Managing ash content and quality in herbaceous biomass: An analysis from plant to product

Abstract

In the bio-based economy, renewable herbaceous biomass such as straw and perennial grasses (e.g. Miscanthus, switchgrass and Reed Canary grass) will become important cellulosic feedstocks for conversion to biofuels, chemicals, electricity and heat. A significant fraction- up to one-fifth- of these herbaceous biomass consists of inorganic constituents, commonly referred to as ash, that can not be converted to energy. This paper reviews the occurrence and origin of mineral nutrients in biomass feedstocks and their impact on the whole production chain, including plant type, growing conditions, harvesting date and ¿method, storage, pre-processing, and conversion systems. The quantity and quality of ash in herbaceous biomass is shown to depend on a large amount of factors that all can be manipulated to a certain extent. Efforts to lower ash content or improve ash quality in herbaceous biomass however may have economic trade-offs that are case specific. An integrative approach to ash management in biomass delivery and conversion systems is proposed that will help develop strategies to reduce or overcome the impact of inorganic constituents in biomass conversion

Full text

MANAGING ASH CONTENT AND -QUALITY IN HERBACEOUS BIOMASS:
AN ANALYSIS FROM PLANT TO PRODUCT
Robert R. Bakker and H.W. Elbersen
Wageningen University & Research Centre (WUR)
Institute Agrotechnology & Food Innovations-Biobased Products
P.O. Box 17, 6700 AA Wageningen, the Netherlands
e-mail: robert.bakker@wur.nl
ABSTRACT: In the bio-based economy, renewable herbaceous biomass such as straw and perennial grasses (e.g.
Miscanthus, switchgrass and Reed Canary grass) will become important cellulosic feedstocks for conversion to
biofuels, chemicals, electricity and heat. A significant fraction- up to one-fifth- of these herbaceous biomass consists
of inorganic constituents, commonly referred to as ash, that can not be converted to energy. This paper reviews the
occurrence and origin of mineral nutrients in biomass feedstocks and their impact on the whole production chain,
including plant type, growing conditions, harvesting date and –method, storage, pre-processing, and conversion
systems. The quantity and quality of ash in herbaceous biomass is shown to depend on a large amount of factors that
all can be manipulated to a certain extent. Efforts to lower ash content or improve ash quality in herbaceous biomass
however may have economic trade-offs that are case specific. An integrative approach to ash management in biomass
delivery and conversion systems is proposed that will help develop strategies to reduce or overcome the impact of
inorganic constituents in biomass conversion.
Keywords: energy crops, ash, biomass resources, miscanthus, switchgrass
1 INTRODUCTION
In the bio-based economy, renewable herbaceous
biomass such as straw and perennial grasses will become
important cellulosic feedstocks for conversion to
biofuels, chemicals, electricity and heat. A significant
fraction of these herbaceous biomass consists of
inorganic constituents, commonly referred to as ash,
which can not be converted to energy. Not only the
amount of ash, also the composition can be of great
importance. Ash content can originate from the biomass
itself or from collection and pretreatment methods and
often lead to high disposal costs resulting in a negative
economic impact on biomass conversion systems. Only
in a few cases and under certain conditions, such as
thermal conversion of rice hulls, have biomass ash
residues become a marketable product. The negative
impact of inorganic constituents in herbaceous biomass
may be aggravated in biochemical conversion systems
(such as production of cellulosic ethanol), where the use
of inorganic chemicals during pretreatment adds to the
total amount of non-convertible inorganic residues in the
conversion system.
This paper reviews research on occurrence and origin
of mineral nutrients in biomass feedstocks in the whole
production chain, including the effects on ash content
and composition of:
• Plant type
• Plant fraction
• Growing conditions
• Harvesting time and -method
• Handling systems
• Pretreatment concepts (i.e. field leaching)
• Conversion system
The objective is to assess the evidence of the role of
ash in biochemical and thermal conversion systems. In
addition, methods to reduce or overcome the impact of
inorganic constituents will be reviewed covering the
whole production chain from agronomy, collection and
handling, to pretreatment and conversion to products.
2 OCCURRENCE AND ROLE OF INORGANIC
CONSTITUENTS IN HERBACEOUS BIOMASS
0
1
2
3
4
5
6
7
8
9
10
Miscanthus-
A
Miscanthus-
B
Switchgrass Rapeseed
straw
Grass seed
straw-A
Grass seed
straw-B
Wheat
straw
Reed
Canary
Grass
Ash content (%) (% )
Figure 1: Ash content (% d.w.) of 8 herbaceous biomass
grown on clay soils in the Netherlands (2005 harvest)
Figure 1 shows the ash content of 8 herbaceous
biomass types that were recently collected from
commercial farms in the Netherlands. All samples were
collected from crops grown on clay soils. Total ash
content (% d.w.) is shown to vary enormously, from 2%
for Miscanthus, to 8.5% for Reed Canary Grass. The
wide variability in ash is in itself a potential bottleneck
for biomass conversion, however, ash content alone
cannot predict the potential impact of ash in herbaceous
biomass types may have on thermo- or biochemical
conversion.
In order to understand better why and at what level
specific inorganic constituents are found in herbaceous
biomass, Table I reviews the occurrence of 5 main ash
components (Si, K, Ca, S and Cl) that are generally
thought to have an impact on biomass conversion system.
Silicon has several beneficial effects to a large number of
plant species. It increases leaf erectness, resulting in a
reduced susceptibility against lodging. In addition, a
high content of silicon in leaves increases the resistance
of the tissue against fungal attacks, blast infection, and
insect pests. Potassium is the macronutrient required by
plants in the largest amount after nitrogen [1]. The

potassium requirement for optimum plant growth is in
the 1-5% dry matter weight range, depending on species,
while the potassium concentration in mature plants
generally does not exceed 2% of dry matter. Potassium
is characterized by its high mobility in plants at all
levels, including between individual cells, between
tissues, and in long-distance transport within the plant
[2]. Potassium is not metabolized and forms only weak
complexes in which it is readily exchangeable.
Potassium is involved in a large number of essential
processes for plant growth, including enzyme activation,
protein synthesis, photosynthesis, regulation of osmotic
pressure, vascular transport, and cation-anion balance
[3]. K-deficiency in plants may lead to a variety of plant
growth-inhibiting factors, including wilting and
sensitivity to drought stress, increased lodging, and
increased susceptibility to frost damage and fungal
attack. Calcium content of plants varies between 0.1 and
> 5.0% of dry weight depending on growing conditions,
plant species and organ [2]. Calcium might be firmly
bound to plant structure or is exchangeable at the cell
walls. Like potassium, calcium is a nutrient that has an
important role in many essential processes including
binding form, cell wall stabilization, secretory processes,
membrane stabilization and regulation of osmotic
pressure. Sulfur is a macronutrient required for plant
growth and is increasingly being recognized as the fourth
major plant nutrient after nitrogen, phosphorus and
potassium [4]. Sulfur is incorporated into many organic
structures, including amino acids, proteins, and enzymes.
It is taken up in particular by the roots in the form of
sulfate, SO42-. The majority of sulfate in the plant is
reduced for incorporation into organic molecules,
however some sulfate can be utilized without reduction
[2]. The functions of sulfur in plant growth are closely
related to the functions of the various organic structures
of which sulfur is a constituent. The sulfur requirement
for optimal plant growth varies between 0.1 and 0.5% of
dry plant weight, and varies among plant species. In
general, graminae require less sulfur for plant growth
than leguminosae.
Table I: Occurrence of inorganic constituents in biomass
(% dry plant matter)
Occurrence (%)
Silica 0.5 - 15%
Potassiuma 1 – 2 %
Calciumb 0.1 - 5.0%
Sulfur 0.1 – 0.5%
Chlorine 0.2 - 2.0%
Notes: a. In young plant shoots, up to 5% potassium may
be found, b. in mature leaves, calcium might reach more
than 10%
Chlorine is a naturally abundant element and is taken up
by plants in the form of chloride ion, Cl-. Chlorine in
plants occurs mainly as a free anion or is loosely bound
to exchange sites. In addition, a number of chlorinated
organic compounds have been found in plants, however,
the functional requirement for plant growth of most of
these compounds is not known [2]. Chlorine plays an
essential role in a number of processes, including the
opening and closure of stomata. Similar to potassium,
chloride has high mobility within the plant, and average
chlorine content in plants ranges from 0.2 - 2.0% of dry
plant weight. The supply of chlorine can be from a
variety of sources including irrigation water, rain,
fertilizers, and air pollution generally exceeds the plant’s
growth requirements, and therefore concerns exist about
chlorine toxicity in plants rather than chlorine deficiency.
3 FACTORS AFFECTING ASH QUANTITY AND –
QUALITY IN HERBACEOUS BIOMASS
In the following section, main factors in the entire
(primary) production to conversion chain are reviewed
that affect the ash quantity and –quality in herbaceous
biomass.
3.1 Plant type and -species
C3 plants (e.g. wheat, sorghum, reed canary grass)
exhibit a higher yield potential in temperate and cold
climates and need more water to produce a similar
amount of plant dry matter as compared to C4 plants. As
a result, C3 plants generally contain a higher ash
concentration as water uptake is directly related to
deposition and uptake of Si and other inorganic
constituents in plant biomass.
C4 plants (sugar cane, maize, Miscanthus,
switchgrass) have a higher yield potential in
Mediterranean and warm climates and use on average
half as much water as C3 plants. As a result, C4 species
exhibit lower ash concentrations compared to C3 plants
(see also 3.3 growing conditions).
3.2 Plant fractions
The distribution of ash and specific inorganic
components in herbaceous biomass may vary
significantly among different plant fractions. For
example, Summers et al. [5] determined total ash and
silica in different botanical fractions of rice straw (leaf,
stem, node, panicle) and concluded that ash and silica
content varied significantly among straw fractions: leaves
contained 18-19% total ash (of which 76% consisted of
silica), whereas stems only contained 12% ash (with 42%
silica). Distribution of inorganic constituents among
plant parts is often specific and can have a direct impact
on the application of the biomass type. For instance rice
hulls, a byproduct of rice grain processing and a high
ash-high silica material, are generally considered a good
biomass fuel for combustion, whereas rice straw is
considered a difficult fuel due to the combination of high
ash, high silica, and high alkali content (leading to ash
agglomeration) in the material.
3.3 Growing conditions
For any given species, soil type, and in particular its
texture, is a very important factor in deposition of
inorganic constituents in biomass. Elbersen et al. [6]
determined total ash and nutrient content of 5 switchgrass
varieties on a clay and a sandy soil in the Netherlands
(Figure 2). Switchgrass grown on sandy soil consistently
showed lower ash (51-73% reduction compared to clay
soils) and potassium content (16-44%), whereas results for
chlorine were variable. The difference in total ash content
among these soil types can be largely explained by the
higher soluble silica level in clay soils, which results in
higher ash levels in crops grown on clay soils.

0
1
2
3
4
5
6
7
Ash (%) K (%) Cl (%) Ash (%) K (%) Cl (%)
Clay soil Sandy soil
Forestburg Summer CIR Blackwell Carthage
Figure 2: Ash, K and Cl content of 5 switchgrass
varieties grown on a clay and a sandy soil in the
Netherlands (1999-2000 harvest)
As discussed earlier, ash content is also highly
influenced by water uptake by the plant-this explains for
instance the high ash content in wetland species (e.g. rice)
and low ash content in C4 species (e.g. Miscanthus) that
are characterized by a high water use efficiency. Finally,
the type and amount of fertiliser affects ash content and –
quality in herbaceous biomass, in particular with regards to
K- and Cl-containing fertilizers.
3.4 Harvest
Both the total amount of ash as well as specific
inorganic constituents in herbaceous biomass can be
manipulated by the timing of harvesting. Extending
harvest dates later in the season generally leads to lower
ash content. A number of constituents (e.g. K, Cl) are
particularly reduced due to effects of increased
senescence and translocation (plant nutrients are removed
from leaf and other tissues to under-ground parts), and
leaching (removal of soluble constituents by rain, mist or
dew). The beneficial effects of leaching on combustion
characteristics have been described for many herbaceous
biomass types, including rice and wheat straw [8], reed
canary grass [9], banagrass [10], and others. Efforts to
include a time window to allow for leaching by natural
means (e.g. rain, dew) are generally referred to as
delayed harvesting, spring harvest, or field leaching.
Delaying harvest however can also have important trade-
offs, such as a high loss of plant matter (which reduces
yields considerably) or an increase in total ash (due to
losses of organic matter) [11].
The selection of mechanical harvesting techniques
may affect ash content and composition as well, in
particular in field harvest operations that include
swathing, raking or curing the biomass prior to
collection, which is often performed to enhance field
drying or optimize harvest operations. Swathing or
raking may increase the amount of soil particles in the
biomass, which add to total ash composition [11]. For
many biomass types including cereal straws, field
operations prior to biomass collection (e.g. irrigation,
combine harvesting of cereals) should be tuned to avoid
submersion or flattening of straw, as both may lead to
contamination with soil.
3.5 Handling and Storage
As with harvest, handling after field operations may
lead to increasing inorganic constituents in herbaceous
biomass. So far, most of the experience with handling
and storage of biomass has been with woodfuels which
have inherently low ash content (0-1% d.w.). Given the
low ash content, the contamination with extraneous
matter in these fuels can be relatively easily monitored
by performing total ash analysis. In herbaceous biomass
however, evidence of contamination is more difficult to
determine without specific elemental ash analysis which
is normally not available at conversion facilities, or too
expensive to carry out at a regular basis.
Impacts of long-term storage of herbaceous biomass
on ash are generally believed to be small, as long as the
biomass is stored at sufficiently low moisture
contents(<20% d.w.). However, if microbial degradation
in stored biomass occurs, the loss of organic matter will
lead to an increase in inorganic constituents on a
volumetric basis. For wet biomass streams, ensilage
techniques have been proposed to allow for longer term
storage prior to conversion, however effects on specific
inorganic constituents and total ash have so far not been
analyzed.
3.6 Pre-processing
Given the beneficial effects of leaching of inorganic
constituents (see 3.3) from biomass on ash quality, a
number of authors have proposed a controlled washing or
leaching step prior to conversion to remove troublesome
elements from herbaceous biomass. Techniques might
include submersion in water, dewatering, and/or drying.
Removing elements after initial, primary conversion (e.g.
char wash) has been proposed as well [12]. For leaching
of wet-harvested, chopped banagrass for thermal
conversion, Turn et al. [13] proposed a combination of
treatments that include crushing, imbibition (i.e. adding
water to the crushed biomass to facilitate extraction), and
dewatering. For leaching of wheat straw prior to
combustion, Knudsen et al. [14] proposed a process
similar to cane diffusion. Constraints for commercial
application of leaching as a pre-processing step are the
incremental costs of leaching, high water requirements,
and reduction of conversion efficiency due to higher fuel
moisture content. Applications where leaching can be
integrated into an existing process would seem
beneficial, and may be more feasible from a practical
point of view. The processing or milling of sugar cane is
an example of such a process: leaching of potassium and
other elements occurs along with the extraction of sugar.
3.7 Thermochemical and biochemical conversion
It is beyond the scope of this paper to fully assess the
impact of inorganic constituents on biomass conversion
systems. For thermal conversion systems, the impact of
total ash and specific inorganic ash components on
combustion or gasification has been extensively reported
on [15, 16, 17].
For biochemical conversion systems, impact of ash
and ash composition on conversion systems has not yet
been widely studied. Specific constituents in herbaceous
biomass may lead to fermentation inhibition (depending
on the tolerance of the microorganism and the dry matter
loading during fermentation), and a higher total ash
concentration will also impact the quality of the non-
fermentable ligneous residue that is made available for
thermal conversion. In Figure 3, an estimate is made of
total ash content in the ligneous residue that is extracted
following pretreatment, enzymatic hydrolysis and
fermentation of the carbohydrate fraction. Ash content is
shown to be directly related to ash content in the biomass

and may reach up to 20% d.w., and is dependent on the
amount of inorganic constituents added to the system
during pretreatment (e.g. sulfate). The amount of ash in
the ligneous residu is also dependent on the selected
upgrading technique for lignin: drying lignin will
inevitably lead to a higher ash content then dewatering as
soluble salts remain in the solid phase.
0
5
10
15
20
25
246810
Ash in Feedstock (% dm)
Ash in ligneous residu
2.0%
1.0%
0.5%
inorganic
matter
added for
pretreatment
:
Figure 3: Total ash composition in ligneous residue
following pretreatment, enzymatic hydrolysis and
fermentation of herbaceous biomass
4 CONCLUSIONS AND RECOMMENDATIONS
The quantity and quality of ash in herbaceous
biomass depends on a large amount of factors including
plant type, plant fraction, growing conditions,
fertilisation, choice of harvest date, harvest techniques,
and conversion systems. Most of these factors can be
managed to a certain extent to reduce total ash in biomass
or to remove undesirable inorganic components, however
measures to improve ash quality in herbacous biomass
however may have economic trade-offs that are case
specific. An integrative approach to ash management in
biomass conversion systems is needed that will help
develop strategies to reduce or overcome the impact of
inorganic constituents in biomass conversion. Efforts
should be made to integrate this approach with beneficial
uses of ash derived from biomass, including the potential
for recycling of nutrients to the field.
5 LITERATURE
[1] Wallingford, W., 1980. Functions of Potassium in
Plants, In: Potassium for Agriculture, Potash and
Phosphate Institute, Atlanta, Georgia
[2] Marschner, H., 1993. Mineral Nutrition of Higher
Plants, 2nd edition; Academic Press, London,
United Kingdom
[3] Munson, R.D., 1980. Potassium availability and
uptake, In: Potassium for Agriculture, Potash and
Phosphate Institute, Atlanta, Georgia
[4] Tandon, H.L.S., 1991. Sulphur research and
Agricultural Production in India, 3rd revised edition,
The Sulphur Institute, Washington D.C.
[5] Summers M.D. et al, 2001. Properties of Rice Straw
as Influenced by Variety, Season and Location,
ASAE paper number 01-6078, ASABE, St. Joseph,
MI, USA
[6] Wallingford, W., 1980. Functions of Potassium in
Plants, In: Potassium for Agriculture, Potash and
Phosphate Institute, Atlanta, Georgia
[7] Elbersen H.W. et al, 2002. Switchgrass as an
alternative energy crop in Europe, Final report FAIR
5-CT97-3701. www.switchgrass.nl
[8] Jenkins B.M. et al, 1996. Biomass and Bioenergy
10:243-260
[9] Burvall, 1997. Biomass and Bioenergy 12: 149-154.
[10]Turn, 1997. Biomass and Bioenergy 12: 241-252
[11]Bakker R.R. et al, 2004. Biomass and Bioenergy 25:
597-614
[12]Jensen P.A. et al, 2001. Biomass and Bioenergy 20:
431-446
[13]Turn et al. Removal of inorganic constituents of
fresh herbaceous fuels: processes and costs,
Proceedings Third Biomass Conference of the
Americas, Montreal.
[14] Knudsen N.O. et al, 1998. Possibilities and
evaluation of straw pretreatment, In: Biomass for
Energy and Industry, Proceedings of the International
Conference, Wurzburg, Germany
[15] Miles et al, 1993. Alkali slagging problems with
biomass fuels, in: Proceedings of the First Biomass
Conference of the Americas, August 30-September 2
in Burlington, VR, National Renewable Energy
Laboratory, Golden, Colorado
[16] Baxter L.L. et al, 1993 Biomass and Bioenergy
4(2):85-102.
[17] Jenkins B.M. et al, 1996. Combustion
Characteristics of Leached Biomass, In: Bridgewater,
A.V. and D.G.B. Boocock (eds.), Developments in
Thermochemical Biomass Conversion, Blackie
Academic and Professional, London, pp.1316-1330