Fluidized Bed Co-gasification of Algae and Wood Pellets: Gas Yields

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Fluidized bed co-gasification of algae and wood pellets: gas yields and bed agglomeration analysis Youjian Zhu, Patrycja Piotrowska, Philip Joseph van Eyk, Dan Bostrom, Xuehong Wu, Christoffer Boman, Markus Broström, Jun Zhang, 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/acs.energyfuels.5b02291 • Publication Date (Web): 04 Dec 2015 Downloaded from http://pubs.acs.org on December 7, 2015

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5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies 14 – 16 December 2015, Wuhan, China

Fluidized bed co-gasification of algae and wood pellets: gas yields and bed agglomeration analysis Youjian Zhu1, Patrycja Piotrowska2, Philip J. van Eyk3, Dan Boström2, Xuehong Wu1, Christoffer Boman2, Markus Broström2, Jun Zhang1*, Chi Wai Kwong3, Dingbiao Wang4, Andrew J. Cole5, Rocky de Nys5, Francesco G. Gentili6, and Peter J. Ashman3*

1

School of Energy and Power Engineering, Zhengzhou University of Light Industry, Zhengzhou,

Henan 450002, China 2

Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics,

Umeå University, 901 87 Umeå, Sweden 3

School of Chemical Engineering, University of Adelaide, Adelaide SA 5005, Australia

4

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, Henan 450001,

China 5

MACRO, the Centre for Macroalgal Resources and Biotechnology, James Cook University,

Townsville Qld 4811, Australia 6

Department of Wildlife, Fish, and Environmental Studies, Swedish University of Agricultural

Sciences (SLU), 901 83 Umeå, Sweden

* Email: [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT Algae utilization in energy production has gained increasing attention due to its characteristics such as high productivity, rapid growth rate and flexible cultivation environment. In this paper, three species of algae, including a fresh water macroalgae, Oedogonium sp., a saltwater macroalgae Derbersia tenuissima and a microalgae species, Scenedesmus sp, were studied to explore the potential of using smaller amounts of algae fuels in blends with traditional woody biomasses in the gasification processes. Co-gasification of 10 wt% algae and 90 wt% Swedish wood pellets was performed in a fluidized bed reactor. The effects of algae addition on the syngas yield and carbon conversion rate were investigated. The addition of 10 wt% algae in wood increased the CO, H2 and CH4 yield by 3-20%, 6-31%, and 9-20% respectively. At the same time, it decreased the CO2 yield by 3-18%. The carbon conversion rates were slightly increased with the addition of 10 wt% macroalgae in wood, but the microalgae addition resulted in a decrease of carbon conversion rate by 8%. Meanwhile, the collected fly ash and bed material samples were analysed using scanning electron microscopy combined with an energy dispersive X-ray detector (SEM-EDX) and X-ray diffraction technique (XRD). The fly ashes of wood/marcoalgae tests showed a higher Na content with a lower Si and Ca content compared to wood test. The gasification tests were scheduled to last 4 hours, however only Wood and wood/Derbersia gasification experiments were carried out without significant operational problems. The gasification of 10 wt% of Oedogonium N+ and Oedogonium N- led to defluidization of the bed in less than an hour and wood/Scenedesmus test was stopped after 1.8 h due to severe agglomeration. It was found that the algae addition had a remarkable influence on the characteristics and compositions of the coating layer. The coating layer formation and bed agglomeration mechanism of wood/macroalgae was initiated by the reaction of alkali compounds with the bed particles to form a low temperature melting silicates (inner layer). For WD/SA test, the agglomeration was influenced by both the composition of the original algae fuel as well as the external mineral contaminations.


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In summary, the operational problems experienced during the co-gasification tests of different algae-wood mixtures were assigned to the specific ash compositions of the different fuel mixtures. This showed the need for countermeasures, specifically to balance the high alkali content, in order to reach stable operation in a fluidized bed gasifier.

Key words: Co-gasification, wood Pellets, algae, fluidized bed reactor, gas yields, agglomeration


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1. Introduction Algae utilization in energy production has gained increasing attention due to its characteristics such as high productivity, rapid growth rate and flexible cultivation environment1, 2. Moreover, algae cultivation process can be effectively combined with the cleaning of CO2 rich gas3 and with wastewater treatment4. The algae utilization technologies can basically be divided into two categories: thermo-chemical and bio-chemical conversion technologies5, 6. Due to the growing conditions, the harvested algae generally contain a high moisture content5, 7. This hinders its utilizations in combustion, pyrolysis, gasification and trans-esterification to biodiesel which requires dry fuels. Therefore previous research has mainly focused on anaerobic digestion8, 9, hydrothermal gasification10 to produce methane, fermentation11 to produce bioethanol and hydrothermal liquefaction12, 13 to produce bio-oil, since these processes allow the use of wet biomass. However, the bio-chemical conversion methods, like anaerobic digestion and fermentation, usually need a long reaction time with a relatively lower conversion rate compared to the thermochemical conversion methods12. Wet thermochemical methods, such as hydrothermal liquefaction/gasification are considered as a promising conversion methods which could convert wet fuel into bio-oil/gas, but these technologies are immature and considerable technical difficulties need to be solved12 before the industrial scale application. Conventional Gasification (hereafter referred to as gasification) is a thermo-chemical method which can convert solid fuels into combustible gases. In contrast to hydrothermal liquefaction/gasification, gasification of biomass has been well developed and has been widely used for several decades12. Some small scale algae gasification experiments have been performed previously to study the operating conditions13 and the additions of catalysts14 on the syngas and tar yields without considering the impact of ash on the operating of the device. However, pure algae gasification is restricted by both high alkali metal content in ash15 and high energy consumption in fuel drying process5.


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The recently introduced algae steam drying method based on heat circulation technology could greatly decrease the energy consumption in algae drying16. Moreover relevant research has indicated that an innovative integration of drying, gasification and combined cycle could achieve a total power generation efficiency of around 40%12. This is comparable or even higher than the conventional coal-fired power plant. Moreover, co-gasification of algae with other solid fuels could change the algae ash chemistry and reduce the ash-related operational problems17 e.g., agglomeration and fouling. Currently, research on the co-gasification of algae with other fuels is rare. Co-gasification of a Kingston coal and a marine microalgae, Tetraselmis sp., was performed in a fluidized bed by Alghurabie et al18 and it was found rapid bed sintering and ash agglomeration in pure algae gasification. Co-gasification of Kingston coal and algae was also not successful because of the blockage of the product pipeline. However, it laid a foundation for the following algae gasification experiment. After the modification of gasification device, co-gasification of four different algae species with an Australian brown coal were successfully performed by Zhu et al17. Increased CO, H2 yields, carbon conversion rate and decreased CO2 yield were observed with the addition of 10 wt% macroalgae, while decreased CO, CO2, H2 yields and carbon conversion rate were observed with the addition of 10 wt% micoralgae due to the particularly high ash content of microalgae. The effects of algae additions on fuel ash chemistry were also investigated and the results indicated that macroalgae addition significantly increased the Na and K content in the fly ash and bed ash. Agglomerations were found in Oedegonium/coal and Scenedesmus/coal tests, the formation of liquid alkali-silicates and the formation of Fe−Al silicate eutectic mixture were proposed to be the main reasons for Oedegonium/coal and Scenedesmus/coal, respectively. Co-gasification of wood and algae were also studied previously. Yang et al.19 performed co-gasification experiments of torrefied and pelletized Eucalyptus globulus and Spirulina platensis (one type of microalgae) in a fluidized bed. The effects of microalgae addition on the syngas composition and heating rate were investigated and it was found that firstly the LHV and the content of CO increased with the increase of microalgae in the fuel blend. Then the content of CO


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started to decline when the fraction of microalgae reached a certain value which is varied with ER. A reverse trend was observed for CO2 and H2 content. However, the influence of the algae ash on the operating of the reactor is not considered in this research. Since algae biomass generally have a high ash content compared to stemwood based fuel assortments, and especially high alkali metal content, its addition to wood would lead to significant ash chemistry change that could lead to operating difficulties. Therefore, a clear and further understanding of the ash chemistry change due to algae addition is necessary to the safe and stable operation of the co-gasification process. The present study is motivated by expanding the fundamental and applied knowledge of using smaller amounts of algae fuels in blends with traditional woody biomasses in the thermochemical processes. The main objectives of the paper are to: (1) investigate the impact of algae addition in wood on the main gas yield; (2) investigate the impact of the algae addition on the change of the ash composition; and (3) investigate the impact of algae addition on bed coating and agglomeration and to determine the possible mechanisms for the agglomeration.

2. Experimental section Three species of algae and a Swedish wood pellet were used in this work. The wood pellet (hereafter referred to as WD) is a commercial softwood pellets of pine and spruce, it is mainly used for small to medium scale heating applications as well as large scale combined heat and power system in Sweden. The freshwater Oedogonium sp. and the saltwater Derbersia tenuissima (hereafter referred to as Deb) are key targets for the treatment of waste water and for their potential as a feedstock for bioenergy application20. This two species macroalgae were cultivated in tanks at James Cook University, Townsville, Queensland, Australia. Oedogonium sp was grown in two nitrogen environments, low (

Fluidized Bed Co-gasification of Algae and Wood Pellets: Gas Yields

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