Lecture_notes - Environmental aspects.pdf

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Växjö university
02-05-21
Lesson 06: 1
Distance learning course
Bioenergy Technology
Environmental aspects - Chapter 6, General considerations
The scope of this chapter is to bring up to discussion some environmental aspects that are not
in a simple way covered by the previous chapters.
The fundamental structure of biomass
For the scope of this chapter it is of interest to subdivide the biomass in another way than we
have previously done. We will now regard the biomass as composed from cellulose, lignin,
hemicellulose, extractives, ash and water. Looking at it from this point of view – and
restricting ourselves to look only at dry woody biomass – Howard (1973) found the following
compositions for pine components:
Bark
Needles
Branches
Top
Root
Stem
Cellulose 1
23.7
42.6
36.9
41.5
44.6
51.1
Hemicellulose 1
24.9
22.3
33.7
31.2
25.6
26.8
Lignin 1
50.0
37.7
35.0
32.5
31.3
27.8
Extractives 2
13.0
26.2
13.6
11.0
11.7
9.1
Ash 2
0.9
2.4
1.2
0.8
1.6
0.3
1
Weight-% of extractive-free oven-dry weight
2
Weight-% of un-extracted oven-dry weight
Consider now that the heating value of the extractives is about 33-38 MJ/kg, of lignin 25.1
MJ/kg, of hemicellulose about 16-17 and of pure cellulose 17.4 MJ/kg. We then find that 1 kg
of stem wood should have a heating value of 20.4 MJ/kg and that the amount of ash becomes
3g/20.4 MJ = 147 mg/MJ. For the needles we find in a similar way a heating value of 22.7
MJ/kg and a specific ash content of 24g/22.7 = 1057 mg/MJ. Thus the specific ash content in
the needles is more than seven times higher than the specific ash content in the stem wood.
Exercise : Do the same calculations for the other components.
This is general: The green parts and the fast growing parts will have higher ash contents per
energy unit than will the stem wood. And not only that: The ash composition is also different.
Material balances
Consider now a full-grown pine tree. This tree will typically be composed of 7-15 %
branches, 3-8 % needles, 3-8 % bark and 65-80 % stem wood. Let us now, for the sake of
discussion, define a generic tree as 10 % branches, 5 % each bark and needles and 80 % stem
wood. Further assume that the primary aim of the forestry is to supply saw-mills with raw
material – Not to supply fuel.
The removal of 1000 kg of stem wood and bark from the forest – leaving branches and tree-
tops and needles – would then mean that 941 kg of stem wood with a mineral content of 0.3
% is taken out together with 59 kg of bark with a total mineral content of 0.9 %. The total
mineral loss would thus be 3 35 kg.
If the felling residues – branches and treetops – are removed together with the stem, we will
now remove a further 118 kg of branches containing a total of approximately 1.18 kg of
mineral.
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Växjö university
02-05-21
Lesson 06: 2
Distance learning course
Bioenergy Technology
What happens is thus that the removal of felling residues – in spite of the fact that they are
only less than 20 % of the total mass – increases the mineral withdrawal from 3.35 to 4.53 kg
corresponding to an increase of approximately 35 %. This very simple example illustrates
how important it is to incorporate into any system for bioenergy utilisation a sub-system to
balance the mineral content of the soil. We call this ash recirculation.
The second thing is that the ash composition differs so that for example calcium – which
might be one important constituent in the formation of submicron particles – is less common
in the stem wood than in the fuel collected from branches and treetops. Potassium – on the
other hand - seems to be present in higher concentrations in the stem-wood ash than in ashes
from residual forest fuels.
The most volatile elements – such as mercury, chlorine, zinc, lead and alike – tend to emerge
from the fireplace in gas phase and then co condense on the fly-ash particles during the
convective pass. Low-volatile elements such as copper tend rather to stay in the bottom ash.
Some elements – like phosphorus – are distributed between fly-ash, gas phase and bottom ash
in a way determined by the combustion technology.
Thus, ash recirculation is much more complex than only collecting all the ashes and shipping
them back to the felling site:
•
What is removed from the forest is determined by the biological needs of the plants in
combination with the technology used for fuel collection. If the green parts are left at the
felling, then the mineral withdrawal is very much different from what it is if the green
parts are also removed.
•
What is left in the ashes after combustion is a very complex function of the technology
used, of the cooling rates in the convective pass and of the efficiency of the particle
separation devices used for flue-gas cleaning.
•
To maintain a mineral balance in the soil, what is needed is to recycle the ashes with an
addition of the material losses in the combustion plant.
Ash properties, preparation and impact
Usually, the ashes have a very high alkalinity, i.e. they will give rise to very high pH-values
when exposed to water. For areas exposed to acid rain, this is generally speaking a positive
property since the ashes may then be used to counteract acidification. However, the porous
ash material dissolves very rapidly which may give rise to a pH-chock, which may – in turn –
promote the leakage of aluminium ions to the ground water. Also may the rapid dissolution of
the ash particles lead to a local eutrofication, which is very harmful to the young plants at a
reforestation site.
For both these reasons, you usually have to take measures to stabilise the ashes, i.e. bind them
so that they dissolve slowly and release their mineral content over a period of 5-10 years.
The binding occurs as a chemical reaction – either with carbon dioxide so as to produce
calcium carbonate (CaCO 3 ) or as a hydration so as to form calcium hydroxide (Ca(OH) 2 ). The
ash is usually watered either to reduce the dust problems or to produce a slurry suitable for
pellet production. In cases of water quenching, the ash may be so wet as to need drying prior
to pelletising or other treatment. Given the wet (moist) ash it may thus either be rolled into
Växjö university
02-05-21
Lesson 06: 3
Distance learning course
Bioenergy Technology
pellets or only spread for drying. The drying may either be natural or forced by the aid of
warm flue gases. In the latter case, the CO 2 -content of the flue gases will promote a
carbonisation while – in the former case – the hydration is more likely to occur.
From a leaching resistance point of view, the carbonised ash is preferable. Ashes that have
only been air-dried may carbonise after they have been spread in the forest, where the CO 2 -
concentration is slightly increased due to the respiration of the plants (see chapter 2 if you get
confused by this). Since CaCO 3 has a larger volume than Ca(OH) 2 , this might lead to a
breakage of the ash particles which will increase the leakage. Too rapid leaking of cat-ions
may cause a secondary effect by increasing the local nitrate losses from the soil and may thus
cause eutrofication in nearby water recipients.
A rule of thumb says that ashes should not be spread on forest land within five years prior to
or after a clerar-cut and that the ash durability should be such that the deterioration of the ash
particles takes 5-25 years.
Limiting factors for ash recirculation are the heavy metals (you will want to refer to the paper
by Denis Dugwell to get more information on this topic). A recommendation by
Skogsstyrelsen (The national board of Forestry in Sweden) sets an upper limit to ash
recirculation to maximum 3 tonnes/hectare and 10 years to avoid most such problems. There
are also limiting values for a number of heavy metals. Please refer to “Rekommendationer vid
uttag av skogsbränsle och kompensationsgödsling” meddelande 2/2001 and other relevant
publications at www.svo.se).
In the textbook, you will find in section 6.3 some diagrams to indicate the concentrations of
various metals in fresh-water lakes as a function of pH. Remembering that the ashes are
alkaline to their nature will now indicate that the solubility of several metals is decreased as
the pH-value is increased. However – this is also dependent on the amount of metals available
and it is not so easy as to say that the metal content in fresh-water is decreased by the
recirculation of ashes.
The bottom ash does usually contain so much unburned fuel as to make it unsuitable for
recirculation. This is not because the unburned fuel has any negative impact but only because
too high contents of fuel will make the ash particles too brittle. Mixing the bottom ashes with
a binder – cement or something like it – may make also these ashes suitable for recirculation.
Particulate emissions from biofuel combustion
In chapter 5 in the textbook some emphasis was put on the formation of gaseous emissions
and on soot. However, also other particles are formed during combustion and the submicron
particles have come more and more into focus during the last few years. Present findings
indicate that one major mechanism of formation of these very small particles is homogeneous
condensation of potassium oxide and/or calcium oxide in the flue gas.
It may be suspected that the formation of these particles has been increased by the use of air-
staging as a method of NO x -abatement, but this has not been investigated in any detail.
Anyway, the mechanism is – roughly – that the pure metal is gasified in the primary
combustion zone where the mean conditions are sub-stoichiometric. Usually, the metal vapour
pressure is higher than that for its oxide and when the metal vapour meets the secondary air
Växjö university
02-05-21
Lesson 06: 4
Distance learning course
Bioenergy Technology
and is oxidised a super-saturated pressure of the metal oxide is established. This means that a
homogeneous condensation takes place resulting in a very large number of very small nuclei.
Typical number concentrations would be in the order of 10 7 particles/cm 3 . Assuming a
particle of CaO, a density of 3000 kg/m 3 and a particle diameter of 10 -7 m, this gives about 3
mg/m 3 only from these very small particles. However, number concentrations exceeding 10 8
are not uncommon in small, biofuel-fired plants and the submicron particles may thus
constitute a significant part of the particulate emissions.
The mass concentration might seem low, but the health effects of these particles may be
severe. This is partially because these submicron particles penetrate the lungs all the way
down to the alveolar sacs but also because they represent a relatively large external area, in
the example above they represent 0.3 m 2 /m 3 of gas. As mentioned previously in this course,
several different contaminants such as heavy metals but also heavy hydrocarbons may
condensate on such surfaces. Substances that condensate on the submicron particles may then
be transported deep into the respiration system and deposited. Even if the submicron particles
themselves would be completely inert, they may thus be regarded as a potential health hazard.
The common systems for particulate removal from flue gas – cyclones, electrical precipitators
or bag filters – are all inefficient for these small particles. Bag filter may certainly remove
them but at a high cost in terms of pressure drop. Some results indicate that flue-gas
condensation may be efficient to transfer them from gas phase to liquid phase but then the
problem with the condensate on the particles remain.
It has been shown that one main formation mechanism is via the presence of calcium,
potassium and other high volatile metals and these metals are – for natural reasons – present
in higher concentrations in biouels than in other fuels. Once again the biological origin of the
fuel has an impact on the effects caused by it.
The question of submicron particles has recently been focussed by the European Union and
regulations for air quality (pm10 and pm 2.5) are currently being developed and put into
action.
Storage
A common way of fuel handling in Sweden is to harvest the fuel during winter/spring and
then to store it until next winter. If you look back into chapter 4 of the textbook (e.g. section
4.5) you will find that “terminal storage” is mentioned at several places. Now, storage of
comminuted material in large piles may seem a good idea – in spite of the degradation taking
place – but this is not necessarily the case. During storage, some of the extractives are
released from the hips. The extractives do certainly smell nice in low concentrations (the are
used in soap and several other hygiene products) but at higher concentrations they may cause
serious problems with allergic reactions and alike to those living in the close vicinity. In
extreme cases, the odour is so intense as to prevent residents in areas close to wood-chips
terminals from garden life during periods.
Växjö university
02-05-21
Lesson 06: 5
Distance learning course
Bioenergy Technology
Discuss:
•
Taking the potential environmental problems with terminal storage into account – what
aspects would you say are important as fuel logistics are concerned?
•
The paper of Denis Dugwell brings into discussion the behaviour of metals during
combustion. Now – how would you say that the metal emission “fingerprint” of a certain
type of biofuel would vary with the moisture content of the fuel? From that: Is it possible
to draw any conclusions concerning metal emissions like the conclusions we drew about
hydrocarbon emissions and alike when figure 5.14 in the textbook was discussed? Or
would you say that the mechanisms are different?
•
Typically, fly-ash particles from a biofuel-fired plant exhibit a bimodal distribution. One
mode is the submicron part and the other mode is particles about a few micrometers in
diameter – up to maybe 100
m at the most. Why is this distribution bimodal and how
would you say that this changed by “normal” particle removal devices?
µ
•
Considering the bimodal size distribution – how would you characterise the health impact
from the particle emissions?

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