Integrated Capture and Conversion

CHAPTER

Integrated Capture and Conversion

14 Turgay Pekdemir

Future Technology Execution, Transverse Technologies, ALSTOM (Switzerland) Ltd, Zentralstrasse, Birr, Switzerland

CHAPTER OUTLINE 14.1 Introduction ................................................................................................... 253 14.2 Routes to CDU ................................................................................................ 254 14.3 Integrated CO2 utilisation processes................................................................ 255 14.3.1 Mineralisation ............................................................................ 256 14.3.1.1 Single-step aqueous processes ........................................... 257 14.3.1.2 Multistep aqueous processes.............................................. 257 14.3.1.3 Alkaline solutions processes ............................................... 260 14.3.1.4 Single-step dry processes ................................................... 260 14.3.1.5 Commercial relevance, market readiness and challenges .... 260 14.3.2 Tri-forming................................................................................. 264 14.3.2.1 Commercial relevance, market readiness and challenges .... 265 References .............................................................................................................268

14.1 Introduction There are several well-known methods with varying maturity for separating and purifying carbon dioxide (CO2) by two or more steps from emission sources of varying concentration (or dilution). These include absorption, adsorption or membrane separation. The separation and purification steps can produce almost pure CO2 from the source gas but at considerable cost. For example, current mature technologies used in upstream oil and gas processing operations can be applied to separating and concentrating CO2 from fossil-fuel-fired power plant flue gas, which can be transported to locations where it can be sequestered underground. This requires significant amounts of energy that consequently reduces the net electricity output of the power plant by as much as 30%. This suite of technologies is referred to as carbon capture and sequestration/storage (CCS). Additionally, for a number of reasons, especially due to public (social) acceptance, transporting large amounts of CO2

Carbon Dioxide Utilisation. http://dx.doi.org/10.1016/B978-0-444-62746-9.00014-1 Copyright © 2015 Elsevier B.V. All rights reserved.

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and sequestrating (storing) underground (land or sea) are perceived as posing a serious risk and might even be a showstopper for CCS. As a possible way for the elimination of this risk, CO2 utilisation, instead of storing it geologically, as a contribution to CO2 storage has lately started receiving considerable attention in many circles. This is called carbon dioxide utilisation (CDU) in which CO2 is used for beneficial purposes, for example, in enhance oil recovery (EOR), conversion to other chemicals and fuels such as methanol, methane, formic acid, etc., solvent for some chemical processes, production of algae and so on. However, the ‘big picture of CDU’ and on which technology the efforts should focus and why are not yet clear. Although the surge of development effort in recent years for new technologies are expected to make both CCS and CDU less costly and publically more acceptable, it is still highly desirable to find ways to integrate CCS and CDU processes seamlessly offering value through synergetic benefits of both energy and material savings. This chapter will analyse some promising concepts of integrated carbon capture and utilisation (CCU) and discuss their commercial relevance including a rough assessment of their readiness to market, and problems that will need to be overcome. However, let us first briefly describe the possible routes through which CO2 can be made available to CDU processes.

14.2 Routes to CDU The routes from the CO2 source to its utilisation can follow alternative routes as shown in Figure 14.1. This also shows the routes for recycling of the CO2 utilisation product. These routes offer different potential for the integration of utilisation

FIGURE 14.1 Some possible routes for the CO2 utilisation. C indicates conversion and R recycle routes.

14.3 Integrated CO2 utilisation processes

processes to the CO2 emitting source and will have various degrees of challenge associated with them. Route 1: describes an ideal situation where source gas is fed directly into the utilisation process without needing a capture plant. This is marked as route C1 in Figure 14.1 where the capture and conversion take place in a single plant (i.e., in-situ conversion) working on the source gas directly and integrated to the source plant. Route 2: due to the transportation related issues, it is desirable to co-locate the utilisation process with both the CO2 source and capture plants. This is marked as route C2 in Figure 14.1 where CO2 is first captured from the source gas and fed into the utilisation process at the same site. This route might be perceived as CO2 utilisation without any serious degree of integration except energy and material flow connections between the three processes (source, capture and utilisation). However, there is a possible situation where the conversion process might act as part of the capture plant. For example, conversion processes can substitute for the regeneration step in a solvent-based capture plant. Route 3: in this case, the utilisation process is off-located with the capture plant being co-located with the source plant. This is marked as route C3 in Figure 14.1 where the captured CO2 is transported to the utilisation process site. Transportation of the source gas directly to the conversion plant can be considered as another variation for off-located utilisation but this would not be a feasible option, except for very short distances, as this will require not only prohibitively large transportation network but also expensive material requirements due to normally reactive (corrosive) nature of the source gas. Following the conversion process, the product or CO2 emitted following the use of the product can be recycled to various destinations: • •

Power (source) plants (i.e., onsite power þ CO2 to fuel utilisation), marked by R1 in Figure 14.1 Capture plants (i.e., CO2 to fuels for mobility þ capture from air), marked by R2 in Figure 14.1

If it is not recycled, the product or CO2 emitted following the use of the product can be either stored permanently or emitted into the environment either completely or partially.

14.3 Integrated CO2 utilisation processes The capture and compression of CO2 is currently the major cost in the CCS chain using underground geological storage. CO2 conversion process that integrates the CO2 capture to a higher degree has therefore a good chance of being costcompetitive against the case without any serious degree of integration. In this section, some example processes and concepts across the routes described earlier will be discussed.

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This section will present two processes with the potential of being applied in the conversion of CO2 to commercially beneficial products, directly from the gas stream containing CO2, namely, mineralisation and tri-reforming.

14.3.1 Mineralisation Mineralisation is based on industrial imitation of the natural weathering process1 and involves conversion of CO2 to solid inorganic carbonates using alkaline and alkaline-earth oxides, such as magnesium oxide (MgO) and calcium oxide (CaO), which are present in naturally occurring silicate rocks such as serpentine and olivine. These oxides are chemically reacted with CO2 to produce compounds such as magnesium carbonate (MgCO3) and calcium carbonate (CaCO3, commonly known as limestone).2 The overall reaction of CO2 mineralisation into carbonates under alkaline conditions, in summary, can be represented as follows (where M: Mg, Ca, Fe, etc.)3: xMO$ySiO2 $zH2 O ðsÞ þ CO2 /x MCO3 ðsÞ þ ySiO2 ðsÞ þ z H2 O þ Heat (14.1) On a smaller-scale, industrial wastes and mining tailings that are readily available and reactive can also be used as alkalinity sources.4 Waste materials that can be considered include pulverized fuel ash from coal-fired power plants, bottom ash and fly ash from municipal solid waste incinerators, de-inking ash from paper recycling, stainless steel slag and waste cement. Brines and mud suspensions, such as those produced from water from natural underground reservoirs (formed as waste products during oil or natural gas extraction), residue from desalination processes and aqueous red mud flows (mixtures of bauxite and saline wastewater from aluminium production), are also considered. The brines may also be extracted from the saline aquifers subsequent to underground CO2 storage. As injected, CO2 will be continuously displacing some of the brine originally available in the same reservoir. Additionally, quite a number of industrial processes also produce large volumes of brines as effluents with potential application for CO2 mineralisation. It has been suggested that it may be possible to use CO2 directly from the source gas and thus implementing capture and sequestration in a single step.5 CO2 mineralisation directly from flue gas (FG) can potentially become cost competitive to the alternative where capture is followed by transportation and geological sequestration (i.e., conventional CCS). Considering the cost of mineral carbonation, especially with the natural silicates as the source material, the integration of the CO2 capture step into a mineral carbonation process is, therefore, perceived to be a promising avenue. Moreover, these processes have an added advantage, and thus economic benefits, in that they can potentially capture SOx and NOx equally to CO2 and other impurities like mercury, and further trace metals partially providing a multicomponent removal method.6e8 There are a few process variations for the mineralisation of CO2 covered in some recent dedicated reviews and research publications.1,2,9e13 This section will concentrate only on those process that can potentially mineralise

14.3 Integrated CO2 utilisation processes

CO2 directly from the flue gas (i.e., integrating the capture plant). Such options are summarised in Table 14.1 and can be divided as follows.

14.3.1.1 Single-step aqueous processes These are relatively simple and use only inorganic and essentially inert additives that require very little make-up. They usually operate above 100  C and require high CO2 partial pressures, normally greater than 15 bar and even greater than 100 bar in some reports.14,21 They also suffer from the need for energy intensive pretreatment of the source material either by fine grinding and/or heating at very high temperatures greater than 600  C to achieve meaningful reaction rates. The most widely available source material for mineralisation processes is serpentine but this unfortunately requires the most costly pretreatment (thermal and fine grinding). On the other hand, the material that requires almost no pretreatment and is able to react at much more moderate temperatures and pressures is unfortunately the least common source, CaO-rich waste material. A potential benefit from the one-step aqueous processes is safely and permanently binding away the asbestos present in serpentine or mine tailings. Only for CaO-rich waste material, it is possible to skip the CO2 capture step and use CO2 directly from the flue gas in this route of mineralisation. These simple processes have been well studied but mainly in small laboratory scale investigations. There appears to be no investigations reported under commercially relevant scales. The energy needs, especially for the solid pretreatment and the reaction conditions, seem to be the largest barriers on the way to being a commercial application. These barriers are not easy to overcome and thus the likelihood of these processes being applied commercially and their readiness to market is very weak.

14.3.1.2 Multistep aqueous processes The most direct mineralisation method is a low-pressure carbonation process that requires other steps prior to mineralisation such as dissolution for extracting the metal oxides from the source material without the acidifying help of high CO2 partial pressures, and separation steps for removing the side-products (i.e., the multistep aqueous carbonation in Table 14.1). In such a scheme, energetic and financial costs need to be invested in the dissolution kinetics via solid pretreatment and possibly high dissolution step temperature which can be offset by avoided CO2 capture costs.22 Mineralisation processes operating at moderate CO2 pressure of 10e20 bar can be operated directly with flue gas compressed to achieve the required partial pressure of CO2.23,24 The multistep processes are more complex because the dissolution or extraction of active metal oxides, needed to reduce the need for energy-intensive high temperature and pressure reaction conditions and pretreatment of the source material (grinding to fine size), is implemented by additives that act as ligands or merely as a pH buffering agent, lowering the pH without the need for high pressure CO2. It is also possible to use an induced pH swing process for accelerating the process without having to employ high temperature and pressure reaction conditions. This approach uses the alkalinity employed in pH swing processes to capture CO2 in an aqueous solution that is then used to precipitate carbonates from it by

257

258

Process

Solid Feed

Single-step aqueous carbonation

Mined minerals, mine tailings, metal oxide rich waste materials Any

Alkaline solution carbonation Single-step dry carbonation

Multistep aqueous carbonation

Operating T ( C)

Operating PCO2 (bar)

100 > T > 150

Additives

Value Addition

Challenges

References

3 > P > 80

None to optional NaHCO3, NaCl

Hazardous waste remediation

14,15a,b

Various

Various

Brines, red mud

Low to moderate

Low to moderate

Strong base Strong acid Acid–base salts (NH4SO4) Alkalinity (NaOH)

Alkalinity and CO2 colocation, alkalinity cost

4,6,17,18

Metal oxide rich waste materials

25–500

Various

Iron-rich compounds Pure carbonates Pure carbonates (e.g., soda ash), Hazardous waste remediation Hazardous waste remediation

Energy demand for feed pretreatment, solids handling and feed scarcity (waste materials) Additive recovery

Only for highly reactive waste materials

15a,b,19,20

None

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CHAPTER 14 Integrated Capture and Conversion

Table 14.1 Overview of the CO2 Mineralisation Options Suitable for Working Directly with Flue Gas. The Value Addition Is in addition to the Possibility of Using the End Products as Materials for Civil and Structural Engineering.

14.3 Integrated CO2 utilisation processes

mixing it with a metal oxide rich solution. Both manufactured alkalinity in the form of ammonia or sodium hydroxide and alkalinity from highly alkaline wastes like red mud or alkaline brines can be used for this. Using ammonia or sodium hydroxide for CO2 capture applications has already been demonstrated as separate processes.19,25 The CO2 capture with ammonia has also been integrated into a pH swing process based on ammonium sulphate ((NH4)2SO4).26e28 This process uses recyclable ammonium salts to overcome two of the barriers for the development of CO2 mineral carbonation, namely the low efficiency of mineral dissolution and high cost recycling of the additives. In this process, the CO2 is captured as NH4HCO3 from the power plant flue gas that is later converted to calcium and magnesium carbonates in a downstream carbonation stage. The energy used is claimed to be about 60% less than a typical capture process, since desorption and compression are not required. NH4HSO4 is used to extract Mg (as MgSO4) from serpentine or Ca from mineral wastes, in the dissolution step. The carbonation is then performed by reacting NH4HCO3/(NH4)2CO3 with MgSO4. In the reaction, MgCO3 is precipitated and (NH4)2SO4 is left in solution, which is then recycled. Since the carbonation requires pH > 7, ammonia water is added to switch the pH from acidic (dissolution step) to basic. In the pH swing step, other elements (e.g., Fe, Al) which are brought into solution during the dissolution step are also precipitated and separated from the solution as hydroxides prior to the carbonation. (NH4)2SO4 is finally regenerated thermally to give NH3 and NH4HSO4. It is also claimed by the developers that the overall process is able to dissolve up to 90% Mg, sequester about 80% CO2 from flue gas and produce three separated materials: silica, iron oxides and magnesite with high purity.26e28 The group is currently working on the optimisation of some of the steps of the process (reduction of the reagents used and alternative regeneration options instead of costly thermal process). Also included in the on-going work is a techno-economic assessment of the optimised process in order to assess the reduction in energy usage, operating costs and plant capital cost. The results will help the acceleration of the deployment of the ammonia-based mineralisation of CO2 directly from flue gas.29 Despite the use of alkaline agents in a pH swing process, this opens the possibility of direct CO2 capture from the flue gas and thus significant energy and cost savings due to omitting the capture step. The use of capture additives normally requires nontrivial regeneration processes or demand for potentially costly make-up for the additives. For example, pH swing processes need to have acid and base regeneration steps unless cheap sources of both are available in the form of industrial wastes or natural brines. For strong acids and bases such as hydrochloric acid (HCl) and caustic soda (NaOH), Chlor-Alkali electrochemical separation processes are normally required for their regeneration. These are generally energy intensive and result in too high energy penalties for the process to be economically viable. However, it has recently been claimed6,30,31 that by modifying the conventional process to produce HCl instead of chlorine gas, it has been possible to reduce the energy needs by up to 80%. For the salts of a strong acid and a weak base, such as (NH4)2SO4, the regeneration can be accomplished using heat alone, but

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nevertheless this results in a significant energy penalty. Compared to their inorganic counterparts, using organic acids and ligands as additives is not favoured as they are not sufficiently stable and, especially at higher temperatures, carbonate precipitation can be inhibited.

14.3.1.3 Alkaline solutions processes This option uses brines or slurries as source material, thus not needing dissolution steps but would normally require addition of alkalinity (i.e., NaOH).30 The processes using alkaline solutions or slurries as the sole source for metals can side-step completely the need for solid mining, transport and pretreatment and dissolution enhancing measures (the latter only partly for slurries). However, as the inherent alkalinity of available brines is generally not sufficient, the alkalinity has to be first provided externally. This unfortunately means a significant energy penalty and the creation of large volume acidic waste streams. The red mud slurries, being a waste from a process that combines manufactured alkalinity with bauxite ore, are probably an exemption for external alkalinity requirement. This means that red mud slurries can potentially be carbonated under mild process conditions with smaller energy input demands. Prominent examples of research work using both manufactured (i.e., electrolytically) and natural alkalinity in the form of brines for the integrated mineralisation of CO2 capture are those from the corporations like Calera, Skyonic and Alcoa.4,6,17,18,30,32,33

14.3.1.4 Single-step dry processes This option, implemented as gasesolid operation, is only feasible for the most reactive source materials, essentially metal hydroxide or oxide (i.e., CaOH, MgOH or CaO, MgO) rich waste materials. Pure gasesolid carbonation is possible at low to moderate CO2 concentrations, but elevated temperatures are still needed for fast kinetics requiring, therefore, a trade-off between energy requirements and reactor size. Unfortunately, direct gasesolid reactions are too slow to be practical and are only feasible at reasonable pressures for refined, rare materials like the oxides or hydroxides of calcium and magnesium. As a result, mineral carbonation without refined materials cannot directly capture CO2 from flue gases, but could be possible in the case of pressurized CO2 rich gases. The energy consumption associated with this approach is expected to be much lower (or negative) in comparison to the wet method due to the suitability of dry carbonation for heat recovery as the temperatures involved are greater than 500  C, where the reaction rates appear to become significant.10

14.3.1.5 Commercial relevance, market readiness and challenges Mineralisation processes potentially offer production of not only pure carbonates of high value but also other pure side-products such as various metal ores. Carbonate product of sufficient purity are currently of high economic value with application as white pigments or fillers for example in paper and plastics manufacturing. Appropriate purity and particle size silica powders are also quite valuable, being in demand for the manufacture of glass, electronics, construction and plastics materials.34 However,

14.3 Integrated CO2 utilisation processes

higher purity products almost always come with a price tag that renders the CO2 mineralisation not commercially competitive to already established manufacturing routes and are thus economically unviable in supplying these materials. It should also be noted that even if some of the products are saleable with a good initial value, considering the volumes that will be produced if the processes were to be widely deployed, it is highly likely that the market will soon be saturated by these products. The disposal possibilities will very likely become essential eventually. The weight of CO2 generated in the coal-combustion process itself is generally more than twice the weight of the coal fired, and the volume of the reservoir required to store the carbonates is greater, for example, than the volume of the mines from which the coal and minerals were removed. However, the eventual above ground storage of the carbonate product from the process is not a major issue because of its stability and environmental neutrality.11 Additionally, the amounts of material needed for carbonation are rather large requiring around 3 tonnes minerals per ton CO2 captured. This equates to handling about 8 tonnes of material (including the coal) per ton coal used. It is stated by Burges et al. that it is important to note the high CO2 partial pressures (40e150 bar) required to achieve reasonable reaction rates and conversion efficiencies for the CO2 mineralisation, especially when considering that at atmospheric temperature, the pressure to which CO2 must be compressed to achieve supercritical conditions for pipeline transportation to geological storage is around 75 bar.11 These aggressive process conditions, of course, suggest relatively high technical and economic risk. It is also pointed out that usually expensive additives cannot be properly recovered and recycled for reuse when employed in the direct carbonation methods.11 Energy input (preheat, crush/grind, etc.) is currently estimated to be in the range of 10e400 kWh/t CO2. Additionally, although high carbonation conversion and acceptable rates have been achieved in the aqueous-based process, it appears that the cost is still too expensive (in 2011 the cost ranged from 30 to 100 Euro per tonnes CO2) for the mineralisation to be applied on a larger scale.9 The anticipated cost range of CO2 sequestration by mineral carbonation processes seems to be, despite the high degree of uncertainties, relatively high compared to other CO2 storage technologies and current CO2 market prices. The main barrier, thus, for widespread implementation of carbonate mineralisation is its relatively high cost. Furthermore, the cost estimates suffer from energy-use analysis errors. Despite the huge volume of investigations and research in the field, the carbonation processes in general and specifically from flue gas directly have been demonstrated mainly as laboratory tests or otherwise small scales using a variety of natural silicates, waste solids and brines of various nature. No work based on natural minerals has been implemented so far or on sufficiently larger scale systems that would convey confidence in the commercial viability of the CO2 mineralisation. However, the mineralisation process on waste solids such as fuel ashes has been tested in a few small pilot scale setups using flue gas slip streams from coal- and biomassfired power plants.7,35,36

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For example, a US-based start-up company Calera, using a combination of waste materials, brine and manufactured alkalinity as source materials, has been testing a pilot unit located in Moss Landing (CA, US) at a scale of 2.5 tonnes per day.7,30 The Calera pilot plant has now been in operation for approximately 3 years and the calcium carbonate produced from the capture and conversion of CO2 from the neighbouring gas power plant has been used in a variety of demonstration and commercial projects.31 The product, a special calcium carbonate, is a free flowing powder in dried form which can then be used to manufacture products such as board-type materials because of the special cementing nature of the calcium carbonate. However, it should also be noted that a plant previously proposed for Yallourn in Victoria, Australia, has now been cancelled due to the unavailability of brines of suitable quality and quantity.7,31 It is stated by Burges et al. that a serious concern with the Calera technology is the quantity and quality of brine that must be processed to provide sufficient alkaline earth metals to sequester industrial quantities of CO2.11 For example, considering that seawater contains approximately 1270 ppm magnesium and 400 ppm calcium ions, it is reported by the authors that even complete depletion of these ions would consume only 2.7 kg of CO2 per tonnes of water. They then estimate that a plant capable of sequestering 5 Mt/a of CO2 would require a minimum of 1851 Mt/a (5 Mt/day) of seawater. Burges go on to say that brines of higher Mg and Ca content would, therefore, be more suitable for the process, but their local availability in the quantities required is in question.11 Additionally, the need for an external source of alkalinity and possibly calcium and magnesium, and also for a disposal option for the generated products (mainly carbonate granules), will possibly limit the scope significantly. This highlights the possibility that feedstock availability, and to a lesser degree the disposal of the products, might carry significant risk for the commercialisation and widespread deployment of the process. Skyonic, another US-based start-up, has been developing a process called SkyMineÒ to mineralise CO2 directly from flue gas using caustic soda (NaOH). It is reported that the SkyMineÒ process with 96% carbon capture efficiency would cause a 25% decrease in the net output for a plant.37 The energy requirement for the SkyMine process based on a 650 MW coal-fired power plant is reported elsewhere to be 234 kWh/t CO2.38 These energy penalty levels are small compared to those of the processes capturing CO2 almost in pure form (99%) with a rate of 90% from flue gas. They are yet to be validated at larger scales but are also unconvincing considering the energy requirements for electrolytically producing NaOH. As the molecular mass of CO2 and NaOH are very close (44 and 40, respectively) and they react with a one to one stoichiometry, capturing 1 tonnes CO2 will require about 1 tonnes of NaOH. It is estimated that the production of 1 tonnes of NaOH would require around 3 MWh electricity.39 If it is assumed that a 500 MW power plant would produce 400 tonnes CO2 per hour, this makes a greater than 1 GW electricity requirements for NaOH production alone. Even if it is assumed that Skyonic can produce a tonne of NaOH with lesser purity than commercial grade NaOH using 1 MWh electricity, the energy demand for NaOH generation is almost the complete electricity output from the power plant. Nevertheless, Skyonic recently announced

14.3 Integrated CO2 utilisation processes

that, having received investments from, among others, BP and ConocoPhilips, they are about to start the construction of a large-scale demonstration plant at a cement factory in San Antonio, Texas, where it already operates a pilot facility, to capture 75,000 tonnes of CO2 per annum. This large-scale plant is planned to be operational in 2014 to produce NaHCO3 (baking soda) and other marketable chemicals, such as hydrochloric acid (HCl (aq)), hydrogen chloride gas (HCl (g)), chlorine (Cl2) and hydrogen (H2). Sales of baking soda and the side products are projected to fully cover the costs.17,40e42 However, it is envisaged that the need for an external source of alkalinity and possibly a disposal option for the generated acid (largely HCl) will probably limit the scope noticeably. Mineralisation of CO2 directly from flue gas can save the substantial costs due the removal of the capture step with added benefits of concomitantly removing other pollutants (i.e., SOx, NOx, mercury and trace metals), the remediation of waste materials (i.e., steel slug, ash, red mud, chrysotile (asbestos containing) and other mine tailings) and the production of value-added main and side-products (filler grade carbonates and silica powders, soda ash, compounds rich in iron, nickel, cobalt, manganese, chromium). However, the production of materials with high value from natural silicates in a cost-effective way has not yet been demonstrated. Although, the integration of the mineralisation process with CO2 capture has been shown to have some benefits through its implementation in the Calera and Skyonic processes, detailed process description and independently peer-reviewed energetic and cost analysis are yet to appear. Once the performance of these processes are independently verified, applications using manufactured alkalinity and/or brines could appear soon if and when encouraging regulations are in place. In fact, the absence of a demonstration processes using widely available natural silicates, combined with the capture step, show that financial and energetic performance can be noticeably improved in comparison to the conventional alternative. Capture followed by transportation and sequestration of supercritical CO2 is perhaps the biggest hurdle to overcome in commercial viability and the market readiness of the process for CO2 mineralisation directly from flue gas. In summary, mineralisation directly from waste (i.e., flue) gas can offer opportunities to the industries with large emissions and high energy and materials intensity (i.e., power plants, iron and steel, cement, glass, waste and minerals and mining).34 Furthermore, evidence from the literature suggests that carbonation of CO2 directly from flue gas into stable solids via mineralisation is technically feasible as far as the material handling and availability of the source material, and disposal or utilisation of the output material are concerned. However, there appears to be considerable challenges and uncertainties in (1) the costs as well as level of energy demand, (2) availability of the raw materials, (3) availability of the disposal capacity, (4) market for the products and their economical values. These aspects also seem to be showing variations depending on the geological locations and makeup. These prevent the likelihood of the widespread deployment of the technology and investments from the potential technology developers and users. Therefore, more accurate estimations are required to determine the economic feasibility of CO2 mineralisation

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directly from flue gas to stable carbonated solids as an alternative to geological CO2 sequestration.

14.3.2 Tri-forming A concept that directly utilises flue gas, rather than pre-separated and purified CO2 from flue gases, for the production of hydrogen-rich syngas from methane reforming of CO2 is a so-called ‘tri-forming’ process.43 This process has been pioneered by a group at Pennsylvania State University (PA, US) under the leadership of Chunshan Song44e48 and the subject of research by many others such as Halmann and Steinfeld,49,50 Kang et al.,51 Cho et al.,52 Jiang et al.43 The tri-reforming process synergistically combines the endothermic CO2 reforming, known also as ‘dry-reforming’ (DRM), steam reforming (SRM) and exothermic oxidation of methane (POM and CCM) in a single reactor. The process makes use of not only CO2 but also the H2O and O2 in the waste flue gas from fossilfuel-based power plants for the production of synthesis gas.48 The reactions involved are presented in Table 14.2, together with the corresponding reaction enthalpies. The process is illustrated schematically in Figure 14.2. The incorporation of O2 in the last two reactions in Table 14.2, which can be supplied by air or an enriched oxygen flow, generates heat in situ that can be used to increase energy efficiency and also reduces or eliminates carbon formation on the reforming catalyst. The demand for methane instead of being satisfied through natural gas can also be met from biogas. If desired, the tri-reforming process can utilise coal, biomass or other carbonaceous materials instead of natural gas.49,50 It is reported by Song and Pan47 that with the tri-reforming process it is possible to achieve greater than 97% methane conversion and around 80% CO2 conversion at equilibrium temperatures in the range 800e850  C under atmospheric pressure. The gas phase tri-reforming reaction without a catalyst has been found by these authors to be negligible at temperatures as high as 850  C. It therefore appears that catalysts play an important role in conversions as well as on the H2/CO ratios of the products from tri-reforming. It is also possible to adjust the selectivity for H2 and CO by controlling the amount of steam and CO2 added to the reaction.48e58 An important observation reported by Song and Pan47 is that CO2 conversion can be maximized

Table 14.2 Main Reactions for Tri-forming Process for Syngas Production43 Process Constituents

Reactions

DH0298 ðkJ=molÞ

DRM: Dry reforming of CH4 SRM: Steam reforming of CH4 POM: Partial oxidation of CH4 CCM: Catalytic combustion of CH4

CH4 þ CO2 4 2CO(g) þ 2H2(g) CH4 þ H2O 4 CO2(g) þ 3H2 (g) CH4 þ 12O2 4 CO(g) þ 3H2 (g) CH4 þ 2O2 4 CO2(g) þ 2H2O(g)

þ247.3 þ206.3 35.6 880

14.3 Integrated CO2 utilisation processes

FIGURE 14.2 Schematic illustration of tri-reforming natural gas using flue gas from fossil-fuel-fired power plants. Adapted from Song.44

by tailoring the catalyst composition and preparation method: certain catalysts can give much higher CO2 conversion than other catalysts under the same reaction conditions with the same reactants feed. It is argued by Halmann and Steinfeld49 that the presence of substantial amounts of N2 in the reactant mixtures should not pose a problem for the process as it has been shown that the selective partial oxidation of methane with air to syngas is highly effective using lanthanideeruthenium oxide catalysts. Almost 100% conversion to CO and H2 has been achieved at around 800  C and atmospheric pressure on a gas mixture of CH4:O2:N2 of 2:1:4 ratio.59 The process, as shown in Figure 14.2, can make use of ‘waste heat’, if available, from the power plant in addition to the recovery of the low grade heat generated in situ from the oxidation of methane (POM and CCM) with the O2 present in the flue gas (Table 14.2). This can potentially reduce the consumption of natural gas by the process whilst meeting the process energy requirement.

14.3.2.1 Commercial relevance, market readiness and challenges The syngas product from the tri-reforming process can be used for the manufacture of hydrogen, methanol (a precursor for polymers), dimethyl carbonate (DMC; an automobile fuel additive and intermediate to polycarbonates), dimethyl ether (DME; a fuel additive and aerosol propellant), hydrocarbons and ammonia. The production of the latter involves about 3% of the world energy consumption.49 Syngas containing nitrogen is perceived to be particularly useful for ammonia synthesis. The proposed processes can also facilitate the transition to a hydrogen fuel economy as the products are effectively energy storage materials.49

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The production of ammonia, as well as urea, with further processing can also benefit from the already high nitrogen content of the flue gas from conventional combustion, using the air and thus avoiding its separation. The great global demand for these potential chemical products offers another substantial commercial advantage to the tri-reforming process. Comparing tri-reforming to DRM and SRM, Song and Pan47 reported that the tri-reforming not only consumes less energy but also has a lower CO2 footprint per unit of desired syngas (H2:CO ¼ 2). It has been shown that tri-reforming uses 45.8% less energy and produces 92.8% less CO2 compared to DRM. When compared to SRM, tri-reforming uses 19.7% less energy and produces 67.5% less CO2.47 This imparts great industrial and environmental potential to the trireforming by enabling not only the production of high value and high demand chemicals (i.e., methanol, DME, DMC, hydrogen, ammonia, urea, etc.) but also through the use of these chemicals in the production of electricity by fuel cells and clean combustion.53, 60e64 An investigation by Halmann and Steinfeld49 of the comparative performance of the tri-reforming process (natural gas and coal) on CO2 emission avoidance, fuel saving, exergy efficiency and % of world capacity for products with coal gasification revealed that the predictions of CO2 emission avoidance were particularly large for methanol and urea production from tri-reformed flue gases emitted from both coaland gas-fired power stations, amounting to 47% and 50% for methanol and to 41% and 43% for urea, respectively. Furthermore, another extensive investigation by Minutillo and Perna65 for the treatment of CO2 from fossil-fuel-fired power plants by the tri-reforming process revealed that the reduction in CO2 emissions could be close to 85%. The results from the investigation of Halmann and Steinfeld50 showed that very high fuel savings, of the order of 75%, could be possible for hydrogen production from tri-reformed flue gases emitted from both coal- and gas-fired power plants. Preliminary evaluations showed that economics were also favourable. The combination of dry reforming with steam reforming offers the advantage in that it does not only produce syngas with controllable H2/CO ratios for the manufacture of a specific product through Fischer-Tropsch (F-T) synthesis, but also mitigates the formation of particulate (solid) carbon deposition through oxidation arising from reactions below: CH4 þ O2 /C þ 2H2 O

(14.2)

2CO/C þ CO2

(14.3)

Carbon deposition is a significant problem in dry reforming and its mitigation may enhance catalyst life,48 thus offering comparatively better commercial value to the tri-reforming process. Despite a couple of decades of investigation into tri-reforming, the data on the process performance have been mainly obtained from laboratory research, with very limited pilot scale tests coming from a development program at the Korean

14.3 Integrated CO2 utilisation processes

Gas Corporation (KOGAS).52 The work at KOGAS began by testing the trireforming reaction in developing a process for the production of di-methyl-ether (DME) with a direct synthesis process using a 50 kg/day pilot plant in 2001. KOGAS later launched a 10 tonnes/day DME demonstration plant project in 2004 at the Incheon KOGAS LNG terminal. Simultaneously, KOGAS has also established a burner tester to investigate the characteristics (catalyst, operation condition and combustion) of the tri-reformer. The overall strategy of KOGAS has been scaling-up the tri-reformer by studying the tri-reforming process and collecting data using the burner tester (25 N m3/h DME) to build and calibrate the KOGAS tri-reformer model at the scale of the burner tester, and then to use the model to design the trireformers at the scale of pilot (33 N m3/h DME) and demonstration (2503 N m3/h DME) units. The operating conditions for the tri-reformer reactor is reported to be 950e1050  C and 13e30 atm. Encouraging results from the KOGAS program seem to have lead to KOGAS recently securing a number of business contracts for building DME plants all over the world (Saudi Arabia, Mongolia, Mozambique, Australia, Russia, Malaysia, Nigeria etc.)66 offering profitable solutions to the gas fields of large CO2 content, instead of flaring. Considering the recent surge in shale gas productions, this is a significant step in the market readiness of the tri-reforming process with further expansion for the progress into commercialisation and widespread deployment. However, as has been recognised by the pioneers, despite the advantages, the tri-reforming process faces a number of other challenges47 that need to be addressed by demonstration scale projects before its widespread deployment which can be achieved with confidence. For example, issues such as effective conversion of CO2 in the presence of O2 and H2O (currently around 80% CO2 conversion); heat management; minimization of the effect of SOx, NOx, particulate material (i.e., fly ash) and heavy metals in flue gas on tri-reforming process; separation and recycling of the unreacted reactants; management of inert N2 gas in flue gas and the integration of the process into power plants have to be shown as being no threat by demonstration scale applications. Furthermore, as noted from the methane oxidation reactions in Table 14.2 (POM and CCM), the tri-reforming process will produce excess CO2 as well as unreacted gas that needs to be recycled into the process. This captive CO2 might significantly reduce the amount of CO2 to be utilised from the non-captive (i.e., flue gas source) process. Depending on the requirements of possible applications for the syngas, the product from the tri-reforming has to be treated to separate unreacted reactants and recycled back to the process. Despite the process being based on conventional processing equipment, it has been pointed out that it has a very complicated mechanism that has not been optimised.67 In summary, despite the need for further work at demonstration scale in order to address issues such as: • •

CO2 conversion efficiency; sensitivity of the process to SOx, NOx, ash and heavy metals;

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•

separation and recycling of the unreacted reactants;

its advantages render the tri-reforming as a very attractive route for direct conversion of CO2 from industrial flue gasses. This is because it offers: • • • • • •

prevention of carbon deposition; controllable H2/CO ratios (for effective syngas production); an autothermic reaction enthalpy; production of not only chemicals of large global demand as commodities but also electricity through the secondary use of these chemicals; suitability of using not only natural gas but also coal and biomass; the use of conventional equipment in the process.

The recent surge in the shale gas production offers another significant advantage for the widespread deployment of the tri-reforming process in enabling the natural gas being made available to the market, being converted to chemicals, fuels and electricity.

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