ReviewHybrid and single feedstock energy processes for liquid transportation fuels: A critical review
Highlights
► Thermochemical-based indirect liquefaction of coal, biomass, and natural gas. ► Energy systems for liquid transportation fuels. ► Contributions for single feedstock and hybrid feedstocks energy systems. ► Studies on stand-alone systems and network-based supply chain analyses.
Introduction
The current global energy sector is mostly driven by fossil fuels (i.e., petroleum, coal, and natural gas), with petroleum being the longstanding, primary energy source. The United States Energy Information Administration also projects that petroleum will remain the primary fuel of choice in its various projection cases up to 2035 (Energy Information Administration., 2011). Increasingly though, the sector is faced with challenges over high energy prices, volatility of the global oil market, and the pressure to reduce greenhouse gas (GHG) emissions from fossil fuel consumption. Alternative energy sources such as solar, wind, hydropower, and nuclear still need major technological developments to play a significant role in replacing fossil fuels and abating greenhouse gas emissions. Additionally, uncertainties such as the recent earthquake and tsunami that damaged several nuclear reactors in Japan may have profound impacts on the future of world nuclear power (Energy Information Administration., 2011).
To address some of these challenges, the use of biomass to produce liquid transportation fuels (biofuels) has emerged as a focus of interest since they provide a renewable carbon-based source that can absorb atmospheric CO2 during photosynthesis (Lynd et al., 2009; NAS, NAE and NRC, 2009; Science & Board, 2008). In the United States transportation sector, which consumes a large majority of the petroleum supply to the country, biofuels can complement and/or replace petroleum, reducing both fuel imports and GHG emissions, provided that the biomass is cultivated sustainably.
Today, corn-based ethanol and soybean-based diesel comprise a majority of the manufactured biofuels. However, their use for fuel production has led to concerns regarding the impact on the price and availability of these feedstocks as sources of food (Lynd et al., 2009). Lignocellulosic plant sources (e.g., corn stover or forest residue) are expected to be a more considerable source of biofuels in the future, though an increase in crop production will be required to generate an appropriate amount of sustainable residue for fuels production (de Fraiture et al., 2008, DOE and USDA, 2005, National Research Council, 2008).
In light of the aforementioned challenges, many research efforts have explored alternative, non-petroleum based processes, including thermochemical liquid fuel production from coal, natural gas, and biomass via synthesis gas (syngas) intermediate and conversion to hydrocarbons through the Fischer–Tropsch (FT) reaction. Syngas is produced via natural gas reforming, coal or biomass gasification, and this intermediate opens opportunities of hybrid processes that combine syngas from multiple feedstock sources. Utilizing coal, natural gas, and biomass as carbon sources will shift dependence away from petroleum and the produced FT liquids can be readily integrated with the current fuel market. The abundance of coal, combined with recent expansions of natural gas from conventional and unconventional sources, will enhance the security of fuel supply, while the incorporation of biomass in the portfolio of energy processes will help reduce GHG emission figures. In order to compete with petroleum-based fuels, many researchers have pursued efforts to optimize the plant design, technology considerations, as well as multiple products to increase the profitability of the process. It is also important to optimize the utilities usage of the plant (e.g., heat, power, and water), the investment for the heat exchanger/power recovery network, and recover additional profit in selling utility byproduct.
With the move towards biomass-based fuels, the demand for agricultural or perennial crops will increase, affecting the national and regional withdrawal and consumption of freshwater needed for irrigation. Moreover, the freshwater input for the biorefineries to make up for process losses will add to the consumption figure, putting additional stress on water resources. As an essential resource to various ecosystems and human activities, the demand of freshwater spans across agricultural, industrial, and residential sectors. While the abundance and distributed nature of the supply thus far has kept the price of freshwater relatively low, future population increases and economic developments will intensify the pressure on water resources. In the United States alone, the total national freshwater in 2005 use was 349 billion gallons per day (Bgal/d) (Kenny et al., 2009), and is expected to increase by 25% in 2030 assuming an average population growth scenario of 0.9%/year (Energy Information Administration., 2010). If a large-scale production of biomass for transportation fuels is realized, the freshwater use will exceed the expected 25% and additional measures will have to be employed to minimize the supply-demand gap.
The intensifying stress on water resources prompts efforts to minimize freshwater usage in energy conversion processes. Water is typically used for washing operations, separation processes, steam and power generation, cooling systems, or as a raw material input to the processes. The discharged wastewater needs to be treated before the final disposal to the environment and the treatment processes can be energy intensive (Department of Energy, 2006; Mielke, Anadon, & Narayanamurti, 2010). Thus, the development of approaches to design efficient water networks for industrial processes, minimizing both freshwater consumption and wastewater discharge from the process, are of major importance (Ahmetovic and Grossmann, 2010b, Ahmetovic and Grossmann, 2010a, Grossmann and Martín, 2010, Karuppiah and Grossmann, 2006).
In addition to process developments for a single plant, studies have also been done that consider a network based approach to identify potential locations of these fuel producing plants. Logistical considerations in transporting the feedstocks and products to and from the plants are taken into account such that the overall cost of production is minimized. The transportation factor for biomass-based systems is especially important due to the diffused nature of biomass resources and its low energy density compared to coal and natural gas, which are more centralized and produced in high amounts. Strategic locations of biorefineries will depend on the regional production of biomass, whether the availability can fulfill a continuous supply of biomass to the plant or not. In hybrid processes where biomass feedstock is combined with coal and/or natural gas, it is important then to investigate the trade-offs of delivering coal, natural gas, and biomass feedstocks in choosing the locations of the hybrid plants.
In this paper, we review and discuss the thermochemical based process designs for the production of liquid transportation fuels from coal, biomass, or natural gas feedstocks. Specifically, studies on the indirect liquefaction of the three feedstocks via syngas intermediate are reviewed to elucidate the contributions already established in literature. The main products considered in this paper include gasoline, diesel, kerosene, methanol, and di-methyl ether (DME), which can be produced using commercially viable technologies. Comprehensive reviews of the challenges and opportunities in energy research and advances in energy process systems engineering (Liu, Georgiadis, & Pistikopoulos, 2011) have recently appeared.
The review will initially detail the single-type feedstock designs in which only one type of feedstock: coal, natural gas, or biomass is used as a raw material for liquid fuel production (i.e., coal to liquids (CTL), natural gas to liquids (GTL), and biomass to liquids (BTL), respectively). Hybrid feedstock energy processes using a combination of two or three feedstocks will then be discussed to highlight the strategic and synergistic benefits of a mixture of different feedstocks (i.e., coal and natural gas to liquids (CGTL), coal and biomass to liquids, (CBTL), natural gas and biomass to liquids (BGTL), and coal, biomass, and natural gas to liquids (CBGTL)). Finally, key challenges and opportunities in the field are outlined. Table 1 lists all the abbreviations used in this review paper.
Section snippets
Single feedstock energy processes
Table 2 classifies the contributions for single feedstock energy processes, namely coal to liquids (CTL), natural gas to liquids (GTL), and biomass to liquids (BTL) processes into two general categories, contributions on (a) stand-alone processes, and (b) network-based energy supply chain analyses. In the first category, research publications that investigate the development of a single plant process are tabulated according to their specific contributions. In this paper, we focus on the
Hybrid feedstock energy processes
The development of the three single feedstock energy processes opens up opportunities to develop hybrid energy processes. Hybrid energy processes provide a compelling alternative in the portfolio of energy resource production, due to their flexibility in converting multiple types of feedstock inputs into a consistent range of products. Moreover, hybrid energy systems can have synergistic effects due to the combined advantages of each individual system. For example, combining coal or natural gas
Research groups contributions
Table 6 presents a representative list of the research contributions on single feedstock and hybrid feedstock energy processes organized by research groups worldwide.
Future challenges and opportunities
Based on the studies present in literature, the following challenges and opportunities are highlighted for the single feedstock and hybrid feedstocks energy processes:
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Hybrid feedstock energy processes. Further development of hybrid feedstock energy processes can be pursued in the future. Based on the current simulation studies, optimization based process synthesis approaches are important tools in designing efficient hybrid systems with multiple feedstocks and technologies. Additional
Conclusions
This review paper has outlined the contributions for single feedstock and hybrid feedstocks energy systems, based on indirect liquefaction of coal, natural gas, and biomass via syngas intermediates, and includes studies on the stand-alone systems and network-based supply chain and facility location analyses. The main products considered are gasoline, diesel, kerosene, methanol, and DME, with optional co-production of electricity, hydrogen, and LPG. Future opportunities are outlined,
Acknowledgement
The authors acknowledge partial financial support from the National Science Foundation (NSF EFRI-0937706).
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