PRIMARY ENERGY SUPPLY AND CONVERSIONS TO
ENERGY CARRIERS
Professor em. Björn Kjellström
Heat and Power Technology
Royal Institute of Technology
Stockholm
NOMENCLATURE
c
cp
cv
F
g
h
H
I
m
p
P
Q
t
T
u
v
V
w
W
x
y
z
speed of light
specific heat at constant pressure
specific heat at constant volume
force
acceleration of gravity
specific enthalpy
heating value
electric current
mass
pressure
power
thermal energy, heat
time
temperature
specific internal energy
specific volume = 1/ρ
electric tension
velocity
energy, general
distance
parameter defining physical state
vertical distance
η
ρ
energy efficiency
density
Subscripts
el
fuel
heat
loss
marg
th
electricity
fuel
useful heat
loss
marginal
thermal
LIST OF ABBREVIATIONS
bbl
cap
GNP
lb
LNG
LPG
SEK
USD
109 barrels (billion barrels)
capita
Gross national product
pound (1 lb = 0,454 kg)
Liquefied Natural Gas
Liquefied Petroleum Gas (mainly propane and butane)
Swedish crown
US Dollar
1. INTRODUCTION
1.1 Scope of these notes
The objective of the notes is to give an introduction to the current issues of energy
engineering with a focus on the role of energy supply for development of the human
societies. Technical issues are treated only to the extent that this is necessary for
understanding of the limitations and potential of the current technologies.
These notes present current estimates of the potential of the different energy sources
that are being used for supply of technical energy to human societies and includes a
review of various processes that are or may be used for conversion of primary energy
from these sources to energy carriers.
Definitions
In section 1.6 of the lecture notes “Energy for human societies”, energy sources and
energy carriers were defined. A distinction was made between “Finite” energy sources
and “Continuous” or “Renewable” energy sources but it was made clear that the
distinction is not quite clear.
Finite energy sources will be defined as those that are stored on our planet and
regenerated at a rate that is many orders of magnitude less than the present rate of use.
The fossil fuels i.e. natural gas, oil, different types of coal, peat and uranium will then
belong to this group.
Renewable energy sources are those that can be assumed to be available for as long as
there is human life on this planet. Solar radiation energy and energy that is
continuously generated by solar radiation like wind energy, wave energy, potential
energy in water located above ocean level and biomass belong to this group and so
does tidal energy and geothermal energy.
Difficulties involved in resource estimates
An important issue discussed in these notes is the amount of energy that can be
utilised from any given energy source. For finite energy sources this would be the
total amount of the particular type that is presently stored on or in this planet
expressed for instance in TWh. For renewable energy sources, the potential would
rather be expressed as the maximum rate at which the energy source can be utilised,
expressed for instance in terms of TWh/year.
In both cases there are three fundamental difficulties.
The first difficulty is simply lack of reliable data. The current knowledge about the
deposits of fossil fuels and uranium is limited simply because complete geological
surveys have not been carried out. Similarly, limitations in hydrological and
meteorological data lead to uncertainties about energy potentials in solar radiation,
wind, waves and hydropower. A distinction must therefore be made between proven
1
and estimated reserves or potentials. Because of the uncertainties involved in the
estimates it must be accepted that these can cover a wide range.
The second difficulty is related to the cost of exploiting the energy source. If this cost
is considered too high, there will be no exploitation. In general, the cost increases
when a larger part of the potential is utilised. Depending on the willingness to pay the
cost for exploitation, the exploitable reserve or potential will be a small or a large
fraction of the proven reserve or potential. Both the cost for exploitation and the
willingness to pay can change with time. The exploitation cost will vary with
economic conditions like salaries or interest rates, may be reduced as a result of
technical development or may increase if more strict environmental requirements are
imposed.
The third difficulty is related to the environmental impacts of the exploitation. If these
are considered unacceptable, again there will be no exploitation. This may limit the
exploitable reserve or potential even further. Since there might be different opinions
about what impacts are acceptable, the numbers presented for exploitable reserves or
potentials may differ a lot.
The problem is illustrated in figure 1.
Figure 1. Uncertainties and limitations affecting energy source estimates
A consequence of this is that all quantifications of energy reserves or potentials must
be used with great caution and particularly so if the quantifications are on a global
level.
2
2. RESERVES
FUELS
AND
EXPLOITATION
OF
FOSSIL
General remarks
Fossil fuels include coal, oil, natural gas and sometimes also peat. As indicated by the
name, fossil fuels have been generated from the remains of plants and animals buried
millions of years ago in swamps, lakes and seabeds. These materials, consisting
mainly of carbon, hydrogen, oxygen, sulphur and some mineral matter were
transformed by heat and pressure in the Earth´s crust into coal, oil and natural gas.
There are several reasons why these fuels have had an increasing importance for the
industrial and technological development during the last two centuries. The most
important are:
− Fossil fuels are relatively easily accessible in the Earth´s crust. They are available
for extraction in large quantities at thousands of locations;
− Fossil fuels are relatively easy to use.
− Fossil fuels have a high energy content (high heating value);
− Fossil fuels are relatively easy to transport and store.
Even though the negative environmental effects of burning fossil fuels with sulphur
were noticed locally long ago at places (like London and Manchester) where the use
was concentrated, the large debate about the role of fossil fuels in the global energy
system is rather new and started in the 1970:s. The main concern now is the effects of
continued emissions of large quantities of carbon-dioxide, the main combustion
product that leads to an increasing content of this gas in the atmosphere. The expected
consequence is climate changes caused by the changes of the radiation balance for the
Earth. This issue will be discussed in the next lecture.
Nevertheless fossil fuels dominate the energy supply to the technical energy system,
see figure 2.
Nuclear Hydropower
energy
2%
7%
Natural Gas
22%
Biofuels
11%
Coal
22%
Oil
36%
Figure 2. Primary energy supply for human use of technical energy
3
Coal
Coal is a heterogeneous mineral material consisting principally of carbon, hydrogen
and oxygen and with lesser amounts of sulphur, nitrogen and ash-forming inorganic
compounds.
Coal originates from wood and other biomass that was later covered, compacted and
transformed into rock over a period of hundreds of thousand years. This process leads
to a gradual reduction of the oxygen content (from typically 42% in biomass to 25 13% in coal) and thereby an increase in the carbon content from typically 50% in
biomass to 68 – 80% in coal. The carbon content and the properties of coal depend on
the age of the coal and there are large differences between coals found at different
sites. Coal are “ranked” with respect to its heating value and the general tendency is
that younger coals like lignite1 have a lower heating value than older coals like
bituminous coals and anthracite. The heating value ranges from 6500 – 8000
kWh/ton.
Coal is mined in surface mines (strip mining) or in underground mines. Strip mining
leads to substantial local temporary changes of the landscape, but restoration appears
as possible, see figure 3. Underground mining is associated with high occupational
hazards.
Some refinement of the coal by washing is sometimes made close to the mine. This
will reduce the ash and sulphur contents.
The proven reserves that can be exploited with the present technologies is about
10 000 Gton and the estimated reserves about 15 000 Gton2. The exploitable reserves
at acceptable cost were about 500 Gton in 1977 and is now, with higher coal prices,
about 1000 Gton.
Table 1 shows the proven and exploitable reserves of coal in 20023.
Table 1.
Exploitable reserves of coal
Region
North America
Latin America
Europa and Eurasia
Russia
Africa and Middle East
Australia
China
Remaining Asia
Total
Exploitable
Gton
258
22
198
157
57
82
114
96
984
reserve
% of total
26
2
20
16
6
8
12
10
100
1
Swedish “brunkol”
Nämnden för Energiproduktionsforskning, planeringsrapport NE 1977:23
3
Energifakta, datablad September 2003
2
4
The coal reserves in Sweden are relatively small, about 30 Mton. The deposits are
located between Ängelholm and Helsingborg. The heating value is low, only about
5
Figure 3. Landscape affects of strip-mining of coal
a. During mining
b. Degraded land after mining
c. Reclaimed land after restoration
6
5500 kWh/kg. About 0,5 Mton was mined annually during the period 1940-45 but the
operation was then stopped because it was uneconomical.
Most of the coal is used not far from where it is mined, but there is an international
trade of coal. The largest exporters of coal are USA, Australia, South Africa, China
and Indonesia. Sweden imports about 3,5 Mton annually. In 2002, 0,5 Mton was used
as fuel in co-generation plants (see section 6.4) and the industry the remaining 3
Mton. About 50% of this is used for production of coke, see section 6.5. The coke is
used in the steel mills.
Oil
Crude oil is a mixture of hydrocarbons with small amounts of sulphur, oxygen,
nitrogen, metals and minerals. Crude oil is generally found trapped in rock formations
that were originally part of the ocean floor. It originates from remains of plants and
marine animals that were encased in rock layers at elevated pressure and temperature
over millions of years. The oil moves slowly up-wards through pores and cracks in
the rock until it is prevented from further flow up-wards by an impermeable layer of
clay or shale. Below such roofs of clay or shale, large deposits of crude oil may be
found.
The deposit can be exploited, after it has been located, by drilling a hole through the
roof and tapping the oil through a pipe. Sometimes the oil is under pressure and flows
spontaneously through the pipe. Otherwise the oil is pumped.
Almost all the crude oil is treated in a refinery where the light and heavy hydrocarbons are separated by distillation into a large number of fractions, see figure 4, that
are suitable for different purposes. The actual product mix depends on the
composition of the crude oil, the refinery process and the way the refinery is operated.
Figure 4. Typical product mix from an oil refinery. Light products at the top
7
The heavy fractions can be converted to light fractions with higher product value like
gasoline and diesel fuel by thermal treatment (cracking) or by treatment with
hydrogen.
The light products are used in particular as fuel for engines or gas turbines. The
heating value of these products is about 11 900 kWh/ton. The heavier fuel fractions
are mainly used as boiler fuel but may also be used in slow speed marine diesel
engines. There heating value is about 11 300 kWh/ton.
The estimated reserves of crude oil are about 260 Gt. The reserves that are now
considered exploitable at reasonable cost are about 143 Gton and are distributed as
shown in table 2.
Table 2.
Exploitable reserves of oil
Region
Exploitable
reserve Gton
% of total
Middle East
93,4
Central and South America
North America
Africa
Europe incl. earlier USSR
Rest of Asia and Australia
Total
14,1
6,4
10,3
13,3
5,2
142,7
65
10
5
7
9
4
100
A large part, almost two thirds, is found in the Persian Gulf countries and the reserves
in Saudi Arabia dominate, see figure 5.
30
26
25
20
Series1
15
10,1
9,9
10
9,5
9,4
5
0,5
0,4
0,4
Neutral
Zone
Oman
Qatar
0
Saudi
Arabia
Iraq
UAE
Kuwait
Iran
8
Figure 5. Oil reserves in the Persian Gulf countries, percentage of total world
reserves.
Most of the oil is not used where it is produced. Most of the production is done in the
Persian Gulf region but the consumption is dominated by the United States, Europe
and Japan, see figure 6 that illustrates the shipping of oil.
Figure 6. International shipment of crude oil
Sweden has only small oil reserves, on Gotland. Production was started but has closed
down for economic reasons. In the early 1970:s, Sweden´s energy system was very
dependant on imported oil, but the oil dependence has since then been reduced, see
Energy in Sweden. The structure of the import has also changed. Earlier the import
relied to a large extent on oil from the Middle East. Now, oil from the North Sea
dominates. The security of supply has improved considerably by this.
As can be seen from figure 2, the global energy supply is very much relying on oil.
The importance is actually greater than the share of the energy supply, 36% could
indicate. Almost the entire transport sector including road vehicles, air planes and
ships can only use a fuel with properties similar to the light oil products. Substitutes
can be produced from coal, natural gas or biomass but these are expensive and it will
take a considerable time to build a production capacity for such substitutes that can
meet the present demand for oil products. It is not surprising that the oil importing
countries are concerned about the political instability in the Middle East.
9
One of the worries is of course major disruptions of the supply, but large increases in
the price is also something to be concerned about because of the disturbances in the
global economy that this could cause.
Figure 7 shows the historic variation of the crude oil price in “USD 2000/barrel4”.
Reasons for major fluctuations in the price are also indicated. Between the 2nd World
War and the outburst of serious political unrest in the Middle East in the early 1970:s,
the crude oil price was stable in the range 15 – 11 USD/barrel. The rapid increases in
two steps during the 1970:s had enormous effects on the global economy and
triggered research on alternative renewable energy sources all over the world. Many,
for instance an official Swedish Government study5 expected the oil prices to increase
even further. The projection in that study for the year 2000 was a crude oil price of 57
USD/barrel. This was an important reason for the recommendation to continue the
expansion of nuclear power generation in Sweden.
In reality this level was reached already a few years after completion of the study, but
the price soon dropped rapidly and has since then fluctuated around 20 USD/barrel,
with some tendency to approach 25-30 USD/barrel recently.
Figure 7. Variation of crude oil price 1947-2003
Natural gas
Natural gas originates from the same material as oil. Therefore oil and natural gas are
often found at the same places, see figure 8.
4
1 barrel of oil is 159 litres = 135 kg. A crude oil price of 20 USD/barrel is then equivalent to 14,7
USD/MWh(fuel)
5
Energikommissionen SOU 1978:17 p. 327
10
Earlier, natural gas was often regarded as just a by-product of oil production. At sites
where no infrastructure for piping the gas to consumers existed, the gas was often
burnt at the site in a flare.
Figure 8. Geological formations associated with oil and natural gas reserves
The main component in natural gas is methane CH4, but there are also smaller
amounts of other light hydrocarbons like ethane, propane and butane. Raw natural gas
may contain water vapour and nitrogen, two incombustible gases. There may also be
some sulphur in the form of H2S. Sulphur compounds, water vapour and heavier
hydrocarbons are normally separated from the gas before distribution of the natural
gas. The gas that is sent out for consumption contains 98 – 99% methane and its
heating value is around 10,8 kWh/Nm3 . The density of the gas at “normal conditions”
1 bar, 0oC is 0,75 kg/m3 i.e. 1 Nm3 is equivalent to 0,75 kg.
The proven reserves of natural gas are about 1500 PWh. As shown in table 3, most of
the reserves are found in the Middle East, the former Soviet Union and Eastern
Europe.
11
Table 3.
Proven reserves of natural gas
Region
Earlier Soviet Union and East Europe
Middle East
Afrika
Asia
Latin America
North America
Western Europe
Australia and New Zealand
Total
Proven reserves
P Wh
Tm3
57,1
617
44,9
485
9,7
105
9,3
100
7,6
82
7,4
80
5,3
57
0,7
8
142
1534
%
40
32
7
7
5
5
4
<1
100
Natural gas is mainly distributed in pipelines. Overseas transport is done in cold liquid
form, as LNG i.e. liquefied natural gas. Figure 9 shows the main European network
that is supplied mainly from the North Sea and from Russia and Eastern Europe. LNG
is received in several ports, for instance in France, Italy and Belgium.
Figure 9. The European natural gas network (source Eurogas)
12
The Swedish network for natural gas is
shown in figure 10. The gas is imported
from Denmark. The network covers the
west coast up to Gothenburg and has a
branch to Jönköping. A larger network is
occasionally discussed but has so far not
materialised.
In Sweden, natural gas is used mainly as
fuel for co-generation plants and industry.
Some is also used for heating of single
family houses or as fuel for cars.
Figure 10. The network for natural gas in
Sweden
Sustainability of fossil fuels
Figure 2 shows that fossil fuels are responsible for 80% of the primary energy supply
to the technical energy system. It is then not surprising that the sustainability of this
supply is a matter worth worrying about.
There are two issues involved here. One is related to the possible depletion of the
reserves. Since the reserves are limited, at least at a given cost for exploitation, they
will certainly run out at some time in the future if the consumption rate is kept
constant or increased. Table 4 illustrates results of a primitive approach to this issue.
The time required for consumption of the fuel in the known exploitable reserves and
the estimated reserves if the consumption rate remains unchanged is shown there.
Table 4. Estimated time required for depletion of fossil fuels
Fuel
Present rate of
consumption
PWh/year
Known
exploitable
reserves
PWh
Coal
24,6
about 7500
Time
for
depletion
of
known
exploitable
reserves
about 300 years
Oil
Natural gas
40,3
24,6
about 1600
about 1500
about 40 years
about 60 years
Estimated
reserves
PWh
Time
depletion
estimated
reserves
about 112000
about
4500
years
about 70 years
about 2900
for
of
The weakness of that type of estimates is that the market price of the fuel in reality
will increase when the sources run closer to depletion. This can be expected to reduce
the consumption rate and make the fuel last longer than estimated when a constant
consumption rate is assumed. Also exploitable reserves and the incentives to look for
new reserves will increase when the price of the fuel increases.
13
It is an interesting observation that the estimated time for depletion of the oil reserves
has remained at about 30 years since the early 1970:s. Some argue that this shows that
the concerns about depletion of the oil reserves are unfounded, but they seem to
ignore that price increases that are inevitable when the oil reserves that can be tapped
at low cost run out, will make it unattractive to use oil for all the present purposes.
There will be a need to find substitutes for oil, simply for economic reasons.
It is also true that local depletion of
sources that can be exploited at low cost
is already happening in USA. USA relied
almost entirely on its own reserves until
about 1970. Since then the production has
stagnated but the consumption has
continued to increase, see figure 11. In
1998, more than 50% of the oil was
imported.
This is, for good reasons, considered a
serious threat to the self-reliance of the
energy system and it is not surprising that
USA is trying to maintain and expand
some kind of control over the oil-rich
countries in the Persian Gulf region.
Weapons of mass destruction and the lack
of democracy in Iraq, were probably not
the only reasons for the recent war there.. Figure 11. Balance between
production and consumption in USA
oil
The other issue related to the sustainability of fossil fuels as dominating energy
sources is the environmental impacts caused by the use of these fuels. As will be
discussed in more detail in a subsequent lecture, the main concerns are related to the
emissions of carbon-dioxide caused by combustion of the carbon in fossil fuels and
the effects on climate by an increasing content of this “green-house gas” in the
atmosphere. The issue is controversial. Some argue that the risk for a significant
climate change sets a limit for the use of fossil fuels. Other mean that the climatic
effects caused by the limited amounts of fossil fuels that remain to be exploited are
not serious and that there is no need to shy away from fossil fuels for environmental
reasons.
Most of the scientists that have studied the issue appear to support the first view. This
has resulted in much discussions about reasonable actions to reduce use of fossil fuels
on high international levels, resulting in the so called Kyoto-protocol, a subject for
another lecture in this course.
14
3. RESERVES AND EXPLOITATION OF NUCLEAR
FUEL
The use of nuclear energy rests on the conversion of mass to energy that results from
fission6 of heavy atom nuclei or fusion7 of light atom nuclei. Despite considerable
research efforts, it has not yet been possible to control the nuclear fusion process8.
The only non-military use of nuclear energy is therefore based on nuclear fission.
Candidates for nuclear fission fuel are the isotopes 235U and 233U of uranium and the
isotope 239Pu of plutonium. The isotope 232Th of thorium and the isotope 238U of
uranium can be converted to fissile material by neutron bombardment. These
materials are therefore potential raw materials for production of a nuclear fuel and are
called “fertile”. Plutonium or 233U does not exist naturally. The only naturally
occurring nuclear fission fuel is 235U that constitutes about 0,7% of naturally
occurring uranium.
The assured and exploitable reserves of uranium at present market price is about 3,1
Mton. The reserves are distributed as shown in table 5.
Table 5. Known exploitable reserves of uranium
Country
Uranium
kton
Australia
Kazakstan
Canada
South Africa
Namibia
Brazil
Russia
USA
Uzbekistan
Other countries
Total
863
472
437
298
235
197
131
104
103
267
3107
reserves
Equivalent
thermal
energy PWh after
fission of 235U
121
66
61
42
33
27
18
15
15
37
435
% of World reserves
28
15
14
10
8
6
4
3
3
9
100
The total reserves have been estimated to about 16,2 Mton, equivalent to about 2300
PWh of thermal energy by fission of 235U. If the dominating isotope in natural
uranium 238U, is used for conversion to fissible plutonium, the energy potential of the
uranium reserves would increase by almost a factor of 100. The present use of
uranium is about 55 kton/year, which means that at this rate of consumption the
known exploitable reserves would be depleted in about 55 years. Sweden uses about
2000 ton natural uranium per year. It is mined mainly in Canada and Australia.
The most frequently used nuclear reactors that use ordinary water as coolant cannot
use natural uranium as fuel. The nuclear fuel must contain about 3-4% of the fissible
uranium isotope. This means that the natural uranium must be processed in so called
enrichment plants where 60-75% of the non-fissile isotope 238U is separated, leaving a
6
Fission = The action of splitting
Fusion = The action of melting together
8
The process is behind the energy conversion in the sun and has been used and demonstrated for
weapons applications in the so called hydrogen bombs.
15
7
composition of the uranium that is suitable as nuclear fuel for these reactors. The
enrichment process is expensive and difficult. Substantial industrial capacity for
enrichment is found only in USA, Russia and France.
In the nuclear reactors of the current types, all the fissile material cannot be utilised.
The contents of fission products increase in the nuclear fuel and after some time these
products will acts as “poisons” and absorb so many of the neutrons that drive the
fission process that further operation of the reactor is impossible. During the time of
operation, some of the non-fissile 238U has been converted to fissile 239 Pu. This fissile
plutonium and the 235U that has not been used can be separated from the fission
products and made into fresh nuclear fuel. This operation is called reprocessing and is
practised in some countries.
The residues from this process, containing the fission products is very radioactive and
must stored isolated from the ecosystem for very long time, 10000 – 100 000 years.
The same applies to the spent fuel if it is not reprocessed. The reprocessing waste and
the spent fuel with fission products must be kept cooled for decades after removal
from the reactor to avoid overheating, melting and possible release of gaseous
radioactive fission products.
The entire nuclear fuel process is illustrated in figure 12.
Figure 12. The nuclear fuel process
16
4. POTENTIAL
AND
HYDROPOWER
EXPLOITATION
OF
Hydropower plants utilise the difference in potential energy between lakes or water
reservoirs located at different altitudes. The potential energy is used to accelerate the
flow of water and the kinetic energy in the water flow is used to drive a turbine,
connected to an electric generator. The water is returned to the lake at the higher level
by solar energy that vaporises water from lakes and oceans that later returns as rain
and hydropower is therefore considered a renewable energy source.
A typical arrangement of a hydropower plant is illustrated in figure 13.
Figure 13. Arrangement of hydropower plant
A close-up of the power plant building is shown in figure 14.
Figure 14. Hydropower plant building
17
The possibilities to use hydropower depend on the annual rainfall and the topography.
The estimated economic potential for hydropower generation of electricity is about
13100 TWh/year.9 The geographical distribution of the utilisation and potential are
shown in table 6. The numbers refer to “average” years with respect to rainfall.
Considerable variations between wet and dry years must be expected. For Sweden the
variation is at least ±20%.
The gross potential is estimated on basis of the topography and the distribution of the
rainfall. Since it is unrealistic to use every small stream of water that flows towards
the oceans, this number represents an unrealistic but absolute limit for the potential.
The economic potential is defined as the hydropower that can be exploited with
generation costs that are competitive with fossil fuel options. The practicable potential
is less than the economic because of environmental restrictions. Such restrictions
apply for instance in Sweden where four large rivers in Northern Sweden has been
declared by the Swedish Parliament as protected against exploitation.
Table 6. Geographical distribution of hydropower utilisation and potential
Region
Sweden
Europe
incl
Turkey
Asia
Africa
North America
Latin America
Oceania
World total
Gross
potential
TWh/year
200
23900
10100
16000
1000
Economic
potential
TWh/year
94
2085
Practicable
potential
TWh/year
64
1670
Developed
potential 1990
TWh/year
6410
745
Remaining
economic
potential %
32
64
3815
2500
970
3530
200
13100
3050
2000
775
2825
160
10480
390
57
587
418
43
2240
90
98
61
88
22
83
It can be noticed that the main part of the remaining unexploited potential is found in
the parts of the world where electricity use is still at a low per capita level.
At most places there is a seasonal variation of the rainfall. The electric power
generation would have to be matched to follow these variations if water cannot be
stored from wet season to dry season. Such storage requires dams and possibilities to
vary the water level in the reservoir that is contained by the dam and the natural
topography.
5. POTENTIAL AND EXPLOITATION
RENEWABLE ENERGY SOURCES
OF
OTHER
Other renewable energy sources will be covered in a special lecture. Separate lecture
notes will be distributed that covers this subject.
9
When comparing this with the potential in uranium and fossil fuels it must be remembered that the
hydropower potential is expressed in electric energy units, whereas the units for fuels and uranium are
equivalent thermal energy units. The efficiency for conversion of thermal energy to electric energy is
about 30% in a nuclear power plant and 30-50% in a power plant using fossil fuels.
10
This number is valid for a year with normal precipitation. The actual generation 1990 was 71,4 TWh
18
6. PROCESSES FOR GENERATION OF ENERGY
CARRIERS
Refined petroleum fuels
The crude oil that is extracted from the oil wells is seldom used as such. The crude oil
consists of a mixture of different hydrocarbons with varying molecular weight and
structure. The crude oil is used as a feedstock for producing hydrocarbons with a
more homogeneous composition and with properties that are suitable for specific
applications. This process is called refining and is carried out in an oil refinery. An
example of a process flow sheet for an oil refinery is shown in figure 15.
The process is based on separation of hydrocarbons with different boiling temperature
by distillation into six fractions, gas, raw gasoline, raw kerosene, gas-oil, lubricant
distillates and heavy residue. The different fractions are then treated further and
blended into the final products.
The scheme does not show a de-sulphurisation step for fuel oils but such a step can be
included. In principle the de-sulfurisation is based on catalytic treatment with
hydrogen at high pressure.
Figure 15. Process flow sheet foe an oil refinery
19
Most of the oil products used in Sweden are refined in the Swedish refineries. The
biggest, Scanraff is located in Lysekil, two other are located in Gothenburgh. There
are also two small refineries making special products like asphalt and special oils,
located in Nynäshamn and Gothenburgh.
The energy balance for a refinery depends on the composition of the feedstock, the
product mix and the extent of de-sulfurisation of fuel oils. Roughly 95% of the energy
input with the crude oil is found in the products. The losses increase if a crude oil with
low content of light hydrocarbons is treated by thermal or catalytic cracking for
production of more light products than can be achieved by distillation. Desulfurisation requires 3% of the product energy per percentage unit reduction of the
sulfur content.
6.2 Heat generation
Thermal energy, heat, is one of the dominating final uses of technical energy. The
final user then generates the heat by either combustion of a fuel, by passing electric
current through a resistor or by using electrical energy to drive a heat pump, a device
that is used to convert thermal energy at low temperature to thermal energy at high
temperature.
Thermal energy as an energy carrier is almost always in the form of hot water that is
piped from the producer of hot water to the users. The district heating systems
operated in most towns of Sweden are examples of this. The principle is illustrated in
figure 16.
Figure 16. Principle of a district heating system
20
Hot water, normally at a temperature of 90 – 120oC, is sent out from the heat
production plant to a pipe network. The heat consumers are connected to this pipe and
taps a part of the flow into a heat exchanger, used for heating of the water circulating
in the radiator system for heating of the building and for hot tap water. From the heat
exchanger flows water that has delivered its heat back to the return pipe in the district
heating network, where water flows back to be heated again in the heat production
plant.
At some places, steam is used as energy carrier instead of hot water.
At the heat production plant, thermal energy is generated either by combustion of a
fuel, by passing electric current through a resistor or by using electrical energy to
drive a heat pump. In Sweden, about 46 TWh(heat) was delivered to the district
heating networks in 2001. About 1,4 TWh where generated by electric boilers, 7.1
TWh by heat pumps and the rest by combustion of a fuel.
When a fuel is used, the combustion takes
place in a furnace, where the fuel reacts at
high temperature with air. The carbon in
the fuel reacts to carbon-dioxide and the
hydrogen to water vapour if the
combustion is 100% complete. In these
reactions chemical energy is converted to
thermal energy. The resulting temperature
in the combustion gases will, depending
on the fuel reach 1400 – 2000oC. The
thermal energy in the hot combustion
gases is picked up by water that is
circulated through pipes that cover the
walls of the furnace, see figure 17.
The design of the combustion equipment
depends on the type of fuel used. For oil,
gas or a dry powderised solid fuel, air and
fuel are injected together into the furnace
as shown in figure 17. Other types of
combustion equipment are being used, for
combustion of solid fuels with larger
Figure 17. Oil or gas fired hot water
particle size see figures 18 and 19.
boiler, output about 46 MW
Figure 18 shows a boiler with combustion on a grate. The fuel and air are introduced
separately into the furnace. The burning fuel is resting on a structure of iron bars, the
grate. Part of the air is supplied from below through the openings in the grate, part is
supplied above the burning fuel bed to complete the combustion. The iron bars are
linked together to form a band that can be moved slowly as indicated by the arrow in
the figure. Ideally, the fuel particles that are transported along the furnace on the grate
are completely burnt when they reach the end of the grate and all that remains is
21
ashes. The ashes fall off the grate at the turning point of the grate and can be collected
for disposal.
Figure 18. Hot water boiler for combine combustion of coal and wood chips,
equipped with a traveling grate
Figure 19. Hot water boiler with fluidized bed combustor
Figure 19 shows a boiler with combustion in a fluidised bed. The fuel particles with
sizes from a few mm to a few cm are introduced into a bed of hot sand. The air is
supplied from below and the air velocity is so high that the particles are carried with
22
the air flow upwards. The fuel particles are burning while transported up-wards
together with the sand particles. The sand and un-burned fuel particles are separated
from the gas flow and returned back to the bottom of the fluidised bed. The hot gases
pass heat exchanger surfaces where the thermal energy is transferred to water.
The efficiency of hot water boilers that use combustion of a fuel for the heat
generation is about 90%.
When electricity is used for generation of hot water, an electric current is passed
trough resistance elements that are positioned in the water flow to be heated. Electric
boilers are compact, easy to control and require low investments. The efficiency is
very high, in the range of 99%.
A more effective way to use electricity for production of hot water is to use a heat
pump. The principle is illustrated in figure 20.
Figure 20. Process flow scheme for a heat pump
The operating principle is explained in the figure. Heat pumps used for heat supply to
district heating systems use water from a lake, sewage water or waste water from an
industry as the heat source and electric energy for driving the compressor. The
electricity consumption depends on the temperature of the heat source but is normally
about 30% of the thermal energy increase from the temperature of the heat source to
the temperature of the hot water delivered to the district heating system.
Distribution losses in district heating systems range from 5-15%. Small and scattered
heat loads lead to higher losses. Normally the heat losses from a big system supplying
a typical town, are about 10%.
23
6.3 Electricity generation
6.3.1 Overview
Electric energy is without competition the most versatile of all the energy carriers. It
is relatively easy to convert to any of the technical energy forms that are important for
final use in the human society, heat, mechanical energy and light. Electricity can also
be transported over long distances without large losses. Its only weakness is that it is
expensive to store in large quantities. As a consequence of this, the generation of
electricity must always match the demand and this leads to high requirements on load
following capability for an electricity supply system.
Four conversion routes are used for generation of electricity:
− Mechanical energy is used to drive an electric generator
− Thermal energy is converted to mechanical energy and the mechanical energy is
used to drive an electric generator
− Chemical energy is converted directly to electric energy
− Radiation energy from the sun is used to generate electric energy
The first two of these routes are the only ones that can be used to generate electricity
at reasonably low cost in large quantities. Each of the conversion routes will be
discussed in the following.
6.3.2 Direct mechanical to electrical energy
Mechanical energy can be extracted from nature in the form of hydropower, wind
power, wave power or tidal power. The principle is simple: The kinetic energy in a
naturally moving fluid is picked up by a turbine or similar device and the shaft of the
turbine is used to drive the electric generator. The principle is illustrated in figure 14.
The conversion efficiency can be high and approach 90% for hydropower plants. For
wind power it is less for reasons explained in a later lecture.
6.3.3 Thermal to electrical energy
Most of the electricity generation in the World is based on conversion of thermal
energy to mechanical shaft power that is used to drive an electric generator. In most
cases the thermal energy is generated either by combustion of a fuel or by nuclear
fission. There are also examples of power plants using geothermal heat or solar heat.
The process for conversion of thermal energy to mechanical energy is the same as that
briefly described in the first lecture, namely:
A working medium (gas or liquid) at temperature T0 is compressed to an elevated
pressure in a pump or compressor.
The pressurised working medium is heated from the temperature T1 obtained after the
compression to a temperature T2 by means of thermal energy generated by
combustion or transferred to the working medium. The average temperature during
heating of the working medium is Thot.
24
The energy in the hot pressurised working medium is used to drive a turbine or a
piston engine. The temperature in the working medium is then reduced – thermal
energy is converted to mechanical work.
After this, the working medium, now at temperature T3 is either exhausted to the
atmosphere or cooled to the initial temperature T0 and returned to the pump or
compressor.
Since only a fraction ηC= (1-T0/Thot) of the thermal energy supplied to the working
medium during step 2 can be converted to mechanical work11 the conversion is
associated with considerable losses. The losses appear as the thermal energy that must
either be exhausted to the atmosphere or be removed in step 4 by cooling of the
working medium. If only mechanical work is considered as useful energy from the
process, the maximum achievable efficiency of a process where thermal energy is
converted to mechanical work is obviously equal to ηC, also called the Carnot
efficiency.12
6.3.3.1
Processes with combustion of a fuel for heat generation
The most important processes for generation of electricity from a fuel that is
combusted to generate heat are:
− The steam process
− The gas turbine process
− The combined gas turbine and steam turbine process
− The internal combustion engine process
Each of these has advantages in some situations and some capacity ranges.
The working principle of a power plant using the steam process is illustrated in figure
21. A fuel is combusted in a furnace of a steam boiler and the heat is used to heat
pressurised water and turn it into steam that is further heated up to a temperature of
400-600oC. The energy in the hot pressurised steam is then converted to mechanical
power in a turbine where the steam pressure drops to something like 0,05 bar. The
steam is then cooled and condensed to water and the water is pumped back into the
steam boiler.
In reality the process is made more complex in order to reach a high efficiency. The
purpose is to design the process in such a way that the average temperature for heat
addition to the working medium becomes as high as possible. This is achieved by preheating of the water before it enters the steam boiler by means of steam extracted
from the turbine and by interrupting the expansion in the turbine at one or two
pressures for re-heating of the steam. Figure 22 shows an example of a process flow
scheme for a modern steam power plant.
11
Please observe that the temperatures must be given in the absolute Kelvin scale in this formula.
Temperatures in K are obtained by adding 273,15 to temperatures in oC.
12
The limitations in conversion of thermal energy to mechanical work were explained by the French
engineer Sadi Carnot in 1824
25
Figure 21. Principles of a steam power plant
Figure 22. Flow scheme for a large steam power plant
26
The steam process is being used for power plants over a wide capacity range from a
few MW to over 1000 MW. Any combustible material can in principle be used as
fuel. The efficiency is limited by the maximum allowable temperature in the heat
exchanger tubes, the “superheaters” where the steam is heated to its entrance
temperature to the steam turbine. Steam temperatures of 580oC are being used in
several commercial plants. Research is in progress with temperatures up to about
650oC. Depending on steam conditions used in the process and the process design,
the net efficiency of large steam power plants can be in the range 35 – 49%.
The principles of a gas turbine used for generation of electric energy is illustrated by
figure 23.
Figure 23. Principles of the gas turbine process
Air is sucked in by a compressor and the pressure is raised to something between 12
and 20 bar. Fuel is burnt in the compressed air and the temperature is raised to
something in the range 1000 – 1400oC. The energy in the hot compressed gases is
converted to mechanical power in the turbine that drives both the compressor and the
electric generator.
Gas turbines require clean fuels and most gas turbines operate on natural gas or light
fuel oil. It is possible to use coal or wood as fuel if the solid fuel is first converted to a
gas in a gas producer and then thoroughly cleaned from dust and corrosive vapours
that may form by reactions between the ash elements in the solid fuel.
Gas turbines are available with capacities in the range from about 100 kW to 250
MW. The efficiency is increasing with size and the inlet temperature to the turbine.
With the simple process gas turbine process efficiencies in the range 30 – 35% can be
achieved. The main energy loss is in form of thermal energy in the hot exhaust gas.
This energy can however be utilised if the hot exhaust gas is used for generation of
steam to a steam process. This combination of the gas turbine and steam turbine
27
process is illustrated in figure 24. Normally 60 – 70% of the electricity is generated by
the gas turbine and 40 – 30% in the steam turbine.
The principles of such a “combined cycle” power plant is illustrated by figure 24.
Figure 25 shows a complete process flow scheme for such a plant.
Figure 24. Principles of a combined cycle power plant
28
The combination of a gas turbine process and a steam process makes it possible to
reach very high efficiencies. Figure 26 shows a comparison between efficiencies for
the simple gas turbine process, the steam process without re-heat of the steam, the
steam process with re-heat of the steam and the combined cycle. The improvement
achieved by introduction of the combined cycle is obvious.
The diagram does not show however the efficiency that is possible with double reheat of steam and extreme steam temperatures that is used in some recently built
steam power plants. In these, efficiencies in the range 45 to 49% has been obtained
for coal fired power plants with output about 410 MW.
Figure 26. Efficiencies of different types of power plants
The combined cycle does not only offer high efficiency, it is also relatively cheap to
build. The necessary investment is about 30% of that required for an advanced steam
power plant.
The weakness of the combined cycle is however that gas turbines are designed for
clean fuels like natural gas. These fuels are normally more expensive than for instance
29
coal that can be used in the advanced steam power plant. It is therefore not at all
obvious that the combined cycle power plant is the best solution for electric power
generation at a given site.
An alternative to the simple gas turbine is the reciprocating internal combustion
engine. The power output ranges from about 10 kW to about 40 MW. Diesel oil or
natural gas is used as fuel for small engines with outputs up to a few MW. Larger
engines may also use heavier fuel oil. The efficiency is in the range 40 – 42 % also for
small capacities.
6.3.3.2
Processes using nuclear fission for heat generation
Most of the nuclear power plants use the steam process for conversion of the nuclear
fission heat to electrical energy. Two types of processes are used, illustrated in figures
27 and 28. The process shown in figure 27 uses steam generated in the nuclear reactor
to drive the steam turbine. Since the nuclear reactor then is cooled with boiling water,
this type of reactor is called boiling water reactor.
Figure 27. Flow scheme for the Oskarshamn boiling water reactor power plant
In the process shown in figure 28, the reactor is cooled with circulating water at a
pressure of about 160 bar. The thermal energy in this water is used to generate steam
by means of a heat exchanger “the steam generator”. The steam is then used to drive
the steam turbine.
30
Figure 28. Flow scheme for steam power plant with a pressurized water reactor
It is not possible to reach very high temperatures for the working medium in these
processes. Typical maximum steam temperatures are in the range 270 – 285oC. As a
consequence, the efficiencies are relatively low about 30%.
6.3.3.3 Processes using solar heat
Use of solar heat for generation of steam in a steam power plant will be discussed in a
subsequent lecture.
6.3.4 Chemical to electrical energy
It is not necessary to use the conversion route via thermal energy for conversion of a
fuel to electric energy. Direct conversion is also a possibility and this makes it
possible to avoid the inevitable losses associated with the conversion from thermal
energy to mechanical energy.
The principle of the so called “fuel cell” process is illustrated in figure 29. Fuel in gas
form is entered into a chamber where one of the walls is an electrode. Oxygen is
supplied to another chamber, also with a wall in form of an electrode. The two
electrodes are in contact with an electrolyte. The difference in chemical potential
between the two chambers results in a flow of fuel ions through the electrolyte from
the fuel side to the oxygen side and also a flow of electrons from the electrode on the
fuel side to that on the oxygen side.
31
Figure 29. Working principles of a fuel cell
The process is successfully applied for relatively exotic applications like electricity
generation in space crafts and propulsion of submarines. The electrolyte is then
phosphoric acid and the fuel is hydrogen. There are also experimental installations for
electric power generation and propulsion of cars. These fuel cells are expensive,
partly because the electrodes use platinum as catalysts. Also hydrogen is not a readily
available fuel. The efficiency for conversion of hydrogen to electricity is about 50%.
Natural gas, ethanol and methanol can be used for production of hydrogen that can be
used as a fuel for fuel cell, but these conversion process is also associated with losses
and this reduces the overall efficiency from commercial fuel to electricity to
something between 35 and 40%. From an energy efficiency point of view, there are
no gains in comparison with advanced combined cycle processes.
Use of other electrolytes is studied in research projects. One approach that is to use a
molten carbonate at about 650oC as electrolyte. Fuel cells of this type can also use the
chemical energy in carbon-monoxide, CO. This means that cheap commercial fuels
like coal can be gasified to generate a gas that can be used as fuel for the fuel cell.
Overall efficiencies of 50 – 55% can be reached with this process.
Another approach is to use a solid oxide at high temperature, about 1000oC as
electrolyte. If this type of fuel cell is combined with a steam process, efficiencies in
the range 65 – 70% are projected.
6.3.5 Solar radiation to electrical energy
Generation of electric energy by direct conversion of solar radiation energy will be
discussed in a subsequent lecture.
32
6.4 Co-generation of electricity and heat
In all the thermal processes for electricity generation described in section 6.3.3, a
significant fraction of the thermal energy is not converted and leaves the process as
thermal energy at a lower temperature.
The human society also can make use of such heat, either for heating of buildings,
where a temperature level of 50 – 90oC is possible to utilise, or for drying in industrial
processes, where temperatures in the range 100 – 150oC are useful. The overall
efficiency of the power plant would increase considerably if such thermal energy
would also be utilised.
The principle is simply to use for instance water for a district heating system as a
coolant for the condenser in a steam power plant. In case of a gas turbine or
reciprocating engine a heat exchanger cooled by district heating water is installed in
the exhaust gas stream. Figure 30 illustrates the principle for a co-generation plant
with a steam process.
Figure 30. Principles of a co-generation plant with a steam process
When the steam process is used for co-generation, the temperature level in the
condenser must be raised from the 25 – 35oC level that is used in plants that generate
only electricity to about 100 – 130oC.
The consequence is that the amount of electricity that can be generated with a given
amount of fuel will be less if co-generation is used. The gain in overall efficiency can
however compensate for this. No reduction of electric power yield will be obtained if
a heat exchanger is installed for recovery of thermal energy in the exhaust gases from
a gas turbine or reciprocating internal combustion engine.
33
The advantage of co-generation is obvious if the fuel consumption per electric energy
unit for cogeneration is compared with the fuel consumption for a process with only
electricity generation. The marginal efficiency ηmarg is defined as13:
P
η m arg = el
ΔP fuel
Some process industries like pulp and paper mills and sugar mills need process steam
at pressures between 12 and 2 bar. This means that a steam boiler must be installed to
supply the process with the steam. These industries also need electric power and it is
then often attractive to use co-generation of electricity and steam. The additional cost
for installation of a steam boiler for higher pressure than that needed by the process is
not high and therefore the electricity can often be produced at a cost much below the
price of grid electricity. Co-generation is particularly interesting if the industry also
generates a combustible residue that can be used as fuel for the steam boiler.
Table 6 shows typical performance data for different co-generation processes. The
marginal efficiency is calculated for an efficiency of 90% in separate heat generation.
All the processes show marginal efficiencies far above the efficiencies of the most
advanced processes for separate electricity generation.
Table 6.
Performance data for co-generation plants14
Process
Fuels
Steam (district heating)
Steam (steam for industry)
Simple gas turbine (district
heating)
Combined gas turbine steam
process (district heating)
Diesel
engine
(district
heating)
Combined fuel cell steam
process (district heating)
Anything
Anything
Natural
gas
Natural
gas
Diesel oil
Natural
gas
Thermal
energy
hot water
steam
hot water
ηel
ηtot
α
ηmarg
0,3
0,25
0,35
0,88
0,88
0,88
0,5
0,4
0,66
0,84
0,83
0,85
hot water
0,45
0,88
1,04
0,86
hot water
0,45
0,89
1,02
0,88
hot water
0,60
0,90
2,0
0,90
The following definitions are used:
P
P + Pheat
Electric yield η el = el ; Overall efficiency η tot = el
P fuel
P fuel
P
El/heat output ratio α = el
Pheat
13
ΔPfuel is the difference between fuel input for co-generation and for heat
generation only.
14
When comparing the performance data in table 6 it must be remembered that the
co-generation plant with the steam process can use any kind of fuel, whereas the other
processes require more expensive fuels.
34
6.5 Production of charcoal and coke
Solid fuels release combustible gases, “volatiles” when the fuel particle passes the
temperature interval from about 250 to about 900oC during the combustion process.
This is normally an advantage because it leads to easier ignition of the fuel. In some
applications this release of gases leads to problems. The problems can be eliminated
by pre-treatment of the solid fuel by heating to the temperature where no more
volatiles are released.
6.5.1
Charcoal production
On a global scale, the most important application where the volatiles cause problems
is the use of biomass as cooking fuel in stoves without a chimney in developing
countries. When burning wood or other biomass directly, there will always be a lot of
smoke. Where cooking is done indoors or outdoors in places with poor ventilation, the
smoke is particularly unpleasant.15 The smoke consists mainly of un-burned or partly
burned volatiles from the wood and one possibility to eliminate the smoke problem is
to convert the wood into charcoal that has much less volatiles and therefore burns
without much smoke. Charcoal is therefore an important energy carrier in these
countries. The efficiency in the conversion process from wood to charcoal is
determining the sustainability of the fuel wood supply. Many African countries show
rapid de-forestation in the neighborhood of the towns. This is partly blamed on the
charcoal production.
The oldest and probably still the most widely used method for charcoal production is
the earth kiln. Two varieties exist, the earth pit kiln and the earth mound kiln, see
figure 31.
Figure 31. Traditional charcoal kilns
15
The smoke is also bad for the health of the women exposed to it
35
An earth pit kiln is constructed by first digging a small pit in the ground. Then the
wood is placed in the pit and lit from the bottom, after which the pit is first covered
with green leaves or metal sheets and then with earth to prevent complete burning of
the wood. The earth mound kiln is built by covering a mound or pile of wood on the
ground with earth. The mound is preferred over the pit where the soil is rocky, hard or
shallow, or the water table is close to the surface. Mounds can also be built over a
long period, by stacking gathered wood in position and allowing it to dry before
covering and burning.
Earth kilns can be made at minimal cost, and are often used near wood resources,
since they can be made entirely from local materials. Earth kilns can be made in any
size, with the duration of the process ranging from three days to two months. Gross
variations in the quality of the charcoal can occur, because in one batch some of the
wood is burned and some of the wood is only partly carbonised. Efficiencies are
generally low, around 10-20% by weight and 15-30% in energy terms. The efficiency
and the quality varies depending on the construction of the kiln (e.g. walls can be
lined with rocks or bricks and external chimneys can be used), and the monitoring of
the carbonisation process.
There are several industrial processes for production of charcoal from wood. One of
the most efficient is the Lambiotte retort process, see figure 32. It gives a yield of
about 60% on energy basis.
The predried wood is lifted to the top of the retort by means of a skip-car or conveyor
and is dumped into a double bell gate, which allows the wood to enter and yet
prevents escape of significant quantities of retort gases. The level of the wood in the
retort is monitored and loading controlled to keep the level constant by means of
automatic controls.
During carbonisation the wood moves slowly down the retort encountering a rising
countercurrent flow of inert hot gas, which dries the wood and raises it to carbonising
temperature. Typically the wood takes about eleven hours to pass through the retort
and emerge as charcoal from the base through a pair of hydraulically operated
interlocking gates, which allow the finished charcoal to be removed in small batches
about every twenty minutes. The time to pass through the retort and the intervals
between removals of charcoal at the base are under the control of the operator and can
be varied to suit the moisture content of the wood, required rate of charcoal output
and fixed carbon requirement in the product. The movement of the charge down the
retort leads to formation of undesired fines but by unloading intermittently as
described above this effect is reduced and the percentage of fines is about the same as
charcoal produced in conventional brick kilns.
The charcoal has to be cool when it leaves the retort as otherwise it would ignite on
contact with the air. The cooling is accomplished by blowing either cool inert or
combustible gas into the bottom of the retort, and as it rises it extracts the heat from
the finished charcoal passing down the retort to the discharge gates. The heated gas is
drawn off at the middle of the retort just below the point where the hot gas for
converting the wood to charcoal is blown in.
36
The correct circulation of the gas streams is assured by close control of the pressures
at critical points. The hot gas is usually produced by burning combustible gas in a
stove with air and this hot neutral gas at around 900°C is blown in just above the exit
point of the cooling gas stream. This strips the remaining tar from the charcoal at this
point and completes the carbonisation step. The gas passes up the retort, giving up its
heat to the descending charge and picking up and 'rinsing' away the volatiles being
given off by the descending wood.
1. Raw material.
2. Drying stage
3. Distillation stage
4. Carbonisation stage
5.
6.
7.
8.
9.
Cooling
Retort
Hot
Cold
stove
inert
stage
Charcoal
gas
gas
gas
Fig. 32 Lambiotte retort - Sectional View
37
The gas issuing from the head of the retort is maintained at a temperature sufficient to
prevent condensation of tars and other volatiles at the top of the retort and in the
associated pipework. The high thermal efficiency of the Lambiotte retort is due to the
fact that the products of the system, charcoal and volatiles leave the retort at about the
same temperature as the wood enters it.
There are a number of options in disposing of the gas steam from the top of the retort.
The most obvious one is to use this gas to heat the wood and to cool the charcoal.
Providing the wood being carbonised is well dried there is enough heat in the effluent
gas when it is finally burned with air to perform this.
6.5.2
Coke production
Most steel mills use coal as the primary energy source. In a blast furnace where the
fuel and iron ore are supplied from the top and the heat is generated in the bottom by
combustion of the fuel the fuel particles must have sufficient mechanical strength to
carry the weight of the charge above. Coal does not meet that requirement, but the
product obtained after de-volatilisation, “coke”16 will.
For this reason the coal is de-volatilised before used in the blast furnace. This is
achieved by heating of the coal in a “coke oven”. The combustible volatile gases
obtained in the coking process are used as fuel for heating of the coal and for other
fuel demands in the steel mill.
6.6 Tri-generation processes
Co-generation of electricity and heat is one example of energy conversion processes
where more than one form of useful energy is produced. Some other such processes
are possible but are not used to a large extent. One example could be the pyrolysis
processes discussed in section 6.5. If these are designed well, the generation of
combustible gases may exceed the needs for the pyrolysis process. The excess gas
may then be used for other purposes, for instance as fuel in a plant for cogeneration of
electricity and heat. This is practised in the steel mills in Luleå and Oxelösund. The
primary energy, coal, is then used for production of three energy carriers, coke,
electricity and heat.
Tri-generation is also planned for a plant for production of fuel ethanol from wood to
be located somewhere in Northern Sweden. In the process, wood is treated with an
acid in order to convert the cellulose and hemi-cellulose to fermentable sugars. The
process is called hydrolysis. The fermentation of the sugars gives ethanol. However,
the lignin in the wood will not be converted and appears as a residue from the
hydrolysis. This residue represents a substantial fraction of the energy input with the
wood.
16
Swedish “koks”
38
The plan is to use part of the lignin as fuel for the process, where heat is required for
distillation of the ethanol to reduce its water content. The remaining residue from the
hydrolysis can be used for production of solid fuel pellets or, together with residual
heat from the ethanol plant as energy input to a co-generation plant.
A simplified, popular presentation of this tri-generation process with energy inputs
and outputs is shown in figure 33.
Figure 33. Bioenergy combine for production ethanol, electricity, fuel pellets and
district heat
The overall efficiency for generation of ethanol, electricity and heat is estimated to be
68%. This is significantly better than the efficiency of 25% for ethanol production
only.
6.7 Production of hydrogen
Hydrogen is considered an interesting energy carrier, mainly because combustion of
hydrogen leaves water as the only combustion product17. It is certainly possible to use
hydrogen as fuel for generation of heat in small and large scale, for using hydrogen as
fuel in all the thermal processes for electricity generation and of course to use
hydrogen as a fuel for fuel cells with small or large capacity.
Unfortunately, sources of hydrogen, ready for exploitation do not exist in nature even
though there are large amounts of water that might be used as raw material for
hydrogen production.
17
This might not be entirely true. If hydrogen is burnt with air and the combustion temperature is high
also nitrogen oxides NOx will form by reactions between oxygen and nitrogen in the air. If however
combustion conditions can be controlled to avoid formation of NOx, the combustion product would be
water only.
39
Hydrogen can be produced from fossil fuels, but this appears as less attractive than
use of electrolysis of water, with electricity generated either by solar energy,
hydropower or nuclear power.
Electrolysis of water requires about 4500 kWh/Nm3 of hydrogen. The effective
heating value of hydrogen is 10.62 MJ/Nm3 and this gives an efficiency for hydrogen
production of about 66%. Storage and handling of hydrogen is not easy. The energy
density is low and the molecules are small. This leads to large storage volumes or
high pressures and risks for hydrogen losses by leakage.
The European Commission appears to be quite optimistic about a future hydrogen
economy and supports several research projects including a pilot project illustrated in
figure 34 where solar energy is used to generate electricity in photovoltaic panels and
the electricity is fed into the grid and used for production of hydrogen by electrolysis.
The hydrogen is used in a bus driven by a fuel cell.
Figure 34. A hydrogen energy pilot project.
Time will show if the optimism on the high political level in Europe is justified. There
are good reasons for being skeptical. The costs for hydrogen production and handling
are high and so are the costs of fuel cells. There are needs for technology development
and mass production of equipment to bring the costs down. The main obstacle will
probably be “a chicken and egg problem” – who wants to develop and mass produce
40
fuel cells when there is no hydrogen , and who wants to invest in hydrogen production
when there are no fuel cells?
Strong government interactions will probably be needed for introduction of the
hydrogen economy.
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