Elsevier

Bioresource Technology

Volume 99, Issue 8, May 2008, Pages 3174-3181
Bioresource Technology

Sunflower shells utilization for energetic purposes in an integrated approach of energy crops: Laboratory study pyrolysis and kinetics

https://doi.org/10.1016/j.biortech.2007.05.060Get rights and content

Abstract

Sunflower is a traditional crop which can be used for the production of bioenergy and liquid biofuels. A study of the pyrolytic behaviour of sunflower residues at temperatures from 300 to 600 °C has been carried out. The experiments were performed in a captive sample reactor under atmospheric pressure and helium as sweeping gas. The yields of the derived pyrolysis products were determined in relation to temperature, with constant sweeping gas flow of 50 cm3 min−1 and heating rate of 40 °C s−1. The maximum gas yield of around 53 wt.% was obtained at 500 °C, whereas maximum oil yield of about 21 wt.% was obtained at 400 °C. A simple first order kinetic model has been applied for the devolatilization of biomass. Kinetic constants have been estimated: E = 78.15 kJ mol−1; k0 = 1.03 × 103 s−1.

Introduction

The reduction of crude oil reserves and the other fossil fuels, in connection with the environmental burden that is given out from their use, in addition to continuously bigger demand for energy, has made necessary the need of finding out renewable energy resources friendlier to the environment. Biomass represents the fountainhead of Renewable Energy Sources (RES) in European Union. Approximately 63% of ‘renewable energy’ is produced from biomass. Biomass is the only renewable source of fixed carbon. Thermochemical conversion drives to a more effective management of biomass.

In practice, there are two types of biomass: firstly, residue types (any kind of plant residues, animal wastes and the organic part of municipal waste) and secondly, biomass which is produced from dedicated energy crops. There are several possible sources of producing biomass for energy. It can be produced either by establishing plantations of bioenergy crops including short rotation woody perennials or by removing residues from cropland (Lal, 2005). Energy crops are traditional crops which can be used for the production of bioenergy and liquid biofuels or plants not cultivated at present for commercial purposes, such as miscanthus, cardoon and reeds whose end product is intended for energy production and biofuels (Lal, 2005).

Energy crops are divided into two categories: annuals such as sweet sorghum (Sorghum bicolor L. Moench), fibre sorghum (S. bicolor L. Moench), kenaf (Hibiscus cannabinus L.), rapeseed (Brassica napus L.), Brassica carinata (B. carinata L. Braun), etc. and Perennials: I. Agricultural: Cardoon (Cynara cardunculus), reeds (Arundo donax L.), miscanthus (Miscanthus × giganteus), switchgrass (Panicum virgatum), etc. II. Forest: Eucalyptus (Eucalyptus camalduensis Dehnh. and E. globulus Labill.), black locust (Robinia pseudoacacia), etc. (CRES, 2006). Crop residue is defined as the non-edible plant parts that are left in the field after harvest.

The current IEA Bioenergy Task (Task 30) dealing with energy crop development is called short rotation Crops for Bioenergy Systems (Wright, 2006). Many different perennial and annual crops can be included under this heading.

The sunflower (Helianthus annus) is an annual plant in the family Asteraceae, with a large flower head (inflorescence). The stem of the flower can grow up to 3 m tall, with the flower head reaching 30 cm in diameter. The term “sunflower” is also used to refer to all plants of the genus Helianthus, many of which are perennial plants. Sunflower can be grown in any area with limited water availability. Sunflower is grown in many semi-arid regions of the world from Argentina to Canada and from central Africa into the Soviet Union. It is tolerant to both low and high temperatures but more tolerant to low temperatures. Sunflower will grow in a wide range of soil types from sands to clays. The demands of a sunflower crop on soil macronutrients are not as great as corn, wheat or potato. It can be found not only in Greece, Turkey, but also in 23 European countries (Venturia and Venturi, 2003, Demirbas, 2005, Demirbas et al., 2006). Sunflower is one of the leading oilseed crops cultivated for the production of oil mainly used for human consumption. It can also been considered as an important crop for biodiesel production, particularly in Southern European countries.

In Greece, sunflower is mainly grown in the Northern part of the country. At the present, the total cultivated area is 26,000 ha. The increased interest for utilizing oil seeds for biodiesel may encourage the sunflower cultivation (Kallivroussis et al., 2002). This cultivation however, has as result an important amount of residues remaining in the field. The production of biodiesel and bioethanol fuels is often accompanied by the co-production of lignocellulosic “waste” by-products. For example, the oil from sunflower seeds (that can be used to manufacture biodiesel fuel) has as “waste” co-products the sunflower shell and its stem (Antal et al., 2006). Those residues can be used for energy production or f bio carbon products.

Production of energy from residues would reduce atmospheric CO2 emissions increase associated with fossil fuel use. Thermochemical conversion can be applied to sunflower residues as well. Not all the residues produced, however, can be or should be used for bioenergy production. Indiscriminate removal of residue can lead to decline in soil quality with long-lasting adverse impacts on the environment (Lal, 2005). A study performed by Teerakun and Reungsang (2005) searching for plant species suitable for accumulating carbofuran residue in rice field soil, indicated that Helianthus annuus L. (sunflower) was the most suitable species for phytoremediation of carbofuran residue in rice field soil because it highly accumulated carbofuran up to 93.4 μg/kg dry weight in its stems and leaves. In addition, H. annuus L. (sunflower) could tolerate carbofuran since it showed similar physical appearance (circumference and height) to control not receiving carbofuran.

The present study is part of a research program aiming the integrated utilization of energy crops suitable to Greek conditions, for the biodiesel production and parallel gasification of the residues for energy production. The ultimate purpose of the program is the evaluation of the benefits to the agricultural sector from the cultivation of energy crops such as Sunflower for biofuels production and additionally the estimation of positive environmental impacts, since less CO2 will be emitted to the atmosphere by the use of biofuels. Sunflower heads (shells) without the seeds have been used for the experimentation. Thermogravimetric analysis was carried out for the purpose of sunflower ultimate analysis estimation and slow pyrolysis for kinetic purposes.

Section snippets

Potential crops for biodiesel production in Greece

Two biofuels are suitable for Greece: biodiesel and bioethanol. Especially for the production of biodiesel the available resources are: Seed oils (sunflower seed, soybean, rapeseed etc.) and used vegetable oils. Four oleiferous crops (groundnut, sesame, soybean and sunflower) are currently cultivated for their seeds. Among them, groundnut, sesame and soybean are cultivated in a relatively small area, while sunflower is cultivated in relatively larger area. These crops are traditionally used

Literature review

There are studies in the international literature on energy production from sunflower residues, mainly performed by Turkish researchers concerning pyrolysis not only of sunflower stalks but also sunflower by-products such as sunflower oil cake and oil bagasse (Gercel, 2002a, Gercel, 2002b, Pütün et al., 1996, Pütün et al., 2001, Yorgun et al., 2001a, Yorgun et al., 2001b, Demirbas, 2003a, Demirbas, 2003b, Demirbas, 2004, Demirbas, 2005, Demirbas, 2006, Demirbas et al., 2006). These studies

Materials

The sunflower shells studied in the present study were originated from central Greece, the area of Lamia, site Varka where 10 hectares were cultivated during the cultivation period of 2005–2006, in the frame of a pilot program funded by the Ministry of Education in Greece and EC and coordinated by the Agricultural University of Athens, (Kallivroussis et al., 2002). The field of 10 hectares has been previously used for cotton cultivation. The sunflower stem reached the high of 1.40–1.60 m and at

Kinetic modelling

Biomass pyrolysis is generally a complex process that is why it is difficult to discover kinetic models that explain the mechanism of thermal decomposition. In many of kinetic formulations of solid state reactions, it has been assumed that the isothermal homogeneous gas or liquid phase kinetic equation can be applied. So, for a single reaction at any time, the kinetic equation to describe the thermal decomposition can be written (Zabaniotou and Ioannidou, 2006)dwdt=k0exp-ERTf(w)where w is the

Results and discussion

A set of experiments of sunflower residues pyrolysis was performed at a temperature range of 300–600 °C. The heating rate was 40 °C s−1. Data on the effect of temperature on the yields of char, gas, oil and water from pyrolysis of sunflower residues are presented in Fig. 3. Fig. 3 shows that the char yield decreases with increasing temperature and above 436 °C it tends to a value of ∼32 wt.% of dry sunflower residue. On the other hand, the gaseous products increase with temperature tending to a

Conclusion

Pyrolysis of sunflower shells in a captive sample batch reactor has showed that the maximum gas yield of around 53 wt.% was obtained at 500 °C, whereas maximum oil yield of about 21 wt.%, was obtained at 400 °C.

A simple first order kinetic model has been applied to the devolatilization of sunflower shells. Kinetic constants have been estimated: E = 78.15 kJ mol−1; k0 = 1.03 × 103 s−1.

Acknowledgements

The authors acknowledge the General Secretariat for Research and Development (GSRT) of Greece and EC the financial support under the PYTHAGORAS II program. They are also grateful to Prof G. Papadakis and his team, from the Agricultural University of Athens (AUA) for providing the Sunflower shells and O. Ioannidou for his help in TGA.

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