Subscriber access provided by SELCUK UNIV
Article
Co-gasification of Australian brown coal with algae in a fluidized bed reactor Youjian Zhu, Patrycja Piotrowska, Philip Joseph van Eyk, Dan Bostrom, Philip Chi Wai Kwong, Dingbiao Wang, Andrew J. Cole, Rocky de Nys, Francesco G. Gentili, and Peter J. Ashman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502422t • Publication Date (Web): 26 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 47
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Co-gasification of Australian brown coal with algae in a fluidized bed reactor Youjian Zhu1,2, Patrycja Piotrowska3, Philip J. van Eyk2, Dan Bostrom3, Chi Wai Kwong2, Dingbiao Wang1.*, Andrew J. Cole4, Rocky de Nys4, Francesco G.Gentili5, Peter J. Ashman2.*
1
School of Chemical Engineering and Energy, Zhengzhou University, Henan 450001, China
2
School of Chemical Engineering, University of Adelaide, Adelaide SA 5005, Australia
3
Thermochemical Energy Conversion Laboratory, Umeå University, Umeå, Sweden
4
MACRO, the Centre for Macroalgal Resources and Biotechnology, James Cook University,
Townsville Qld 4811, Australia 5
Department of Wildlife, Fish and Environmental studies, SLU Umeå
1 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract: Recently the use of algae for CO2 abatement, wastewater treatment and energy production has increasingly gained attention worldwide. In order to explore the potential of using algae as an alternative fuel as well as the possible challenges related to the algae gasification process, two species of macroalgae, Derbesia tenuissima and Oedogonium sp., and one type of microalgae, Scenedesmus sp. were studied in this research. In this work, Oedogonium sp. was cultivated with two protocols: producing biomass with both high and low levels of nitrogen content. Cogasification of 10 wt% algae with an Australian brown coal were performed in a fluidized bed reactor and the effects of algae addition on syngas yield, ash composition and bed agglomeration were investigated. It was found that CO and H2 yield increased and CO2 yield decreased after adding three types of macroalgae in the coal, with a slight increase of carbon conversion rate, compared to the coal alone experiment. In the case of coal/Scenedesmus sp, the carbon conversion rate decreased with lower CO/CO2/H2 yield as compared to coal alone. Samples of fly ash, bed ash, and bed material agglomerates were analysed using scanning electron microscopy combined with an energy dispersive X-ray detector (SEM-EDX) and X-ray diffraction (XRD). It was observed that both the fly ash and bed ash samples from all coal/macroalgae tests contained more Na and K as compared to the coal test. High Ca and Fe contents were also found in the fly ash and bed ash from the coal/Scenedesmus sp test. Significant differences in the characteristics and compositions of the ash layer on the bed particles were observed from the different tests. Agglomerates were found in the bed material samples after the co-gasification tests of coal/ Oedogonium N+ and coal/ Oedogonium N-. The formation of liquid alkali-silicates on the sand particles was considered to be the main reason of agglomeration for the coal/ Oedogonium N+ and coal/ Oedogonium N- tests. Agglomerates of
2 ACS Paragon Plus Environment
Page 2 of 47
Page 3 of 47
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
fused ash and tiny silica sand particles were also found in the coal/ Scenedesmus sp test. In this case, however, the formation of a Fe-Al silicate eutectic mixture was proposed to be the main reason of agglomeration. Debersia was suggested to be a potential alternative fuel which can be co-gasified with brown coal without any significant operating problems under the current experimental conditions. However, for the other algae types, appropriate countermeasures are needed to avoid agglomeration and defluidization in the co-gasification process.
Key words: Co-gasification, brown coal, algae, fluidized bed reactor, gas composition, agglomeration and defluidization
3 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 47
1. Introduction Due to the grand challenges of environmental pollution and green house gas emissions, renewable energy has gained increasing attention. Among all the renewable energy resources, biomass is the only renewable energy that can replace fossil fuel in all energy utilization areas, including heat production, electricity generation, chemicals and liquid fuel synthesis1. If it grows in a sustainable basis, biomass fuel can be considered carbon neutral; it could also be carbon negative if combined with the carbon capture and storage. An additional benefit of biomass utilization is that conventional emissions, e.g. SO2 and NOx, generated during biomass combustion or gasification, are much lower compared to fossil fuel2. However, currently biomass utilization is still under-developed. Due to large costs associated with collection, handling, treatment and storage, the supply is often limited and availability can vary significantly with seasons and other local factors3. Because of these factors, biomass utilization is usually restricted to small scale applications and has lower efficiencies than coal fired plant4. Moreover, the low heating value, high alkali content5,
6
and high tar yield7
characteristics also restrict its utilization. Currently coal is still the major energy resource in many countries, especially in China and Australia. Co-utilization of coal and biomass could provide many advantages, e.g. reductions of CO2 and other gaseous pollution from electricity generation8, and improvement of the overall efficiency via synergistic effects3. It can also smooth out the biomass supply strain due to seasonal fluctuations and moderate the influence of lower heating values and higher alkali content of biomass. Co-utilization of up to ~10% biomass could also help establish biomass supply chains in the market before it would be feasible for biomass-only systems3.
4 ACS Paragon Plus Environment
Page 5 of 47
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Gasification is one of the thermochemical process which convert solid fuel to a combustible gaseous product using a gasification medium such as air, oxygen, CO2 or steam9. This technology has been widely used due to its high efficiency and low emissions. In addition, fluidized bed gasification is considered as a flexible technology which can convert reactive solid fuels of varying quality10. Co-gasification of different types of coal and biomass have been conducted in fluidized bed reactors by numerous researchers focusing on the effects of the blending ratio11, 12 and operating parameters13,
14
on gas compositions/yields, carbon conversion rate, and cold gas efficiency.
Effects of the blending ratio on tar contents and NH3 and H2S emissions have also been studied in the literature12. Some pilot scale tests have also been reported, i.e., co-gasification of sawdust with subbituminous/bituminous coal was performed in fluidized bed by McLendon et al.15 to provide operational data for numerical simulation verification. No synergies were found for the subbituminous coal and biomass mixture. However for the bituminous coal and biomass mixture, it was found that the addition of biomass can prevent clinker formation and agglomeration which happened during gasification of bituminous coal alone. Moreover, the transport properties for both mixtures were significantly improved compared to coal only experiments. Saw and Pang16 studied the effect of lignite to biomass ratio on the syngas composition and tar content by performing co-gasification of lignite and wood pellets in a 100 kW dual fluidized bed gasifier. The non-linear relationship between syngas composition and lignite/wood ratio indicated the existence of synergistic effects during the co-gasification experiments. It was also found that as the lignite to biomass ratio increased, the H2/CO ratio increased and tar yield decreased. The optimal blending ratio of 40wt% lignite and 60 wt% wood for Fischer-Tropsch synthesis was also suggested. 5 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 47
Most biomass used in co-gasification processes are woody biomasses, agricultural residues and municipal wastes. However the recent use of aquatic biomass, such as algae, for energy production17 has received attention due to the short growth cycle, high production yield, high CO2 fixation and flexible growing conditions18 since it can be cultivated using low-grade water and non-arable land17. The current use of algae for energy production is mainly focused on biological methods such as anaerobic digestion19 and liquefaction20. Most of the research in thermochemical conversion of algae is related to pyrolysis21,
22
and bio-char reactivity23,
24
.
Research on algal ash in combustion process has also been performed by Lane et al25. The occurrence of the main inorganic elements in four types of algae as well as the interaction of algal ashes and quartz bed material were investigated. It was found that no chemical reactions happened between the ashes and the bed materials for the algal fuels tested. Melt-induced agglomeration was proposed to be the main agglomeration mechanism for all the algae types. However, research into the co-gasification of algae with other fuels is more limited26,
27
.
Alghurabie et al.26 performed co-gasification experiments of Kingston coal and marine microalgae, Tetraselmis sp., in a fluidized bed reactor. Effects of air/fuel ratio, steam/fuel and bed temperature on the product gas compositions were investigated and optimal operating parameters were selected for coal gasification based on heating value of the product gas, carbon conversion rate and H2/CO ratio for Fischer-Tropsch synthesis. Pure algae gasification was unsuccessful due to the rapid bed sintering and ash agglomeration, however, the cause of ash agglomeration was not studied. Co-gasification of coal and algae was also not successful because of a blockage of the product pipeline. Yang et al.27 performed co-gasification experiments of torrefied and pelletized Eucalyptus globulus and Spirulina platensis (one type of microalgae) in a fluidized bed. The effect of temperature, equivalence ratio (ER), blending ratio and steam
6 ACS Paragon Plus Environment
Page 7 of 47
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
injection on the syngas composition, lower heating value of the syngas and tar content were investigated. They found that LHV, and the CO and H2 contents increased with increasing temperature, whereas the CO2, CH4 and tar content decreased. Additionally, with increasing equivalence ratio all gas constituents except of CO2 increased, and with injection of steam all gas constituents other than CO2 and CH4 increased. The influence of increasing microalgae ratio in the fuel blend had a more complex effect. At first LHV, and the content of CO increased with the increasing fraction of microalgae in the fuel blend. However, when the fraction of micoralgae reached a certain value (this value is varied with ER), the content of CO started to decline. A reverse trend was observed for CO2 and H2 content. The authors27 also indicated that more understanding about the role of ash should be considered for the future work due to the high ash content of algae. From the above it is clear that most of the co-gasification work was performed with the aim of investigating the blending ratio and operating parameters on gas compositions and tar content. However, due to the high ash content and high alkali metal content of algae compared to other solid fuels (e.g. coal and woody biomass), ash related problems such as deposition, agglomeration and defluidisation are expected, which make the operation difficult to control and lead to unscheduled shutdown of the gasification plant28. The mechanism and formation of agglomeration in fluidized bed combustion or gasification of coal29, 30 and biomass31-33 have been studied extensively. Two main mechanisms34, 35 can be simplified to explain bed sintering and agglomeration. These are melting-induced (non-reacting mechanism) and coating induced (reacting mechanism). In the first mechanism, melted ash is formed at the operating conditions in the bed and glues the ash/sand particles together. In the second mechanism, the alkali compound reacts with the sand particles (mainly SiO2) to form a melting phase, followed by the adhesion 7 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
and reaction with fine bed particles. However, the agglomeration mechanism for fluidized bed gasification of algae or algae/coal mixture is still unclear. The aim of this paper is hence to explore the feasibility of co-gasification of small amount (10 wt%) of algae with brown coal to produce syngas. Co-utilization of a small amount of algae with coal could make the most of their respective advantages and also help build the algae market infrastructure and lay a foundation for the future algae-only system. The resultant syngas can be used for heat and electricity generation, as well as chemical and liquid fuel production depending on its composition. The main objectives of the paper are: 1) to investigate the effects of the addition of different types algae in brown coal on the product gas yield; 2) to investigate the effects of the addition of algae on the ash composition; and 3) to investigate the effects of addition of algae on bed agglomeration and to identify the possible mechanisms for these ash related problems.
2. Experimental section 2.1. Fuels A Victorian brown coal, Loy Yang (hereafter referred to as LY), and three species of algae were used in the experiments. Two species of macroalgae, one freshwater Oedogonium sp. and one saltwater Derbersia tenuissima (hereafter referred to as Deb), were cultivated in tanks at James Cook University, Townsville, Queensland, Australia. Oedogonium sp. was cultured in two nitrogen environments, low (
