Saturday, February 29, 2020

The Challenges of Fluidized Bed Combustion, Coal Gasification, Coal Liquefaction and Carbon Dioxide Sequestration

Coal is plentiful in the United States, but technologies must be developed to make it burn more efficiently and to sequester the carbon dioxide it generates.

The United States is, by any measure, the “Middle East” of coal resources. The readily available (least expensive to recover) portion is estimated to be close to 400 billion tons, enough to generate electricity as well as produce fuels for about 250 years. Today, coal combustion accounts for more than half of the nation’s electric power generation. However, burning coal produces large amounts of ash and carbon dioxide, and sulfur-containing coal emits sulfuric acid, the main constituent of acid rain. To address these issues, the U.S. Department of Energy has collaborated with industrial partners and university researchers to develop a range of technologies that improve plant efficiency and reduce emissions. They are also working on coal tech- nologies that can produce hydrocarbon fuels for vehicles. This article discusses fluidized bed combustion, coal gasification, coal liquefaction, and carbon dioxide sequestration.

*Fellow of ASM International

Fluidized bed combustion

Fluidized beds suspend solid fuels on upward- blowing jets of air during the combustion process (Fig. 1). The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective chemical reactions and heat transfer. The mixing action of the flu- idized bed brings the flue gases into contact with a sulfur-absorbing chemical such as limestone or dolomite. More than 95% of the sulfur pollutants in coal can be captured inside the boiler by the sorbent. Fuel is burned at temperatures of 760 to 925°C (1400 to 1700°F), well below the threshold where nitrogen oxides form. (At ~1370°C (2500°F), the nitrogen and oxygen atoms in the combustion air combine to form nitrogen oxide pollutants).

The popularity of fluidized bed combustion is due largely to the technology’s fuel flexibility and the capability of meeting sulfur dioxide and nitrogen oxide emission standards without the need for expensive add-on controls. Almost any combustible material can be burned, from coal to municipal waste.

1. First Generation: The DOE Clean Coal Technology Program (www.fossil.energy.gov) led to the initial market entry of First-Generation pressurized fluidized bed technology, with an estimated 1000 megawatts of capacity installed worldwide. These systems pressurize the fluidized bed to generate sufficient flue gas energy to drive a gas turbine and operate it in a combined cycle.

The First Generation pressurized fluidized bed combustor is based on a “bubbling-bed.” A relatively stationary fluidized bed is established in the boiler, in which low air velocities fluidize the material, and a heat exchanger immersed in the bed generates steam. Cyclone separators remove particulate matter from the flue gas prior to entering a gas turbine, which is designed to accept a moderate amount of particulate matter.

2. Second Generation: The pressurized fluidized bed combustor is based on “circulating fluidized-bed” technology, as well as several measures to enhance efficiency. Higher air flows entrain and move the bed material, which is recirculated with adjacent high-volume, hot cyclone separators. The relatively clean flue gas goes on to the heat exchanger. This approach theoretically simplifies feed design, extends the contact time between sorbent and flue gas, and reduces likelihood of erosion in heat exchanger tubes. It also improves sulfur dioxide capture and combustion efficiency.

Another innovation is the integration of a coal gasifier (carbonizer), which produces a fuel gas that is burned in a “topping combustor.” The gasified coal adds to the combustor’s flue gas energy as it enters the gas turbine, which is the more efficient portion of the combined cycle. The topping combustor must exhibit flame stability in combusting low-Btu gas, and must demonstrate low- NOx emission characteristics.

Fig. 1 — This schematic shows the parts of a pressurized fluidized bed boiler. The popularity of fluidized bed combustion is due largely to the technology's fuel flexibility and the capability of meeting sulfur dioxide and nitrogen oxide emission standards without the need for expensive add-on controls. Almost any combustible material can be burned. Diagram courtesy of U.S. Dept. of Energy, www.fossil.energy.gov. 

3. Materials developments: Tests of promising new hot-gas filter components and systems are continuing at the Power Systems Development Facility (PSDF) in Wilsonville, Alabama. (http:// psdf.southernco.com/tech_papers.html) Advances made to date in this critical technology area include the development of clay-bonded silicon carbide candle filters and the associated filter vessel. Efforts are currently focused on improved candle filter materials for enhanced durability under extreme temperatures and a highly corrosive environment. New ceramics and ceramic- metallic composites show promise in this area.

Coal gasification

An alternative to coal combustion is coal gasification. Rather than burning coal directly, a coal gasifier reacts coal with steam and controlled amounts of air or oxygen under high temperatures and pressures. The heat and pressure cause chemical reactions with the steam and oxygen to form a gaseous mixture. This mixture is called a synthesis gas (or syngas), and is made up primarily of carbon monoxide and hydrogen. The syngas is then combusted in a gas turbine to generate electricity.

In gasification-based systems, impurities can be separated from the gaseous stream before combustion. As much as 99% of sulfur and other pollutants can be removed and processed into com- mercial products such as chemicals and fertilizers.

Combined cycle system

If the syngas is burned to produce electricity, it typically serves as a fuel in an Integrated Coal Gasification Combined Cycle (IGCC) system. The IGCC system has two basic components. A high efficiency gas turbine burns the clean syngas to produce electricity, and exhaust heat from the gas turbine is recovered to produce steam for traditional high-efficiency steam turbines.

IGCC power generating systems are being developed and operated in Europe and the United States. These systems raise efficiency by taking the heat from the gas to produce steam to drive a steam turbine. Existing commercial systems can achieve a thermal efficiency greater than 40%. With further advances in gas turbine technologies, these systems are capable of reaching above 50% efficiency. IGCC systems can be designed to produce little solid waste and low emissions of SOx and NOx. In addition, over 99% of the sulfur present in the coal can be recovered.

Furthermore, carbon monoxide in the synthesis gas can be reacted with steam to make hydrogen and carbon dioxide. This creates the potential to separate a relatively pure stream of carbon dioxide for “sequestration,” thus avoiding its emission into the atmosphere. The resulting hydrogen can be burned in a gas turbine for electricity generation, or as a fuel in other applications, such as hydrogen-powered vehicles. Instead of burning it to generate electricity, the syngas can be processed with commercially available technologies to produce a wide range of fuels, chemicals, fertilizer, and industrial gases. (For more details, visit http://www.fossil.energy.gov/programs/powersystems/gasification/index.html.)
Coal may be converted into liquid fuels by several different processes.
Fig. 2 — A schematic diagram of a generic hybrid combination of a coal gasifier and a fluidized bed combustor. Diagram courtesy U.S. Dept. of Energy, www.energy.doe.gov.

Coal liquefaction

Coal may be converted into liquid fuels by several different processes, such as Bergius, Fischer- Tropsch, Schroeder, and Low-Temperature Carbonization.

  1. Bergius: Hydrogen at 700 atm pressure is injected into a heavy paste of crushed coal at a temperature of 400°C (750°C) with an iron/molybdenum-oxide catalyst. The process requires about 7000 cubic feet of hydrogen per barrel of oil it produces.
  2. Fischer-Tropsch: This process involves first treating white-hot hard coal or coke with a blast of steam, producing carbon monoxide and hydrogen. This is followed by a catalyzed chemical reaction in which carbon monoxide and hydrogen are converted into liquid hydrocarbons of various forms. Typical catalysts are based on iron and cobalt.
  3. Low-Temperature Carbonization: LTC is a pyrolysis process that involves heating coal, shale, or lignite to about 425°C (800°F) in the absence of oxygen. Oil is thus distilled from the material, rather than burned as it would if oxygen were present. Also known as the Karrick process, a ton of coal yields up to a barrel of oil, 3000 cubic feet of rich fuel gas, and 1500 pounds of solid smokeless char (semi-coke).
  4. Schroeder: The coal is pulverized and dried, and then saturated with methanol. The methanol- saturated coal is slurried in benzene, and the slurry is exposed to microwave energy of a frequency and for a period of time sufficient to cause hydrogenation of the coal. The principal product is benzene. Unreacted coal and char may be recycled or subjected to gasification with steam and oxygen to produce synthesis gas that can be converted to methanol.
All of these liquid fuel production methods release carbon dioxide (CO2), far more than is released in the extraction and refinement of liquid fuel production from petroleum. Therefore, carbon dioxide sequestration is needed to avoid releasing it into the atmosphere.

Hybrid generators

Linking a coal gasifier and a fluidized-bed combustor arranged in a “topping cycle” could be an ideal combination of lower-cost capital equipment, high-performance fuel combustion, and improved environmental performance for future power plants (Fig. 2). The combination may be particularly suited for smaller power stations, those in the 200 to 300 megawatt range.

In this hybrid system, coal is partially gasified in a pressurized gasifier. This produces a fuel gas that can be combusted in a gas turbine, the “top” of the cycle, hence the name. Left behind in the gasifier is a combustible char that can be burned in a fluidized bed combustor or advanced high-temperature furnace that produces steam to drive a steam-turbine power cycle and to heat combustion air for the gas turbine. Heat from the gas turbine exhaust also can be recovered to produce steam for the steam turbine.

This highly integrated system of gasifiers, combustors, gas, and steam turbines results in a high overall fuel-to-electricity efficiency, exceeding 55% in many advanced concepts. (The average efficiency of today’s coal-burning power plant typically is around 33 to 35%).

Hybrid systems may also lead to less expensive power plants. Because it is not required to break down coal completely into synthetic gas, a partial coal gasifier can be a relatively simple, compact, and low-cost component. The char combustion system likewise can be a relatively low cost module, and unlike many older coal combustors, which are designed to fire a specific type of coal, fluidized bed combustors can accept a wide range of fuels and would have no trouble burning chars produced from a variety of different coals. (This information is from http://www.fossil.energy. gov/programs/powersystems/combustion/index. html.)

The challenge

Coal produces solid waste (ash) when burned, and coal from the eastern United States contains sulfur, which produces sulfuric acid, the genesis of “acid rain.” However, it is neither the ash nor the sulfur that is the major environmental concern regarding coal as fuel for electrical power generation: The biggest challenge is the carbon dioxide that is produced by burning coal.
The combustion of carbon produces carbon dioxide, the most important of the “greenhouse” gases. Coal derives all of its energy from the combustion of carbon, producing significantly greater quantities of carbon dioxide than does the combustion of natural gas for a given amount of energy. Therefore, if coal is to become a truly viable solution, the carbon dioxide will have to be “sequestered.”

Sequestration is a process by which carbon dioxide is stored in underground sites such as porous rock beds, for a 25% premium. It is estimated that new electric power plants can sequester 80 to 95% of the carbon dioxide emissions. However, the cost of the addition and maintenance of sequestration equipment to a current generating facility results in a 60% premium over current costs. (For more details on sequestration, visit http://www.fossil.energy.gov/programs/sequestration/overview.html.)

We will be able to get as much coal as we need in the near to intermediate future, but the cost of processing to remove contaminants will go up as more is burned. Coal liquefaction and gasification are methods to explore as means of cleaning before burning, and methods of disposing of the ash after burning. It is estimated that coal gasification, which leads to cleaner burning and is ash-free, can be achieved at a premium of about 20 to 25% over current coal costs.

Because coal accounts for over 50% of the electricity generated in the United States today, a sig- nificant increase in the cost of coal-generated energy would be felt throughout the economy. For example, an environmentally acceptable conversion to all-coal electrical generation would likely result in about an 80% increase in the price per kilowatt-hour in the near term. However, the pre- mium would fall as new technologies supplant those to which the carbon-trapping equipment is added after construction.

In sum, coal can provide large reserves of energy. However, coal must be cleaned before it is burned, and the products of coal combustion must likewise be cleaned. Whether the cost of environmentally benign electricity is too high or not is a question that people concerned with local, national, and global economics and environmental issues will have to answer.

Source: Dennis R. Hardy - Naval Research Laboratory Washington, Bhakta B. Rath* - Naval Research Laboratory Washington, James Marder* - ASM International Materials Park

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A Cradle-to-grave Life Cycle Assessment (LCA) of a Carbon Fiber Reinforced Polymer (CFRP) Intensive Multimaterial Bodies in White (BIW) for a Passenger Vehicle

ABSTRACT

Vehicle lightweighting strategies must deliver sustainable returns to customers and society. This work evaluates the sustainable return on investment (SROI) of lightweighted advanced high strength steel (AHSS) and carbon fiber reinforced polymer (CFRP)-intensive multimaterial bodies in white (BIWs) for automobiles. The SROI depends on the lightweighted BIW’s manufacturing cost and the difference in sustainable cost between a baseline (mild steel) BIW and the lightweighted alternative. The sustainable cost is the sum of the customer’s lifetime fuel (or electricity) costs and the costs of environmental externalities. A cradle-to-grave life cycle assessment (LCA) was conducted to quantify the environmental impacts of CFRP and AHSS BIWs in gasoline-fueled cars, bioethanol (E85)-fueled cars, and battery electric vehicles (BEVs) driven for a lifetime distance of 200 000 km. For cars fueled with gasoline- or corn-based bioethanol, the CFRP BIW yielded the lowest SROI; the AHSS BIW performed best for BEVs and cars fueled with wood bioethanol. However, the commercial availability of recycled carbon fiber should increase the SROI of the CFRP BIW in the future. Additionally, the SROI of CFRP BIWs is maximized when carbon fiber production is done using energy from a low carbon-intensity electric grid or decentralized sources such as waste-to energy incineration plants.


INTRODUCTION

Because the automotive sector is a major source of greenhouse gas (GHG) emissions, regulatory authorities around the world have introduced stringent emissions standards for new vehicles. For example, the European Union will require new passenger vehicles to have GHG emissions of no more than 95 g/km by 2021.1 Similarly, the United States Environmental Protection Agency (USEPA) and National Highway Traffic Safety Administration (NHTSA) jointly issued a standard that requires car makers to make significant fuel economy improvements in order to reduce fleetwide GHG emissions of US light duty vehicles.2 Although currently being reviewed by US  policymakers,3  the  standard  requires  a  reduction of fleet-level GHG emissions from 225 g/mile (140 g/km) in the base year of 2016 to 143 g/mile (89 g/km) by 2025.2 Other countries have developed similar emissions targets.4 To improve the fleetwide fuel economy of passenger cars and satisfy new regulatory standards, automotive original equip- ment manufacturers (OEMs) have made significant R&D investments (US $100 billion/year),5,6 focusing particularly on the development of alternative powertrain technologies, fuels from renewable feedstocks (biofuels), and lightweight automotive components.

Unlike fuel-side initiatives (i.e., new powertrains and biofuels), lightweighting approaches can be easily integrated into traditional automotive material supply chains, and can be applied to any vehicle, irrespective of its powertrain and fuel. Steel  parts  account  for  ∼56%  of  the  weight  of  a  typical passenger vehicle;7 replacing them with lightweight alternatives can yield part-level weight savings of 10−70%7 and reduce fuel consumption by 6−42%.8 Materials commonly used in the design and construction of lightweight automotive components include advanced high strength steel (AHSS), aluminum alloys, and carbon fiber reinforced polymer (CFRP) composites. Replacing conventional mild steel automotive parts with lightweight alternatives can reduce vehicle weight by 10− 20% if the replacement parts are made from AHSS, 40% if they are made from aluminum, and 60% if they are made from  CFRP.9 CFRP composite parts offer the greatest weight reduction potential and thus the greatest weight-related fuel savings in passenger vehicles.10,11

Metal parts are widely used in the automotive industry, but CFRP composites have not achieved comparable acceptance. Their limited use is partly due to their comparatively high manufacturing costs (CRFP composite parts for passenger vehicles cost around 20 times as much as steel equivalents12) and poor end-of-life recyclability. The high cost of manufactur- ing CFRP composite parts is largely due to (i) the high price of polyacrylonitrile (PAN), which is the main raw material used in CF synthesis, and (ii) the need to cure CFRP composite parts, which increases manufacturing cycle times (and thus costs). Considerable efforts are being made to reduce cycle times in CF manufacturing.13

The costliness of CFRP composite parts is also partly due to the poor circularity of carbon fiber in the technosphere. Today, CFRP composite scrap is typically disposed of by incineration with energy recovery. However, EU regulations require producers to adopt hierarchical waste management practices that prioritize reuse and recycling.14 The use of secondary carbon fiber with the strength and functional performance of virgin carbon fiber would reduce manufacturing costs and the need to synthesize virgin CF. Pyrolysis, solvolysis, and fluidized bed processes could all potentially be used to recover carbon fiber from CFRP composite scrap and thereby increase the circularity of CF without compromising its mechanical properties. These technologies could make recycled CF an inexpensive alternative to virgin CF,15,16 enabling large scale automotive utilization of CFRP composites.10

Previous life cycle assessment (LCA) studies have assessed the environmental performance of lightweight materials when used in individual components (i.e., structural parts such as the body in white (BIW), engine hood, chassis, cross-members, and front-end parts)17−23 and entire cars, examining both single-material and multimaterial designs.18,24−26 However, these studies only   examined   metal-based lightweighting materials. Although the environmental performance of CFRP composite parts has been studied extensively, few LCA studies in the open literature comprehensively assess the cradle-to- grave environmental performance of lightweighted CFRP auto parts. The scarcity of such assessments of CFRP automotive parts is partly due to the heterogeneity of CF production, which introduces considerable uncertainty into LCA models,27 and partly because OEMs prefer to maintain confidentiality regarding the formulation of CFRP composites. The few published LCAs for CFRP composite parts were restricted to cradle-to-gate assessments28 or evaluated a limited subset of impacts (cumulative energy demand and global warming potential) and only addressed the use of CFRP composite parts in internal combustion engine (ICE)-powered ve- hicles.18,29 There are thus no published assessments of the environmental performance of CFRP automotive components in vehicles with ICEs using renewable fuels (e.g., E-85) or alternate powertrains such as battery electric vehicles (BEV). This work is thus the first report describing the environmental performance of CFRP BIWs in bioethanol-driven ICEs and BEVs.

This study presents a cradle-to-grave LCA of a CFRP- intensive multimaterial BIW for a passenger vehicle. A BIW was chosen because it is the structural component that offers the greatest potential weight savings. The environmental performance of a CFRP BIW in gasoline-driven ICE vehicles was compared to that of a state-of-the-art mild steel (MS) BIW. In addition, scenario analyses were conducted to assess (a) how the environmental performance of the CFRP BIW would be improved if the circularity of CF increased, (b) how the environmental impact of the CFRP BIW compares to that of a BIW made from AHSS, and (c) the environmental performance of the CFRP BIW in BEVs and vehicles with ICEs using bioethanol (E85). The LCA results were used to estimate the sustainable return on investment (SROI) of AHSS and CFRP BIWs. We expect that the findings presented herein will help OEMs identify optimal material lightweighting strategies for passenger cars operating under different propulsion modes.

LIFE CYCLE ASSESSMENT METHODOLOGY

Goal and Scope Definition. The goal of this LCA is to quantify the cradle-to-grave environmental performance of a CFRP-intensive multimaterial BIW in a midsize passenger car and compare the results to those for a conventional mild steel BIW. It is assumed that the CFRP components of the BIW are manufactured in Sweden and that all other life cycle stages  occur within Europe.

Functional Unit. The equivalent functional unit (FU) is a BIW for a compact car with a service life of 200 000 km traveled over 12 years. Some studies suggest that the mass of a CFRP BIW is 171 kg;30,31 others use a value of 139 kgs.32 The latter value is used here. Table 1 shows the CFRP BIW’s assumed material composition. The reference component, that is, the MS BIW, is assumed to have a mass of 280 kg,33 50% greater than that of the CFRP BIW.

Table 1. Weight and Material Composition of the CFRP and MS BIWs

System Boundary. Figure 1 shows the system boundary representing the cradle-to-grave life cycle stages of the CFRP- BIW. The life cycle is divided into five stages: (1) raw material acquisition; (2) manufacturing of the individual BIW components including the stamped steel, extruded aluminum, casted  magnesium,  and  CF-reinforced-epoxy−resin (CFRP) parts; (3) assembly of the BIW and its integration into the passenger car; (4) the car’s use phase, with an assumed service life  of 200 000  km; and  (5)  end-of-life management.  In this final stage, the BIW is removed from the car and its metal and CFRP components are separated. The metal parts are shredded and recycled while the CFRP components undergo incineration with energy recovery.

Figure 1. Cradle-to-grave life cycle stages of a CFRP BIW for a lightweight passenger car (life cycle stages are indicated by numbers).

Table 2. Midpoint Impact Categories Chosen for the Study

The CFRP parts of the CFRP BIW are assumed to be sourced from Sweden, which has three major advantages as a supplier of CFRP parts. First, CF production from PAN is energy-intensive, requiring 116 MJ energy per kg CF synthesized.27 This is the single largest contributor to the environmental impact of manufacturing CFRP auto parts. High manufacturing stage impacts may dilute or even outweigh the use-stage environmental benefits of lightweighting. The low carbon intensity of Sweden’s electric grid makes it a good location for manufacturing CFRP components with low greenhouse gas (GHG) emissions.34 The overall impact of using electricity from the Swedish grid for CF production must however be evaluated. Second, Sweden offers good opportunities for symbiotic colocation of CF production sites with facilities such as waste to energy (WtE) plants; Sweden generated 2 TWh of electricity and 14.6 TWh of heat from WtE plants in 2014.35 Third, Sweden has a feedstock advantage because it produces large quantities of low energy- intensity materials (e.g., lignin) that can serve as precursors for PAN synthesis. European boundary conditions were assumed for all other life cycle stages.

Life  Cycle  Inventory  (LCI)  Data  for  CFRP-BIW. Foreground  LCI  data  pertaining  to  the  life  cycle  stages  of the CFRP-BIW were obtained from the academic literature and reports produced by industry associations. The SimaPro LCA software package (version 8.2.0)36 and the Ecoinvent data- base37 were used to obtain background data for the life cycle model. LCI data for cold-rolled steel coils (including end of life credits) were obtained from the World Steel Association. LCI data for aluminum (Al) alloys and CFRP composites were obtained from the Ecoinvent database and the academic literature, respectively. The Swedish (SE) electrical grid’s composition was used to estimate the emissions due to the manufacturing of CFRP composite parts. Details of the LCI modeling process (i.e., the major assumptions made and the key data sources used to develop the LCI data) are provided in section S1 of the Supporting Information together with LCI data for individual BIW components (i.e., stamped steel, cast aluminum, and fabricated CFRP parts) and the MS and AHSS BIWs.

Calculation of Mass-Induced Fuel Consumption for a Gasoline-Driven ICE. Use-stage fuel consumption values for the reference component (MS BIW) and the CFRP BIW were estimated using eqs 1 and 2.


Here FCSteelBIW and FCCFRPBIW are the fuel consumption of the specified component in liters; MIF is the mass-induces fuel consumption (0.27 L/(100 km × 100 kg)); FRV is the fuel reduction value with powertrain adaptation (0.32 L/(100 km× 100 kg)); WtSteelBIW is the weight of Steel BIW (280 kg); WtCFRPBIW is the weight of CFRP − Instensive multimaterial BIW (139 kg); and LD is the lifetime distance (200 000 km) Details of the fuel consumption calculations are provided in section S2 of the Supporting Information.

The mass-induced fuel consumption (MIF) (0.27 L/(100 km × 100 kg)) and fuel reduction value (FRV) (0.32 L/(100 km × 100 kg)) used here are based on the averages for six compact car variants.38 The fuel consumption of the reference (MS BIW) and lightweight (CFRP BIW) components was calculated using previously reported methods.39,40

Life Cycle Impact Assessment Methodology. Environmental impacts were quantified in terms of 10 midpoint categories shown in Table 2.

Uncertainty Analysis. The LCA results for the baseline CFRP BIW are sensitive to four key modeling parameters: (a) the difference in weight between the CFRP and MS BIWs, which is a key determinant of the environmental benefit of lightweighting-induced fuel savings; (b) the composition of the materials used to produce the CFRP BIW; (c) the processing conditions during fabrication of individual CFRP BIW components (e.g., material efficiencies); and (d) the FRV for the CFRP BIW. Variation of these key parameters introduces uncertainty in LCA results. Therefore, an uncertainty analysis was performed to determine how variation in four selected parameters (see Table 3) influenced the predicted overall environmental performance of the CFRP and MS BIWs. The analysis was conducted  by performing Monte Carlo simulations in SimaPro with 5000 steps and a 95% confidence level.

Table 3. Key Modeling Parameters Varied in the Uncertainty Analysis

a The 280 kg lower limit was chosen because it is the value yielding a weight saving of 50%; the upper limit of 325 kg was chosen based on the literature30,31 and gives a 57% weight saving. CFRP components can potentially yield weight savings of 70%. bThe material efficiency (i.e., the proportion of material not lost as offcuts during cutting) for precured CF fabric is 89%44 (which was used as the baseline value); a conservative estimate of 80%45 was used as a lower bound for the uncertainty analysis. The postcuring material efficiency was taken to be 100% as a baseline, representing the best-case scenario (techniques such as resin infusion molding cause zero material waste46), and a lower bound of 80%45 was used in the uncertainty analysis. c0.21 is the FRV of a compact car without powertrain adaptation (worst case scenario).38,47

The energy consumed during CF production is the dominant contributor to the GWP of CFRP composites. Therefore, it was assumed that the CFRP composite parts  would be manufactured in Sweden, the electrical grid of which has the lowest carbon intensity in Europe (46 g CO2/KWh).48 To further evaluate the potential GWP reduction achievable by lightweighting with CFRP, an additional uncertainty analysis was performed to assess the GWP impact of varying the source of electricity used in CF synthesis and CFRP composite production.

Scenario Analysis. The purpose of the scenario analysis was to compare the environmental performance of steel and CFRP BIWs under optimal conditions, that is, accounting for likely future developments in the metal and composite industries. The steel industry is strongly advocating the use of advanced high strength steel (AHSS) in structural parts such as BIWs. On the other hand, the composites industry is trying to improve the circularity of CF by exploring the potential to introduce recycled (secondary) CF into the market. CF can be recovered from postconsumer CFRP composite scrap by chemical or thermal treatment (solvolysis and pyrolysis, respectively) without significantly reducing its mechanical strength. The resulting recovered CF can be used to make structurally competent lightweight parts for diverse applications, reducing the need for virgin CF derived from the energyintensive precursor PAN.

Our LCA compared the environmental impacts of the AHSS and CFRP BIWs with postconsumer recycling (PCR) for passenger vehicles with conventional and alternative propulsion systems. The AHSS BIW was treated as the reference component in this scenario. The weight of the AHSS BIW is 235 kg, which is 16% lower than the typical weight of an MS BIW. The CFRP composite scrap was assumed to be recycled chemically by the solvolysis technique. LCI data for the solvolysis of CFRP composite scrap were obtained from the literature49,50 and are shown in section S3 of the Supporting Information. These data account for both the energetic cost of solvolysis and a credit for not using virgin CF synthesized from PAN. The proposed scenarios are summarized in Table 4.

When considering the impact of the lightweighted AHSS and CRFP BIWs in BEVs, the use-stage electricity consumption attributed to the BIWs was calculated as described by Kim and Wallington.47 In this case, the AHSS BIW served as the reference component.


where EAHSSBIW and ECRFPBIW denote the electricity consumption of respective components (KWh); MIFBEV is the mass induced fuel equivalent of the reference component; FRVBEV is the fuel reduction value equivalent of the lightweight component. For BEVs, MIFBEV and FRVBEV is taken as (0.05 Le/(100 km × 100 kg)) from Kim et al., study.47 WtAHSSBIW is the weight of steel BIW (235 kg); WtCRFPBIW is the weight of CFRP instensive multimaterial BIW (139 kg). The factor of 9.1 in these equations is used for unit conversion (1 L equiv petrol = 9.1 KWh). Details of the electricity consumption calculations are presented in section S2 of the Supporting Information.

Table 4. Scenarios Considered When Comparing the Environmental Performance of AHSS and CFRP BIWs with Postconsumer Recycling (PCR)

Figure 2. Environmental impacts of the MS BIW and the CRFP BIW in a gasoline-burning ICEV based on a cradle-to-grave LCA. Results are shown for 10 midpoint environmental impact categories. The maximum score for each category is shown at the top of the figure.

Sustainable Return on Investment (SROI) of Passenger Cars with Lightweighted BIWs. In financial terms, the return on investment (ROI) is the ratio of the net gains from an investment (i.e., the difference between the revenues due to the investment and the investment’s cost) to the investment’s cost. The SROI is a similar quantity that is used to evaluate an investment based on its sustainable returns, i.e. its benefits in terms of reducing both the cost to the consumer and the environmental externalities imposed on society. A transition from mild steel (MS) to lightweighted parts would  be a major strategic move for automotive OEMs. Therefore, before the transition is made, it is essential to properly evaluate the SROI of the available lightweighting strategies.

An earlier assessment of the sustainability of replacing MS with lightweight materials determined the breakeven ratio,39 that is, the ratio at which the environmental impact (assessed in terms of GHG emissions) of a lightweighted solution is identical to that of the market incumbent. The time taken to reach this breakeven point is referred as the payback time.39,52 The payback time is shorter for lightweight material options such as AHSS;52 longer driving distances and/or higher FRVs are required for CFRP composites because of the environ- mental impact of their production. In this work, the SROI of CFRP lightweighting was estimated using a method inspired by the concept of the breakeven ratio. However, instead of calculating breakeven in terms of GHG emissions, the SROI metric uses cost as an indicator. Our decision to use SROI as an indicator metric was motivated by the expectation that it would help OEMs evaluate the sustainable returns of lightweighting solutions, allowing them to be reported in tandem with and compared to investment costs.

From a societal perspective, the benefits of lightweighted BIWs (or indeed any lightweighted car components) are 2- fold. First, customers benefit from lower driving costs due to fuel savings. Second, lightweighting can reduce environmental externalities by reducing greenhouse gas emissions and air pollution. The SROI accounts for these benefits and is calculated using eq 5.
where sustainable costsbaseline = FC + CCC + APC represents the sustainable cost of an MS BIW in a car operating on gasoline (which was taken as a baseline in this work); sustainable costslightweight = FCLWX + CCCLWX + APCLWX represents the sustainable cost of a lightweight (AHSS or CFRP) BIW in the same car. FC and FCLWX represent the fuel costs of baseline and lightweight BIWs, respectively, and are given by the expression (lifetime fuel consumption × cost of fuel in €/liter) for the relevant fuel.

CCC and CCCLWX represent the climate change cost of baseline and lightweight BIWs, respectively, and are given by the expression lifetime GHG emissions in tons × costs of climate change in € /ton GHG) for the relevant fuel.

APC and APCLWX represent the air pollution costs of baseline and lightweight BIWs, respectively, and are given by the expression (exhaust emissions in tons of PM2.5 during the vehicle’s use-stage × cost of PM2.5 in €/ton) + (nonexhaust emissions in tons of PM10 during the manufacturing and EOL stages of the BIW’s life cycle × cost of PM10 in €/ton) + (lifetime NMVOC emissions in tons × costs of NMVOC in €/ton) + (lifetime SO2 emissions in tons × costs of SO2 in €/ton) for the appropriate fuel.

For BIWs in BEVs, only nonexhaust emissions (in ton of PM10) were considered over the component’s life cycle (including the use stage).

Cost of BIW to OEM is the manufacturing cost of the BIW incurred by the OEM.

Figure 3. Results of a contribution analysis showing the environmental impacts of manufacturing individual CFRP BIW components as well as the impacts due to the use of the CRFP BIW in a gasoline-powered ICE and its end-of life processing by incineration with energy recovery.

RESULTS AND DISCUSSION

Baseline Scenario: MS-BIW vs CFRP-BIW in ICEVs. The CFRP-BIW outperformed the MS BIW with respect to 5 of the 10 chosen midpoint environmental impact categories (Figure 2). The life cycle impacts of the CFRP-BIW were 12−84% lower than those of the MS BIW with respect to HH-CP, HH- NCP, GWP, ODP, and CED. On the other hand, the life cycle FETP, AP, FEP, and PMFP impacts of the MS BIW were 30− 48% lower than those of the CFRP BIW.

Contribution Analysis of the CFRP-BIW. A contribution analysis of the CRFP-BIW was performed to identify hotspots for potential improvement at each stage of its life cycle (Figure 3). The manufacturing stage impacts were assessed by considering the contributions of five materials (mild steel, aluminum, thermoplastics, structural adhesives, and CFRP composites) used in the construction of a CFRP BIW.

During the manufacturing stage, mild steel, thermoplastics, and structural adhesive collectively contribute only 1−13% of the total impact. The environmental burden of the manufacturing stage is thus largely due to the fabrication of CFRP composites and the production of aluminum components.

The contributions of CFRP composite parts ranged from 28% to 58% in 7 out of 10 impact categories. These impact scores were mainly due to the synthesis of PAN-derived CF, which is a raw material for CFRP composite fabrication. CF synthesis is environmentally impactful for three reasons. Its AP impact is primarily due to air emissions from CF manufacturing facilities, particularly releases of ammonia.

The PMFP impact is also partly due to these releases of ammonia. Atmospheric ammonia concentrations correlate strongly with particulate matter concentrations53 and acid deposition.54 The use of advanced emissions control systems such as regenerative thermal oxidizers (RTO)55 as an abatement control strategy could significantly reduce the burden in these categories. The HH-NCP, ODP, and FEP impacts of CRFP composite parts were largely due to the composition of Sweden’s electrical grid. The CF and CFRP composites were assumed to be manufactured in Sweden to exploit the country’s low carbon-intensity grid. The Swedish electricity generation mix is dominated by hydro (42%) and nuclear (41%) power.56 The ODP impact is due to nuclear power plants specifically, the coolants used in uranium enrichment. The Ecoinvent data set37 for nuclear power production in Sweden uses global average values for enriched uranium as key raw material inputs, which is important because some coolants used for uranium enrichment in certain geographical locations are ozone-depleting (e.g., CFCs).57 Conversely, hydroelectric power is a major source of FEP and HH-NCP impacts. Hydroelectric power plants alter the nutrient budgets of surface water systems and may increase eutrophication.58 Hydropower may also exacerbate non- carcinogenic human health effects due to assimilation of neurotoxic pollutants such as methylmercury.59,60 CF manufacturing also accounted for 30% and 50% of the total GWP and CED impacts of the CRFP-BIW, respectively. The CED impact is due to the fossil energy used in PAN synthesis and the nuclear energy used in CF synthesis. The GWP of CF manufacturing was primarily due to fossil energy consumption during PAN synthesis.


Figure 4. (a) Uncertainty analysis for the MS and CFRP BIWs; (b) Results of a differential uncertainty analysis comparing the GWP impacts of steel and CRFP BIWs assuming CF and CRFP production using energy sourced from the energy grids of different countries (SE, Sweden; FR, France; FI, Finland; BE, Belgium; AT, Austria; EU, EU-27 Average; and UK).

Aluminum components also contributed significantly to the environmental burden of the manufacturing stage, accounting for 31−67% of the overall impact in the FEP, FETP, HH-NCP, and HH-CP impact categories. This was primarily due to the consumption of primary aluminum to produce wrought aluminum components (sheets and extrusions), which comprise 16% of the mass of a CFRP BIW. The HH-CP   and HH-NCP impacts are attributed to waste streams from primary aluminum production plants. Red mud is a byproduct of bauxite ore processing that has been linked to both carcinogenic   and   noncarcinogenic   (genotoxicity)   risks in humans.61,62 Interestingly, however, the HH-CP and HH-NCP impacts of the CFRP BIW are 82% and 59% lower, respectively, than those of the market incumbent, that is, the MS-BIW (Figure 1). This was due to the impact of recycling of scrap steel at the end of life and particularly the disposal of slag from electric arc furnaces (EAF). The FEP impact was linked to the geography of material supply chains: much of the primary aluminum used in wrought components is sourced from China (China accounts for 56.3% of the world’s primary aluminum production according to the Ecoinvent, Rest of the World data set), which has a coal-intensive electrical grid. This is the main reason why aluminum components contribute 46% of the BIW’s FEP impact. The contribution of aluminum components is reduced to 41% if the primary aluminum is assumed to be sourced from the EU-27 and countries in the European Free Trade Association (EFTA). If the primary aluminum for wrought component production is sourced exclusively from Canada, the contribution of aluminum components to the total FEP impact of the BIW is reduced further still, to just 21%. Sourcing primary aluminum from countries other than China thus reduces the FEP impact of the CFRP BIW by between 10% and 40%.
Aluminum components are also responsible for 31% of the FETP impact. The cast aluminum parts of the BIW are made from AlMg3 alloy, whose aluminum consists of 20% primary and 80% secondary metal ingots. The FETP impact is due to the production of secondary aluminum, specifically the alloying additive (copper) used to prepare postconsumer aluminum scrap for melting.63 Studies on the production of secondary  aluminum have demonstrated the ecotoxicity and ecotoxico- logical potency of copper.64−66 The end of life stage also accounts  for 48% of  the FETP  burden,  which  was  mainly attributed to the incineration of CFRP composite scrap and the recycling of steel parts in EAF.

The use stage impacts of the CRFP-BIW are significant (37−83%) for GWP, ODP, PMFP, POFP, and CED, and moderate for other impact categories (9−28%). The use stage accounts for 83% of the POFP impact. Despite the low manufacturing stage impact on POFP, the CRFP-BIW’s lifecycle impact marginally exceeds (by 4%) that of the MS BIW because POFP impact is engine-dependent (it relates to NOx formation due to incomplete combustion) rather than fuel-dependent. Lastly, as expected, the end of life credits for the CFRP-BIW in eight impact categories are small (<10%), highlighting the need for better methods of recycling CF from CFRP composite scrap.

Figure 5. Environmental impact values based on a cradle-to-grave LCA of AHSS-BIW and CFRP-BIW-PCR (using the Swedish electrical grid for CF manufacturing) and CFRP-BIW-PCR (using electricity obtained from a WtE plant).

Uncertainty Analysis of the Mild Steel and CFRP BIWs. Figure 4A shows the results of the uncertainty analysis for the MS and CFRP BIWs.

This analysis revealed some interesting trends in the environmental performance of MS and CFRP BIW. For the MS BIW, the HH-NCP, HH-CP, ODP, and FEP impacts varied significantly (by 66−79%). A contribution analysis for the MS-BIW (shown in section S4 of the Supporting Information)  showed  that  scrap  steel  processing  in  EAF during the end of life stage was primarily responsible for its HH-NCP and HH-CP impacts. This implies that these impacts are sensitive to variation in the weight of the MS-BIW. The high uncertainty associated with the ODP impact was also attributed to variation in the weight of the MS BIW because heavier components increase gasoline consumption during the use stage. The variation in FEP was due to both the manufacturing and end of life stages. The FEP impact of manufacturing (which stems from the energetic cost of stamping) increases with the weight of  the BIW but  results in a correspondingly large end of life recycling credit. The CED varied by 45% upon varying the weight of the MS BIW, which was attributed to differences in weight-induced gasoline consumption during the use stage. The CED, FETP, FEP, and ODP impacts of the CFRP-BIW varied only modestly (by 23− 36%) compared to the variation in these impacts for the MS BIW. The predicted variation was attributed to variation in the material efficiency of CFRP composite parts and changes in fuel consumption based on the absence of powertrain adaptations (a worst-case scenario without such adaptations was considered in the uncertainty analysis). Overall, the results suggest that CFRP BIWs exhibit superior environmental performance to a degree that exceeds the uncertainty in the estimates with respect to GWP, HH-CP, and HH-NCP, whereas MS BIWs perform better with respect to PMFP, AP, and FETP. For the other studied impact categories, there was significant overlap between the results for the two BIWs, suggesting that the results obtained are highly sensitive to variations in the modeling parameters.

An additional differential uncertainty analysis was performed to assess the extent of variation in GWP when the electricity used to produce CF and CFRP composites originated from the grids of European countries other than Sweden. The results are shown in Figure 4B.

The carbon intensity of the Swedish electrical grid (46 g CO2/KWh) is lower than that of France (107 g CO2/KWh), Finland (240 g CO2/KWh), Belgium (259 g CO2/KWh), Austria (359 g CO2/KWh), the EU-27 average (482 g CO2/ KWh), and the UK (612 g/KWh). The results clearly show that the likelihood of CFRP BIW having a lower GWP impact than MS BIW decreases as the carbon intensity of the electricity used to manufacture CFRP composite parts increases. The results (Figure 4B) indicate that for a given    set of LCA modeling conditions, the CFRP BIW should have a lower GWP impact than the MS BIW if the carbon intensity of the electricity used to produce CFRP parts is below 360 g CO2/KWh. Above this threshold, uncertainty increases and the MS BIW may have the lower GWP impact.

Figure 6. Environmental impact values based on a cradle-to-grave LCA of the AHSS-BIW and CFRP-BIW-PCR for an ICE fueled with E85 containing bioethanol produced from (A) Swedish woody biomass; and (B) corn from the USA. The maximum score in each impact category is listed at the top of the figure.

Scenario Analysis. The results obtained under the three scenarios summarized in Table 4 are discussed below.

Scenario 1: ICE Optimization. The environmental impacts of the AHSS BIW, the CFRP BIW-PCR(SE), and the CFRP BIW-PCR (WtE) are compared in Figure 5.

The absolute scores for the AHH BIW, CFRP BIW-PCR (SE) and CFRP-BIW-PCR(WtE) are lower than those for the corresponding baseline variants (i.e., the MS-BIW and CFRP- BIW with incineration). However, the trends in the impacts resemble those for the baseline cases: both CFRP BIW variants exhibit superior environmental performance with respect to GWP, CED, ODP, HH-CP, and HH-NCP, while the AHSS BIW performs better with respect to PMFP, POFP (by a small margin), AP, FEP, and FETP. A notable difference is that the FEP impact burden of CFRP-BIW-PCR is higher than that for incineration of CFRP composite scrap for energy recovery. This increase is due to (a) the use of acetic acid in solvolysis, because acetic acid is known to increase eutrophication,67 and (b) the energy burden of solvolysis. The use of a decentralized energy source (a WtE plant) reduced environmental impacts by  1−28%  compared  to  using  the  Swedish  grid,  and had particularly beneficial effects on the CED, ODP, and HH-NCP impacts. The collocation of a CF production site with a WtE incineration plant would allow the former to use electricity generated by the latter for CF synthesis. This could be a good sustainable business model for CF manufacturing, especially in countries with highly carbon-intensive electrical grids.

Scenario 2: Alternative Fuel (ICE Burning E-85). As shown in Figures 6A,B, the life cycle impacts of the AHSS BIW and CFRP BIW-PCR were also evaluated under the assumption that they would be used in ICE-powered vehicles fueled with bioethanol from woody biomass and corn-based feedstocks.

The AHSS BIW exhibited superior overall environmental performance to the CFRP-BIW-PCR for E85-fueled cars when the bioethanol portion of E-85 was sourced from a woody biomass feedstock. Using the CFRP BIW in woody bioethanol-fueled automobiles would be environmentally unwise because it negates the climate change benefits of the CRFP and introduces significant trade-offs in other impact categories. The CFRP BIW-PCR had a lower impact on PMFP, FETP, AP, and FEP, suggesting a pattern resembling that seen with gasoline-fueled ICEs. As discussed in the contribution analysis, these impacts are primarily due to the manufacturing stage, so changing the fuel employed in the use stage is unlikely to affect them greatly. However, the CFRP BIW-PCR performed poorly with respect to ODP and POFP, which was not seen in the gasoline-fueled case. This is because the well-to-wheel environmental impacts of wood bioethanol are lower than  those of gasoline. Consequently, the weight-induced fuel benefits (including GWP) of the lightweight CFRP BIW are lower for wood bioethanol than for gasoline.

Figure 7. Environmental impact values based on a cradle-to-grave LCA of the AHSS BIW and CFRP BIW-PCR for a BEV charged with electricity generated using a grid mix corresponding to the EU27 average. The maximum score in each impact category is listed at the top of the figure.

The   use   of  CFRP  composite  lightweight parts in automobiles fueled with corn-based bioethanol presents an interesting value proposition. The CFRP BIW-PCR achieved better environmental performance than the AHSS BIW in all impact categories other than ODP and FETP in a vehicle running on E85 containing corn-derived bioethanol. The end of life stage of the AHSS BIW is responsible for its high HH-CP and HH-NCP impacts (which are due to slag disposal from EAF, as mentioned previously). However, its higher impacts in other categories are due to the environmental impact of corn bioethanol. The FEP impact of corn ethanol is particularly high because of fertilizer-laden agricultural runoff from  cornfields. Unlike biofuels derived from woody biomass, biofuels derived from agricultural crops are often criticized because of their effects on eutrophication and terrestrial and freshwater ecotoxicity.68−70 Energy consumed during crop harvesting and drying also increases the GWP, AP, POFP, and PMFP impacts of the AHSS BIW in the corn bioethanol case; the AHSS BIW increases fuel consumption relative to the CFRP BIW-PCR and so exacerbates these impacts. Overall, the LCA results indicate that lightweighting with CFRP composites may be environmentally beneficial in regions (e.g., the USA) where corn bioethanol is a prominent ICE fuel.

Scenario 3: Alternate Powertrain BEV. The life cycle environmental impacts of the AHSS BIW and CFRP BIW- PCR for a BEV are shown in Figure 7.

For a BEV, the absolute impact scores of both the AHSS and CFRP BIW variants are lower than those for the ICE variants. However, the AHSS BIW has a clear environmental advantage over the CFRP BIW PCR in the BEV case. Because of the BEV’s low FRV, weight-induced energy savings do not provide significant impact reductions for BEVs as they do in gasoline- fueled ICEs. The high manufacturing impact of the CFRP BIW and the minimal benefits of lightweighting during the use stage explain the poor environmental performance of the CFRP BIW PCR in seven impact categories in the BEV case; it only outperforms the AHSS BIW with respect to HH-CP, HH- NCP, and (marginally) FEP.

The SROI of Lightweighted BIWs for Various Fuel Options. SROI values for replacing an MS BIW in a gasoline-fueled car with one of the lightweight BIWs considered in this work (and potentially also replacing the gasoline powertrain with a greener alternative) were determined using eq 5, yielding the results shown in Table 5. Manufacturing costs71 and social costs72,73 were calculated using literature data; details of the calculations are presented in section S5 of the Supporting Information

Table 5. SROI of Lightweighted BIWs (per Functional Unit)

The SROI values represent the sustainable returns (€) per euro spent by an OEM on manufacturing lightweight BIWs, and range from 0.54 to 3.13. These values indicate that sustainable returns are maximized by replacing an MS BIW with an AHSS BIW in ICEs fueled with woody biomass- derived bioethanol and BEVs. Conversely, a CFRP BIW is a superior replacement for passenger cars fueled with gasoline or corn bioethanol. Although the social costs of the CFRP BIW are lower than those of the AHSS BIW for both E85 variants, its SROI is lower because of its high manufacturing costs. However, commercialization of CFRP recycling technologies is expected to increase the SROI of the CFRP BIW when used with alternative fuels by increasing the usage of secondary CF and thus reducing manufacturing costs.

In summary, we have conducted a detailed cradle-to-grave LCA of AHSS and CFRP BIWs for three different vehicle propulsion modes. Four key insights were obtained. First, the CFRP BIW exhibits worse environmental performance than the MS BIW with respect to the PMFP, AP, FEP, and FETP impact categories. Its higher impact scores are predominantly due to the release of atmospheric pollutants such as ammonia during CF production, the electricity source (assumed to be the Swedish electric grid in this work) used in CF synthesis, and the alloying additives used to recycle postconsumer aluminum scrap.

Second, the GWP of the CFRP BIW was lower than that of the MS BIW for gasoline-fueled ICEs, which is encouraging from a climate change perspective. This was attributed to the source of electricity used for CF production. Sweden, with its low carbon-intensity electrical grid, should be a favored location for CF production. The likelihood of a CFRP BIW having a lower GWP than an MS BIW is highest for electrical grids with carbon intensities below 400 g CO2/KWh. Moreover, our scenario analysis showed that operating a CF production facility in symbiosis with a decentralized energy source such as a WtE plant would be more beneficial from a climate change perspective in countries where the carbon intensity of electricity is high.

Third, the AHSS BIW exhibited superior overall performance in cars fueled with woody biomass-derived bioethanol and BEVs, but the CFRP BIW performed better for vehicles fueled with corn bioethanol. This is mainly due to the high well-to- tank impact of corn bioethanol, which reduces the impact scores of the CFRP BIW because of its high fuel savings potential.

Finally, the SROI values calculated for the AHSS and CFRP BIWs under different propulsion modes ranged from 0.54 to 3.13. On the basis of these values, the AHSS BIW should be preferred for BEVs and the CFRP BIW for gasoline-fueled ICEs. For ICEs fueled with E85, the CFRP BIW should be generally preferred to the AHSS BIW if its manufacturing costs can be reduced sufficiently. This would require the availability of recycled CF with properties suitable for producing lightweight structural components or the development of a method for preparing CF from an inexpensive lignin-based precursor.

Source: Kavitha Shanmugam - Umeå University, Gadhamshetty Venkataramana - South Dakota School of Mines and Technology, Pooja Yadav - Swedish University of Agricultural Sciences, Dimitris Athanassiadis - Swedish University of Agricultural Sciences

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Synthesis of Zeolite From Coal Fly Ash for Ceramic Membranes and Ceramic Materials

Abstract

The fly ash, from the combustion of coal to produce energy and heat, is industrial waste, which large accumulations represent a serious environmental threat. To reduce the environmental burden and improve the economic benefits of energy production the science and industry focus on the transformation of coal combustion by-products into new functional materials. The fly ash was studied by modern analytical methods. As a result of the hydrothermal reaction several types of zeolites were synthesized from the fly ash: analcime, faujasite (zeolite X) and gismondine (zeolite P). It was shown that the experimental conditions (temperature, reaction time and alkali concentration) have a significant influence on the type of zeolite and its content in the reaction products. The series of experiments resulted in building approximate crystallization field of zeolites and other phases as the first stage of the formation of ceramic membrane and other materials. 

Introduction

Fly ash is bulk industrial waste of coal combustion in thermal power plants (TPP), steel mills, etc. Therefore the problem of utilization of this technogenic waste, occupying large areas and causing damage to the environment, is very important. Many papers were published on the properties of fly ash and possibilities of its use [1-5]. Nevertheless the development of advanced technologies for the utilization of fly ash remains an important task. One of their solutions may be represented by the synthesis of zeolites from fly ash. Conversion of the fly ash in zeolites has many applications, including ion exchange, molecular sieves and adsorbents [6-8]. Identification of new applications has a real commercial interest: the list of marketable products is expanding and energy costs are reduced, environmental risks are reduced, the efficiency of sustainable development in the region is increased. Coal fly ash contains significant amounts of SiO2, Al2O3 and other oxides, which are regarded as cheap raw for the ceramic industry. Technologies of zeolite synthesis from fly ash are being constantly improved in both the experimental (variations of temperature, pressure, co-reagent and other methods of exposure) sphere, and the material composition of the initial raw [9-11]; the quality, function and cost of final product depend on it. The synthesis of zeolites from fly ash is the first stage in the formation of ceramic materials (ceramic membranes), which defines the significance of this trend of research.

The aim of this work is to develop the scientific basis for the formation of ceramic materials with given properties, in order to achieve that it is important to determine material composition of the fly ash produced from coal combustion at thermal power stations of Pechora coal basin as raw for ceramic membranes and ceramic materials.

Objects and Methods

For the experiments we used the fly ash from thermal power plants of Pechora coal basin. The synthesis methods of zeolites were based on [9]. Firstly, using a magnetic separator we removed ferriferous phases, which do not participate in the synthesis of zeolites. Dry fly ash is mixed with the solution of sodium hydroxide (NaOH) in a certain ratio, mixed thoroughly, and the suspension was placed in an autoclave. The resulting products of hydrothermal reaction was washed with distilled water and dried. This resulted in powders consisting of the mixture of zeolite and unreacted residue in various proportions.

The chemical composition of the fly ash and the products of hydrothermal reaction was determined as follows: Na2O, K2O, FeO, LOI, CO2, obtained by the complete silica analysis, the other components - with the help of X-ray fluorescence analysis (energy dispersive spectrometer MESA500W, Horiba). 

The phase composition studies were performed on powder diffractometer (Shimadzu XRD 6000, radiation CuK𝞪, Ni filter) within range 2 - 65° 2𝞱 angle with rate 2° 2𝞱/min. Identification of zeolites was carried out using databases «WWW-MINCRYST» (http://database.iem.ac.ru/mincryst/rus/index.php) and International Zeolite Association (http://www.iza-structure.org/databases). To study the morphology and chemical composition of the fly ash we used a scanning electron microscope TESCAN VEGA 3 LMH with energy dispersive Oxford Instruments XMax. 

Fig. 1: SEM photographs of surface of aluminum silicate (a, b) and iron-containing globules (c,d).

Results and discussion

Initial fly ash. X-ray diffraction (Fig. 2) showed quartz, mullite, magnetite and hematite in the fly ash. The broad "hump" (area of increased background) on the diffraction pattern in the area 15–35° 2𝞱 indicates the presence of amorphous phase (probably silicate or aluminosilicate glass). The main components of the chemical composition are oxides of silicon (57.78 %) and aluminum (18.25%), iron oxide content is about 9.0 %, oxides of other elements - 7.42 %, LOI - 7.90 % (Table 1). The fly ash is represented under the electron microscope by globules (Fig. 1), which are divided by the chemical composition to oxide-aluminosilicate and oxide-ferriferous. The globules composition is predominated by silica (from 41.82 to 61.27 %) and alumina (from 17.03 to 22.8 %), the oxides of iron (up to 8.31 %), magnesium (up to 4.83 %), potassium (up to 3.05 %), titanium (up to 1.04 %) and sodium (up to 0.93 %) are also present. Globule size varies from the first to about hundred micrometers; on the surface bubbles and elongated structures (Fig. 1b) are observed. On the surface of iron oxide globules (Fig. 1 c, d) both flat areas and skeletal forms are observed, which are significantly different from each other by their chemical composition. The skeletal forms have a high content of iron oxides (68.14-74.66 %) and low silica (1.06-6.22 %), alumina (1.33-4.17%) and calcium oxide (0.48-3.59%) contents. On the flat areas iron oxide content is greatly reduced (19.29-31.81 %), silica and alumina content increases (27.12-37.86 and 2.06-6.22 % respectively); calcium is present in large amounts (10.45-25.3 %). Globule size ranges from several to tens micrometers. Globules, which contain smaller globules within, are often present (Fig. 2b).

Table 1: Chemical composition of the fly ash.

Fig. 2: Diffraction patterns of products synthesized at 80, 95, 140 and 180 °С for 12 hours (Q - quartz, X – zeolite X, P – zeolite P, A - analcime, K - cancrinite). The interplanar distances are given in Å.

Hydrothermal synthesis. There were two sets of experiments. In the first set the effect of temperature of hydrothermal reaction on zeolite synthesis was studied (reaction temperature 80, 95, 140 and 180 °С, reaction time 12 hours, the ratio of NaOH: fly ash = 1:1, NaOH concentration 3.0 mol/dm3). The second set of experiments studied the influence of reaction time and concentration of alkali on synthesis process (reaction temperature 140 °С, duration 2, 4, 6 and 8 hours, ratio of NaOH: fly ash = 1:1, NaOH concentration 1.5, 3.0 and 4.5 mol/dm3). The process of transformation of the fly ash to zeolites can be represented in the following way:

where n in zeolite formula – oxidation degree, which is equal to 1 for Na.

According to [10, 11], this process consists of three stages: dissolution, condensation and crystallization. When fly ash interacts with sodium hydroxide it is dissolved, and Si and Al are released to the solution. Then the condensation of silicon and aluminum ions occurs, followed by gelling and nucleation (forming nuclei or crystallization centers) and crystallization of zeolites.

The synthesis results in powders consisting of the mixture of zeolite and unreacted residue in different proportions, which output was 70-80 % of the weight of the initial fly ash. The bulk density of powder is 0.78-0.83 g/cm3.

Effect of reaction temperature on the synthesis of zeolites. In the result of the reaction at 80 °С the intense reflections of quartz were diagnosed; no newly formed phases were detected (Fig. 2). Electron microscopic studies revealed numerous globules destroyed by alkaline solution.

By increasing the reaction temperature to 95 °С silica the intensity of quartz reflections decreased, i.e. it was dissolved in alkaline solution. Alongside with quartz reflections the intense reflections were determined, which are characteristic for faujasite zeolite (zeolite A), and weak reflections characteristic for gismondine zeolites (zeolite P). By Si/Al ratio the zeolites are low silica: silica-aluminum module of zeolite X varies from 1.51 to 1.57, zeolite P - from 1.65 to 1.69. SEM images present numerous crystals of zeolite X with octahedral shape with the size of 1-3 mm (Figure 3 a, b). Zeolite P crystals have a rounded shape, their size is about 5 microns (Figure 3 b).

Fig. 3: SEM photographs of products synthesized at 95 °С: a - accumulation of zeolite X crystals, b - crystals of zeolites X and P.

The diffraction patterns of the reaction products obtained at 140 °С showed zeolite P and analcime, weak quartz reflections were also present. Zeolite P is more high-silica compared to the phase obtained at 95 °С: Si/Al ratio varies slightly from 1.93 to 1.94. Silica-aluminum module of analcime varies from 2.12 to 2.21. SEM images showed that zeolite P formed skeletal crystals with size of 10-15 microns (Fig. 4 a). Analcime crystals with size of 15-20 microns were observed (Fig. 4 b).

Fig. 4: SEM photographs of products synthesized at 140 °С: a - single crystal of zeolite P, b - analcime and zeolite P crystals

The reaction at 180 °С resulted in the formation of analcime and cancrinite; no quartz reflections were diagnosed. Si/Al ratio of analcime varies from 2.00 to 2.15. As seen in Fig. 6a, the analcime crystals are formed by tetragonal faces, their size ranges from 15 to 25 microns. Cancrinite columnar crystals with length of up to 2 micrometers and about 200-300 nm in diameter (Fig. 5 b) are often observed on the surface of analcime, indicating later crystallization of cancrinite.

Fig. 5: SEM photographs of products synthesized at 180 °С: a - single crystal of analcime, b - accumulation of crystals of cancrinite.

These results indicate that the reaction temperature influences the type of synthesized zeolite, which differ by the efficient diameter of entrance windows, and according to classification [12], they are divided into narrow, medium and wide porous types. It is determined that increasing reaction temperature results in the formation of narrow porous zeolites; at 95 °С zeolites X formed that are related to wide porous type, at 140 °С – zeolite P, related to medium porous type, and at 180 °С – analcime related to narrow porous zeolite.

Fig. 6: Approximate field of crystallization of zeolites from the fly ash with varying times and NaOH concentrations (X – zeolite X, P – zeolite P, ANA - analcime, HS – hydroxysodalite).

Effect of reaction time and alkali concentration on zeolite type. The set of experiments resulted in the apporoximate crystallization field of zeolites and other phases (hydrosodalite) at 140 °С, the reaction time from 2 to 8 hours, NaOH concentration 1.5, 2.9 and 4.5.

As seen in Fig. 6 the wide porous zeolites X are formed by 4 hours of reaction at a high concentration of alkaline solution (4.5 mol/dm3). Longer reaction leads to the disappearance of the metastable phases of zeolite X and the occurrence of more thermodynamically stable - zeolite P and then analcime.

Zeolite P is crystallized under a wide range of reaction conditions. At the same time, the fields of crystallization of analcime and zeolite P are significantly overlapped, that is, at the same conditions of the hydrothermal reaction, the mixture of zeolites in various quantitative relations is formed. Higher concentrations of alkali results in the increase of the content of narrow porous phases (analcime) compared to zeolite P, and contributes to the formation of non-zeolitic phase - hydrosodalite. 

Conclusions

Several types of zeolites: analcime, faujasite and gismondine zeolites were synthesized as the result of hydrothermal reaction at 80 – 180 °С from the fly ash of power plant (Pechora coal basin, Russia) by adding sodium hydroxide at concentration from 1.5 to 4.5 mol/dm3. The final product was powder: mix of zeolite and unreacted residue in various ratios with yield 70-80 % to the weight if initial fly ash. The bulk density was 0.78-0.83 g/cm3. We determined that the reaction temperature affects the type of synthesized zeolites: wide porous zeolites were formed at 90-100 °С, increasing reaction temperature results in the formation of medium and narrow porous types. It is shown that the type of zeolite and its content in the reaction products are significantly affected by the reaction time and the concentration of alkali. The experiments resulted in the approximate crystallization field of zeolites and other phases as the first stage of ceramic formation.

Source: О.B. Kotova1, I.N. Shabalin2, D.А. Shushkov1, L.S. Kocheva1
1 IG KomiSC of UB of RAS, Russia
2 The University of Salford, United Kingdom

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