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Mineral composition and ash content of six major
energy crops
Andrea Monti
a,
Ã
, Nicola Di Virgilio
b
, Gianpietro Venturi
a
a
Department of Agroenvironmental Science and Technologies (DiSTA), University of Bologna, Viale G. Fanin, 44, 40127 Bologna, Italy
b
Institute of Biometeorology, National Research Council, Via P. Gobetti, 101 I, 40129 Bologna, Italy
article info
Article history:
Received 30 January 2007
Received in revised form
4 September 2007
Accepted 21 September 2007
Available online 25 October 2007
Keywords:
Miscanthus
Arundo
Sorghum
Switchgrass
Giant reed
Cynara
Ashes
Mineral
Bioenergy
abstract
The chemical composition of biofuels has not received adequate attention given that it is
an important aspect in the introduction of energy crops. In this study, the ash content and
mineral composition (C, N, Al, Ca, Cl, Fe, K, Mg, Na, P, S, Si) of stems, leaves and
reproductive organs of some promising energy crops were determined and compared with
the respective recommended thresholds reported in literature. Overall, cynara exhibited
the highest ash and mineral contents, which indicate high slagging, fouling and corrosion
tendencies. However, cynara also showed the lowest Si content, both in leaves (4.3 g kg
À1
)
and in stems (0.9 g kg
À1
).
Sweet sorghum and giant reed exhibited the highest N content (up to 16 g kg
À1
), which
greatly exceeded the recommended limits in leaves. Importantly, Cl always exceeded the
recommended limits (up to 18 mg kg
À1
in cynara), both in stems and in leaves, thus
resulting in a major stumbling block for all crops. Several significant correlations among
elements were found at a single plant part; conversely these correlations were generally
very weak considering different plant components, with the exception of K (r ¼ 0.91**),
P(r ¼ 0.94**) and ashes (r ¼ 0.64**). Generally, leaves resulted in a significant deterioration of
biofuel quality when compared with stems and flower heads. Therefore, agricultural
strategies aimed at reducing the leaf component (e.g. by delaying the harvest) may
considerably improve the suitability of biofuels for current combustion plants.
& 2007 Elsevier Ltd. All rights reserved.
1. Introduction
The development of energy crops is a central aim of the
European Commission’s energy policy [1], with million
hectares being expected to be allocated to dedicated energy
crops in the short term. So far, much effort has been
addressed to evaluate the potential yield of dedicated crops
[2–5], while the quality of biofuels have not received as much
attention [6,7]. Biomass quality can drastically lower the net
energy output, both limiting the effectiveness of conversion
plants [7] and decreasing the heating value. It has been
demonstrated that heating values are negatively related to
ash content, with every 1% increase in ash concentration
decreasing the heating value by 0.2 MJ kg
À1
[8]. Furthermore,
ashes and inorganic elements (e.g. alkali) produced during
combustion may cause a number of serious problems to
power plants through slagging, corrosion and fouling. The
basic mechanisms of these processes have been investigated
and are now reasonably well understood [9]. Fouling is
generally defined as the accumulation of unwanted materials
on the surfaces of processing equipment leading to a decrease
in the exchanger efficiency; slagging is related to the low
melting point of deposits, which causes the formation of a
glassy layer that must be removed. Finally, corrosion is
caused by the interaction between deposits and metal surface
of the exchanger, which involves extra costs in maintenance,
ARTICLE IN PRESS
0961-9534/$ - see front matter & 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biombioe.2007.09.012
Ã
Corresponding author. Fax: +39 51 2096241.
E-mail address: a.monti@unibo.it (A. Monti).
BIOMASS AND BIOENERGY
32 (2008) 216– 223