Introduction.pdf

- Bioenergy chapter 1 -

Biomass as energy source: thermodynamic aspects

1. Thermodynamic aspects................................................................. 2

1.1 Introduction .........................................................................................2

1.2 Sunlight as a resource...........................................................................2

1.2.1 Potential energy from solar radiation.............................................2

1.2.2 Photosynthesis..............................................................................3

1.2.3 The total biomass potential ...........................................................4

1.3 Energy conversion................................................................................5

1.3.1 The 1 st law of thermodynamics......................................................5

1.3.2 The 2 nd law of thermodynamics and the Carnot efficiency..............5

1.4 Conversion of biomass .........................................................................7

1.4.1 Efficiency calculation for combustion............................................7

1.4.2 Gasification of biomass ...............................................................11

1.5 References..........................................................................................12

Krister Sjöström

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- Bioenergy chapter 1 -

Biomass as energy source: thermodynamic aspects

1. Thermodynamic aspects

1.1 Introduction

In this chapter we shall take a closer look at the thermodynamical aspects of

biomass and its conversion into other forms of energy. Special emphasis is given

to calculations in order to give an idea concerning the possibilities and

limitations of biomass as an energy source.

Because all biomass originates from the sun, solar radiation and its potential for

photosynthesis will be discussed first. The process of photosynthesis is briefly

introduced, with the main focus on the quantitative aspects. Once we know the

potential of biomass it is interesting to see how much of the energy stored in it

can be converted into electricity, heat or other forms of energy. In section 1.3,

an introduction to the theory of thermodynamics is given. This theory will be

used in the last parts of this chapter, where some basic calculation examples of

biomass conversion are presented. The first example is an efficiency calculation

of a biomass combustion process, whereas the second part concerns the process

of biomass gasification.

1.2 Sunlight as a resource

1.2.1 Potential energy from solar radiation

All the energy on earth, regardless of the form it is in (fossil fuels, heat, power,

potential and kinetic energy etc.) originates from the sun: the earth has a

continuous power input of 1.73 x 10 17 . On an annual basis this is 1.52 x 10 18

kWh. It is difficult to imagine the significance of this number, but it is in fact

about a factor 10.000 larger than the annual world consumption in 1995: 6.64 x

10 13 kWh, according to the International Energy Agency. One could say that

enough solar energy is therefore available to cover the world energy demand,

but this form of energy is unfortunately spread out over the whole area of the

earth, resulting in an average energy density of only 340 W/m 2 (Dunn, 1986).

The atmosphere has great influence on the energy flow to and from the earth.

The atmosphere reflects about 30 % of the solar radiation and a further 20 % is

absorbed on its way through the atmosphere. The remaining 50% arrives at the

earth’s surface. In figure 1:1 the global energy balance is shown. Notice that,

from the energy coming to the biosphere, a great deal is used for evaporation

while (almost all of) the rest is lost in the form of long wave radiation. This

means that the earth is actually heating up its own atmosphere due to absorption

of the radiation by particles, water vapour and the so-called greenhouse gases.

Only 0.05 % of the incoming solar energy is available for photosynthesis, but

that is still more than 10 times the annual world energy consumption. Some

authors state a number of 1 %, but that is the percentage based on solar

radiation entering the biosphere.

Krister Sjöström

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- Bioenergy chapter 1 -

Biomass as energy source: thermodynamic aspects

Figure 1:1 Global energy balance (Dunn, 1986)

1.2.2 Photosynthesis

The biological utilization of solar energy is accomplished by photosynthesis.

Solar energy is trapped by chlorophyll (the compound that makes a plant green)

and the energy is used to convert carbon dioxide and water into organic

material. This chemical reaction can be written as follows:

H 2 O + CO 2 + light energy ã C n (H 2 O) n + O 2

(1.1)

Besides forming carbohydrates, C n (H 2 O) n , this reaction yields oxygen (all

oxygen in the earth’s atmosphere comes from this source). In reality, the

photosynthesis reaction that takes place is considerably more complicated.

However this is adequate at present. More about photosynthesis can be found in

chapter 2.

Figure 1:2 Losses in the photosynthetic process (Dunn, 1986)

Figure 1:2 shows the losses in the photosynthetic process. Of all incoming

radiation the plant itself reflects 20 %. When we consider the solar spectrum,

we know that it consists of light of different wavelengths: too long to be seen by

Krister Sjöström

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- Bioenergy chapter 1 -

Biomass as energy source: thermodynamic aspects

the naked eye (infra red), the visible part of the spectrum, and wavelengths

which are too short to be seen (ultra violet). A further discussion of the

characteristics of solar radiation is beyond the scope of this text, but it is

important to understand that not every wavelength in the solar spectrum

contains the same amount of energy.

Figure 1:3 Spectral distribution of solar energy (Dunn, 1986)

Figure 1:3 gives the spectral distribution of solar energy. Because not all

wavelengths can be used by photosynthesis, this causes losses. Dunn, 1986,

gives a maximum conversion efficiency of 50 %. The remainder is used for the

actual photosynthesis reaction which itself has an efficiency of 23 %. All these

losses result in around only 10 % of the incident radiation becoming converted

into fixed carbon. The plant itself will use up 40 % of this fixed carbon, mainly

by respiration. Organic material is then burnt in order to release energy. This

process is the reverse of the photosynthesis reaction (1.1).

This all leads to an overall theoretical efficiency of 5.5 %. In practice, however,

there are other sources of loss which further reduce the overall efficiency of a

field crop. These are:

- partial cover by the crop (particularly at the seedling stage)

- inability to convert high light intensities

- shortage of water and nutrients

- damage by animals or disease.

This all results in the following overall efficiency of photosynthesis:

0.5 - 1.3 % for temperate areas,

0.5 - 2.5 % for sub-tropical areas.

1.2.3 The total biomass potential

Earlier we have seen that of all solar energy arriving at the earth’s surface,

80 x 10 12 W is used for photosynthesis. This includes the aforementioned

average overall photosynthesis efficiency of 1 %. It is unrealistic, though, to

think that all energy stored in biomass can be converted into e.g. electricity.

Food and some building materials also originate from biomass. A very low

estimate is that only 10 % of all the energy stored in biomass can be converted

to electricity. This results in a total potential for biomass of 80 x 10 11 W, or on

an annual basis: 7 x 10 13 kWh. This means that the total potential of biomass as

an energy source is as big as the total world energy consumption in 1995:

6.64 x 10 13 kWh.

Krister Sjöström

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- Bioenergy chapter 1 -

Biomass as energy source: thermodynamic aspects

1.3 Energy conversion

In this section we take a closer look at the conversion of one form of energy

into another. We have already seen that energy can exist in different forms, e.g.

electricity, heat, chemical, potential, kinetic, radiation etc. In general:

Energy can be defined as the ability to perform work .

Unfortunately it is not possible to convert any form of energy into another form

without losses. These phenomena are described in the so-called “laws of

thermodynamics”. The two most important laws will be discussed in this

section.

1.3.1 The 1 st law of thermodynamics

One of the most fundamental discoveries in physical science was the

understanding that

the total sum of all forms of energy in a closed system is constant .

If in a closed system a process occurs which increases one form of energy, the

other forms of energy must be decreased by the same amount. Another, more

popular way, of stating this first law of thermodynamics says that

energy is neither created nor destroyed .

A very common mistake is to say that energy has been generated. This is

contrary to the first law of thermodynamics. What is usually meant is that

electricity has been generated or that energy has been converted (from e.g.

chemical energy into electricity).

The first law also implies that devices which have been claimed to continuously

produce more energy than they absorb, known as perpetual motion machines,

are impossible to construct; all examples suggested contain some kind of fallacy.

Very often this is a form of resistance (e.g. friction), which converts some of the

mechanical energy into heat. This latter form of energy can only partly be

converted back to mechanical energy, which implies irreversible losses. This is a

consequence of the second law of thermodynamics.

1.3.2 The 2 nd law of thermodynamics and the Carnot efficiency

Consider a cylinder-piston system, filled with gas under the piston. When the

gas is heated up, it will expand and push the piston and with that produce some

mechanical energy. The second law of thermodynamics says that when energy is

transferred or transformed, part of the energy assumes a form that cannot be

passed on any further, i.e. that can do no work. Very often this low quality

energy is heat.

An energy convertor which operates in a cyclic manner and takes in energy

from a high temperature source, converts some of this energy to useful work,

and rejects the remainder to a cooler temperature sink, is called a heat engine

(figure 1:4).

Krister Sjöström

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