Biomass direct chemical looping process: A perspective
Introduction
The carbon dioxide emission reduction is a major concern for Annex I countries. They are expected to meet the new environmental requirements of green house gas (GHG) emission for the 2008–2012 period as set by the Kyoto Protocol. Emission of GHG, especially CO2, has continued an upward trend since 1970 [1], [2]. The R&D on clean, safe, and cost-effective power conversion processes and the adoption of renewable energy sources, such as hydro, wind solar, geothermal and biomass, has been promoted and supported by the governments of these countries in an attempt to reduce CO2 emission.
The CO2 can be emitted from a number of different human activities. One of the major sources of this GHG is from the combustion of fossil fuels used in power generation, transportation, and industrial processes [3]. Thermo-chemical power production systems generally utilize coal or natural gas as their feedstock. Due to its relatively high energy density and well understood processing techniques, fossil fuels are considered dominant candidates for supplying the world electricity requirements. However, combustion of fossil fuels, especially coal, significantly contributes to carbon dioxide emissions. Several steps are being taken to reduce its release into the atmosphere [4].
The following section will review the conventional CO2 reduction scheme via high efficiency process operation and the current carbon capturing technologies.
There are several methods currently being developed to reduce the CO2 emissions from conventional power plants. The reduction of fossil fuel/coal consumption by improving the energy conversion efficiency represents a first step approach to reducing GHG emissions. The development of efficient power generation systems, such as IGCC and ultra-high temperature alloys for steam turbine applications, has contributed significantly to this cause [5]. Additionally, CO2 capture and storage (CCS) methods are being developed to further stabilize its concentration in the atmosphere from fossil fuel processing impacts [6], [7].The CCS involves carbon dioxide capture, transportation, and sequestration [8], [9]. Here, transportation has been proven feasible with pipeline construction. Geological sequestration refers to storing a large amount of CO2 underground [10], [11]. This method is currently under development in several countries such as Norway and Canada. However, methods of sequestration have not yet been proven reliable and safe for long-term storage.
There are a number of methods for CO2 capture. These techniques vary depending on its application to the power plant of interest. Overall, each method must be capable of capturing the carbon in a high purity stream to be pressurized and transported to a sequestration site. Though numerous, carbon capture techniques can be divided into three categories; post-combustion pre-combustion and oxy-fuel combustion capturing systems [4], [12], [13], [14]. Here, the post-combustion capture system refers to separating carbon from the exhaust stream after its combustion with air. The use of a chemical solvent to scrub the CO2 from the flue gas is an established method in coal-fired power plants [15], [16], [17]. More cost-efficient solvents are continuously under development [18], [19]. Alternatively, membrane and solid sorbent separation techniques are also being studied for this application [13], [20], [21]. In the pre-combustion capture system, CO2 is removed before the combustion step. Natural gas reforming process is an example of the technique [22]. Finally, the oxy-fuel combustion system uses high purity oxygen instead of air to combust the primary fuel to produce a flue gas comprising mainly of water vapor and CO2.
Many of the carbon capture systems currently developed are capable of reducing CO2 emissions from power generation plants [4], [23], [24]. However, a significant amount of parasitic energy is required for its removal [4], [7], [13], [16], [24]. Thus, integrating carbon capture with coal combustion plants reduces the overall efficiency of the industry lowering its profitability. Further research is required to achieve better system integration, thereby increasing the energy conversion efficiency and hence reducing the CO2 capturing costs. The coal thermo-chemical processes are well developed fields for energy production. Coupling these systems with carbon capture technologies will potentially allow the respective countries to meet the GHG emissions requirement, but reduce the overall profitability of the power plant. Additionally, the use of coal continues our dependence of fossil fuels, a depleting resource. An energy source that is both renewable and environmental would be a highly attractive alternative to the current situation.
Biomass is considered a fuel source to partially replace the use of fossil fuels in many thermo-chemical processes [25], [26], [27], [28], [29]. This renewable resource consists of bio-residues from plants such as lignocellulosic materials. Biomass is considered carbon neutral, because the amount of carbon it can release is equivalent to the amount it absorbed during its life time. There is no net increase of carbon to the environment in the long-term when combusting the lignocellulosic materials. Therefore, biomass is expected to have a significant contribution to the world energy and environment demand in the foreseeable future [3], [7]. Replacing some of the current coal feedstock in thermo-chemical processes with biomass will result in an atmospheric carbon reduction. Additionally, if this system is capable of utilizing carbon capture technologies, a net reduction of CO2 is achieved. A power plant that integrates both biomass and CCS technology will receive carbon credit making this process potentially more economically attractive.
Several systems are being developed to apply biomass for energy production. Fig. 1 summarizes the application processes. The biomass systems can be divided into two fields of study; biochemical and thermo-chemical processes. Biochemical processes for biomass application consists of the use of a cell culture to produce a desired product. Glucose is derived from the hydrolysis of biomass as the feed for the fermentor [30]. There are three main thermo-chemical systems currently under research; direct combustion, gasification, and pyrolysis. Biomass combustion research is similar to coal combustion technologies. The biomass directly reacts with air to produce heat in the form of steam. The product stream will be used to drive a steam turbine [31]. Ideally, this technology can serve as a substitute to coal in the conventional combustion system. However, due to the high moisture content, relatively low energy density and highly distributed-resource when compared to coal, replacing this fossil fuel with the renewable fuel significantly reduces the overall efficiency of the power plant. In order to offset the process inefficiencies, a co-combustion system is proposed where a combination of biomass, waste plastic and coal is used as the feedstock to the combustor [32]. This technology is being proven to be cost-effective and practical, because it uses the currently established power plant systems to combust the renewable fuel. Several countries have begun to integrate co-combustion process with their respective power plants [33], [34], [35], [36]. The use of biomass for our energy demand is gradually increasing every year [37].
Pyrolysis refers to the heating of a substance in the absence of an oxidizing agent [38]. Using biomass in this process yields char, liquid and gas. Bio-oils with a wide range of heavy and light hydrocarbon chains [39], [40] is able to produce by manipulating pressure, residence time, temperature and heating up rate. This technology has the potential of producing valuable liquid fuels that may replace the conventional petroleum stations. However, this extensive and highly complicated process requires further research before industrialization can be considered.
With lower operating temperatures compared to combustion systems, gasification process are becoming a potential option for improving the biomass energy conversion efficiency. Furthermore, this technology is capable of producing a variety of chemical products, such as methanol and other hydrocarbons, allowing for flexible marketability [32], [41], [42], [43]. The biomass gasification system has been successfully demonstrated under fluidized and circulating fluidized bed operation. Secondary tar cracking catalyst and steam-reforming techniques are established down stream processing methods that have shown promising results in this system [44], [45], [46], [47]. Additionally, a biomass and coal co-gasification may offset the biomass low energy density issue allowing for the production of both valuable fuels and power [48], [49].
Biomass energy is potentially capable of reducing fossil fuel consumption and CO2 emission. However, the biomass energy conversion systems are relatively more inefficient and cost intensive when compared their coal counterpart processes. Table 1 compares the efficiencies of coal systems to biomass for the thermo-chemical processes [50]. Furthermore, coupling CCS methods to biomass would reduce the process efficiencies. The low efficiencies compared to coal hinder the further development of renewable resource. A highly efficient process is needed in order to replace our current coal dependence.
The thermo-chemical conversion technology based on the chemical looping process (CLP) has attracted much attention as it requires little energy for CO2 separation [51], [52]. This system differs from other processes in that the GHG separates through reaction path design. The CLP technology utilizes metal oxides instead of gaseous oxidants as the oxygen carrier. In one section of the CLP, this chemical intermediate is reduced with carbonaceous fuels to metal producing CO2 and H2O [53], [54], [55], [56]. Condensing the steam from this product stream yields a sequesterable CO2 stream. The reduced metal can then be oxidized to produce electric power and/or chemical.
This paper will provide a perspective on the biomass direct chemical looping (BDCL) scheme which directly converts biomass into hydrogen and/or electricity. The discussion will involve the preliminary design, feasibility, and potential challenges with using biomass in this application.
Section snippets
Biomass direct chemical looping with CO2 capture
The biomass direct chemical looping process is a strong candidate for using this renewable resource to achieve a higher energy conversion efficiency as well as CO2 capture. This process utilizes an iron oxide composite particle to directly react with the biomass in an oxidation/reduction reaction scheme. Li et al. [57] calculated the energy conversion efficiency of the BDCL process, using ASPEN plus, to be over 38% on the higher heating value (HHV) basis for power generation with 99% CO2
Conclusion
With regulations pressing for cleaner energy conversion processes, several technologies are being developed and employed. A first step to reducing carbon emissions is increasing the efficiencies of the current fossil fuel fed power plants. CCS techniques are also being developed to sequester the GHG before being emitted into the atmosphere. However, such methods require a certain amount of parasitic energy to purify the CO2 stream reducing the overall process efficiency. Furthermore, with
Acknowledgement
We would like to express our appreciation to Andrew Tong, Fanxing Li, Hyung Kim and Liang Zeng for their technical and editorial assistance during the preparation of this manuscript.
References (88)
- et al.
Sleipner vest CO2 disposal-injection of removed CO2 into the Utsira formation
Energ Convers Manag
(1995) - et al.
Carbon dioxide in enhanced oil recovery
Energ Convers Manag
(1993) - et al.
Advances in CO2 capture technology – The U.S. Department of energy’s carbon sequestration program
Greenhouse Gas Control
(2008) - et al.
Co-production of hydrogen, electricity and CO2 from coal with commercially ready technology, Part A: performance and emissions
Int J Hydrogen Energy
(2005) - et al.
Co-prodction of hydrogen, electricity and CO2 from coal with commercially ready technology. Part B: economic analysis
Int J Hydrogen Energy
(2005) - et al.
Pushing the limits on possibilities for large scale gas separation: which strategies?
J Membr Sci
(2000) - et al.
Polymeric CO2/N2 gas separation membranes for the capture of carbon dioxide from power plant flue gases
J Membr Sci
(2006) - et al.
Exploration of the ranges of the global potential of biomass for energy
Biomass Bioenerg
(2003) Energy production from biomass (part 1): overview of biomass
Bioresour Technol
(2002)- et al.
Features of promising technologies for pretreatment of lignocellulosic biomass
Bioresour Technol
(2005)