Biomass – Biological Hydrogen and Electricity Generation Potential

Proceedings International Hydrogen Energy Congress and Exhibition IHEC 2007
Istanbul, Turkey, 13-15 July 2007
Biomass . Biological Hydrogen and Electricity Generation Potential
Olga A. Bereketidou1, 2 and Maria A. Goula1*
1Pollution Control Technologies Department, TEI of Western Macedonia, Greece
2Engineering& Management of Energy Resources Dept., University of Western
Macedonia, GR
* corresponding author: mgoula@kozani.teikoz.gr
ABSTRACT
The need to address waste streams and environmental concerns about our current
fossil-based energy system has provided new incentives for using biomass to produce
energy. Mill residues and other wood residues are used to generate electricity,
avoiding landfill disposal costs while generating power for onsite use. Agricultural
residues are used as biomass power feed stocks as a waste control strategy and have
been encouraged, in part, to reduce the air quality impacts of open-field burning. The
pulp and paper industry has been using on-site power systems to recover valuable
chemicals from the black liquor and generate steam and electricity for the plant.
Landfill and manure methane projects utilize methane that would otherwise be vented
or flared, while displacing the need for conventional electricity generation. Using
biomass for energy generation also offers a number of other benefits such as
greenhouse gas reduction and air-quality benefits compared to open burning and coalfired
power plants. Additionally, because many biomass feed stocks are concentrated
in rural areas, biomass energy facilities can provide rural economic development
benefits by creating jobs and tax revenues. Finally, biomass energy offsets fossil fuel
consumption and helps to diversify the nationfs energy supply mix.
As a country, Greece is rich in natural resources, a majority of which are in the
agricultural and forestry sectors. As a consequence of these resources, significant
quantities of residual biomass are available. A study was conducted to determine the
total amount of biomass-derived hydrogen and electricity that could be produced in
Greece from its energy crops and residual biomass. Biological production of hydrogen
using wastewater and other biomass as raw material has been attracting attention as an
environmentally friendly process that does not consume fossil fuels. Additionally, the
percents of todayfs gasoline consumption and electricity consumption were
calculated, as well as the resulting reductions in greenhouse gas emissions.
Keywords: bio-hydrogen production, biomass utilization, electricity generation
Goula and Bereketidou
1. INTRODUCTION
Dependence on fossil fuels as the main energy sources led to serious energy crisis and
environmental problems, resulting from fossil fuel depletion, pollutant emissions and
global climate change. Therefore, it is extremely important to explore the
opportunities for clean and renewable energy for long . term supply. Hydrogen offers
tremendous potential as a clean, renewable energy currency. It is an environmentally
ideal fuel, producing only water upon combustion and it is believed to replace fossil
fuels as the energy source of next generation [1]. The major problem in utilization of
hydrogen gas as a fuel is its unavailability in nature and the need for inexpensive
production methods. Hydrogen can be produced sustainably by electrolysing water
using electricity from renewable energy sources (e.g. wind, marine and solar), through
the utilisation of energy crops and waste streams or other methods [2]. Biomass has
the potential to contribute to the mix, provided appropriate technologies can be
developed for its conversion to hydrogen. Pyrolysis, gasification, bio-photolysis,
biological water gas shift reaction and digestion are feasible procedures for hydrogen
production from biomass and a great deal of interest is expected to be focused on
them in the near future, concerning hydrogen production [3, 4]. The thermo chemical
pyrolysis and gasification hydrogen production methods are economically viable and
will become competitive with the conventional natural gas reforming method. Woody
biomass with low water content is suitable for gasification/pyrolysis while wet
biomass with high carbohydrate content can be converted to hydrogen and organic
acids through the action of fermentative bacteria. In contrast to biomass typically used
in thermo chemical processes, substrates for fermentation need to be low in lignin,
high in carbohydrates and have high moisture content. The ideal energy crop would
be characterised by the highest possible yield (dry matter per hectare) with the
maximum carbohydrate content, low energy input to produce the crop, low cost of
production and low nutrient requirements [5]. These features can be dependent on
climate, soil, water consumption, pest resistance and fertiliser requirements which are
country specific.
Figure 1 Possible biomass utilization
Goula and Bereketidou
2. BIOLOGICAL HYDROGEN
Biological production of hydrogen using wastewater and other biomass as raw
material has been attracting attention as an environmentally friendly process that does
not consume fossil fuels [6]. Biological systems, producing biohydrogen, include
direct biophotolysis, indirect biophotolysis, photo . fermentations and dark
fermentation [7].
2.1. Direct photolysis
The process uses the photosynthetic capability of green algae and cyanobacteria to
split water by the directly absorbed light energy and concomitant transfer of electrons
to a hydrogenase or a nitrogenase for H2 production. In order to collect enough
energy, large bioreactor surface area is needed, which increases the cost for
production of hydrogen fuel. Photosynthetic energy conversion efficiency can be as
high as 10%. This value is close to the practical maximum value because the overall
process is limited primarily by the efficacy of the dark reaction and not by the light
energy capture process. A great proportion of the absorbed photon energy is wasted
by the photosynthetic apparatus and dissipated as heat or fluorescence. Efficiencies
close to 10% value have been demonstrated under low-light and low oxygen partial
pressure conditions [8].
2.2. Indirect biophotolysis
Indirect biophotolysis [9] has the advantage of separating in time the O2 and H2
evolution steps. It involves a photosynthetic biomass production step and an anaerobic
dark fermentation of the biomass to produce H2. If the oxygen and hydrogen evolution
steps can be separated in time, the apparently inherent O2 sensitivity of the H2
evolving process is circumvented. Several models to achieve indirect biophotolysis
have been developed. These systems use algae in most cases and intend to exploit
their capability to produce high biomass yield per surface area photoautotrophically,
to be able to carry out dark fermentation as well as photoheterotrophic growth.
2.3. Dark fermentation
Fermentative processes can use biomass ultimately obtained in a photosynthetic solar
energy conversion system. Much is presently known about the molecular biology of
the H2 producing enzymes [10] and metabolic engineering to direct fermentation to H2
production is scientifically feasible [11]. Dark microbial H2 production is driven by
the anaerobic metabolism of the key intermediate, pyruvate. The complete oxidation
of glucose would yield a stoichiometry of 12 mol H2 per mole of glucose but in this
case no energy is gained to support growth and metabolism of the producing organism
[12]. Under carefully chosen conditions thermophiles produce up to 60.80% of the
theoretical maximum demonstrating that higher hydrogen yields can be reached by
extremophiles than using mesophilic anaerobes [13].
2.4. Photofermentation
Photosynthetic bacteria have long been studied for their capacity to produce
significant amounts of hydrogen [14]. The advantage of their use is in the versatile
metabolic capabilities of these organisms and the lack of PhotosystemII, which
automatically eliminates the difficulties associated with O2 inhibition of H2
production. Phototrophic bacteria require organic or inorganic electron source to drive
their photosynthesis. This is a small disadvantage because they can utilize a wide
range of cheap compounds. The significant disadvantages in these systems are due to
mostly of their use of the nitrogenase enzyme as H2 generation catalyst (see above), to
the requirement for elaborate and expensive anaerobic photobioreactors covering
large areas, and to the low photosynthetic efficiencies.
Goula and Bereketidou
3. DARK FERMENTATION
3.1 Process Description
Hydrogen can be produced sustainably by dark, anaerobic bacterial growth on
carbohydrate-rich substrates, resulting in organic fermentation end products, H2 and
CO2. The fermentation end products are suitable for anaerobic digestion, yielding
methane [15]. Fermentation reactions can be operated at mesophilic (25 . 400C),
thermophilic (40 . 650C), extreme thermophilic (65 . 800C) or hyperthermophilic
(>800C) temperatures. Pure cultures known to produce hydrogen from carbohydrates
include species of Enterobacter, Bacillus and Clostridium [15]. Carbohydrates are the
proffered organic carbon source for hydrogen . producing fermentations. Glucose,
isomers of hexoses or its polymers in the form of starch or cellulose, result in different
amounts of hydrogen per mole of glucose, depending on the fermentation pathway
and end . products [15].
Glucose in biomass gives a maximum yield of 4 mole H2 per mole of glucose when
acetic acid is the end . product:
C6H12O6 + 2H2O2¨ 2CH3COOH + 2CO2 + 4H2 (1)
When butyrate is the fermentation by . product, a theoretical maximum of 2 moles H2
per mole of glucose is obtained:
C6H12O6 + 2H2O¨ CH3CH2CH2COOH +2H2 +2CO2 (2)
The optimum H2 yield can be achieved with acetate as the fermentation end .product.
In practise, high hydrogen yields are associated with a mixture of acetate and butyrate
fermentation products and low hydrogen yields are associated with propionate and
reduced end products, such as alcohols [15]. Vanillin [16] gave the overall equation
for the production of propionate from hexose. The equation involves the consumption
of hydrogen and should be avoided:
C6H12O6 + 2H2¨ 2CH3CH2COOH +2H2O (3)
Hydrogen acts as an acceptor of surplus electrons during acetate/butyrate formation.
In the case of micro flora in anaerobic digestion sludge, the amount of hydrogen gas
recovered is lower than in the case of the micro flora in sludge compost, due to the
consumption of produced hydrogen as a reducing agent in the formation of methane.
Hydrogen production by anaerobic bacteria is depending on the process conditions,
such as pH, hydraulic retention time (HRT) and gas partial pressure. Therefore, the
fermentation products produced by a bacterium depend on the environmental
conditions in which it grows.
In dark fermentation processes, a major drawback is the fact that the gas produced is a
mixture of primarily H2 and CO2, but may also contain other gases, as CH4, H2S or
ammonia. To maintain continuous H2 synthesis and remove diluting (CO2, CH4), and
contaminating CO gases, rapid removal of the gases and purification of the H2 are
essential. Dark fermentation systems appear to have the potential to become practical
bio hydrogen systems. Thus various improvements should be made through gas
removal and separation, reactor design and genetic modifications in the micro
organisms. The produced hydrogen can be used in an internal combustion engine or a
fuel cell for electricity production.
3.2 Bio hydrogen from cellulose by anaerobic bacteria
Cellulose is the most abundant biopolymer on earth and it is the major component of
plant biomass [17]. Lignocellulose in plant cell walls contains cellulose,
hemicellulose and lignin. Cellulose and hemicellulose content of wastes can be
hydrolyzed to carbohydrates which are further processed for organic acid and
hydrogen gas production [18].
Goula and Bereketidou
The conversion of cellulose to hydrogen by microbial fermentation is an answer to the
depletion of hydrocarbon fuel reserves and carbon dioxide release. Cellulose acts as a
substrate for hydrogen production by anaerobic heterotrophic fermentation. Hydrogen
is a major intermediate in anaerobic fermentation by fibrolytic and fermentative micro
organisms. Recent researches introduce bio hydrogen generation from cellulose by
mixed batch cultures, using natural anaerobic micro organisms, obtained from
digested sludge and sludge compost [18, 19]. Anaerobic micro flora in sludge
compost which is made by forced aeration of aerobic activated sludge converts
cellulose to hydrogen with high efficiency (2, 4 mol per mole hexose) [19]. The
absence of methane to the evolved gas is due to the lack of methanogenic activities in
the sludge [17].
In the case of sludge compost, Ueno [19] observed rates of evolved gas production of
up to 3.325 ml/l-culture, with a gas composition of 58 % in H2, after 120 h of
cultivation. Carbon dioxide composition was 42 % and no methane was detected. The
composition of cellulose was 10 g/l and was calculated as hexose. Thus, the rate of H2
production was estimated to193 m3/t-cellulose. Micro flora in sludge compost may be
the most useful micro organisms in the anaerobic production of hydrogen from
biomass resources such as cellulose.
3.3 Hydrogen from cellulosic biomass feedstocks
Biological methods offer distinct advantages for hydrogen production such as
operation under mild conditions and specific conversions. However, raw material cost
is one of the major limitations for bio-hydrogen production. Utilization of some
carbohydrate rich cellulose containing solid wastes is an attractive approach for biohydrogen
production. Cellulosic biomass feedstocks that have been pre-treated can be
used for hydrogen production. Acid and enzymatic hydrolysis breakdown the
cellulose and hemicellulose fibers into constituent sugars that may be easily
metabolized by H2 producing bacteria [20]. It is extremely important that biomass use
for bio . energy is related to significant economic and environmental benefits.
Unused, discarded biomass residues could become an energy resource, competitive to
fossil fuels.
4. RESULTS AND DISCUSSION
4.1. Calculation of waste biomass in Greece
The main biomass resources in Greece are:
Residues from: agriculture in .field, forestry activities, agro- industries and municipal
waste water treatment.
Energy crops: During the last decades, several energy crops have been tested under
Greek climatic conditions. High biomass yields, up to 30 odt/ha/year have been
observed in experimental trials [21]. The quantities of residues from agricultural crops
in Greece in tones of dry matter per year were estimated using data from the annual
Agricultural Statistics, on the cultivated areas and the quantities of the main product
produces per year for each crop for the years 1996 . 1998 [21].
The main types of agro industries in Greece are: rice industries, cotton-ginning
factories, corn industries, fruit industries, wine factories, seed oil industries, olive oil
and olive kernel factories [21]. The main types of agro-industrial residues that can be
used for energy production are the residues from the fruit canneries, rice mills, olive
oil and olive oil kernel factories and the cotton ginning factories.
The annual production of agro- industrial residues in Greece in dry tones per year was
estimated. Greece, located at the southern end of the Balkan Peninsula, is mostly hilly
or mountainous and dry and rocky country.
Goula and Bereketidou
Forest biomass resources will represent the most important biomass resource in the
long term in Greece. Forestlands occupy an area of 6.513.068 hectares, which
represent the 49, 3 % of the total Greek land area. The 90 % of the total fuel wood
production derived from broadleaved species and the rest from conifers [21].
Most of the studied crops performed high yielding potential under Greek climatic
conditions. However, differences have been observed so far depending on the crop
species, the climate and the cultural practices. The annual herbaceous energy crops
contain sweet sorghum with dry matter yields from 13 to 45t/ha, fiber sorghum with
high dry matter yields, up to 27t/ha, kenaf, with dry matter from 7,6 to 23,9t/ha and
rapeseed with dry matter yields from 3 to 8t/ha.
Table 1 Biomass potential in Greece
Source of biomass Quantity (DT/year)
Agriculture: crop residues 3, 8 E 106
Agro-industrial 5, 94E 105
Fuel wood 1, 4 E 106
Municipal: MWWT 3, 16 E 105
Energetic cultivationa 23 E 106
Total 29, 1 E106
aIn Greece the total agricultural land is about 3.8 million ha, from which about 60 % is arable
area and can give an average yield of about10t/ha.
4.2. Potential of hydrogen production
The calculation of the fermentable substrate for H2-production was based on the
assumed cellulose and hemicellulose content in total dry weight for each category of
biomass. The volume of hydrogen derived from all biomass residues was based on
H2-production rate of 193m3/t-cellulose (Ueno, 19).
In agricultural crop and agro-industrial residues, the rate of hydrogen production was
based on an assumed cellulose content of 40 % for the fermentable substrate. In
forestry residues the cellulose content was assumed as 40 % in total dry weight. For
the MWWT the fermentable substrate was based on an assumed cellulose content of
40 %. Energy crops contain large amounts of cellulose, up to 13 % [22].The density
of H2 in standard conditions is 0, 09 Kg/m3, which is used to calculate the total mass
of H2 produced.
Table 2 Potential of hydrogen production from residual biomass . energy crops in
Greece
Total biomass Usable substrate Volume H2 Mass H2 Heating value
(DT/yr) (t/yr) (m3/yr) (t/yr) (GJ/yr)b
Crop residues 3, 8 E 106 1, 5E 106 2, 89 E 108 2, 6E104 3, 7 E 106
Agro-industrial 5, 94E 105 2, 37E 105 4, 57 E 107 4, 1E102 5, 82 E 104
Fuel wood 1, 4 E 106 5, 5 E 105 10, 6 E 107 9, 6 E103 1, 4 E 106
MWWT 3, 16 E 105 1, 26 E 105 2, 43 E 107 2, 2 E 103 3, 12 E 105
Energy crops 23 E 106 2, 99 E 106 57, 7 E 107 5, 19 E 104 7, 4 E 106
Total 29, 1 E 106 5, 4 E 106 10, 4 E 108 9E 104 6, 2E 106
b For H2, the gravimetric heating value used for calculations, was 142 GJ/t
Goula and Bereketidou
4.3. Gasoline displacement with biomass . derived hydrogen
The annual gasoline consumption in Greece is approximately 1,2 billion gallons (data
from energy. ga.gov/gasoline/statistics/manage consumption by country).On a lower
heating value basis, the energy content of a gallon of gasoline is approximately equal
to the energy content of a kilogram of hydrogen. The amount of gasoline that could be
displaced by biomass . derived hydrogen can be calculated using the amount of
hydrogen for transportation applications, the ratio of efficiency of hydrogen use to
gasoline use and the total amount of gasoline consumed in Greece [23]. The produced
hydrogen can be used in an internal combustion engine or a fuel cell for electricity
production. The efficiency in fuel cell is up to 66 %, compared to the efficiency in
internal combustion engine, which is about 33 %.The amount of CO2 emissions from
gasoline- burning mobiles is equal to 9, 1 g per gallon of gasoline consumed. No CO2
is produced by hydrogen combustion, therefore the direct CO2 emissions savings are
equal to 9, 1 g per gallon of gasoline used [23]. Table 3 shows the amount of gasoline
that could be displaced by hydrogen and CO2 reductions for hydrogen fuel, assuming
that the future hydrogen fuel in fuel cell vehicle has the double efficiency, comparing
to the efficiency of todayfs gasoline vehicles.
Table 3 Hydrogen potential, amount of gasoline that could be displaced and CO2
reductions for hydrogen future fuel for transportation.
Total biomass Hydrogen potential %gasoline could CO2 reductions
(t/yr) (Kg/yr) be displaced for hydrogen fuel (t/yr)
Crop residues 1, 5 E 106 2, 6E107 4 % 452, 3
Agro-industrial 2, 37 E 105 4, 1E105 0,065 % 7, 4
Fuel wood 5, 5 E 105 9, 6 E106 1, 5 % 169, 70
MWWT 1, 26 E 105 2, 2 E106 0, 4 % 45, 22
Energy crops 2, 99E 106 5, 19 E 107 8, 5 % 961,097
Total 5, 4E 106 9 E107 15 % 1368,618
Assuming that the efficiency of hydrogen car is twice the one of gasoline fuelled car
4.4. Electricity generation potential from biomass
Today lignite is a precious natural resource and this most important energy carrier
almost exclusively used in Power generation, covers more than 75 % of total
electricity production in Greece. Lignite . fired power plants in Greece use coal
combustion technology, which leads to significant greenhouse gas emissions. The
technology of converting the biomass to electricity by direct-fired systems and the
resulting reductions in greenhouse emissions has been studied. The electricity
consumed in Greece in 2005 is approximately 49. 760.000 MWh (data from Public
Power Corporation in Greece, for the year 2005). The burning of fossil fuels is the
primary source of anthropogenic CO2 emissions. The total Greece anthropogenic
carbon dioxide emissions in 2003 from electricity generation, expressed as carbon
equivalents, were estimated to be 35 million metric tonnes. For a coal-fired power
plant of 600 MW, the coal system reveals about 847 g CO2-equivalent/kWh of
electricity produced and the energy balance reveals that 12, 5 MJ of fossil energy is
consumed per kWh of electricity produced [24].
Goula and Bereketidou
The biomass power system examined in this analysis is representative of todayfs
current technology and employs a direct-fired biomass power plant of 600 MW
electric capacity using biomass residues. Just like the co fired system, the biomass is
assumed to be produced by urban sources and diverted from normal land filling and
mulching operations. The system results in negative greenhouse emissions of -410 g
CO2-equivalent/kWh of electricity produced and the fossil energy consumption is
0.1MJ/kWh of electricity produced [24]. Therefore, avoided CO2 emissions from this
biomass power plant are estimated, taking into account that 248, 2 Kg CO2 are
avoided per 100 kg of biomass usage. Table 4 shows the electricity generation
potential for the direct-fired biomass plant and the greenhouse gas savings that could
result by using the total amount of biomass in Greece in direct-fired power plant.
Table 4 Electricity generation potential and greenhouse gas savings by using the total
biomass residues in Greece in direct-fired power plant
Source of biomass Electricity % of Greece Direct CO2
Potential (MWh/yr) electricity use savings for
could be met with biomass power
biomass power(%) tons CO2/yr
Agriculture: crop residues 5, 4 E106 10 % 943 E104
Agro-industrial 8, 4 E105 2 % 147E104
Fuel wood 1, 97 E106 4 % 347 E104
Municipal: MWWT 4, 5 E105 1 % 8, 4 E104
Energetic cultivation 32 E106 65 % 5708 E104
Total 40, 7 E106 82 % 7223 E104
Assuming a conversion efficiency in the direct-fired power plant of 1.4
5. CONCLUSIONS
Significant amounts of ewastef biomass are produced from the forestry, agricultural
and municipal sectors across Greece. The total mass of residual biomass and energy
crops generated per year reaches at 29,1 E 106 dry tones approximately, which if it was
collected and treated in a direct-fired biomass plant, it could produce up to 82 % of
the total electricity currently used in Greece, while the direct CO2 savings would be
approximately 7223E 104 tons CO2/year. Regarding bio-hydrogen, it can be efficiently
produced by dark, anaerobic bacterial growth on carbohydrate-rich substrates,
resulting in organic fermentation end products. Cellulose and hemi cellulose content
of wastes can be hydrolyzed to carbohydrates which are further processed for organic
acid and bio-hydrogen production. If this hydrogen was used to supply a fuel cell and
assuming that the efficiency of a hydrogen fuelled car is twice the gasoline fuelled
one, the total amount of bio-hydrogen produced could replace up to 15% of the gasoline
currently used in Greece, while the CO2 reductions could be 1368,618 tons/year.
Concluding, if the total amount of residual biomass and energetic cultivation in
Greece would be used as raw material for biological hydrogen production or for
electricity production, a great percentage of Greece electricity use could be met with
biomass power and moreover a great amount of CO2 emissions in the atmosphere
could be saved. However, the above results cannot be fully appreciated unless
economic analysis will be performed in order to distinguish the technology which
would be the most functional and cost effective for each case.
Goula and Bereketidou
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