Economics and Rural Development
Vol.9, No.1, 2013 ISSN 1822-3346
Assessment of the Amount of Greenhouse Gases in Biogas Production in Latvia
Kaspars Naglis – Liepa1, Modrite Pelse2
The provision of energy, its availability and sufficiency at economically justified costs are the issues of economic growth, life
quality, and security for any country. The factor of life quality and security is closely related to a clean environment that does not
threaten human health. One of the indicators for assessing the surrounding environment is emissions of greenhouse gases (GHGs).
GHGs play a significant role in the energy sector, including producing energy from agricultural crops. Agriculture is a significant
producer of greenhouse gases, therefore, if advancing towards sustainable development, it is important to reduce emissions of greenhouse gases both in agriculture and in the entire national economy. Not only production of greenhouse gases, but also their reduction
through photosynthesis is specific to agriculture.
By successfully managing the factors causing the greenhouse gas effect, it is possible to substantially reduce emissions of harmful gases, especially if energy is produced from renewable sources. The aim of the present paper is to calculate a GHG balance and
compare it with the amount of CO2 absorbed through photosynthesis as well as a fossil fuel comparator.
Key words: greenhouse gas emissions, biogas, renewable energy, photosynthesis
JEL Classification: Q01, Q53
Introduction 12
One of the main issues of global policy agenda is
how our mankind deals with energy resources and an
analysis of the causes and effects of its behaviour. The
experts of various fields engaged in the UN Intergovernmental Panel on Climate Change believe that the human
activity affects climate change. The scientists admit that
although natural processes are cyclical, which can play a
role in climate change, the human activity, however, significantly affects the planet’s climate. Without taking
measures that limit the effect of human activity, the even
and sustainable development of our mankind might be
endangered. Even if some measures are taken to adapt to
climate change, the total amount of emissions of greenhouse gases (GHGs) has to be reduced by at least 50%
until 2050 compared with 1990, as it is believed that the
planet’s climate changes will not be irreversible at such a
level of emissions.
Within the context of energy production, the production of energy from fossil resources affects the climate
change most, however, the use of various renewable energy sources cause GHG emissions as well. If the climate
1
Assist. professor dr. oec. Kaspars Naglis-Liepa
Field of scientific interest: sustainable development, alternative energy
Mailing address: Department of Economics, Faculty of Economics, Latvia University of Agriculture, Svetes iela 18, LV3001, Jelgava, Latvia
E-mail: [email protected]
2
Professor, dr. oec. Modrite Pelše
Field of scientific interest: sustainable development, alternative energy
Mailing address: Department of Economics, Faculty of Economics, Latvia University of Agriculture, Svetes iela 18, LV3001, Jelgava, Latvia
E-mail: [email protected]
18
change is viewed within the context of power industry, a
power plant generating electricity and thermal energy can
affect the environment by both its economic activity and
emissions into the atmosphere that arise from burning
fuel. Yet, the supply of energy, its availability, as well as
its sufficiency at economically feasible costs are the issues of economic growth, life quality, and security of any
country. The factor of life quality and security is closely
related to a clean environment making no threats to human health. One of the indicators for assessing the environment is greenhouse gas emissions.
In searching for alternatives for fossil energy resources, a certain role in the entire energy potential is
nowadays played by energy gained from crops of agricultural origin. As it is known, agriculture is a significant
producer of greenhouse gasses; therefore, it is important
to reduce GHG emissions both in agriculture and in the
whole economy if moving towards sustainable development. It is a positive fact that agricultural production features not only the production of GHGs, but also the reduction of GHGs through photosynthesis.
It is possible to significantly reduce emissions of
hazardous gases by successfully managing the factors
causing the greenhouse effect. The aim of the present paper is to calculate a GHG balance by comparing the
amount of GHGs produced with the amount of CO2 absorbed through photosynthesis as well as with a fossil energy comparator. Maize as an energy crop and its use for
biogas production at cogeneration power plants was included in the calculation.
Methods
In the paper, the calculation was based on the balance
calculation method. Growing maize for silage used in energy production at a cogeneration power plant is selected
Vol.9, No.1, 2013 ISSN 1822-3346
as the research object (the plant’s electric capacity is 0.26
MW), while the research subject is a GHG balance for
producing maize silage on Latvian farms according to
technologies of two types. An energy balance was calculated in the paper: energy (electricity) produced and energy consumed for growing maize for silage per 1 ha if it is
used for electricity production at a cogeneration power
plant. The calculation is performed for two yields of
green mass of maize: 30 and 50 t ha-1.
The calculations include the emissions produced in
the production process of a substrate and the emissions
arising from burning biogas used in energy production.
Besides, the calculations allow assessing a GHG balance,
depending on different maize production technologies resulting in different crop yields.
Discussion and Results
The goal of searching for alternative energy sources
is the necessity of generating energy from renewable or
practically inexhaustible natural sources and phenomena,
paying attention to the environmental and economic aspects of these energy sources. As science and technology
freely develops, the existence of alternatives is unavoidable. Multipolar scientific development provides increases
in the efficiency of the kinds of energy which lost their
dominance and new fundamental discoveries (Milciuviene et al., 2006; Moriarty, Honnery, 2009). It has to be
also admitted that a market is not a sufficiently effective
instrument to adjust prices (according to A.Smith’s theory); there are external benefits/costs or externalities.
From the economic point of view, fossil energy has
higher variable cost, the reasons of which are the fact that
it is difficult to access fossil reserves as well as the inclusion of cost of externalities in variable cost. For instance,
the prices of GHG quotas can double the cost of coal
(Heal, 2009).
Integrating negative externalities into the market system is partially based on increasing the use of alternative
energy. Templet offers an empirical analysis of externalities at national level. He measures the amount of subsidies, which have to offset the externalities caused by pollution, and costs of energy and taxes. He concludes that
pollution control makes a positive effect on income inequality and unemployment as well as forecasts that the
cost of pollution will tend to increase rather than decrease
(Cacho, 1999). Essential characteristics of alternative energy are the fact the product is produced and consumed in
regions as well as its positive and negative externalities.
Spatial externalities are related to the effect of economic activity on an ecosystem, biological diversity, and
genetic manipulations. Environmental benefits are gained
by growing a forest, as the forest absorbs CO2 and offsets
the effect caused by industrialisation (Wesseler, 2004).
Flora is not a less significant absorber of CO2, including
crops. Growing energy crops is not anything new, while
Economics and Rural Development
the history of growing crops for biogas production is
quite short, for instance, the first biogas facility in Latvia,
in which energy crops are used, started operating only in
2008.
The European Union has set ambitious targets in its
environmental and power sectors, which resulted in passing the Third Energy Package that composes a part of the
Europe 2020 Strategy. The Strategy continues and updates the EU policy whose goals are to reduce GHG
emissions by 20% compared with 1990, to increase the
proportion of renewable energy in its final energy consumption up to 20%, and to promote an increase in the efficiency of energy use by at least 20%. Latvia has also
accepted these goals.
According to the Climate Change Reduction Program
for the period 2005-2010, the GHG emissions will be reduced 35% in Latvia in 2020 compared with 1990 (GHG
emissions …, 2012). The situation in Latvia corresponds
to the development pattern of an economy in transition: a
sharp decrease in its GHG emissions followed by a stability period which, in its turn, is followed by an increase in
its GHG emissions (Huang, Lee, 2008). It is forecasted
that an increase in the consumption of energy and the related increase in its GHG emissions will correspond to
the emission amount set in the Kyoto Protocol if the so
called scenario with measures is implemented. Owing to
the generous allocation of GHG emission quotas for Latvia within the EU emissions trading system, the country
is not forced to take emergency measures for reducing its
GHG emissions.
The inclusion of externalities of GHG emissions in
the market system is an important production prerequisite
to reach overall efficiency in the distribution of resources.
Therefore, even if GHG emissions are not permanent
goods – they are a compulsory component in the cost
formation of goods – the cost amount and the physical
amount consumed are important. To determine the sustainability of regulated production, it is necessary to find
the amounts of energy consumed and GHG emissions
produced in order to strictly define the cost of including
externalities important to society (indirectly, through various types of support, for instance, purchase prices) in the
market price. To do it, it is necessary to calculate a GHG
balance for the extraction and use of energy resources.
Assessment of the Greenhouse Effect in Biogas
Production in Latvia
In agriculture, there are three gases and their sources
of origin that cause the greenhouse effect:
CO2 emissions: their causes are the use of agricultural land, vehicles, as well as the raising of livestock;
N2O emissions: use of fertilisers containing nitrogen
and of manure as well as microbiologic change in soil
caused by other sources of nitrogen;
CH4 emissions: rising of livestock and irrigation
measures (IFA, 2012). To compare the effects caused by
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Economics and Rural Development
Vol.9, No.1, 2013 ISSN 1822-3346
GHG emissions more objectively, values of CO2 equivalent (CO2eq) are used, which a unified unit of measurement for hazardous GHGs. It characterises the globalwarming potential (GWP) – the extent to which a certain
mass of CO2 affects global warming over a certain period
(100 years). Three main GHGs that affect global warming
are CO2 (1 GWP value), CH4 (25 GWP values), and N2O
(298 GWP values). Sometimes GHG emissions are expressed in units of carbon equivalent which is 3.663 times
greater value (Dick, Smith et al., 2008).
Biogas production and its conversion into energy
provide a neutral output of carbon dioxide, according to
academician A.Kalnins. Biogas production collects methane (CH4) by not exhausting it into the atmosphere, and
it amounts to 1500 kg CO2eq per cattle unit a year (35 kg
CO2 equivalent/per kg CH4) (Kalnins, 2009). In 2009,
LLU scientists conducted an extensive research “Application of Sustainability Criteria to and the Elaboration of
Measures for the Use of Biomass” (Adamovics et al.,
2009) in which, among other tasks, a GHG balance was
calculated for biogas. In that research, an approach which
provides a calculation of the pollution caused by fuel
combustion and electricity generation and its comparison
with the amount of CO2eq absorbed by plants was applied.
The consumption of fuel for technological processes,
which totalled 161 l ha-1 of diesel fuel, was calculated,
and for the purpose of calculating CO2equi emissions it
was assumed that 1 litre of diesel fuel produces 6484g
CO2eq. In the same way, the emissions caused by the consumption of electricity were calculated, assuming that 1
MJ of electricity produces 363 g CO2eq. Other factors
causing emissions, for instance, the use of fertilisers,
were not calculated, as the authors believed that these
factors are offset by the effect of using digestate. It was
found in the research that additional 160.45 g CO2eq MJ-1
of energy is generated by producing biogas (Adamovics
et al., 2009). Yet, it has to be admitted that the amount of
fertilisers, which depends on agricultural and climatic
conditions, not always may be fully offset by the amount
of digestate produced. Therefore, the authors of the present paper believe that it is necessary to include the effect
of using fertilisers, too, in the calculation, which may be
reduced after a precise effect of using digestate is determined. The fuel emission factor (6484g CO2eq) is also a
great value.
Therefore, the authors calculate a GHG balance for
two the most popular maize production technologies. The
method of calculating a GHG balance is presented in
Fig.1.
Factors of the quality of
activity
Amount of fuel consumed, l ha-1
Fuel emission factor,
kg CO2eq l-1
Amount of fertilisers
consumed, kg ha-1
Fertiliser emission factor,
kg CO2eq kg-1
Amount of plant protection products consumed,
kg ha-1
Amount of electricity
consumed, kWh ha-1
Amount of biogas burnt,
m3 ha-1
multiplied
Values of the
quantity of activity
Emission factor of plant
protection products,
kg CO2eq kg-1
Value of qualitative
quantity
Amount of energy consumed to produce a
maize substrate and
process it into biogas,
MJ ha-1
Electricity emission factor, kg CO2eq kWh-1
Biogas emission factor,
kg CO2eq m-3
Fig.1. Scheme for Calculating the Amount of Energy Consumed for Producing a Substrate and Processing it into Biogas
per 1 ha of Agricultural Land
20
Vol.9, No.1, 2013 ISSN 1822-3346
The indicators showing the amount of activity are the
same as in the energy balance calculation method, only
the indicators of activity differ, as in this case they are related to the amount of GHG emissions.
In the formula, the addends consist of a multiplier for
the quantity of activity (litre, kg, kWh etc.) and a multiplier for the quality of activity (kg CO2eq kg-1, kg CO2eq
m-3 etc.). A GHG emission balance may be calculated by
multiplying an indicator of activity level (amount per
unit) by an emission factor expressed in units of CO2
equivalent relative to the amount of CO2 absorbed by
plants (EMabs ha-1). The analytical formula is as follows:
EMabs ha-1 / Qfuel ha-1 * EFfuel + Qfertil ha-1 * EFfertil +
+ Qprot ha-1 *EFprot. + Q el *EFel + Qbiog * EFburn
where
EMabs ha-1 – amount of CO2eq absorbed by maize, kg CO2eq ha-1
Qfuel ha-1 – amount of fuel necessary for growing and harvesting
maize and transporting it to a biogas facility, l
EFfuel – fuel emission factor, kg CO2eq l-1
Qfertil ha-1 – amount of fertilisers necessary for growing maize, kg
EFfertil – fertiliser emission factor, kg CO2eq kg-1
Qprot ha-1 – plant protection products necessary for growing maize,
kg
EFprot – emission factor of plant protection products, kg CO2eq l-1
Qel – amount of electric energy, kWh
EFel – electricity emission factor specific to Latvia, kg CO2eq kWh-1
Qbiog – amount of biogas produced from 1 ha if growing maize,
m3 ha-1
EFcomb – amount of CO2 emissions (normative value) produced by
biogas combustion, kg CO2 m-3.
The results are summarised according to the pairs of
multipliers, namely, emissions caused by the use of fuel,
emissions caused by the use of fertilisers, emissions
caused by the use of plant protection products, and emissions caused by the consumption of electricity which all
make up a total GHG amount. To obtain a balance (ratio),
it is necessary to calculate the amount of CO2 absorbed
by plants through photosynthesis. An intermediate result
is calculated, and a balance (a ratio of the amount of CO2
absorbed to the amount of CO2 produced) is obtained.
Calculation of the Emissions Caused by Automobiles and Tractors
Land tillage, the sowing and harvesting of crops, as
well as the ensilage of green mass and its loading into a
fermentor are related to exploiting machinery, which in
this case means that greenhouse gases are produced. To
calculate the emissions caused by fuel combustion per
hectare of agricultural land, the authors multiplied the
amount of fuel consumed with the amount of emissions
caused by combusting 1 l of fuel. According to the authors, 157.7 l ha-1 of diesel fuel at a crop yield of 50 t ha-1
and 129.6 l ha-1 at a yield of 30 t ha-1 are needed to produce a maize substrate.
The value of fuel emission factor, according to the
scientific literature, is quite large. In the mentioned pro-
Economics and Rural Development
ject of Adamovics, the diesel fuel emission factor is set at
6484 g CO2eq l-1 (Adamovics u.c., 2008), which is much
larger than it is mentioned in other sources of information. In the “Guidance on Measuring and Reporting
Greenhouse Gas Emissions from Freight Transport Operations” developed by the government of Great Britain
(DEFRA, 2012), the diesel fuel emission factor is set
within a range from 2630 g CO2eq l-1 to 2670 g CO2eq l-1.
In the website of the Canadian government’s agency Environment Canada, the diesel fuel emission factor is set at
2663 g C02 l-1 (Environment Canada, 2012). An emission
calculation method developed by the agency Greenfiniti
is based on an assumption that the emission factor is
2850 g Co2eq l-1 (Greenfiniti, 2012). According to a report
of the project “BioGrace – Harmonisation of GHG Calculation for Biofuels” funded by the EU, the diesel fuel
emission factor is much larger or 3140 g CO2eq l-1. Given
the general trend of increasing the factor, the authors will
use the value of BioGrace or 3140 g CO2eq l-1.
To determine the GHG effect for fuel at two different
yields of green mass of maize, it is necessary to multiply
the consumption of fuel (129.6 l ha-1 and 157.7 l ha-1) by
the diesel fuel emission factor (3140 gCO2 eq l-1).
EMfuel ha-1 = 129.6 (or 157.7) l ha-1 * 3.140 kg CO2 eq l-1
Result: the GHG effect for fuel is 406.9 kg CO2eq ha-1
at a yield of 30 t ha-1 and 495.2 kg CO2eq ha-1 at a yield of
50 t ha-1.
Calculation of the Emissions Caused by the Use of
Fertilisers
The determination of GHG effect for the use of fertilisers is related to two aspects. First, fertilisers are products of fossil resources, the production and transportation
of which significantly affect CO2 emissions. Of the total
output of nitrogen fertilisers, 97% is produced by processing ammonia, and 70% of phosphorus fertilisers are
gained by using phosphorus acid. In terms of amounts of
production and consumption, NH3, HNO3, H2SO4, and
H3PO4 are the most important substances (Eiropas
Komisija. 2007). Second, in the result of fertilising soil, a
part of fertiliser substances evaporate or run off into
ground waters. In practice, the use of fertilisers is strictly
regulated in legal enactments (Nitrogen Directive, Cabinet of Ministers Regulation No.33 of 1 November 2011).
Despite the fact these effects are known and politically
regulated, the authors have not found studies, conducted
in Latvia, which set the emission factors for fertilisers.
Several authors, for instance, Smith and his colleagues, in
a report of the UN Intergovernmental Panel on Climate
Change point that climatic conditions, soil properties, geographic position, and other factors significantly affect
the nitrogen fertiliser emission factor (Smith, Bouwman,
2000). Therefore, a certain dilemma exists in assessing
the negative aspects of use of fertilisers: on the one hand,
21
Economics and Rural Development
an amount of GHGs caused by energy production is an
essential factor in assessing the sustainability of production, on the other hand, such an assessment is approximate if fertiliser emission factors are not objective. In accordance with European Parliament and Council Directive 2009/28EC, Article 19, the Member States have to
assess the amount of GHGs absorbed in biofuel production according to a methodology described in Part D of
Annex 5 of the Directive and submit a report (Directive
2009/28/EC). The response submitted on behalf of Latvia
contains a general conclusion that the expected total
emissions might be greater than it is mentioned in Annex
5 of Directive 2009/28EC (Ministry of Economics,
2010). According to maize production technologies, the
following amounts of fertilisers are needed to gain a
maize yield of 30 t ha-1: ammonium nitrate (N 34.4) - 200
kg ha-1 and 300 kg ha-1 of complex fertiliser NPK (16-1616). To increase the yield to 50 t ha-1, the necessary
amount of ammonium nitrate is 300 kg ha-1 and that of
complex fertiliser NPK (16-16-16) is 520 kg ha-1.
There are several researches contributed to determining GHG amounts, for instance, researches by the ecoinvent Centre (Swiss Centre for Life Cycle Inventories) – in
its reports available at the Centre’s database (Ecoinvent,
2010). Topical standard values may be obtained in the report “BioGrace – the List of Additional Standard Values”
of the project “BioGrace – Harmonisation of GHG Calculation for Biofuels” funded by the EU which was published at the end of 2011 (BioGrace, 2011). Besides, there
are various regional studies. The overall trend, according
to various findings, is an increase in the standard emission factor values, which is related to the calculation formula of GHG equivalent, as it includes an assessment of
GHG emissions over time. Therefore, the authors will use
the BioGrace standard values in their further calculations.
The production technologies, included in the calculation,
contain two types of fertilisers: ammonium nitrate (N
34.4) whose emission factor equals 6209 g CO2eq/kg and
complex fertiliser NPK (16-16-16) which is not available
in any information source reviewed by the authors; instead of it, NPK 15-15-15 is available, but the authors
will not use it due to possible errors. The value of emission factor is 7107 g CO2eq/kg. An amount of emissions
from the use of fertilisers can be calculated as follows:
EMfertil ha-1= (200 (or 300) kg ha-1 * 6.209 kgCO2eq l-1 +
(300 (or 520) kg ha-1) * 7.107 kgCO2eq l-1
The GHG effect for the use of fertilisers equals
3373.9 kg CO2eq ha-1 at a yield of 30 t ha-1 and 5558.34 kg
CO2eq ha-1 at a yield of 50 t ha-1.
The emissions from the use of nitrogen oxide (N2O)
due to its runoff and evaporation as a part of the nitrogen
cycle are also regarded as a significant emission factor.
This GHG effect value is comparatively small if compared with practically usable nitrogen fertilisers; the av22
Vol.9, No.1, 2013 ISSN 1822-3346
erage emission amount ranges within 1.25+-1% of the
amount of nitrogen used (Smith, Bouwman, 2000). Yet,
given the “comparative hazard” or scales, nitrogen oxide,
according to Part C of Annex 5, Directive 2009/28/EC, is
almost 300 times more hazardous. As a result, the pollution caused in soil becomes an essential factor of hazardous emissions in agriculture.
A simplified way of how to calculate emissions caused
by the use of nitrogen oxide without performing field
measurements is to assume that the amount of damage is a
percentage (1.25%) of the amount of pure nitrogen. Actually, to determine the pollution of nitrogen oxide, it is necessary to know only the amount of pure nitrogen.
The necessary amount of pure nitrogen (Npure) may
be calculated as follows:
Npure = N 34.4(200 (or 300) kg ha-1) * 0.344 + NPK 1616-16((300 (or 520) kg ha-1) *0.16
According to the maize production technologies
used, 116.8 kg of pure nitrogen is needed to gain a green
mass yield of 30 t per hectare and 186.4 kg of pure nitrogen is required for a yield of 50 t ha-1. An amount of pollution may be calculated as follows:
EMN2O = 116.8 (or 186.4) * 0.0125 = 1.46 (or 2.33) kg
N20 ha-1
To convert a value in units of CO2eq, the value obtained has to be multiplied by 296, as it is assumed that a
unit of nitrogen dioxide is 296 times more hazardous than
a unit of CO2.
Based on the mentioned, the GHG emissions from the
use of nitrogen dioxide at the yields of 30 and 50 t ha-1 are
432.2 kg CO2eq ha-1 and 689.7 kg CO2eq ha-1, respectively.
Result: after summarising the GHG effects calculated
for fertilisers and N2O, it is found that 3806.1 kg CO2eq
ha-1 at a yield of 30 t ha-1 and 6248.0 kg CO2eq ha-1 at a
yield of 50 t ha-1 are produced.
Calculation of the Emissions Caused by the Use of
Plant Protection Products
Using any plant protection product is related to GHG
production. Since the production of plant protection products
is associated with aggressive and hazardous chemical compounds, the use of plant protection products has to be prudent. In terms of GHG emissions, plant protection products
are three to four times more hazardous than fertilisers.
Two types of plant protection products are used in
growing maize: a pesticide, Glyphogan, (4 l ha-1) and a
herbicide, Ararat, (0.2 l ha-1). The amount of plant protection products used, according to the technologies chosen,
has no impact on yields. In the cases of analysis, 4.2 l of
plant protection products are needed.
The amount of GHG emissions is estimated at 20.5 kg
CO2eq l-1 (Hillier, Walter et al. 2011). By using the men-
Vol.9, No.1, 2013 ISSN 1822-3346
tioned values for calculating GHG emissions from the use
of plant protection products, one can gain an equation:
EMprot = 4.2 l ha-1 * 20.5 kg CO2eq l-1
Result: the total amount of emissions from the use of
plant protection products equals 86.3 kg CO2eq ha-1.
Calculation of the Emissions Caused
by the Consumption of Electricity
A certain amount of emissions is produced if electricity is consumed, which is mainly needed to provide the operation of cogeneration power plants. Theoretically, these
emissions are offset, as the electricity produced has a positive balance, therefore, it might not be calculated or it may
be assumed to be zero. Yet, in this case, it is important to
know the amount of electricity consumed, so that it is later
subtracted from the total amount of energy generated in
cogeneration, and to get a precise CO2 balance.
To determine the necessary amount of electric energy, the total electric capacity of electric equipment at biogas facilities has to be multiplied by the number of
hours of their operation. By employing such an approach,
it is found that the necessary amount of electricity to produce biogas from maize silage equals 415 kWh ha-1 a
year (Adamovics et al, 2009).
Yet, in this case, a part of energy necessary for repairing and servicing equipment, which is a value to be hardly
found, would not be taken into account. The authors obtained practical data by using performance indicators of the
training and research farm “Vecauce”. In 2010, 150 069
kWh of electricity was consumed for biogas production,
which equals 802 kWh ha-1 at a yield of 30 t ha-1 and 1339
kWh ha-1 at a yield of 50 t ha-1 if calculated per necessary
area of land for the farm (Vecauce, 2011).
Emissions caused by the consumption of electricity are
a regional indicator and depend on the type of resources
exploited in electricity production. In Latvia, the emissions
from electricity production are assumed to be on average
400 g CO2eq kWh at a power grid energy loss of 11% (European Bank for Reconstruction and Development, 2009).
EMel = 802 (or 1339) kWh ha-1* 0.4 kg kWh-1
Result: the amount of emissions from electricity production is 320.8 kg CO2eq ha-1 at a yield of 30 t ha-1 and
535.6 kg CO2eq ha-1 at a yield of 50 t ha-1.
Calculation of the Emissions Caused by Biogas
Combustion
To convert biogas energy into mechanical energy, a
process of combustion is necessary. In the process of biogas combustion, oxygen is consumed to burn methane existing in biogas, and, in the result, carbon dioxide and vapour (water) is produced.
Economics and Rural Development
To determine the amount of emissions from burning
biogas per ha of agricultural land, the following values
are necessary:
1) amount of CO2 emissions from burning biogas
(normative value), EFcomb kg CO2 m-3
2) amount of biogas produced per ha if growing
maize, * Qbiog m3 ha-1.
According to Weber and his colleagues, 2.75 kg of
CO2 is produced from burning 1m3 of methane. A total
amount of CO2 produced from burning biogas may be
calculated according to the following formula:
kg CO2 total = 1m3 biogas (x%CH4*pCH4*2.75 + pCO2 (1-x%CH4))
(Weber, Cuellar, 2008)
x%CH4 – proportion of methane in biogas,
pCH4 – density of methane, 0.65 kg m-3
pCO2 – density of carbon dioxide, 1.8 kg m-3
According to Weber, it is assumed that irrespective
of the content of methane in biogas, 1.8 kg of CO2 is produced from the combustion of one cubic meter of biogas
(Weber, Cuellar, 2008).
In determining the emissions from biogas combustion
per 1 ha of agricultural land, the second necessary value is
an amount of biogas to be produced from 1 ha of agricultural land. This value depends on the yield of substrate per
unit of area, which is 30 and 50 t ha-1 in the present calculation. For simplicity, it is assumed that the output of biogas per ton of green mass of maize equals 180 m3 t-1. Using
these values, the following equation is derived:
Qbiog, m3 ha-1 = Qgr. mass * Ybiog,
Qgr. mass – yield of green mass of maize, t ha-1
Ybiog – output of biogas per ton of green mass of maize
After inserting the numerical values, the equation is
as follows:
Qbiog, m3 ha-1 = 30 (or 50) t ha-1 * 180 m3 t-1
In the result, it is found that 5400 m3 of biogas per
hectare at a yield of 30 t ha-1 and 9000 m3 at a yield of
50 t ha-1 can be produced.
The values obtained may be inserted in an equation
for calculating an amount of CO2 emissions produced
from combusting biogas:
EMcomb = 1.8 kg CO2 m-3 * 5400 (or 9000), m3 ha-1
Result: by combusting the amount of biogas produced from 1 ha of agricultural land, the emissions of
9720 kg CO2 ha-1 at a yield of 30 t ha-1 and 16200 kg CO2
ha-1 at a yield of 50 t ha-1 are produced.
Total GHG Emissions from Biogas Production if
Maize Silage is Used as a Substrate
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Vol.9, No.1, 2013 ISSN 1822-3346
By summing up the total amount of GHG emissions
during the entire cycle of biogas production, starting with
growing energy crops and ending with combusting biogas,
it is found (in whole numbers) that 14340 kg CO2 ha-1 at a
maize yield of 30 t/ha
yield of 30 t ha-1 and 23565 kg CO2 ha-1 at a yield of 50 t
ha-1 are produced. The results broken down by CO2
amount are presented in Fig.2.
maize yield of 50 t/ha
18000
16200
16000
kg CO2/ha
14000
12000
9720
10000
8000
6248
6000
3806
4000
2000
0
406 495
diesel fuel
86
fertilisers, incl.
N2O
plant
protection
products
320 535
electricity
biogas
combustion
Fig.2. Amounts of GHG Emissions Produced in the Process of Biogas Production from Maize Silage, kg CO2eq ha-1
Source: authors’ construction
Biogas combustion produces the largest part of emissions: 9720 kg CO2 ha-1 at a yield of 30 t ha-1 and 16200
kg CO2 ha-1 at a yield of 50 t ha-1, which accounts for
68% of the total emissions. The next largest group of
emissions is comprised of the emissions from the production and use of fertilisers, which accounts for 23% of the
total emissions regardless of yields. After assessing the
emissions per unit of green mass of maize, one has to
conclude that the amounts of emissions differ insignificantly. The GHG emissions per ton of green mass of
maize amount to 478 kg CO2eq t-1 at a yield of 30 t ha-1
and 471.3 kg CO2eq t-1 at a yield of 50 t ha-1.
Based on the fact that the main product is energy, it is
of great importance to calculate the total amount of GHG
emissions per unit of energy. In this case, it is necessary to
divide the total amount of GHG emissions (14339 kg
CO2eq ha-1 at a yield of 30 t ha-1, 23614 kg CO2eq ha-1 at a
yield of 50 t ha-1) by the amount of energy generated
(116 640 MJ ha-1 at a yield of 30 t ha-1, 194 400 MJ ha-1 at
a yield of 50 t ha-1). In the result, to generate 1 MJ of energy, 122.9 g CO2eq MJ-1 at a yield of 30 t ha-1 and 121.2 g
CO2eq MJ-1 at a yield of 50 t ha-1 are required. One has to
conclude that yields are not a factor significantly impacting
the amount of GHG emissions. The values obtained
stressed the great impact of fertilisers on sustainable biogas
production, therefore, the use of digestate is a significant
factor in growing maize, which is the necessary substrate
for biogas production, to make not only economic, but also
energy and environmental gains.
Calculation of the Amount of CO2 Absorbed
Through Photosynthesis
A significant environmental gain of growing energy
crops is not only the replacement of fossil resources, but
also the absorption of GHGs through photosynthesis oc24
curring in plants. There are three biochemical mechanisms that provide the absorption of carbon in plants.
They are: C4, C3, and CAM photosynthesis. C4 and
CAM plants have special chemical compounds allowing
them to absorb CO2 more effectively. The most typical
plants of C3 photosynthesis are wheat, barley, fruits, rice,
and tomatoes; they account for 89% of the world’s flora.
Plants of C4 photosynthesis are: maize, sorghum, and
sugarcane; they are comparatively little widespread, grow
mostly in southern areas, and account for less than 1 % of
the world’s flora. Plants of CAM photosynthesis are
pineapples and prickly pears that are widespread in tropical areas and account for about 10% of the world’s flora
(Carvajal, 2012).
The amount of CO2 absorbed can be calculated as
follows:
EMabs kg CO2 ha-1 = Qdry matter* PFmaize
Qdry matter – dry matter yield of maize, t ha-1
PFmaize – CO2 absorption factor for maize, kg CO2 t-1 dry matter ha-1
It is assumed that the content of dry matter in the
green mass of maize accounts for 33%, therefore, a dry
matter amount is calculated as follows:
Qdry matter = 30 (or 50) t ha-1 * 0.33
In the result, it is found that 9.9 t dry matter ha-1 at a
yield of 30 t ha-1 and 16.5 t dry matter ha-1 at a yield of
50 t ha-1 are obtained.
The second necessary multiplier, CO2 absorption factor (PFmaize), can be calculated, but in this paper the authors use the CO2 absorption factors available in the scientific literature. The CO2 absorption factor for maize silage equals 17745 kg CO2 ha-1 at a dry matter yield of
Vol.9, No.1, 2013 ISSN 1822-3346
Economics and Rural Development
13 t ha-1 (Chianese, Rotz, Richard 2009) or if measured
per unit of dry matter mass – 1365 kg CO2 t DM ha-1, but
if measured per unit of energy, a value of 0.114 kg CO2
MJ-1 ha-1 is obtained. Regardless of the fact that the authors base their research on observations in the USA, this
emission absorption factor is lower than that in the mentioned research by Adamovics and his colleagues (0.1767
kg CO2eq MJ-1 ha-1 at a green mass yield of 40 t ha-1). It
means that theoretically maize in Latvia absorbs more
CO2 than in the USA, which is difficult to explain. According to the scientific literature, the amount of absorption of carbon dioxide depends on moisture, fertilisation,
solar radiation, and other factors. It has to be noted that
regardless of the fact that the yield mentioned in a research conducted in the USA is expressed in units of dry
matter, but in the research by Adamovics – in units of
green mass of crop, the results have to be approximately
equal if it is assumed that the average content of dry matter in the green mass of maize is about 33%. Being precautious in order not to create a pretentious viewpoint on
biogas as a sustainable product owing to a high CO2 absorption factor, the authors chose the smallest value
which was 1365 kg CO2 t DM ha-1. The values obtained
may be inserted in an equation to calculate the amount of
CO2 absorbed by maize:
EMabs ha-1 = 9.9 (or 16.5) dry matter ha-1 *
* 1365 kg CO2 t-1 dray matter ha-1
The calculation led to a result that the green mass of
maize absorbs 13513.5 kg CO2 ha-1 at a yield of 30 t ha-1
and 22522.5 kg CO2 ha-1 at a yield of 30 t ha-1.
CO2 balance
In a carbon dioxide balance, the amount of CO2 produced is compared with other values, which would allow
us to ascertain the effect of biogas production on global
warming.
The authors will calculate:
1) amount of CO2 produced relative to the amount
of CO2 absorbed, which allow assessing the sustainability
of biogas cycle;
2) amount of CO2 produced from burning biogas in
energy generation relative to fossil CO2 emissions.
The results obtained are summarised in Table 1. It
was taken into account in the calculation of GHG amount
per MJ that 116640 MJ of energy at a yield of 30 t ha-1
and 194400 MJ at a yield of 50 t ha-1 are generated.
The fossil fuel comparator for cogeneration, when calculating energy savings, in accordance with Article 19 of
Part C, Annex 5, Directive 2009/28/EC is 85 g CO2eq MJ-1.
Table 1. CO2 Balance for Biogas Produced from a Maize Substrate per ha and MJ
Source of CO2 emissions
Vehicles and tractors
Fertilisers (N 34,4, NPK 16-16-16)
Fertilisers (N2O)
Plant protection products
Electricity
Biogas combustion
CO2 produced in TOTAL:
CO2 absorbed through photosynthesis
BALANCE 1
Fossil energy comparator
BALANCE 2
Yield 30t ha-1
Produced,
Produced,
kg CO2eq ha-1
g CO2eq MJ-1
406.9
3.5
3373.9
28.9
432.2
3.7
86.3
0.7
320.8
2.8
9720.0
83.3
14340.1
122.9
-13513.5
-115.9
826.6
7.0
-85.0
-78.0
According to the CO2 balance, 7 g CO2 MJ-1 at a
yield of 30 t ha-1 and 5.3 g CO2 MJ-1 at a yield of 50 t ha-1
are additionally produced in the cycle of biogas production from a maize substrate and its use. Negative values
are obtained if the amount CO2 produced from biogas
combustion is compared with the fossil fuel comparator,
which means that in a full energy production cycle, more
CO2 is absorbed than is produced.
Conclusions
Yield 50t ha-1
Produced,
Produced,
kg CO2eq t-1
g CO2eq MJ-1
495.2
2.5
5558.3
28.6
689.7
3.5
86.3
0.4
535.6
2.8
16200.0
83.3
23565.1
121.2
-22522.5
-115.9
1042.6
5.3
-85.0
-79.7
emissions. A saving of emissions accounts for 91% at a
yield of 30 t ha-1 and 93% at a yield of 50 t ha-1 if compared with fossil energy. The use of digestate – a byproduct of cogeneration – in fertilising crops should be
also taken into account; therefore, the total amount of
CO2 can be reduced given the considerable impact of fertilisers on GHG emissions. Thus, one can conclude that
biogas production from energy crops (maize) is sustainable and make a small effect on the environment compared
with fossil resources.
Although an additional amount of CO2 produced exceeds that absorbed through photosynthesis, it is a comparatively small amount if compared with fossil fuel
25
Economics and Rural Development
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