsequently split and ionized and then diffuses through the membrane in ionic form (Fig. 1). To preserve electro-neutrality, there is a simultaneous flux of electrons in the opposite direction. The oxygen can be extracted as a pure product and nitrogen, in principle, would only enter via pinholes or defects in the sealing. The energy consumption for oxygen production using MIEC (mixed ionic-electronic conducting) membranes is anticipated to only depend weakly on the plant size, and values of 185kWh/tO2 [26] and 147kWh/tO2 [27] have been reported from system/BOP simulation studies.

A significant benefit of ceramic membrane technology is that it allows for thermal integration in the process, so the energy consumption numbers can become significantly lower [28], [29]. For instance, when supplying oxygen for a biomass gasifier in a thermally integrated arrangement the power need may be as low as 100kWh/tO2 [28]. Future prospects reported for the membrane route to pure oxygen include [30], [31]:

  • -

    Potential for 25–35% reduction in capital requirements over conventional cryogenic plants.

  • -

    A 30% reduction in operating costs for the process.

  • -

    35–60% reduction in power consumption (depending on product pressure) up to 1000 psig.

  • -

    No net electricity consumption in certain applications.

  • -

    Possible integration with other high temperature processes to produce electrical power and/or steam from depleted air.

  • -

    Substantial reduction in cooling water consumption.

  • -

    Compact and modular design with significantly smaller footprint than cryogenic plants.

Due to important future potential of MIEC membranes, there is significant literature on the integration of MIEC membranes in oxyfuel combustion processes and in IGCC (integrated gasification combined cycle) coal-fired power plants [29], [32], [33], [34], [35], [36], [37], [38]. However, so far, integration of MIEC membranes in cement production plants has not been examined in detail. So, this paper presents a new approach for oxygen production in cement plants at lower energy consumption compared to cryogenic distillation, by using MIEC membranes. The integration of these membranes could favor the reduction of the carbon footprint of one of the most CO2 emitting industries in the world. The present study also quantifies the achievable potential process improvements of this integration. A model of a single tubular MIEC membrane is implemented and the flux characteristics obtained for the single tubular membrane are then assumed to be identical to that of tubes inserted in a bunch into a membrane module. Different configurations utilizing such membrane modules for full oxy-fuel and oxygen enriched combustion in cement plants are considered and analyzed with respect to energy efficiency and potential for process improvements. All the configurations considered are thermally integrated with the clinker production process. Hot air or CO2 are used on the permeate side of the membranes. There is no need for a vacuum pump as the low oxygen partial pressure in the CO2 establishes a driving force versus ambient air or mildly compressed air on the feed side of the membrane. Besides the membrane reactor, heat exchangers, a compressor and an expander are added to the system to ensure the required operating temperature and pressure for the membrane process. An economic analysis is included to compare the viability of these MIEC membranes configurations with a cryogenic plant used as a reference and to point out the most important parameters for the competitiveness of the technology.

Section snippets

Modeling of MIEC membranes

In general, oxygen MIEC membrane materials can be classified into two main groups depending on crystal structure: fluorite and perovskite. In an attempt to create membranes with both high oxygen permeability and good mechanical stability, dual-phase composite MIEC membranes have recently been developed [39]. A detailed review on MIEC membrane materials can be found in Jiang et al. [40] or in earlier reviews [41], [42].

The necessary oxygen partial pressure difference across the membrane can be

System analysis of oxygen production with MIEC membranes

Different system configurations are presented and compared to investigate the energy consumption for oxygen generation as well as the membrane area required for a specific cement plant site. The investigated configurations can be divided into two groups. One includes configurations that involve only oxygen enrichment of the tertiary air (i.e. hot air from the clinker cooler) for combustion in the pre-calciner (oxygen enriched air cases). The other group includes configurations with full

Results

The amount of oxygen required depends on the specific fuel consumption and capacity of the plant. In this study, a modern plant producing 4000 t/day of clinker with an energy consumption of 3000 kJ/kg of clinker is considered. The fuel used is coal (82.5% C, 5.6% H, 9% O, 1.8% N, 1.1% S, 0.03 Cl) with an ash content of 12.2% and a moisture content of 9.5% [3]. The LHV of this coal is 25.9 MJ/kg and 60% is burned in the pre-calciner while the 40% left is used in the kiln. The excess of oxygen

Conclusions

The present paper describes the integration of oxygen membranes in a cement plant both from an energy, exergy and economic point of view. The economic figures of merit are compared with those from a standard cryogenic plant. Two groups of configurations are considered: one includes configurations that involve only oxygen enrichment of the tertiary air for combustion in the pre-calciner (oxygen enriched air cases). The other group includes configurations with full oxy-fuel combustion in both

Acknowledgments

The authors thank DSF (Danish council for Strategic Research) for the financial support received as part of the project “ENEFOX – Energy Efficient Oxygen Production for a Sustainable Energy System” (11-116387). The European Cement Research Academy (ECRA) is also acknowledged for financial support through the CCS Project Phase IV, WP A4 Future Oxygen Supply.

References (67)

  • R. Castillo

    Thermodynamic analysis of a hard coal oxyfuel power plant with high temperature three-end membrane for air separation

    Appl Energy

    (2011)
  • T. Burdyny et al.

    Hybrid membrane/cryogenic separation of oxygen from air for use in the oxy-fuel process

    Energy

    (2010)
  • A. Schreiber et al.

    Environmental assessment of a membrane-based air separation for a coal-fired oxyfuel power plant

    J Membr Sci

    (2013)
  • M. Vellini et al.

    CO2 capture in advanced power plants fed by coal and equipped with OTM

    Int J Greenh Gas Control

    (2015)
  • S. Engels et al.

    Simulation of a membrane unit for oxyfuel power plants under consideration of realistic BSCF membrane properties

    J Membr Sci

    (2010)
  • H. Stadler et al.

    Oxyfuel coal combustion by efficient integration of oxygen transport membranes

    Int J Greenh Gas Control

    (2011)
  • A. Leo et al.

    Costa JCD da. Development of mixed conducting membranes for clean coal energy delivery

    Int J Greenh Gas Control

    (2009)
  • M. Den Exter et al.

    Viability of mixed conducting membranes for oxygen production and oxyfuel processes in power production

    Energy Proc

    (2009)
  • A.J. Samson et al.

    (Ce,Gd)O2−δ-based dual phase membranes for oxygen separation

    J Membr Sci

    (2014)
  • J. Sunarso et al.

    Mixed ionic–electronic conducting (MIEC) ceramic-based membranes for oxygen separation

    J Membr Sci

    (2008)
  • S. Baumann et al.

    Ultrahigh oxygen permeation flux through supported Ba0.5Sr0.5Co0.8Fe0.2O3−δ membranes

    J Membr Sci

    (2011)
  • J. Kotowicz et al.

    The influence of membrane CO2 separation on the efficiency of a coal-fired power plant

    Energy

    (2010)
  • F. Zeman

    Oxygen combustion in cement production

    Energy Proc

    (2009)
  • L.V. Van der Ham et al.

    Exergy analysis of two cryogenic air separation processes

    Energy

    (2010)
  • N.S. Siefert et al.

    Exergy and economic analyses of advanced IGCC–CCS and IGFC–CCS power plants

    Appl Energy

    (2013)
  • A. Hussain et al.

    A feasibility study of CO2 capture from flue gas by a facilitated transport membrane

    J Membr Sci

    (2010)
  • B.J.P. Buhre et al.

    Oxy-fuel combustion technology for coal-fired power generation

    Prog Energy Combust Sci

    (2005)
  • IEA GHG IGGRP

    CO2 capture in the cement industry

    (2008)
  • N. Mahasenan et al.

    The role of carbon dioxide capture and storage in reducing emissions from cement plants in North America

  • C.A. Hendriks et al.

    The reduction of greenhouse gas emission from the cement industry

  • N. Rodríguez et al.

    CO2 capture from cement plants using oxyfired precalcination and/or calcium looping

    Environ Sci Technol

    (2012)
  • D. Leeson et al.

    A systematic review of current technology and cost for industrial carbon capture

    (2014)
  • IEA/World Business Council for Sustainable Development (WBSCD)

    Cement technology roadmap 2009 – carbon emission reductions up to 2050

    (2009)
  • Cited by (0)

    1

    Current address: Danish Gas Technology Centre, Dr. Neergaards Vej 5B, 2970 Hørsholm, Denmark.

    View full text