Source:
"Advanced techniques for generation of energy from biomass and waste" (Pdf
24,1 kb) by H.J. Veringa
Large-scale utilisation of fossil fuels makes the CO2 concentration
in the
atmosphere rise due to the enhanced green house effect. There
are a number
of ways to reduce the emission of green house gases like
large application
of wind energy or solar energy as well as storage of CO2
in the deep sea or
underground. Another way is the utilisation of biomass. Although
biomass
contains carbon and the generation of energy out of this
fuel releases CO2,
this CO2 is also taken out of the atmosphere during growth
of the plant.
Therefore we call biomass a green house gas emission neutral
energy source.
In contrast to the carbon in fossil fuels the carbon in biomass
has a cycle
period from plant to the atmosphere and back of between one
and some tens of
years. Therefore it is called a short cycle carbon whereas
fossil carbon
exists in this form already for millions of years. Actually
it was taken out
of the atmosphere when CO2 was the most abundant gas.
Development of a sustainable society, which
can maintain its
present level of prosperity and preferably enhance the quality
of life of
underdeveloped areas and countries in the world in a situation
of ever
diminishing availability of resourses and without significant
negative
effects of exhaustion and pollution due to today's pattern
of consumption is
possibly one of the great, if not the greatest, challenge
of mankind in this
century.
For a number of reasons this movement towards sustainability
is manifesting itself particularly in the area of energy
production. On the one hand this is due to unprecedented
speed of exhaustion of energy resources, and on the other
hand since, already at this very moment, severe negative
effects due to, unwanted and at present societal conditions
unavoidable, harmful emissions to the environment. This negative
development is expected to enhance and lead to more catastrophic
occasions in the next decades, and certainly require severe
measures
to be taken already now and to be intensified in the coming
years.
Looking at the primary energy consumption by individuals,
at a global scale
along with the availability of energy, the following picture
emerges: For primary energy consumption to maintain life,
each individual needs only 100 watts continuously. When more
prosperity related commodities are taken into account, this
required level of consumption increases rapidly, but also
the time span during which the resource is being used reduces;
we are consuming more and more intermittently. Ultimately
the power level for transportation by car requires a power
of about 100 kilowatt but on the average only 3/4 hours per
day. In spite of the short duration of this use of power,
transportation is by car is the highest energy consumption
of present day mankind.
Globally speaking there is also substantial unbalance in
availability: The
earth receives annually 3 millions of Exajoule (3*1024J)
from the sun. The
reserves of stored energy are estimated at 3*1023, 10 % of
the yearly supply
of energy to the earth by the sun. This Insulation (energy
supplied by the
sun) comes availably as hydropower at 90 Exajoule, 630 Exajoule
as wind and
1250 Exajoule via biosynthesis. This has to be compared to
an annual
consumption of energy, which amounts to 400 Exajoule: 4*1020J.
In fact we
can say that there is by far sufficient of energy from the
sun and related
conversion processes to fulfil all our needs in energy. However
disclosing
this wealthy resource is the major problem due to which we
so much depend,
and will be depending, on fossil fuels now and in the coming
decades.
The availability of oil, including the proven reserves is
up to 8.5*1021
joule (8500 Exajoule) with a yearly consumption level of
150 Exajoules, so
that we can soon expect that crude oil prices will increase
and have severe
negative influence on the global economic stability.
All organic material produced by plants or any conversion
process involving
life is called biomass. Biomass can be produced by dedicated
cultivation for
the purpose of energy production. For this application, obviously,
only fast
growing plants, which give a high yield per hectare. Typical
examples are
Miscanthus, Sweet sorghum and Willow. After harvesting, replanting
of the
same species or similar plants is necessary of course.
When co-products from agricultural business are being used
for energy
purposes, we talk about biomass waste streams. The main product
of
agricultural business is food and biomass for energy production
and should
in no way interfere negatively with food production. Next
to this there are
biomass streams which have in common that they contain more
or less organic
material which has ever been produced by plants, Examples
are waste streams
origination from maintenance work in parks, thinning wood
from forestry,
grass from shoulders and many others. Also there is waste
from household,
industry and in general industrial processes. Examples are
vegetable fruit
and garden waste, demolition wood, slip, saw dust, cocoa
nuts and coffee
waste.
If we suppose that sun and wind will be the main source for
renewable electricity generation due to the fact that the
technology for conversion, PV and windmills, are most suitable
for this purpose, then we will have to, but will be also
be in a position to, produce carbon based materials like
feedstock's and fuel out of photosynthesis processes which
will be the
main substitute for present day raw materials and fossil
fuels. However any conclusion based upon the foregoing arguments
that there is sufficient availability (1250 Exajoule) against
a global need of 400 Exajoule is a too easy conclusion.
The technical potential is that part of the 1250 Exajoule,
which with present day state of the art technology, can be
made available. This technical potential is evaluated at
120 Exajoule, but can increase rapidly if technology progresses.
This number has
to be confronted to a global use of energy out of biomass
of 50 Exajoule.
A growth of the use of biomass as primary energy resource
is feasible, but
breakthroughs are necessary to bring this estimated potential
within reach.
A number of circumstances are important to keep in mind when
setting
strategic targets at any level, starting from national policy
to regional
economic and technological development.
The largest use of biomass takes place in Asia, which is
a low value use
since the main purpose is for space heating and food preparation.
Biomass is particularly suited for production of secondary
fuels. The
technical feasibility (120 Exajoule) is of the order of the
global consumption of fossil fuels (150 Exajoule), but this
high added value use will have to be developed next to low
value use, and certainly not conflict this use, since this
latter can be any primary condition of life in undeveloped
areas of the world.
Like in the area of fossil fuels, we see the same with biomass,
although
less pronounced: high availability is not necessarily at
places where high consumption occurs or will be expected
in the near future. Transportation of
biomass in any pre-processed form to ease transportation
will have to be
considered.
At a national and international level, targets are formulated
and constantly
adapted. In the Netherlands the targets are 10% of renewable
input of
primary energy by the year 2020. It is however not stated
how we will have
to develop the resources for this. It is left open to the
free market to
develop the necessary resources, but no doubt that more than
50% of this
target will be due to implementation of biomass. At the EU
level, the target
is to grow to an input of biomass resources for production
of transportation
fuel up to 5,75% by the year 2010. The present pace of development
is
however too low to comply with any of these targets mentioned.
The ambition for CO2 emission reduction is even more severe
than the targets for renewable energy generation. The actual
target as laid down in the Kyoto agreement for Netherlands
is 6% less emission of any accumulated green house gasses
compared to the year 1990, to be achieved in 2010. Some mechanisms
are under development to create conditions is countries,
which cannot easily match up to these targets like emission
trading and joint implementation.
In Austria, there has been an increase in the use of biomass
for district
heating by a factor of six, and in Sweden by factor of eight
during the last
ten years. In the USA, more than 8,000 MWe of installed generating
capacity is based on the use of biomass.
In France, 5% of heat used for space heating is produced
from biomass.
In Finland, bio-energy already contributes about 18% of total
energy production and the aim is to further increase this
to 28% in 2025.
In Brazil ethanol is produced on a large scale as a fuel
for automobiles.
The total quantity of ethanol produced for haulage purposes
is already 15 to
17 million tons per year. A new EU Directive will stimulate
a similar
development in Europe. As a result, the production of bio-oil,
or
Fischer-Tropsch diesel, and possibly methanol will increase
significantly.
The development of biomass is also important from a social
and environmental
perspective.
The
life cycle of biomass as a renewable
material has a neutral effect
on CO2 and SO2 emissions. The latter type of emissions is
already very low anyway because by nature biomass contains
very little sulphur. Large-scale use of biomass also enables
closure of the mineral and nitrogen cycles.
in Europe
a new purpose needs to be found for land
taken out of production. It is estimated that 200 million
hectares of
agricultural land and 10 tot 20 million hectares of land
with marginal
production possibilities, can be used for the production
of biomass, for
materials, feedstock and energy.
roughly speaking,
11 new jobs are created per megawatt
installed production capacity. Translating this number to
the situation in
Europe, where 5% of energy demand must be derived from biomass,
results in
160,000 new jobs.
biomass can
be used as a decentralised source of energy,
where conversion can take place close to production. This
can lead to social
stability at the regional level, and especially in areas
that are considered
economically disadvantaged.
Of course, the above describes a number of conditions that
are valid
worldwide, but can still vary from country to country and
from region to
region. In the Netherlands, for example, not much land has
been taken out of
production and no large role has been assigned to the cultivation
of biomass
for energy purposes. On the other hand, energy production
in the Netherlands
is greatly, decentralised in large to medium-size units,
and the high degree
of urbanisation results in a large amount of organic waste,
which is
well-suited for energy production. If one considers waste
a mankind induced
residue stream, then the Netherlands produce 65 million tons
annually. Five
million tons of this waste is burned and partly used for
energy production.
Four million tons are land-filled and the rest is reused.
In terms of waste
treatment, Dutch policy can be characterised as very effective.
Nevertheless, the above figures indicate that further utilisation
of waste
is possible, and much could be gained by further increasing
energy
conversion efficiency. Another important aspect is that the
Netherlands has
outstanding facilities for supply and transfer, as a result
of which the
treatment and processing of biomass could lead to new developments.
A large portion of electricity in the Netherlands is generated
from coal.
The coal-fired power plants in our country are extremely
modern and have
flue gas cleaning facilities with state-of-the-art technology.
These power
plants are suitable for co-combustion of biomass or organic
waste.
Meanwhile, most of the sustainably generated energy is produced
by
co-conversion or cofiring of biomass with coal. In this regard
the
Netherlands can be considered as a leading country. By taking
advantage of
national and international developments in the field of biomass,
ECN Biomass
has grown into the largest Dutch R&D group in this
sector, and can measure
up to the top institutes of its kind in Europe.
Co-firing is the simplest form of biomass use in a coalfired
power plant. In
fact, the biomass is carried with the pulverised coal to
the boiler. In this
way a part of the coal used is replaced by biomass, and the
proportionate
part of the calorific value of the biomass used can be considered
as
renewable energy. However, the biomass should be given such
properties that
it can be carried along with the coal without any difficulty.
It has to be
ground into very small particles and in the process acquire
flow properties
that are the same as those of coal. The fibrous structure
of biomass makes
this grinding more difficult, as a result of which much effort
has been
invested in development of the grinding technology. On the
other hand, as
soon as the biomass contributes to heat production in the
boiler, this
energy used by the existing installation, e.g. the steam
boiler and the
turbine, is converted into electricity. The additional investments
are
therefore limited.
A new development that ECN is working on with a number of
electricity-related companies, is torrefaction. This is a
thermochemical
treatment of the organic fuel, i.e. biomass, which requires
a temperature of
only 200-300ºC, in which material embrittles, loses its moisture
and becomes
water repellent too. In this way a nonhomogeneous stream
obtains homogeneous properties needed for its purpose. The
material acquires the same grinding properties as coal and
will no longer absorb any water. These improvements offset
the extra pre-treatment, and are expected to increase the
possibilities for simple co-combustion.
For the time being, power plants are rightfully cautious
to accept a large
share of co-firing. One reason is that the total efficiency
of the power
plant reduces somewhat, which entails losses. Another reason
is the residue,
i.e. the ash released during the process. In the Netherlands
the ash is
fully utilised, for example, in road construction. Very strict
demands are
made with respect to the quality of the ash from an environmental
perspective among other things. Co-firing of biomass, which
also produces
ash that mixes with the coal ash, must not affect its market.
Therefore, a
prudent approach should be maintained and co-firing limited
to a few percent
only. There is no fundamental reason why this cannot be increased;
together
with ECN, the companies are searching for the best option
to gradually
increase the percentage.
Co-conversion is a more advanced use of coal, with the possibility
that it
can also be utilized in gas turbine power plants. For this
application
biomass is first converted to a combustible gas (a mixture
of carbon
monoxide and hydrogen) in a separate gasifier, after which
this gas is blown
and burned in a coal boiler. In this way the mixing of biomass
ash and coal
ash can be avoided and there is more freedom in choosing
the coconversion
percentages. However, an investment in a biomass gasifier
makes the process
more expensive compared to co-firing. In this area the Netherlands
are
playing a pioneering role with a gasifier that was already
built for this
purpose, namely the Amer plant in Geertruidenberg. As this
is still a
completely new technology, it is not surprising that a few
problems have
emerged. ECN and energy utility Essent have cooperated intensively
to solve
these problems. In the meantime, the biggest problems have
been resolved,
and confidence in the reliability of the entire system is
growing, including
the gasifier, gas cleaning, coal boiler, and a steam bottoming
cycle. As a
consequence of ECN’s recent and future contributions,
ECN has become an
important party in the areas of pre-treatment, gas cleaning,
residues and
process control of co-conversion and cofiring at electricity
generating
plants.
The main area of technological development at ECN Biomass
is the
gasification technology. Gasification has already been mentioned
for
co-conversion in coal power plants and as an important option
for
co-conversion in gas turbine power plants. However, the product
of
gasification, i.e. synthesis gas, is also a gas that can
be converted into
electricity in a turbine, a gas engine or a fuel cell. In
the long term,
synthesis gas is vital for the production of green fuels.
With the help of a
catalytic process, diesel fuel or synthetic natural gas
can be produced and mixed with conventional fuels, for
which
a high quality infrastructure already exists. Because engines,
turbines and catalytic processes in particular, are extremely
sensitive to the quality of the fuel – gas in this
case – the
gasification process, which in fact uses waste as raw material,
must also provide a gas that satisfies very strict requirements.
Therefore, gas cleaning is the Achilles heel of this technological
development that is so important for the future.
Besides nitrogenous impurities and particulate matter, tar
is the most
important contaminant. The costs of tar removal have led
to the failure or
early ending of many initiatives. ECN has developed a new
technology named
OLGA, in which the tar is effectively removed. This is done
in a few process
steps with the use of an inexpensive agricultural washing
fluid. The new
technology has the advantage that the condensation temperature
of the
residual tar in the gas remains far below 0ºC. This eliminates
the risk of
the tar condensing, which would be undesirable. The need
for costly
treatment of tar-containing wastewater, which is one of the
big
disadvantages of conventional tar removal, is also eliminated.
The next step
in the research is upscaling the process developed thus far
to the scale of
a gasifier that can power a large gas engine. There is great
interest for
this application in existing installations. Of course, the
research on fuel
production from cleaned synthesis gas will also benefit from
the merits of
the new process.
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