Lecture_notes - Biofuel_combustio.pdf - 05BiofuelCombustion - mmarmour

Växjö university

2002-12-17

Lesson 05: 1

Distance learning course

Bioenergy Technology

Biofuel combustion - Chapter 5, Energy releasing aspects

The formation of biofuels is basically the binding of solar energy in solid, organic, material by

the photosynthesis CO 2 + H 2 O + (Solar) energy

CH 2 O + O 2 , where CH 2 O represents

glucose C 6 H 12 O 6 , which in turn represents cellulose. The energy storage capacity of the solid

material, on a dry basis, is approximately 20 MJ/kg.

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The solid material may then be treated - for example dried or compacted - whereby the energy

content per kg dry substance is not altered, or it may be chemically or biologically converted

into another material. In these latter cases the energy content per kg of dry matter may be

increased but there will inevitably be a loss of carbon involved in the conversion process. If -

for example - 1 kg of solid material (CH 2 O) is converted into methanol (CH 3 OH), and if we

assume that all the carbon originally present in the solid (400 g/kg dry substance) is converted

into carbon-in-methanol, then we will find that we produce 1067 grams of methanol from

1000 grams of pure cellulose. This may be achieved by adding the necessary hydrogen into

the process. What is more interesting: The original material represented - say - 20 MJ whereas

1.067 kg of methanol represents 43.5 MJ. So obviously, the process is strongly endothermic

(it needs energy input) and an absolute minimum would be to supply 23.5 MJ/kg of incoming

dry substance. In reality this energy is taken by partly combusting the raw material itself and

if we limit the amount of produced methanol from 1 kg of input to represent the same energy

as the raw material we find that 20 MJ equals 0.49 kg of methanol. 37.5 % of the methanol is

carbon so we find the upper limit for the carbon conversion into methanol to be 184 / 400 =46 %

on a weight basis. If we finally assume that 10 % of the energy throughput in the process is

lost, we find a production limit of 0.441 kg of methanol/kg of raw material and a carbon

conversion of 41.4 %. If the gain in later steps in the fuel-to-energy chain is big enough to

carry the losses thus experienced - and please beware of the difference between carbon loss

and energy loss - then fuel conversion such as liquefaction or gasification is worthwhile.

Biological conversion is in some cases more efficient than thermochemical conversion, but as

long as no extra energy is supplied the total energy output from the process does not exceed

the energy input via the raw material.

Whatever has been the treatment of the raw material, we end up with a chemical substance

containing in it what was originally solar energy bound by the photosynthesis. We now want

to release this energy and we want to distribute and use it. It can be used either in the form of

concentrated exergy (high quality energy that may be transformed to other types of energy) or

in the form of low-quality energy (anergy, energy that can effectively not be transformed into

any other form of energy). If you are not comfortable with this, please refer back to the

introduction to this course, section 1.3, or to textbooks on thermodynamics.

To understand the limitations in energy conversion you need to be familiar with a few

fundamental things concerning combustion and steam cycles, so lets first go through these

fundamentals.

Växjö university

2002-12-17

Lesson 05: 2

Distance learning course

Bioenergy Technology

Combustion and energy “production“ in general

All gas volumes referred to are given at 0 o C and 10135 Pa unless otherwise stated.

The energy releasing chemical process is the reverse of the photosynthesis reaction

CH 2 O + O 2

H 2 O + CO 2 + Energy

and we will find the energy released as latent heat in the hot gaseous products H 2 O(g) and

CO 2 (g). This process is called “combustion“.

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Usually the combustion process is not supplied with pure oxygen but with air, and since air

can be regarded as 20.8 % oxygen (O 2 ) + 79.2 % inert gases (here designated N 2 , nitrogen),

we get

CH 2 O + O 2 + 3.81 N 2

→

H 2 O + CO 2 + 3.81 N 2 + Energy

The introduction of the inert components into the reaction means that we do not only have

two molecules of combustion products from the reaction (one CO 2 , carbon dioxide, and one

water, H 2 O) as in the first reaction, but that we have 5.81 molecules. This will “dilute“ the

energy released so that instead of getting 20 MJ of energy from one kg of CH 2 O released into

0.747 m 3 of CO 2 plus 0.747 m 3 of H 2 O we will have 20 MJ stored in 0.747 m 3 , CO 2 , 0.747

m 3 of H 2 O and no less than 2.847 m 3 of N 2 . Thus the concentration of energy in the

combustion products becomes 20 / 4.341 =4.607 MJ/m 3 instead of 20 / 1.494 =13.387 MJ/m 3 . The

concentration of energy is a direct measure of the temperature of the combustion products and

thus we have just seen that the use of air for combustion instead of pure oxygen radically

lowers the attainable temperature.

Now look back into section 1.3.2 and you will find that this has an impact on the Carnot

efficiency. Prior to elaborating further on the Carnot efficiency, let us do one more example:

Let us assume that the fuel is not only dry substance (CH 2 O) but also contains some moisture.

We may for example assume a moisture content of exactly six molecules of water per

molecule of glucose, which corresponds to a composition like CH 2 O+H 2 O. If you can not

figure it out yourself you will have to take my word that this represents a moisture content of

37.5 % on a weight basis.

In lesson 3 it was taught that the heating value of moist biofuel (ash content 1.5 %) may be

estimated from

19.2 . (1- f ash - f water ) - f water . 2.679 whereby we obtain 11.0 MJ/kg in this

case. (Yes - 10.995 if you want to be greedy...)

∆

≈

The chemical reaction in this case becomes

(CH 2 O+H 2 O) + O 2 + 3.81 N 2

2 H 2 O + CO 2 + 3.81 N 2 + 11 MJ

where you will notice that the water molecule contained in the fuel, within the parenthesis,

has only been transformed to the right-hand side so that there are now two of them. Thus the

energy concentration in the combustion products becomes 11 MJ split over 1.494 m 3 of water

vapour, 0.747 m 3 carbon dioxide and 2.847 m 3 of nitrogen. Total 11 / 5.088 =2.162 MJ/m 3 . You

will realise that this represents a significant lowering of the temperature.

→

Växjö university

2002-12-17

Lesson 05: 3

Distance learning course

Bioenergy Technology

−

Hot

Cold

Remembering that the Carnot efficiency could be expressed

η =

, introducing the

Hot

combustion temperature as T Hot and finally stating that the cold temperature in a process can

not be lower than the ambient temperature (unless you input extra energy for cooling, of

−

course), we now find that

η =

Comb

Amb

Comb

If we just assume the combustion temperature at 1600 o C and the ambient temperature at 10

o C (we use a nearby creek for cooling purposes) and insert 1873.15 and 283.15 for T Comb and

T Amb in the equation, we find

85 %. That means that - theoretically - approximately 85 %

of the energy contained in the combustion products can be transformed into other forms of

energy, i.e. into mechanical work. This convertible part of the energy is called exergy . The

remaining 15 % of the energy can not be transformed into mechanical work and this part of

the energy is called anergy . Lets - just for fun - assume the combustion temperature at 1150

degrees and the cooling agent at 80 degrees. We now find approximately 75 % exergy and 25

% anergy in the combustion products.

≈

Obviously, the fraction of exergy is a strong function of the peak temperature as well as of the

temperature in the cooling agent. In practical cases, with today’s technology and materials, we

can not count upon getting much more than about half the total exergy into electricity. The

most common process is the steam cycle: Water is boiled into steam in a fuel-fired boiler,

typically at 50-200 bar, the saturated steam is further heated up to 350-550 degrees in

superheaters and the steam is then allowed to expand through turbines connected to

generators. After the turbine passage, the steam is condensed in condensers, cooled by an

agent, and the water is returned to the boiler again. The material in the superheaters sets the

temperature limit for the steam. These will have to withstand not only an internal pressure of

maybe 200 bar but also the oxidising potential of water vapour at up to 550 degrees C.

Common steels (ferritic steels) set a temperature limit at about 550 degrees while austenite

steels may be used up to 650 o C. The outside of the superheater tubes is also exposed to the

flue gases from the combustion. In case of oil firing, the vanadium content limits the material

temperature to just below 550 degrees (high temperature corrosion) whereas coals with a low

vanadium content may be used for steam temperatures up to approximately 570 degrees. For

biofuels the ash content of alkali metals may provide corrosive slags on the superheater tubes

usually limiting the steam temperature to the interval 450-500 degrees. Since the electricity

(=exergy) production is based on the steam, its the steam temperature that enters into the

Carnot efficiency as T Hot , not the combustion temperature.

Thus there are several limits to the steam data - and thereby to the possibility to produce

electricity - set by the fuel quality and by the materials used. The combustion temperature

itself is not a limiting factor. Assume a steam temperature of 500 o C and a condensing process

where the cooling agent is at 10 o C. The Carnot efficiency in this case becomes 63 %. Now

instead assume the cooler to be a district heating system where the temperature level typically

may be 70 o , and we get 56 %. If everything else is constant we then see a reduction in the

electricity production by 11 % only from the fact that we use a commercial heat sink (we sell

the anergy and get paid) instead of just cooling it away. The further reduction in total

efficiency - from 63 % down to the “real“ 40 % - occurs from flow losses in the turbine,

mechanical losses in the gearbox, electrodynamical losses in the generator and other factors.

Växjö university

2002-12-17

Lesson 05: 4

Distance learning course

Bioenergy Technology

Usually the estimate of total efficiency contains 5-10 different efficiencies on top of the

Carnot efficiency, each in the order of magnitude 90-95 %. These may thus be estimated as

0.93 7 =0.602 and applying that to our total Carnot efficiency 63 % we find the final, total

efficiency limit in condensing power plants at about 38 %. If we assume 94 % in each of the

steps we instead arrive at 40 % total... Anyone interested in more details in this field is

referred to textbooks on turbines and steam cycles.

If the sole aim with the combustion process is to “produce“ electricity (i.e. to transform the

chemically bound solar energy into exergy [=electricity]) then the conclusions become:

Use a fuel giving a sufficiently hot flue gas that contains a minimum of corrosive compounds

and

Use the coldest possible cooling medium.

So what has been done frequently has been to use fossil fuels (typically coal, 85 % carbon, 11

% hydrogen, 4 % ash, low vanadium content, low alkali content, heating value 35 MJ/kg) and

to put the power stations near the coast or near rivers where cold water can be used for the

cooling. The chemical composition of this coal can be expressed as C 71 H 110 and the

combustion reaction for dry coal becomes

C 71 H 110 + 98.5 O 2 + 375.285 N 2

→

55 H 2 O + 71 CO 2 + 375.285 N 2 + Energy

From 1 kg of dry coal we obtain 1.233 m 3 of water vapour, 1.591 m 3 of (fossil) carbon

dioxide plus 8.412 m 3 of inerts (N 2 ). The total becomes 30 MJ per 11.236 m 3 = 2.67 MJ/m 3 .

This is almost 24 % higher than the 2.16 MJ/m 3 we obtained from biofuel at 37.5 % moisture

but it is only 58 % of the 4.61 MJ/m 3 we obtained from absolutely dry cellulose. Coals will

usually have a comparatively (compared to biomass) low moisture content, typically below 10

%. Though biofuels thus can produce very high combustion temperatures it is easier to obtain

these high temperatures from coal and consequently the superheater surfaces become smaller.

Also, the flue gases contain less corrosive components than flue gases from biomass or waste

and the ash usually exhibits higher softening and melting temperatures thus reducing the risk

for molten (corrosive) ash (slag) on the superheater tubes.

Historically the power stations have been put close to large heat sinks like rivers or at the

coastline so that cold water is abundant. Doing that you may keep a low cooling temperature

throughout the year - say never exceeding 25 degrees. During the last half of the 1900’s - and

also today - cooling towers have been erected to intensify the cooling and to use the

atmosphere as heat sink.

Combining the relatively inert flue gases from fossil fuels with a low-temperature, large

capacity heat sink maximises the steam temperature and the Carnot efficiency and power

stations built along this concept will exhibit total efficiencies about 40. The use of fossil gas

in combined-cycle processes (steam turbines combined with gas turbines) may provide total

efficiencies approaching 45 % but are expensive and therefore restricted only to very large-

scale applications.

Biofuels, which have a comparatively low energy content per volume, would pose huge

logistic problems if used in this type of low-efficiency applications. The volumes needed to

provide a certain amount of energy become almost 10 times bigger if a typical biofuel is to

replace coal in a specific plant. Therefore biofuels are preferably used in plants where the

anergy can be made commercially available instead of just cooled off. This type of plants are

Växjö university

2002-12-17

Lesson 05: 5

Distance learning course

Bioenergy Technology

called co-generation plants or CHP -plants ( CHP = Combined Heat and Power) and are

characterised by a commercial heat sink. To maximise the electricity production you want to

have a heat sink ( T Amb ) at the lowest possible temperature. If the anergy can be delivered to a

space heating system (temperature levels below 80 degrees C) that is preferable to if the heat

sink is an industrial process where the temperature demand may be above 2-500 degrees. A

typical biofuel-fired CHP -plant will exhibit a total efficiency of 90-95 % whereof

approximately 30-35 % electricity and the remainder as heat, if the heat sink is a space-

heating system. If the heat sink is a high-temperature process, the electricity production will

drop down to maybe 25 or even 20 %. To increase the ratio of electricity-to-heat (exergy-to-

anergy, the

-value) integrated biofuel-gasification-combined-cycle processes are in the

development stage. This BIGCC -process shows promising theoretical data but has so far not

come into commercial use.

Let us work a simple example for what is conventional technology today:

Assume we have a community needing a total of 70 MW of space heating plus a total of 25

MW household electricity. Further assume a 100 MW CHP plant fired with biofuel with energy

content 10 MJ/kg. With an efficiency of 95 %, at full load this plant needs a supply of 10 kg

of fuel per second. This makes up for 36 tonnes per hour or - assuming a bulk density of 600

kg/m 3 - 60 m 3 /h.

Now assume the electricity to be produced in a coal-fired power station, total efficiency 40 %,

coal heating value 35 MJ/kg and density 1350 kg/m 3 . Then the electricity production demands

a total of 1.79 kg coal/second, 6.4 tonnes or 4.8 m 3 of coal/hour. The heat production is

assumed to be coal-based but to have a total efficiency of 100 % (I know this is too high, but it

is only an example). Then the heat production will demand a total of 2.0 kg/second, 13.6

tonnes/hour or 10.1 m 3 /h.

The total amount of coal needed for this community thus becomes 14.9 m 3 or 20.0 tonnes per

hour if the electricity is produced in a condensing plant while the same community could be

supplied by 60 m 3 or 36 tonnes of biofuel per hour if modern technology was used. Thus the

total volumes needed are “only“ four times bigger in spite of the low density and the low

heating value of the biofuel as compared to the coal. The difference in weight is even smaller

- “only“ less than double. Now go back to the previous lecture and think about what happens

to the number of lorries to transport this fuel. Will the transport be weight-limited or will it be

volume limited? What will happen if the biofuel used is pellets with their heating value and

density? The example serves to show how important the choice of technology is when the

energy system is planned.

Exercise : Work through the above example yourself until you really understand!

Combustion characteristics of biofuels and implications for boiler design

Fuels can be considered composed of four main parts: Ash, Moisture, Volatile compounds

and Char. During the heating of a fuel particle, mainly two different processes may be

distinguished, namely drying occurring at about 100 o C and pyrolysis occurring over the

interval 150 - 700 o C. Solid phase combustion occurs at higher temperatures and you will

realise that the final, solid phase combustion occurs with a dry fuel regardless of how much

moisture was present originally.

Lecture_notes - Biofuel_combustio.pdf

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