Available online at www.sciencedirect.com

Bioresource Technology 99 (2008) 2620–2627

Energy and CO2 balance of maize and grass as energy cropsfor anaerobic digestion

Patrick A. Gerin a,*, Francois Vliegen b,c,1, Jean-Marc Jossart d

a Unit of Bioengineering, Universite catholique de Louvain, Croix du Sud 2/19, B-1348 Louvain-la-Neuve, Belgiumb Agra-Ost, Centre de Recherche et de Formation Agricole pour l’Est de la Belgique, Klosterstr. 38, B-4780 Sankt-Vith, Belgium

c GLEA, Koordinationsstelle Grunes Land Eifel-Ardennen, Brodenheckstr. 3, D-54634 Bitburg, Germanyd Laboratory of Crop Ecology, Universite catholique de Louvain, Croix du Sud 2/11, B-1348 Louvain-la-Neuve, Belgium

Received 25 June 2004; received in revised form 11 December 2006; accepted 19 April 2007Available online 18 June 2007

Abstract

Energy crops can be used to feed anaerobic digesters and produce renewable energy. However, sustainability of this option requiresthat it contributes to a net production of renewable energy and a net reduction of fossil CO2 emission. In this paper, the net balance ofCO2 emission and renewable energy production is assessed for maize and grass energy crops produced in several agricultural systemsrelevant for Southern Belgium and surrounding areas. The calculated net energy yields are 8–25 (maize) and 7.4–15.5 (grass) MWhof renewable CH4 per MWh of fossil energy invested, depending on the agricultural option considered. After conversion to electricity,the specific CO2 emissions range from 31 to 104 kgCO2

MWh�1electricity, depending on the case considered. This corresponds to a significant

reduction in CO2 emissions compared to the current reference gas-steam turbine technology which produces 456 kgCO2MWh�1

electricity.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Silage; Co-substrate; Biogas; Renewable electricity; Green certificate

1. Introduction

Following the Kyoto Protocol, 160 countries agreed toreduce their emissions of CO2 and five other greenhousegases. The European Union has made a commitment toreduce its emissions by 8% in 2010 compared to 1990. Coal,natural gas and oil fired energy production plants aremajor contributors to CO2 emissions in the atmosphere.Mitigating the current trend of increasing CO2 emissionsrelies on taking measures to reduce final energy consump-tion, to encourage a more rational use of primary energysources and to exploit renewable energy sources moreintensively. Specific measures have been taken at the Euro-pean level to encourage the production of electricity from

0960-8524/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.biortech.2007.04.049

* Corresponding author. Tel.: +32 10 47 36 46; fax: +32 10 47 30 62.E-mail address: patrick.gerin@uclouvain.be (P.A. Gerin).

1 Present address: PsPc, Chaussee de Dinant, 50, B-5530 Spontin,Belgium.

renewable sources (‘‘green electricity’’). Several countrieshave implemented a ‘‘green certificates’’ market in orderto support this production. In such a market, green certif-icates are issued by a regulating authority to the green elec-tricity producers. The producers can then sell them toelectricity distributors. As the latter need to acquire greencertificates up to a certain percentage of the electricity soldto final customers, and as there are penalties for electricitydistributors with insufficient green certificates, supply anddemand in the green certificates market determine theirprice.

The green certificate system of the Walloon Region ofBelgium (i) considers the amount of avoided fossil CO2

emission (and not saved energy) compared to reference‘‘fossil’’ technologies and (ii) includes in its scope CHPinstallations. In the Walloon system, one green certificateis allocated for each 456 kg of fossil CO2 avoided. Indeed,the reference technology considered is a standard GasSteam Turbine plant with an efficiency of 55%, which emits

P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627 2621

456 kg of CO2 to generate 1 MWhelectricity (CWaPE,2002a). In the case of combined heat and power generation,valorised heat is compared with a gas furnace with a ther-mal yield g = 90% and a specific emission of 279 kgCO2

MWh�1heat in areas connected to the natural gas grid and

with a oil furnace with a specific emission of 340 kgCO2

MWh�1heat in other areas. Accordingly, one green certificate

is delivered for each MWh electricity delivered by windor hydroelectric power plants, as these technologies donot produce CO2 and prevent the emission of 456 kgCO2

MWh�1 by the reference fossil technology. When some fos-sil fuel is used in the renewable electricity production pro-cess, e.g. for cropping, harvesting, conditioning andtransporting biomass, or for combined heat and power(CHP) production from biogas with a dual-fuel enginecycle ignited with fossil diesel, the corresponding emissionof fossil CO2 is taken into account. The net reduction ofCO2 emission has to be calculated. All the operationsrequired to convert an existing resource to the electricitydelivered on the grid have to be considered in the energyand CO2 balance. The green certificate system currentlyrunning in the Walloon Region of Belgium considers onlythe electrical and heat energy and CO2 balance. This sys-tem does not take into consideration the emission of otherpollutants, other environmental impacts (e.g. N2O or CH4

emissions), the recycling of nutrients and organic matter ora more complete life cycle assessment. It also does not takeinto account the construction and maintenance ofmachines, digester plant and buildings.

Biomass, both residual (organic domestic, garden andagro-industrial wastes, crop residues, manure, wastewatersludges. . .) and specifically grown energy crops, offers ahuge potential for the production of renewable energyand electricity. Anaerobic digestion can convert humid, fer-mentable biomass to biogas. Biogas can then be convertedto electricity and heat in a CHP unit. Manure is a resourceeasily available in many farms. Unfortunately, the limitedproduction rate and yield of biogas from manure, andthe high investment costs, make the production of biogasfrom manure uneconomic without significant subsidies,especially for farm-scale plants. Similar situations havebeen observed in Germany and Switzerland, and lead tothe implementation of subsidies systems. The biogas pro-ductivity can be greatly improved by introducing energy-rich co-substrates to the anaerobic digester, which canresult in a better economic situation. While large amountsof energy-rich co-substrates are available as domestic orindustrial wastes, the use of such co-substrates is stronglyrestricted for farm-scale plants due to waste-related regula-tions and precautions regarding the dissemination of con-taminants and pathogens. In the agriculture sector,energy crops can represent a valuable alternative to exoge-nous energy-rich wastes (Weiland, 2003; Mahnert et al.,2002; Lemmer and Oechsner, 2001; Karpenstein-Machan,2001; Linke et al., 1999; Baserga, 1998). This option isimportant, especially when considering the biomass pro-duced on set-aside land and forage surplus resulting from

less intensive animal production, as encouraged by the cur-rent European Community agriculture policy. However,producing energy crops for feeding anaerobic digestersrequires that this option contributes to a net reduction ofCO2 emission and a net production of renewable energy,i.e. the yield of renewable energy per unit of fossil energyrequired to produce it, YR/F, must be significantly greaterthan 1 for the system to be considered sustainable.

Depending on local agricultural conditions, severalenergy crops and cropping systems can be envisaged. Selec-tion of the most appropriate option by the operators andcalculation of the net reduction of CO2 emission for attrib-uting green certificates requires data on the specific bal-ances of CO2 emission and renewable energy productionof each option. The aim of this paper is to determine thesebalances for the energy crops and the agricultural optionsthat are the most relevant for southern Belgium and sur-rounding areas, i.e. maize and grass. The use of these cropsin both farm-scale and centralised anaerobic digestionplants is considered.

2. Methods

2.1. Cropping systems

The most relevant energy crops for feeding anaerobicdigesters in the southern part of Belgium are grass andsilage maize.

Silage maize can be grown in most areas. The dry matter(DM) yields range from 15 to 18 tDM ha�1 y�1 dependingon soil, climate and fertilisation (CIPF, 2003). The yieldcan be as low as 12 tDM ha�1 y�1 in the Ardenne uplandswhere the weather is colder, the vegetation period is shorterand the soils are less fertile. The quantities of N requiredfor the production of 12, 15 and 18 tDM ha�1 y�1 are 195,220 and 240 kgN ha�1 y�1, respectively (De Neuville,2003). In the areas considered, about 80 kgN ha�1 y�1 istypically supplied by the soil owing to the mineralisationof soil organic matter (50 kgN ha�1 y�1) and mineral nitro-gen left by previous crops (30 kgN ha�1 y�1) (Oost, 2001;De Neuville, 2003). Additional mineral and/or organic fer-tiliser is used to complement the crop requirements. Infarms with animals, maize is usually fertilised using man-ure. The application of nitrogen from organic fertilisers islimited to 210 kgN ha�1 y�1 by the Nitrate Directive (Wal-loon Government, 10 october 2002, following the Directive91/676/CEE). Due to the limited rate of manure minerali-sation, this corresponds to 126 kgN ha�1 y�1 available forthe maize crop during the first year after application. Inthe case of anaerobic digestion, maize would be fertilisedwith the digestate. Currently, the Nitrate Directive consid-ers digestate as an organic fertiliser, does not take intoaccount its higher proportion of mineral, directly availableN, and restricts its use to 210 kgN ha�1 y�1. However, theNH4–N in normal slurry is about 40–50% of total N, whilethe directly available NH4–N in digestate reaches 55–65%of total N (Luxen and Vliegen, 2002).

2622 P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627

Four cases will be considered in this study:

M1. Maize is grown and harvested for energy purposes.It is fertilised with manure that cannot be spread onother crops due to the limitation imposed by the NitrateDirective (‘‘excess manure’’ very common in the areaconcerned). Manure is used at the maximum allowedrate of 210 kgN ha�1 y�1, which corresponds to126 kgN ha�1 y�1 available for maize. The soil provides80 kgN ha�1 y�1. To support the maximum productionof maize and compensate for potassium deficiency inmanure, 34 kgN ha�1 y�1 and 111 kgK2O ha�1 y�1 areprovided as mineral fertilisers. The expected yield is17 ± 1 tDM ha�1 y�1. Harvested maize is silaged andfed to a decentralised anaerobic digester on the farm.M2. Maize is grown as an energy crop with nutrientrecycling, i.e. maize is fertilised with digestate at a ratecorresponding to the nutrient exported by harvest. Dig-estate is used at the maximum allowed rate of210 kgN ha�1 y�1 in addition to the 80 kgN ha�1 y�1

supplied by the soil. In nutrient recycling, no additionalfertiliser is added. Owing to the better availability of Nin the digestate compared to manure (case M1), a yieldof 15 ± 1 tDM ha�1 y�1 is still expected. Harvested maizeis silaged and fed to a decentralised anaerobic digesteron the farm.M3. Maize is grown as in case M2. Harvested maize istransported, silaged and fed to a centralised anaerobicdigester 10 km from the farm.M4. Maize is grown as an energy crop using mineral fer-tilisers only. It is fertilised with 115 kgN ha�1 y�1 min-eral nitrogen in addition to the 80 kgN ha�1 y�1

supplied by the soil. P and K are also supplied as min-eral fertilisers. The maize yield is 12 ± 1 tDM ha�1 y�1.Harvested maize is silaged and fed to a decentralisedanaerobic digester on the farm.

In all cases, the maize organic matter (OM) is assumedto be 94% of the DM and the expected biogas productivityof the silage is 0.35 m3

CH4kg�1

OM.Case M1 is a realistic case corresponding to the present

situation in the area considered. Cases M2 and M3 corre-spond to the best agricultural practice that can realisticallybe implemented to approach a sustainable cropping systemin the area considered, with the option of central (case M3)and farm-scale (case M2) digesters. Case M4 can be consid-ered as the worst realistic way envisaged for producingmaize as an energy crop.

Grass is the dominant crop in the Ardenne uplandswhere the maize productivity is the lowest. Grass is alsoproduced in all other areas. It is grown either as permanentgrassland or as a temporary crop. The latter is ploughedevery 2 or 3 years. Depending on intensity, grass can beharvested 2–4 times a year. The first harvest has the highestfeeding and energy value. Yields are typically in the rangeof 8–10 tDM ha�1 y�1 (Luxen and Vliegen, 2002). It will beassumed that grass is fertilised with digestate only, used at

the maximum allowed rate of 210 kgN ha�1 y�1. Due tonutrient recycling, no additional mineral fertiliser isrequired.

Five cases will be considered in this study:

G1. Grass is grown as an intensive permanent energycrop harvested 4 times a year. All the harvested grassis silaged after partial tedding and fed to a decentralisedanaerobic digester on the farm. The grass yield is8.5(±10%) tDM ha�1 y�1 and the methane productivityof the silage is 0.35 m3

CH4kg�1

OM.G2. Grass is grown as an intensive permanent crop har-vested 4 times per year. The first two harvests with thehigher feeding values are tedded and silaged for animalfeeding. The last two harvests are tedded, silaged andfed to a decentralised anaerobic digester on the farm.The yield of grass harvested for energy purposes is3.8(±10%) tDM ha�1 y�1. Due to the lower energy con-tent and digestibility of this grass, the methane produc-tivity of the silage is 0.32 m3

CH4kg�1

OM.G3. Grass is grown and harvested as in case G2. Thetedded grass is transported, silaged and fed to a centra-lised digestion plant 10 km from the farm.G4. Grass is grown as an intensive temporary crop har-vested 3.5 times per year (3 cuts in the first year and 4 inthe second). Compared to permanent grass, temporarygrass requires an additional soil preparation, sowingand weed control every two years. The first two harvestswith the higher feeding values are tedded and silaged foranimal feeding. The last two-three harvests are tedded,silaged and fed to a decentralised anaerobic digesteron the farm. The yield of grass harvested for energy pur-poses is 3.8(±10%) tDM ha�1 y�1. Due to the lowerenergy content and digestibility of this grass, the meth-ane productivity of the silage is 0.32 m3

CH4kg�1

OM.G5. Grass is grown as an extensive permanent energycrop harvested twice a year. The harvested grass is ted-ded, silaged and fed to a decentralised anaerobic digesteron the farm. Due to the extensive cropping system, theyield is only 5(±10%) tDM ha�1 y�1 and the poor digest-ibility limits the methane productivity of the silage at0.23 m3

CH4kg�1

OM.

In all cases, the grass organic matter is assumed to be 88%of the DM.

Case G1 is a realistic case corresponding to the currentreduction of the numbers of both farmers and animals; forthe purpose of landscape conservation, the excess grasslandis maintained and converted to intensive energy produc-tion. Cases G2 and G3 correspond to the best agriculturalpractice that can realistically be implemented to approacha sustainable animal production system and landscape con-servation in the area considered, bearing in mind the ongo-ing reduction of the number of animals. The farm-scale andcentral digester options are represented by cases G2 andG3, respectively. Case G4 corresponds to the introductionof temporary grass in the usual crop rotation. Case G5

P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627 2623

corresponds to the valorisation of grass grown on land setaside for biodiversity conservation. Late harvest results inpoor digestibility and biogas productivity.

Storage losses of 10 ± 5% have been assumed for maizeand tedded-grass silage in the area considered. The bestvalues of Vanbelle et al. (1981) are presently the rule, owingto practice and technical improvements (G. Foucart, per-sonal communication). It is worth noting that a significantfraction of losses are in the form of silage leachates that canbe valorised in the digester, but also that some practicescommonly used differ from the best conservation rulesand result in losses in the higher part of the rangeconsidered.

2.2. Energy systems

Diesel fuel is required to run the machines involved inthe main steps of the crop production. Table 1 presentsthe estimated diesel fuel consumed for each operation.Table 2 summarises the fuel consumed for the combina-tions of the different operations involved in each case con-sidered in this study. For cases G2, G3, G4 and G5, thediesel fuel consumption is roughly allocated according tothe use of grass, either energy production or animal feed.For transportation to the farm-scale digester, an averagedistance of 2 km between the fields and digester was con-sidered. An additional transport over 10 km was consid-ered for the central digester option. In this case, thenumber of loading and unloading operations was kept tothe realistic minimum for each case: direct transport of har-vest from the field to the silo next to the digester, and directtransport of the digestate from the digester to a storagearea at 2 km average distance from the field.

The fossil energy consumed for production and trans-port of fertiliser and pesticides leads to the emission offossil CO2. The fossil energy and CO2 content of eachraw material used for crop production are summarised in

Table 1Fuel consumption by tractors, harvesters and trucks for cropping (calculated

Operation Diesel fu

Maize

A Transport and spreading of manure or digestate and fertilisera 15B Soil ploughing and crumbling 32.7C Grass complementary seeding and maintenanceD Sowing 9.4E Weed control 3F Harvest 27.4G Harvest transporta 7.3H Silo compaction 5.6I Digester feeding 47.3J Additional transport to central digester (harvest)b,d 119K Additional transport from central digester (digestate)c,d 32

a 2 km transport between farm or storage and field.b Additional transport of harvest over 10 km from the farm to central digeste

unloaded.c Additional transport of digestate over 10 km by a 180 kW truck with a 27d Fuel consumption restricted to the part of the harvest actually transported

Table 3. The fossil energy and fossil CO2 contents of man-ure are considered to be zero as, under the conditions con-sidered, this material is produced anyway and its use onenergy crops does not limit other uses. The fossil energyand fossil CO2 contents of digestate are also consideredto be zero as digestate and its nutrient content are totallyrecycled to the energy crops.

Maize and grass can be converted to methane either asco-substrate mixed with slurry manure (widespread anaer-obic digestion technology) or as pure substrate (anaerobicdigestion technology to be developed). The approach of thepresent paper considers the balance of nutrients, carbonand energy related to maize and grass, separately fromthe other substances with which they can be mixed. There-fore, only the contribution of maize and grass in the bal-ances of the digesters is considered, even when they aremixed with other substances. Anaerobic digester operationrequires heat and electricity. A survey of anaerobic digest-ers fed mostly with slurry manure under mesophilic condi-tions has shown that in a practical situation the heat andelectricity requirements correspond to 22–27% and 2.5–3% of the energy in the biogas produced, respectively(Demuynck and Nyns, 1984). With good insulation ofthe digester, the most significant part of the heat is requiredfor substrate heating (data from practice indicate that heatlosses represent only about 20% of the energy required forsubstrate heating in the climatic zone considered). Consid-ering that maize, grass and slurry manure have similar spe-cific heat capacities due to their high water contents, theenergy required to heat each ton of raw material is similar.However, the specific methane production of maize andgrass silage (30–35% DM) is 70–100 m3

CH4/tsilage (calculated

from Weiland, 2003; Lemmer and Oechsner, 2001; Steffenet al., 2000; Linke et al., 1999; Baserga, 1998), i.e. 6–10times higher than the specific methane production of slurrymanure (7.5% DM) of 10–12 m3

CH4/tmanure (calculated from

Weiland, 2003; Steffen et al., 2000; Baserga, 1998;

from Achilles et al., 2002)

el consumed (l ha�1 y�1)

Permanent grass 4 harvests/year Temporary grass 7 harvests/2 years

18 1832.7

6.211.1

0.275 047.2 47.226 22.48.8 8.8

23 27396.6

r with bunker silo by a 180 kW tractor and a 50 m3 trailer, and return trip

m3 slurry tank, and return trip unloaded (additional loading included).(case G3).

Table 2Fuel consumption for the cropping systems considered (calculated from Table 1)

Cropping systema Operationsb Diesel fuel consumed (kg ha�1 y�1)

M1, M2, M4 [A + B + D + E + F + G + H + I]maize 123.3M3 [A + B + D + E + F + G + H + I + J + K]maize 249.4G1 [A + C + E + F + G + H + I]permanent grass 108.1G2, G5 [0.5(A + C + E + F + G + H + I)]permanent grass 54.1G3 [0.5(A + C + E + F + G + H + I) + J + K]permanent grass 92.1G4 [4/7(A + B + D + E + F + G + H + I)]temporary grass 79.8

a See text for description.b See Table 1.

Table 3Fossil energy and CO2 content of raw materials for crop productiona

Energy consumeda

(MJ/kg)CO2 emitteda

(kgCO2/kg)

Mineral fertilisera

N 70 ± 34 2.5 ± 0.1P2O5 12 ± 4 1.1 ± 0.4K2O 7.5 ± 2.5 0.67 ± 0.19Weed controlab 200 ± 20 15.45 ± 1.5Diesel fuelbc 56.3 ± 5.6 3.64 ± 3.6

a Scharmer and Gosse (1998).b Range set to 10% of the mean.c Calculated from CWaPE (2002b); includes both the energy and CO2

content of the diesel fuel and the energy and CO2 involved in diesel pro-duction and distribution.

2624 P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627

Demuynck and Nyns, 1984). It follows that the heatrequirement for maize and grass conversion to methanewould be 2–5% of the biogas energy produced. In mostcases, the heat and electricity required by the digester willbe produced by running a combined heat and power(CHP) plant on the biogas. In real systems, the digesterconsumption will be incorporated in the energy balanceof the system when measuring the net energy output ofthe plant. For the sake of simplicity, the heat and electricityrequired for digestion of the energy crop will be neglectedin the present study when calculating the energy and CO2

balance of the methane produced. Incorporating these con-sumptions would affect the results presented below by 5%at most, i.e. below the margin of uncertainty of the results.

3. Results

3.1. Energy and CO2 balance of maize and grass

The energy and CO2 balances of the various croppingsystems considered for maize and grass are summarisedin Figs. 1 and 2, respectively. The inputs refer to the rawmaterials required for the crop production. The corre-sponding fossil energy and fossil CO2 involved are calcu-lated on the basis of the data in Table 3. The outputcorresponds to the energy after conversion of maize andgrass to methane.

For maize, there is in all cases a net production ofrenewable energy as methane, with values in the range

111–171 GJrenewable ha�1 y�1 depending on the croppingsystem. The renewable to fossil energy ratios YR/F rangefrom 7 to 25 GJrenewable/GJfossil and the specific fossilCO2 content of the methane produced is between 10 and27 kgCO2

MWh�1CH4

. The range of uncertainty for most val-ues is not higher than 25%.

For grass, the net production of renewable energy is inthe range of 29–79 GJrenewable ha�1 y�1 depending on thecropping system. The renewable to fossil energy ratiosYR/F range from 7 to 14 GJrenewable/GJfossil and the specificfossil CO2 content of the methane produced ranges from 17to 35 kgCO2

MWh�1CH4

. The range of uncertainty for mostvalues is not higher than 25%.

3.2. Impact of transportation

Figs. 1 and 2 show that the diesel fuel represents a signif-icant contribution to the CO2 balance in all cases investi-gated. In order to assess the impact of fuel for additionaltransportation, the specific fuel consumption and corre-sponding balances for transportation of grass and maizewere calculated under the hypotheses specified in Table 4.In many cases, energy crops are mixed with slurry manurefor anaerobic digestion. Transportation of the manure hasthen to be considered, especially because a common dige-state is obtained and has to be transported back to thefields. For the sake of comparison, Table 4 also presentsthe specific fuel consumption for manure transportation.Differences in the specific fuel consumption result from dif-ferences in the specific volume and weight of the materialstransported and the corresponding limitations in the trans-ported weight or volume. Road transportation by truckappears to be significantly more efficient than by tractor(smaller load, slower speed and longer travel time).

Fig. 3 presents the influence of transportation distanceon YR/F and the net reduction of CO2 emission associatedwith the production of electricity from anaerobic digestionof energy crops and manure. Fig. 1 shows data for casesM3 and G3 with increasing distance to the central digesterabove 10 km. In each case, the fossil CO2 emission ratherthan YR/F limits the transportation distance. Under thesimple hypotheses considered here, the limiting distancesare 150 and 86 km for maize and grass, respectively (trans-port by tractor). Manure transport is limited at 116 km and

0

50

100

150

200

M1 M2 M3 M4 G1 G2 G3 G4 G5

Ene

rgy

(GJ

ha-1

y-1 )

Cropping system :

Input as fertiliser

Input as weed control

Input as fuel

Output as CH4

17.5±

3.9

22.5±

4.9

11.2±

2.4

7.4±

2.2

13.9±

3.2

11.4±

2.7

6.7±

1.6

7.6±

1.8

10.8±

3.1

YR/F (GJrenewable / GJfossil) :

Fig. 1. Energy balance and energy ratio (YR/F) for biogas production from maize and grass energy crops, calculated from the data in Tables 1–3 under thehypotheses specified in the text. The intervals were calculated assuming that the uncertainty for each parameter is independent of the uncertainty for theother parameters.

0

10

20

30

40

M1 M2 M3 M4 G1 G2 G3 G4 G5

Fos

silC

O2

emis

sion

(kg C

O2

MW

h-1C

H4)

Cropping system

Fertilisers

FuelWeed control

Fig. 2. Specific fossil CO2 emissions induced by the production of biogasfrom maize and grass energy crops, calculated from the data in Tables 1–3under the hypotheses specified in the text. The intervals were calculatedassuming that the uncertainty for each parameter is independent of theuncertainty for the other parameters.

P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627 2625

29 km for transportation by truck and by tractor,respectively.

4. Discussion

Figs. 1 and 2 summarise the results for a broad range ofrealistic cases, including extreme situations that cannot beconsidered as examples of sustainable agriculture in theconditions encountered in Belgium and related areas. In

all cases, the biogas production from maize and grassgrown partly or totally as energy crops results in a net pro-duction of renewable energy with YR/F much higher than 1.Assuming that the biogas produced is used in a CHP unitwith a net yield of 30% biogas energy consumed releasedto the grid as electricity, the resulting YR/F range from2.8 to 9.4 GJrenewable electricity/GJfossil fuel. The specific CO2

emissions range from 31 to 104 kgCO2 MWh�1electricity,

depending on the case considered. This corresponds to asignificant reduction in CO2 emission compared to the ref-erence emission of 456 kgCO2

MWh�1electricity by gas-steam

turbines. The relative reduction in CO2 emission is evenhigher when assuming that the heat produced by theCHP unit, if appropriately used, substitutes for fossil-fuelderived heat and avoids the corresponding CO2 emission.

4.1. Grass vs maize energy

The energy yield per cropped area is much higher formaize than for grass (Fig. 1), despite the higher require-ment for fuel (Table 2). This results from the significantlyhigher biomass productivity of maize crops. However,grass can be grown with high yield in places where maizewould not reach the minimum yield of 12 tDM ha�1 y�1

considered here. Except for case G1, which results in agood energy productivity, it should also be noted that thepoorer energy balance of grass results from the prioritygiven to other uses of the biomass produced: forage in cases

Table 4Specific energy and CO2 balances for transportation of maize, grass and slurry manure (calculated from Achilles et al. (2002) and Table 3)

Transportation Fuel consumption (lfuel tFM�1 km�1) Energy consumption (MJ tFM

�1 km�1) CO2 emission (kgCO2tFM

�1 km�1)

Maizea 0.28 12.9 0.84Grassa 0.39 18.6 1.20Slurry manure by tractorb 0.19 8.8 0.57Slurry manure by truckc 0.05 2.2 0.14

Distance is expressed as the distance between field and digestion plant.a Harvested maize (30% DM) and grass (35% DM) transported using a 180 kW tractor with 50 m3 trailer and resulting digestate transported using a

180 kW slurry truck with a 27 m3 tank, unloaded return trips included.b Manure (7.5% DM) and resulting digestate transported using a 180 kW tractor with a 15 m3 trailer, all trips loaded.c Manure (7.5% DM) and resulting digestate transported using a 180 kW slurry truck with a 27 m3 tank, all trips loaded.

0

50

100

Net

avo

ided

CO

2em

issi

on

(kg C

O2t F

M-1

)

5

10

15

0 50 100 150

YR

/F(G

J ren

ewab

le/G

J fos

sil)

Transportation distance (km)

1

Fig. 3. Influence of transportation distance on the net avoided CO2

emissions (top) and the renewable to fossil energy ratio (YR/F) (bottom)for anaerobic digestion of maize (square), tedded grass (circle) and manuretransported by tractor (down triangle) or truck (up triangle). Calculationswere performed with the following hypotheses: (1) 30% DM and350 m3

CH4tOM

�1 for maize silage (case M3), 35% DM and 320 m3CH4

tOM�1

for tedded-grass silage (case G3), and 7.5% DM and 12.15 m3CH4

tFM�1 for

manure; (2) specific fuel consumption presented in Table 4; (3) conversionof biogas to electricity performed with a gas engine with a net electric yieldto the grid of 30% (fossil fuel consumption and CO2 emissions from adual-fuel engine would reduce the values presented slightly).

2626 P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627

G2, G3 and G4 and biodiversity conservation in case G5.In both cases, the grass with the poorest energy contentis used for energy production. Even if not optimalfrom the energy point of view alone, this is consistent withagriculture being a provider of food, renewable raw mate-rials, environmental conservation, etc., and not only ofenergy.

While energy yield per cropped area is higher for maizethan for grass, the ranges of YR/F (Fig. 1) and specific CO2

emissions (Fig. 2) are similar for maize and grass. From theenergy point of view, both crops are adequate for renew-able energy production.

4.2. Central vs farm-scale anaerobic digester

Comparing cases M2 and G2 to M3 and G3, respec-tively, shows that the net energy balance per ha is affectedby only 4–6% (Fig. 1). However, YR/F is decreased by 40–50% (Fig. 1) and the specific CO2 emission is raised byabout 70–100% (Fig. 2) in the centralised option comparedto the farm-scale option. The poorer efficiency of the cen-tralised option results from the additional energy requiredfor transportation. As shown in Fig. 3, maize (30% DM)and tedded grass (35% DM) can be transported over longdistances (150 and 86 km, respectively) before the netCO2 emission becomes positive and YR/F reduces to 1. Atsuch distances, transportation by truck would be muchmore satisfactory and allow even longer distances. How-ever, any unnecessary km decreases the energetic and envi-ronmental attraction of energy crops as a substitute forfossil fuels. In particular, YR/F decreases quickly withtransportation distance; the latter should thus be kept toa minimum. It should also be stressed that in most cases,energy crops will be mixed with slurry manure and theircommon digestate will have to be transported. Then, thedata presented in Fig. 3 for manure are more relevant.Transport of manure and related digestate is energeticallyless favourable due to its low specific energy content.Fig. 3 also shows that the choice of the transport mode(tractor, truck) has a huge impact on the energetic andenvironmental balance of the system. While the energy fig-ures presented here favour the farm-scale option, manyother economic, social and technical factors also need tobe considered.

4.3. Environmental assessment

The present study focuses on the energy and CO2 bal-ance of the biogas production, as the core of calculatingspecific green certificates. The fossil energy and CO2

involved in seed production and construction and mainte-nance of machines, digester plant and buildings should betaken into consideration for a more general life cycleassessment. A more general environmental assessmentshould complement the present study to include also theenvironmental aspects of managing and recycling organicmatter and nitrogen fertilisers and their relationships with

P.A. Gerin et al. / Bioresource Technology 99 (2008) 2620–2627 2627

the emission of greenhouse gases such as methane, ammo-nia and N2O. As an example, as the greenhouse effect ofmethane is 21 times higher than CO2, leakage of 5% ofthe methane produced totally annihilates efforts to reduceCO2 emissions through the use of maize or grass as asource of renewable energy. Adequate coverage of the dig-estate storage tank and good agricultural practices forspreading the digestate are therefore highly important.Other aspects to be included in a more general assessmentinclude the social and economic contributions of energycrops in maintaining agricultural and economic activitiesin rural areas.

5. Conclusions

In spite of the fossil energy consumed for their produc-tion and transformation to biogas, maize and grass energycrops allow a net production of renewable energy togetherwith a significant reduction in fossil energy related CO2

emission. Owing to their high energy contents, maize andtedded grass can be transported to a central anaerobicdigestion plant while keeping the energy and CO2 balancepositive. However, increasing the transportation distancedecreases the advantage of the energetic and environmentalbalance. Maize presents a better renewable energy produc-tivity and yield. Grass, while being less productive, offersgood energy balances and has several other agriculturaland environmental advantages. In most cases investigatedhere, grass with the highest energy content was used foranimal feeding while methane was produced from grasswith the lowest energy content. The present paper focusesmainly on the fossil CO2 and energy balances of maizeand grass energy crops, as related to the green certificatesystem developed in the Walloon region of Belgium, whichis based on reduction of CO2 emissions. However, the datapresented here offer a useful background for a moredetailed environmental assessment of the optionsconsidered.

Acknowledgements

The authors gratefully acknowledge Pepin Tchouate He-teu for fruitful discussion and critical review of the dataand the manuscript.

References

Achilles, W., de Baey-Ernsten, H., Frisch, J., Fritzsche, S., Froba, N.,Funk, M., Grimm, E., Hartmann, W., Kloepfer, F., Peters, R., Sauer,

N., Schwab, M., Siegel, F., Weiershauser, L., Witzel, E., 2002.Betriebsplanung Landwirtschaft 2002/2003, Datensammlung, Kurato-rium fur Technik und Bauwesen in der Landwirtschaft - KTBL, 18.Auflage, Darmstadt, ISBN 3-7843-2141-0, p. 379.

Baserga, U., 1998. Installations agricoles de co-digestion pour la produc-tion de biogaz: biogaz issu de dechets organiques et d’herbe energe-tique. Station federale de recherches en economie et technologiesagricoles, CH-8356 Tanikon, Switzerland. Rapport FAT 512, pp. 1–11.

CIPF, 2003. Resultats des essais comparatifs de varietes de maıs ensilageet grain: annee culturale 2002-2003. Centre Independant de ProductionFourragere, Unit of Crop Ecology, Universite catholique de Louvain,Louvain-la-Neuve, Belgium. p. 27.

CWaPE, 2002a. Le regime des certificats verts. Commission Wallonnepour l’Energie, Namur, Belgium. Document available on <http://www.cwape.be>, p. 12.

CWaPE, 2002b. Coefficients d’emission de CO2 de differentes sourcesd’energie primaire, et emissions de la filiere electrique classique.Commission Wallonne pour l’Energie, Namur, Belgium. Documentavailable on <http://www.cwape.be>.

De Neuville, M., 2003. Optimisation de la fumure azotee organique etminerale: bilan de 3 annees d’essais. Le Sillon Belge 3072 (8/03/2003),34–35.

Demuynck, M., Nyns, E.J., 1984. Biogas plants in Europe. Solar EnergyR&D in the European Community Serie E Energy from Biomass, vol.6. D. Reidel Publishing Company, Dordrecht, Boston, Lancaster,ISBN90-277-1780-X, p. 339.

Karpenstein-Machan, M., 2001. Sustainable cultivation concepts fordomestic energy production from biomass. Critical Reviews in PlantSciences 20 (1), 1–14.

Lemmer, A., Oechsner, H., 2001. Kofermentation von Gras und Silomais.Landtechnik 56 (6), 412–413.

Linke, B., Baganz, K., Schlauderer, R., 1999. Nutzung von Feldfruchtenzur Biogasgewinnung – Use of crops for biogas production. Agrar-technische Forschung 5 (2), 82–90.

Luxen, P., Vliegen, T., 2002. Etude de la biomethanisation du lisier. In:Rapport des essais et des activites de Agra-Ost 2002. Agra-Ost –Centre de recherche et de formation pour l’est de la Belgique, SanktVith, Belgium, p. 25.

Mahnert, P., Heiermann, M., Plochl, M., Schelle, H., Linke, B., 2002.Verwertungsalternativen fur grunlandbestande: Futtergraser alsKosubstrat fur die Biomethanisierung. Landtechnik 57 (5), 260–261.

Oost, J.F., 2001. Valorisation des engrais organiques par le maıs. Culturede maıs 30/03/2001, CIPF – Universite Catholique de Louvain,Louvain-la-neuve, pp. 28–33.

Scharmer, K., Gosse, G., 1998. Ecological impact of biodiesel - Produc-tion and use in Europe. In: Proceedings of the 2nd European MotorBiofuels Forum, 22–26 September 1996, Graz, Austria, pp. 317–328.

Steffen, R., Szolar, O., Braun, R., 2000. Feedstocks for anaerobicdigestion. In: Anaerobic digestion: making energy and solving modernwaste problems. Ed. Ortenblad., H., AD-Nett report 2000. AD-Nett-A network on anaerobic digestion of agro-industrial wastes. pp. 34–48.

Vanbelle, M., Arnould, R., Deswysen, A., Moreau, I., 1981. L’ensilage, unprobleme d’actualite. Institut pour l’encouragement de la RechercheScientifique en Industrie et Agriculture (IRSIA), Brussels, Belgium,p. 89.

Weiland, P., 2003. Production and energetic use of biogas from energycrops and wastes in Germany. Applied Biochemistry and Biotechnol-ogy 109, 263–274.