Is grass biomethane a sustainable transport biofuel.pdf

September 15, 2017 | Autor: D. Nizami | Categoría: Engineering, Technology, Sustainable Transport
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Modeling and Analysis

Is grass biomethane a sustainable transport biofuel? Nicholas E. Korres, Anoop Singh, Abdul-Sattar Nizami and Jerry D. Murphy,* Biofuels Research Group, Environment Research Institute, University College Cork, Ireland Received December 15, 2009; revised version received February 8, 2010; accepted February 11, 2010 Published online in Wiley InterScience (; DOI: 10.1002/bbb.228; Biofuels, Bioprod, Bioref. 4: 310–325 (2010) Abstract: Grassland is a beneficial landscape for numerous reasons including potential to sequester carbon in the soil. Cross compliance dictates that grassland should not be converted to arable land; this is particularly interesting in Ireland where 91% of agricultural land is under grass. Biogas generated from grass and further upgraded to biomethane has been shown to offer a better energy balance than first-generation liquid biofuels indigenous to Europe. The essential question is whether the gaseous biofuel meets the EU sustainability criteria of 60% greenhouse gas emission savings. The base-case scenario investigated included: utilization of electricity from the grid; over-sizing heated digestion tanks to hold digestate in the winter period; vehicular efficiency 82% of that of a diesel vehicle; and no allowance for carbon sequestration. The analysis of the base case showed a reduction in emissions of 21.5%. However by varying the system, using electricity from wind, improving digester configuration, and by using a vehicle optimized for gaseous fuel, a reduction of 54% was evaluated. Furthermore allowing for 0.6 t carbon sequestration per hectare per annum the reduction increased to 75%. © 2010 Society of Chemical Industry and John Wiley & Sons, Ltd Keywords: biomethane; grass silage; greenhouse gas emissions; life cycle assessment


approximately 55% methane (CH4) is upgraded to 97% CH4, compressed to 300 bar and sold at the farm gate as a transport

Grass as a source of transport fuel rassland sequesters carbon into the soil which is not released on harvesting leading to a potential for sustainable biofuel production from grass.1 Biogas produced from grass may be upgraded to biomethane and used as a transport fuel.2,3 The model is quite simple; it does not demand the scale or complexity that would be associated with, for example, a grain ethanol facility. The authors visited a 150 ha farm near Salzburg in Austria. At this farm grass is cut, and silage is produced and stored in a silage pit or in bales. The silage is fed into an anaerobic digester throughout the year. Biogas is produced on a continual basis. The biogas which has a composition of


fuel. In the facility modelled here in an Irish context, 137.5 ha would generate the same quantity of grass silage as the 150 ha Austrian farm (Table 1). In Ireland, grassland occupies approximately 91% of agricultural land.3 The 2009 EU Renewable Directive,4 recognizes the potential for biogas as a transport fuel in attributing a typical greenhouse gas (GHG) savings of 83% to compressed biomethane generated from residues. It does not specifically mention grass as a feedstock. However grass must be considered as a bioenergy feedstock due to cross compliance,5 which states that the ratio of arable land to permanent grassland (based on 2003 levels) must not change by more than 10%, due to the associated release of carbon to the atmosphere should this

Correspondence to: Jerry D. Murphy, Biofuels Research Group, Environment Research Institute, University College Cork, Ireland. E-mail: [email protected]


© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Table 1. Size of the digester tanks and biomethane yield (adapted from Smyth et al.3). Component


Farm size (ha)


Silage production (tDM ha–1yr –1) –1

Silage biomass (t ha



yr )


Silage yield (t yr –1)


Dry solids (DS) (t yr –1) @ 22%DS



Volatile dry solids (VS) (t yr ) @90% of DS


Loading rate (kg VS m–3 day–1)

1.44 3

Working volume of each digester (m )


Volume of digester 1 (m3) @ 80% working volume


Volume of digester 2 (m3)

3531.7 3


Amount of feed stock added (m day ) @ 10% DS


Retention time (days)


Biogas production VS destruction



yr ) @ 55%


Biomethane production (mn3 yr –1) @ 97% methane

463105 9262

Losses in upgradation and compression process (mn3yr–1) @ 2% loss Net biomethane production (mn3 yr –1)

453843 –1

Energy in net biomethane produced (GJ yr ) Energy in net biomethane produced (GJ ha–1 yr –1) Digestate yield (t yr –1) –1

(t ha


yr )

who examined the energy balance. Sustainability, however, is defined by emissions savings. Thus the aim of this paper is to determine the GHG emission savings of grass biomethane as a transport fuel when compared to the fossil fuel it replaces (diesel in this instance) on a whole life cycle analysis. To be deemed sustainable according to the Renewable Directive,4 the reduction in emissions needs to be 35% if operated before 2017, 50% after 2017 and 60% after 2018. The methodology employed involves a Life Cycle Assessment (LCA) of current agricultural practices for reseeded Lolium perenne (perennial ryegrass) pastures for silage; the process technology investigated includes for continuous stirred tank reactors (CSTR), and biogas upgrading for biomethane production.

Methodology and description of process

VS degraded (t yr –1) @ 55% degradation (mn3

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16631 121 6683.25 48.6

change occur. Biomethane as a transport fuel is a mature technology with abundant alternative feedstock including municipal solid waste, sludge, slaughter waste, biofuel residues, and energy crops such as maize.3 The natural gas grid may be utilized for distribution of the produced biomethane; end uses include electricity, thermal and transport energy. Ambition of paper Previous work by the research group has shown that grass biomethane used as a transport fuel is a beneficial industry for Ireland,2 with a net energy superior to first-generation indigenous European liquid biofuel systems (wheat ethanol, rapeseed biodiesel) and comparable to tropical biofuel systems such as palm oil biodiesel (Fig. 1).3 But is grass biomethane a sustainable biofuel as defined by the Renewable Directive?4 This paper builds on the building blocks of Smyth et al.3

Overview of process Smyth et al.3 defined the building blocks of the energy balance of grass biomethane. They concluded that grassland was able to produce 122 GJ ha-1 yr-1 as biogas. In the base case assessed which allowed for the use of biogas to satisfy thermal parasitic demand (raising the temperatures of the digesters to the mesophilic range), and for losses of methane in upgrading to biomethane, the gross energy available for sale as biomethane was 104 GJ ha-1yr-1. Furthermore, allowing for energy in agriculture – both direct and indirect – the net energy of the system was 69 GJ ha-1 yr-1 (Fig. 1). An energy balance, however, is not directly related to a GHG balance. A biofuel system that generates significant quantities of fuel per hectare GJ ha-1 yr-1, may not necessarily be sustainable. Emissions associated with agriculture, source of fertilizer, sources of parasitic energy demand, and efficiency of vehicle may deem it unsustainable. This paper will build on the building blocks of a 137.5 ha Irish farm as previously described by Smyth et al.3 but investigate GHG balance. Life Cycle Assessment (LCA) LCA is a methodology that can be used for evaluation of sustainability of biofuel production.6-9 A ‘cradle-to-grave’’ or ‘field to wheel’ process is used here. The cradle is the production of grass silage in the field and the grave the use of the resulting compressed biomethane in a vehicle. All emissions (direct and indirect) are calibrated in the analysis for agriculture, transport, and process. According to 2009/28/EC Directive,4 Annex V, C-13 ‘emissions from the fuel in use shall be taken to be zero

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb


NE Korres et al

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Grass biomethane base case: excludes the use of digestate as a fertiliser and uses biogas as a source of thermal energy Grass biomethane digestate fertiliser: allows for the use of digestate as a substitute for mineral fertiliser Grass biomethane wood chips: allows for the use of wood chips to generate thermal energy to satisfy parasitic demand Figure 1. Comparison of gross and net energy output of selected energy crop biofuel systems3.

for biofuels and bioliquids’ hence emissions from combustion of biomethane in vehicles are not taken under consideration in this study. The functional unit is defined as m3 biomethane yr-1 and the environmental impacts are expressed as g CO2 equivalent (CO2e) MJ-1 energy replaced. The energy replaced must be based on a field-to-wheel analysis rather than a field-to-tank analysis. This must include both the fuel produced in terms of GJ (field to tank) and the relative efficiency of the vehicle on biofuel as compared to operation on the transport fuel displaced (field to wheel). This is further discussed in more detail in the section Transportation and utilization of biomethane. CO2 with GWP of one (1) for 1 kg CO2 is used as a reference gas for the measurement of the emissions. The GWP of 1 kg of N2O and CH4 is 296 and 23 respectively.4 The volume of GHG emissions in terms of CO2e can be calculated using Eqn 1.4

GHG(t of CO2e) = CO2(t) + 23 × CH4(t) + 296 × N2O(t)


Building blocks in agronomy These building blocks are based upon the work of Smyth et al.3 They include: •


Reseeding frequency for perennial ryegrass of 8 years at a rate of 25 kg ha-1.

• • •

Lime addition to acidic soils, typically 10 t ha-1 in eight years. Herbicides; glyphosate before ploughing and three treatments of asulam during life cycle of crop. Harvesting twice per year, at the end of May and beginning of July.

Fertilization is dealt with in more detail due to its importance in GHG emissions. The growth of 15 t of grass dry matter (DM) requires the uptake of approximately 450 kg N ha-1 from the soil;10 approximately 360 kg N ha-1 is required for the production of 12 tDM of grass. According to the same author10 the background availability of N from Irish grassland soils (which originates from approximately 7000 kg N ha-1 of soil organic matter of which about 2% is available N for plant uptake), averaged approximately 135 kg N ha-1 yr-1. Background N is the nitrogen content in the soil when no external N is added10 due to the mineralization of soil organic matter.6 Biomethane production results in the generation of digestate which can replace conventional fertilizers providing further environmental benefits to the biofuel process chain.6 In this study, each hectare produced 12 tDM of grass per year, equivalent to 54.5 t of grass silage @ 22% dry solids content. The resultant

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

digestate production is 48.6 t ha-1 yr-1 (Table 1). The Nitrate Directive (91/676/EEC) considers digestate as an organic fertilizer without taking into account its higher proportion of directly available mineral N: its use is restricting to 170 kg N ha-1 yr-1. Data regarding the chemical composition of digestate are rare. 3,9 Smyth et al. 3 reported a value of 2.1, 0.087 and 3.08 kg of N, P and K t-1 of digestate, respectively. Based on these values digestate provides 102 kg N, 4.2 kg P and 149.7 kg K ha-1 yr-1. Digestate in this study was used at the maximum rate to provide 102 kg N ha-1 yr-1 (Table 2); thus supplementary mineral fertilizers, based on recommended fi xed N application rates by Teagasc, the Irish Agricultural Authority, to cover the nutritive needs of the crop (for the production of 12 t DM ha-1 yr-1) are reduced to 123 kg N ha-1 yr-1. The digestate is spread in the field twice a year, along with mineral fertilizer, after every harvesting operation. According to Humphreys et al.12 and Aavola and Karner,13 N uptake in grasslands, under various mineral N application rates of approximately 225 kg N ha-1 yr-1, is 75%, of which 60% is allocated to above ground biomass. In this case the supply of 102 kg Ndigestate ha-1 yr-1 indicates 60% N uptake by above ground material, a value similar to these authors.

Table 2. Nitrogen fertilization management. Nitrogen Fertilization


N-Soil organic matter (kg N ha-1) (2% available for plant uptake)a


Average background N (kg N ha-1 yr-1)a

135 -1

-1 b

Recommended mineral N fertilization (kg N ha yr ) Establishment (first) year

300 -1



Subsequent year (125 and 100 kg N ha yr after 1 and 2nd cut respectively)


Total N required for 15 t DM ha-1 (kg N ha-1)a




Total N required for 12 t DM ha (kg N ha )

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Biomethane production Anaerobic digestion (AD) is a ubiquitous technique for converting organic, wet biomass into renewable energy in the form of biogas which may be upgraded to biomethane.14 The transportation distance is assumed very conservatively as 10 km to the AD plant. If the AD plant is located in the centre of the 137.5 ha area, the radius of the circle is only 0.66 km. Diesel is assumed to be used in tractors and transport vehicles. A CSTR system with two digesters working in series operating at 10% DS is assumed; the digester operates in the mesophilic temperature range at 38°C (Fig. 2). The temperature of the incoming feedstock is 10°C, which is typical for the south of Ireland; this feedstock temperature must be raised by 28 °C. The annual silage yield from the 137.5 ha (Table 1) farm is 7500 t yr-1 The loading rate for wet digestion of grass silage is taken as 1.44 kg VDS m-3 day-1, similar to Smyth et al.3 At this loading rate, the working volume of each digester is calculated as 1413 m3, assuming digesters are cylindrical in shape, with diameter:height ratio 1:1.5.15 The volume of the first digester is 1766 m3 assuming an 80% working volume, and the volume of the second digester is 3532 m3, assuming half of the digester volume is used as for storage (Table 1). Approximately 45.2 m3 feedstock (@ 10% DS) is fed into the first digester every day. The total retention time is 62.5 days, with the substrate spending about half of the time in each digester. The substrate flows by gravity from the first to the second digester and the liquid is circulated back to the first digester. The recirculation of liquid digestate reduces the water demand and increases the microbial populaion improving the efficiency of the the AD facility.16


N from digestate (kg N ha-1) First year: 24.28t digestate [email protected] kg N t-1 digestate (assume half production in first year) Subsequent years: 48.56t digestate [email protected] kg N t-1 digestate

51 102

Supplementary mineral N fertilizer (kg N ha-1) First year


Subsequent years



Data from Humphreys10. b Coulter and Lalor11.

Figure 2. Daily mass balance of anaerobic digester3.

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb


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Maceration of the silage is carried out before insertion of feedstock into the first digester. Maceration reduces particle size to prevent physical obstruction of pipes and pumps by the fibers, and it also increases the surface area available for microbial attack, and thus speeds up the digestion process.16 Mixing may allow better digestion of grass silage by keeping the material homogenous and hindering the settling of silage particles. The optimal DS content for grass silage digestion in CSTR is reported as 10%.3,17 The produced grass silage (7500 t yr-1) is mixed with 9000 t of water to obtain the desired DS level. The water demand is fulfi lled by the recirculation of the liquid digestate. Biomethane yield The destruction of 1 kg VS produces about 1mn3 of biogas at 55% methane content.3 A methane yield of 550 mn3 biogas per tonne VS (302 mn3 CH4 t-1 VS) added to the AD plant is assumed on the basis of 55% destruction of VS.3 Total biogas production is 816,750 mn3yr-1 (Table 1). For vehicular use, biogas has to be upgraded to achieve 97% methane content.18 In addition, biogas has to be compressed to 250 bar.15 Methane losses during the upgrading and compression of biogas vary between 0.2 and 13% but were normally under 2% and during the rest of the system the losses are minimal.15,17 A net biomethane yield of 453, 843 mn3 (@ 97% methane) is calculated. Each hectare of grass land produces a gross energy of 121 GJ energy per annum (Table 1). This is in line with the value obtained by Smyth et al.3 Upgrading of biogas comprises the removal of CO2 , H2S and other possible pollutants from biogas.18 De Hullu et al.19 compared five techniques for upgrading of biogas viz. chemical absorption, high pressure water scrubbing (HPWS), pressure swing adsorption, cryogenic separation, and membrane separation. They found that membrane separation and HPWS are the simplest processes to operate because they do not need special chemicals or equipment to run. HPWS also provides maximum purity up to 98% with minimal cost.19 In addition to scrubbing, compression to 300 bar is required to utilize the biomethane as vehicular fuel.15 Transportation and utilization of biomethane The on-site option for the use of biomethane as a transport fuel is to compress to 300 bar and discharge to the vehicle through cascading pressure reduction to 250 bar.


Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Alternatively, the existing natural gas infrastructure may be used as a distribution system to a service station at a remove from the facility. A European Commission report 20 states that the energy required for natural gas local distribution is zero. This is because the high pressure trunk lines (typically operating at between 35 and 70 bar) that feed low pressure networks (typically operating at 7 bar) provide sufficient energy to supply local distribution. Thus in either case there is a necessity to scrub and to compress to 300 bar. As the natural gas infrastructure may not cover all the country, bi-fuel cars are used in many regions as the vehicle of choice for use of biomethane. Bi-fuel vehicles are able to run on either liquid fossil fuel (petrol or diesel) or compressed gas (either natural gas or biomethane). The fuel source is chosen by pushing a button in the car. The bi-fuel car is tuned and optimized for the liquid fossil fuel and thus is not optimized for the gaseous fuel due to the lower flame speed of the air-gas mixture compared to air-petrol mixture. Power and Murphy21 reported that existing bi-fuel vehicular engines are 18% less efficient (km MJ-1) operating on gaseous fuel than liquid fuel. Th is is supported by Murphy et al.18 who stated that a Volvo V70 bi-fuel obtained 9.8 km/l on petrol (0.327 km/MJ) but 9.6 km/mn3 operating on compressed biomethane (0.262 km/MJ); approximately a 20% reduction. They18 found a similar value when comparing with diesel. In an Australian public discussion paper, 22 it is also suggested that biogas is 18% and 29% less efficient than diesel and petrol fuels respectively. The data is engine specific and in the present study a bi-fuel car is assumed with an 18% reduction in engine efficiency when compared to diesel. The produced biomethane can thus replace diesel equivalent to 99.2 GJ ha-1 yr-1.

Energy and related emissions from crop production Direct energy consumption and related emissions Agronomic operations The primary fuel input into ryegrass production system is diesel for tractors and trucks. The estimation of GHG emissions from operations in the field requires knowledge of energy consumed during these operations. Murphy et al.18 reported GHG emissions from diesel consumption as 2.688 kg CO2e L-1 whereas the GHG emissions in diesel produc-

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

tion are 0.51 kg CO2e L-1.23 The gross energy of the diesel equals 36 MJ L-1 of the fuel4 thus the GHG emissions equal 88.8 g CO2e MJ-1. The direct energy consumed during field operations was estimated based on Eqn 2.24

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value to obtain a more conservative interpretation was chosen. GHG emissions then were estimated based on Eqn 3. GHG emissions (kg CO2e ha-1 yr -1) = FE (MJ ha-1) × 0.0888 (kg CO2e MJ-1)



FE = ∑ ( Fci × fc ) / Oci



where FE=fuel energy consumed (MJ ha-1), Fci= fuel consumption (L h-1) for i field operation, ƒc=heating value of the fuel & Oci= work capacity for i operation (ha h-1). The individual values for each component for each field operation in Eqn 2 were selected from an extensive range of publications suitable for grass production and the highest

The results from the energy consumed during the whole crop cycle and the subsequent GHG emissions are reported in Table 3. The average direct energy consumption was estimated as 2.98 GJ ha-1 yr-1. Smyth et al.3 recorded very similar values (2.9 GJ ha-1 yr-1). Harvesting, spreading of digestate, ensiling and ploughing are the operations (in that order) which require the highest energy inputs and thus emit the highest amounts of CO2e. Direct emissions during the crop cycle equals 2.67 gCO2e MJ-1 of energy replaced (Table 3).

Table 3. Direct energy consumption and related CO2 emissions during the eight year crop cycle (emissions from diesel production are included).


Energy Consumed (MJ ha -1 yr -1)

Average Energy consumed (MJ ha -1 yr -1)

CO2 Emissions (kg CO2 ha -1 yr -1)

g CO2 e MJ -1 Energy Replaced

Year 1

Year 2-8


























Year 1

Average Emissions (kg CO2 ha -1 yr -1)

Year 2-8


















0 (22.5)



0 (2.0)




0 (27)



0 (2.4)












Spreadinge Transporte Harvestingf a,g

Ensiling Total

























2787.9 (2814.9, 2810.4)h



247.7 (250.1, 249.7)h




Data of energy consumption for rolling and ensiling from Smyth et al.3 b Fertilizer is applied four times during the first year of the crop cycle and twice every other year after each harvesting . c Lime is applied in two intervals during the crop cycle, the first and fifth year after establishment. d Herbicides are applied before ploughing and after sowing to favor crop against weed competition at the first year and twice during the rest of crop cycle, the 3rd and 6th year (corresponding values for energy consumption and related emissions are shown in the parentheses). e Transport and spreading were estimated based on the assumption that each load carries 16 t digestate, hence 418 loads needed per year of which 250 were assumed with excluding empty return. The energy consumption for transport is assumed as 1 and 1.6 MJ t-1 km-1 with excluding and including empty return, respectively.9 Energy required for loading and spreading of digestate is assumed as 2.5 and 17 MJ t-1, respectively.21 f Harvesting includes operations such as cutting, mowing and turning the grass. g Ensiling is comprised of operations such as silage collection, unloading and inlaying. h First number in the parentheses represent values for the 3rd and 6th year and second number for the 5th.

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb


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Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Herbicide volatilization Volatilization emissions from pesticides occur up to 48 h after their application and it is a significant pathway for losses to the atmosphere.25 One of the most important factors which influence pesticide volatilization losses from treated surfaces is herbicide vapor pressure which is strongly correlated to volatilization emissions.25 Herbicides need to be considered more for the acute effects on human health and the possible harmful effects to adjacent crops due to herbicide drift.26 The total quantity of active ingredient (a.i.) (i.e. mecoprop, 2, 4-D, asulam, MCPA and glyphosate for pre-planting or grassland destruction and re-seeding) of the most common herbicides applied to the crop is calculated based on their application rate. The vapor pressure for each a.i. at 25 °C was obtained.27-28 Then each a.i. was classified based on data provided by Baas and Lekkerkers29 and the corresponding emission factor due to volatilization was obtained. Derivation of herbicide emission due to volatilization was estimated from Eqn 4.29 i

EHerbicide = ∑ mHerbicide , i × EFHerbicide , i



where Eherbicide = total emission of pesticide (kg yr-1) due to volatilization mherbicide = mass of individual herbicide applied (kg yr-1) EFherbicide = emission factor for individual herbicide (kg kg-1) The energy consumed for the production of one kg of a.i. as proposed by Saunders et al.30 and their corresponding emission factors (i.e. kg CO2e MJ-1) was adopted. The final result equated to 5.44 kg CO2e ha-1 yr-1 or 0.054 g CO2e MJ-1 energy replaced (Table 8) was insignificant in terms of GHG emissions.

Lime dissolution and related emissions Agricultural lime is commonly used in the management of grasslands to neutralize soil acidity. In this analysis, it is assumed that lime application in the pasture is in the form of crushed limestone (CaCO3). Carbon dioxide emissions from liming are calculated from the amount of CaCO3 applied per year (10 t ha-1 yr-1 over 8 years = 1250 kg yr-1) using Eqn 5. i

Elim e = ∑ mlim e , i × EFlim e , i 1



where Elime = total emission of C or CO2 from liming (t C yr-1) mlime = mass of individual liming agent applied (t CaCO3 yr-1) EFlime = emission factor (carbon conversion factor) for individual liming agent (t C t-1 CaCO3). The emission factor equals to 0.12 t CO2-C t-1 of CaCO3.29 A value of 550 kg CO2e ha-1 yr-1 is calculated, which equals to 5.55 g CO2e MJ-1 energy replaced (see Table 8). Nitrous oxide emissions Plant nitrogen is of central importance for the yield and protein content of grass leys and for forage quality. Nitrous oxide (N2O), a long-lived trace atmospheric gas, functions as a GHG with a strong capacity to absorb infrared radiation. The fraction of applied N actually emitted as N2O varies on a site specific basis.31 Coefficients of variation for N2O emissions measurements typically range from 0.003 to 0.03.32 Modelling nitrogen dynamics in the soil is based on numerous processes for litter input, including N deposition, microbial decomposition, N mineralization, and other processes which are beyond the scope of this paper.33 Emissions of nitrous oxide (N2O) from the use of fertilizer were estimated using Eqn 6.34 Equation 6 includes for both direct and indirect emissions (Eqns 7 and 8) for the estimation of N2O.

N2O(t) = 0.0125 × N applied (t) + 0.01 (NH3 + NO) emitted (t)


NH 3 (t ) = ∑ cfi × N fertilizeri applied (t )


NO (t ) = 0.007 × Total N applied (t )



where cƒi values for the most common fertilizers used in Ireland (ammonium nitrate, calcium ammonium nitrate (CAN) and urea) are 0.02, 0.02 and 0.15 t of NH3 t-1 of N applied yr-1 respectively.34 CAN is the most prevalent source of nitrogen fertilizer used in Ireland; thus a value of 0.02 is chosen. Indirect emissions of nitric oxide (NO) are estimated based on Eqn 8. Both NH3 and NO emissions are summed as indirect emissions as shown in Table 4 and derived below for each of years 2–8: • N2O = 0.0125 * 123 kg N ha-1yr-1 + 0.01 (2.46 + 0.861) = (1.54 + 0.03) kg ha-1yr-1 (6) -1 -1 -1 -1 • NH3 = 0.02 * 123 kg N ha yr = 2.46 kg ha yr (7)

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

emissions during crop production cycle equates to 6.34 CO2e

Table 4. Direct and indirect N2O emissions. Direct



Year 1 (kg CO2e ha-1 yr-1)




Years 2–8 (kg CO2e ha-1 yr-1)




Average (kg CO2e ha-1 yr-1)



Emissions (g CO2e MJ-1 energy replaced)



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MJ-1 energy replaced (Table 5).

Emissions from transportation

525 5.29

• NO = 0.007 * 123 kg N ha-1yr-1= 0.861 kg ha-1yr-1 • CO2e= (1.54 + 0.03) kgN2O ha-1yr-1 * GWP (296) = (456 + 9.7) kg CO2e ha-1yr-1


The total N2O emission from fertilizer applications is estimated as 525 kg CO2e ha-1yr-1 (Table 4). This result is in the lower level of the range observed by Kiely et al.35 who reported N2O emissions in Irish grassland ecosystems between 2 to 8.4 kg N2O ha-1 yr-1, (equivalent to 592 to 2486 kg CO2e ha-1 yr-1).

Indirect inputs and related emissions Indirect emissions result from the energy (and the associated CO2) invested for the production of the primary inputs into the crop (i.e. fertilizers and lime, herbicides and seeds) along with the energy required for their transport and application. Table 5 charts the analysis of these emissions. Lime production results in CO2 emissions from the calcination of limestone to produce different lime types.39 More specifically, limestone or chalk or calcium carbonate (CaCO3) is transformed into quicklime or calcium oxide (CaO) after heating, then into hydrated lime or slaked lime or calcium hydroxide (Ca(OH)2) after adding water. The amount of CO2 released into the atmosphere was estimated as a direct emission; hence no indirect emissions from lime production are included in Table 8. Herbicides are more of an issue in overall sustainability; GHG emissions are negligible. Seed production: West and Marland38 calculated the emissions for 1kg of ryegrass seed to be equal 0.54 kg C kg-1 seed. Allowing for 25 kg seed ha-1 an input of 13.5 kg C (or 49.5 kg CO2) is required in the establishment year (Table 5). Nitrogen and potassium have the highest percentages of indirect energy consumed in the silage crop (i.e. 71.4 and 13.3% respectively) and average CO2 emissions during crop production period (i.e. 71.7 and 16% respectively). Indirect

Berglund and Börjesson7 reported that transportation of grass by truck required 0.7 MJ energy per tkm excluding empty return. The transportation of 7500 t grass yr-1 from field to AD plant (10 km distant) requires 52.5 GJ yr-1, equivalent to 1458.3 L diesel (3.19 kg CO2e L-1). Total emissions in transportation of grass is calculated as 4.66 t CO2e yr-1, equivalent to 0.34 g CO2e MJ-1energy replaced. In this study, lime is imported from Derbyshire, UK. According to Harrison et al.40 transport of limestone occurs by rail. The quarries in Derbyshire are rail-linked. The distance from the limestone production plant to the port at Southport is 130 km, from where it is transported by ship 218 km to Dublin. The average transport distance in Ireland is 207 km. Typical energy requirements for 1000 km transport is 0.1 GJ t-1 for bulk sea carriers, 0.7 GJ t-1 for rail and 1.9 GJ t-1 for 28 t truck.41 Accordingly, the energy consumed is 0.091, 0.0218 and 0.39 GJ t-1 for rail, sea and land transportation respectively. The summed emissions are 0.5 GJ t-1 lime with subsequent CO2 emissions equal to 0.044 kg CO2e kg-1 of limestone (88.8 g CO2e MJ-1 diesel). At 10t of limestone per hectare in an 8 year cycle on 137.5 ha displacing 0.625 GJ ha-1yr-1, this equates to 55 kg CO2e ha-1 yr-1, which equals to 0.55 g CO2e MJ-1 energy replaced.

Emissions from biomethane production Direct energy consumption and related emissions Heating digesters The heat requirement of digestion is calculated by summing the energy lost from the digester tank and energy required to heat the feed stock. Equations 9 and 10 are used.9 hl ⫽ UAΔT q ⫽ CQΔT

(9) (10)

where, hl, heat loss (J s-1); U, overall coefficient of heat transfer (W m-2 °C); A, cross sectional area through which heat loss is occurring (m2);

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb









0 (5000)

0 (4.4)





Year 2-8







Energy required (MJ kg-1)









Year 1


5 12630.2

9835 (11199, 12835)







Average Energy Consumed (MJ ha-1 yr -1)


0 (3000)

0 (1364)





Year 2-8

Energy Consumed (MJ ha-1 yr -1)

1.98e, g







Emission Factora kg CO2e MJ-1


Saunders et al. Kelm et al.36 c Styles and Jones37. d Energy and emissions in parentheses for the application of herbicide (asulam) in the 3rd and 6th year of the crop cycle. e West and Marland.38 f Smyth et al.3 g kg CO2e kg-1 seed.















Year 1


Fertilizer a,b,c

Crop Production

Dose Applied kg ha-1 yr -1









Year 1


510.1 (2670, 592)



0 (81.8)





Year 2–8

CO2 emissions (kg CO2e ha-1 yr -1)

Table 5. Indirect energy consumption and related CO2 emissions during the eight-year crop cycle.









Average Emissions (kg CO2e ha-1 yr -1)









g CO2 e MJ-1 Energy Replaced

NE Korres et al Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

ΔT, temperature drop across the surface (°C). q, heat required to raise feedstock to digester temperature (kJ s-1); C, specific heat of the feedstock (kJ kg-1 °C-1); and Q, volume to be added (kg). The coefficient of heat transfer for the wall, floor and roof of the digester is taken as 0.8, 1.7 and 1 W m-2°C, respectively.42 The temperature drop across the surface and the temperature difference between the feedstock and digester temperature is taken as 28°C. The volume of feedstock added daily is 45.21 m3 at 10% DS (Table 1). As the feedstock has a low solids content, its specific heat is assumed to be similar to that of water (4.2 MJ t-1 °C).3,42 The authors have visited a number of facilities where the developer is loathe to burn the biogas produced to satisfy thermal parasitic demand. The product (renewable gaseous transport fuel) has an elevated revenue associated with it due to carbon taxes and green image. The developer does not want to minimize this return. Thus in this model the production of thermal energy is assumed to be by natural gas; an emission factor of 240 g CO2e kWh-1 thermal energy43 is used. This analysis may be considered conservative as temperature drop from the floor of digester must be less than 28°C and recycled leachate will be warmer than 10°C. The amount of heat provided by metabolic generation is uncertain and therefore neglected, giving a more conservative result. The annual thermal energy demand is calculated as 3703 GJ, which emits 248.8 t CO2e yr-1, equivalent to 18.25 g CO2e MJ-1 energy replaced (Table 6). These values are conservative; Smyth et al.3 generated only 63% of the energy required to heat the same feedstock (2351GJ). The conservativeness of the approach employed here relates to modest loading rates, effective volumes of only 80% of the tank and the oversizing of the second chamber to provide storage. Thus the stored mate-

NE Korres et al

rial is also heated. This is discussed further in the sensitivity analysis. Biogas losses and CO2 emissions Even moderate losses of methane can significantly affect the emissions from the biomethane production process, since methane is 23 times a more potent GHG than CO2. Methane losses during the upgrading and compression of biogas are taken as 2% of biomethane;17 losses during the rest of the system are considered negligible. According to Murphy and McKeogh43 each m3 of biogas which escapes and is not combusted, produces 9.16 kg of CO2e. Thus this escape equates to 147.5 t CO2e yr-1or 10.82 g CO2e MJ-1energy replaced. Indirect emissions from biomethane production The energy required for maceration, mixing, and water pumping activities is supplied by the electric grid. At present 542.8 kg CO2e MWh-1 is produced in the Irish Grid.44 The electrical energy requirement for maceration is taken as 2 kWh t-1 of silage added, 3 for mixing slurry digesters (operating at about 12% DS) 10 kWh t-1 of slurry digested18 and for pumping of water from the second digester to the fi rst is taken as 0.2 kWh m-3, (assuming 4 kW pump with capacity of 20 m3 h-1).15 The total emissions from the maceration, mixing and water pumping are calculated as 98.68 t CO2e yr-1, equivalent to 7.24 g CO2e MJ-1 energy replaced (Table 7). The electrical demand for biogas scrubbing and compression is ranged in between 0.3 and 0.6 and 0.35 and 0.63 kWh m-3 upgraded biomethane, respectively.2,20,46 A value of 0.35 kWh m-3 is assumed for each operation, which equates to 317.7 MWh electricity demand per year. Approximately 172.4 t CO2e is emitted during the production of the required electricity. The upgradation and compression of biomethane emitted 12.6 gCO2e MJ-1 energy replaced (Table 8).

Table 6. GHG emissions from digesters. Energy (GJ yr -1)

Energy (GJ ha -1 yr -1)

GHG Emission (kg CO2e yr-1)

GHG Emission (g CO2e MJ-1 Energy Replaced)

Heat loss from digester 1





Heat loss from digester 2





Heating of the feedstock digester 1




Heating of the feedstock digester 2 Total

0.00 3703

0.00 26.93


© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

9.56 0.00 18.25


NE Korres et al

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Conservative assessment of sustainability of grass biomethane Table 8 outlines the summary of the GHG emissions carried out in this paper. Transport as expected in an LCA is a small element; typically transport is a major issue for financial concerns, but not for energy balances or emissions. Agronomy contributes 28.5% of the emissions. Indirect agricultural emissions are the biggest concern in agronomy. This as outlined in Table 5 is dominated by production of nitrogen. The biggest issues are:

The whole life cycle emission of diesel is 88.8 g CO2e MJ-1; the production of GHG emissions for grass biomethane is 69.74 g CO2e MJ-1(Table 8); thus the GHG emission saving from the use of grass biomethane as opposed to diesel on a field-to-wheel basis is 21.5%. Vehicular inefficiency is included as the GHG emissions are expressed on the basis of energy displaced. If this fuel is to be considered a sustainable transport biofuel at least a 35% savings must be effected, after 2018 a saving of 60% must be effected. Another issue is carbon sequestration, which demands a section on its own due to the complexity of the carbon cycle.

1. Electricity (plant and the upgrading process termed indirect emissions)


Carbon sequestration in grassland

2. Heating of the digesters (direct emission)


3. Vehicle inefficiency


4. Biogas losses


5. Indirect agricultural emissions (dominated by production of N)


Grasslands remove CO2 from the atmosphere via photosynthesis and emit CO2 to the atmosphere via respiration. The sum of these two natural processes may classify grassland either as a source or sink for CO2 . Irish researchers35 found that Irish grasslands are C sinks sequestering 2 t carbon ha-1

Table 7. Emissions in maceration, mixing and pumping activity during biogas production. Quantity (t yr-1) Maceration Mixing Water pumping

Energy Required (kWh t-1)

GHG Emission (kg CO2e yr -1)











GHG Emission (g CO2e MJ-1 Energy replaced) 0.60 6.57





Table 8. Summary of GHG emisions (gCO2MJ-1energy replaced) as examined in this study. Parameters



Sub Totals

Crop production




Herbicide volatilization




Lime dissolution




N2O emission





Percentage of Total


Total agricultural emissions










7.24 12.64


Total processing emissions






Biogas Losses






Transportationa Biomethane production process AD plant Upgrading





Transportation includes emissions from lime and silage transportation to the field.


© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb


Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

yr-1 (above and below-ground);47 this is equivalent to 7.33 t CO2e ha-1 yr-1. The growing period has a significant effect on C sequestration; a longer growing period extends potential for C sequestration.48 In Ireland, growing seasons are longer in the south-west and shorter in the north-east. Consequently, C sequestration is a site-specific attribute with typically more in the south-west and less in the north-east. The allocation of the carbon to the stem which is cut as opposed to the root structure which remains is very important. Perennial grasses allocate more carbon to the root structure (as opposed to the stem) than annual grasses. Swinnen et al.49 estimated that a typical winter wheat crop grown in temperate W. European conditions may transfer over 1.6 t ha-1 of C below ground within a growing season. Saggar et al.50 found that C translocation in various grass species ranged between 2.45 and 4.43 t of C ha-1.

Scenarios to increase sustainabilty of grass biomethane The big issues in determing the GHG associated with grass biomethane are listed in order of importance. These can be explored as ways as improving the sustainability of the fuel. Electricity and wind energy The most significant GHG emissions avoidance can be made from technology substitution or improvement. Using electricity from wind greatly reduces the emissions associated with the grass biomethane system. Assessment of GHG per kWeh varies greatly in the literature. Values quoted range from 8 to 46.4 g CO2e kWeh-1 from wind energy51-52 If we consider the highest value, i.e. 8.5% of the value from the Irish grid used (542.8 g CO2e kWhe -1), a 26% reduction in emissions from grass biomethane (from 69.74 to 51.55 g CO2e MJ-1) is obtained. With electricity from wind energy, biomethane from grass silage effects a 42% reduction in emissions (Fig. 3), which would lead to a classification of sustainable biofuel for the years 2010 to 2017. Heating of the digester and digester configuration Half of the volume in the second digester is used as a storage tank. The heat requirement to maintain a constant temperature within it for maximum microbial activity can be reduced significantly, if the second digester is used only for

NE Korres et al

digestion and digestate is stored in a separate tank. This leads to a further drop of 1.97 g CO2e MJ-1 (due to reduced surface area and associated reduced heat loss; refer Table 6), a 2.7% drop in GHG emissions. This leads to an overall production of 49.7 g CO2e MJ-1 generating a cumulative saving of 44.2% compared to diesel (Fig. 3). This would lead to a classification of grass biomethane as sustainable for the years 2010 to 2017. Vehicle efficiency The modification of a spark ignition engine is comparatively easy as the engine is designed to operate on an air/fuel mixture. The basic modification is based on a gas-air mixer instead of the carburettor. The engine control is performed by the proportion of the fuel mixture. Spark ignition engines converted to use natural gas as a fuel show a power decrease of 18% (used in this paper) due to decreases in volumetric efficiency (the gaseous fuel and the lower flame speed of airgas mixture compared to air-gasoline mixtures).53 This power loss can be decreased by utilising a higher compression ratio and advancement in spark timing.53 Improvements in engine efficiency to a similar km MJ-1 as diesel will improve the emissions by 18%. This leads to an overall production of 40.66 g CO2e MJ-1 instead of 88.8 gCO2e MJ -1 for diesel, generating a cumulative saving of 54.2% compared to diesel (Fig. 3). Biogas losses and indirect agricultural emissions A 2% loss would be considered quiet good for a biogas system. There is potential for these losses to be conveyed to a combined heat and power (CHP) plant or a boiler which would reduce the fugitive emissions. Biogas that is emitted to the atmosphere has a GWP of 9.19 kg CO2 m-3; biogas that is combusted has a GWP of 1.96 kg CO2 m-3;43 this a fourfold improvement is possible. Emissions from agriculture are difficult to reduce. One potential is to utilize permanent pastureland and/or reseed through injection rather than ploughing. These scenarios will not be studied here but offer further opportunities to improve sustainability. Net carbon sequestration Three scenarios are examined. The first is the expected value for net carbon sequestration; the second and third scenarios are included to allow for the variability in the values found in the literature and to outline the relative sensitivity of the GHG savings to net carbon sequestration values:

© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb


NE Korres et al

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Figure 3. Percent CO2 savings over fossil diesel under a range of C sequestration and various scenarios in biomethane production (The scenarios are cumulative left to right, for example improving heat includes for elect & wind and base case scenario).

(1) The amount of C sequestered by grassland as recorded by Byrne et al.47 is 2 t C ha-1 yr-1 and 0.6 t soil C ha-1 yr1. This is in agreement with Freibauer et al.54 and Jones and Donnelly 55 where a minimum amount of 0.6 t C ha-1 yr-1 was reported as potential soil carbon sequestration rate (or net carbon sequestration rate) for perennial ryegrass and permanent crops under European agricultural conditions. Additionally, according to Kiely et al.56 net carbon sequestration in grasslands in Cork, Wexford and Kerry (counties in the south and south-east of Ireland) was found to be between 0.3 to 0.75 tonnes of C ha-1 yr-1. A value of 0.6 t soil C ha-1 yr-1 (net carbon sequestration) yields a savings in GHG emissions of grass biomethane as compared to fossil fuel substituted of 75%. (2) Various researchers54-56 outline a wide range of carbon sequestration from grassland under different grassland cropping systems and locations. These values range from 0.1 to 3 t C ha-1 yr-1.54-56 An upper boundary of net carbon sequestration of 2.8 t C ha-1 yr-1 is chosen. This yields a GHG savings of 150%. (3) An intermediate case is chosen for net carbon sequestration. For simplicity this is equated to the amount of carbon contained in the digestate 2.2 t C ha-1 yr-1(Table 9). This yields a GHG savings of 129%. All of these scenarios allow sustainability after 2017. The authors suggest that the initial value is more likely generating a savings in GHG emissions of grass biomethane as compared to fossil fuel substituted of 75%.


Further research/limitations of this paper Further research (beyond the scope of this paper) is required to ensure correct analysis of the emissions associated with grass biomethane. The main issues are: •

Carbon sequestration varies considerably; in particular it may be better not to plough the land but undertake the reseeding process through no-till. Volatilization of digestate is not adressed in this paper. Table 9 suggests 2.2 t C ha-1 yr-1 is applied to the land as digestate. The assumption here is that this is slow to volatilize as 55% of volatiles already destroyed over 62 days in two digesters. This paper has not considered land-use change. Landuse change would be beneficial to the system as belching cattle are removed from the system. The exact benefit depends on numerous considerations including: the agricultural practice (beef, milk); the cattle feed; the slurry collection method; and the stocking rate.

Table 9. Estimated carbon balance for this study. Parameter

t C ha–1

Silage: 12 t DM ha-1 @45% Ca


Biogas: 5940 mn3 ha-1 @1.96 kg CO2b


Digestate (no storage losses were assumed):



Prochnow et al45 Murphy et al.18


© 2010 Society of Chemical Industry and John Wiley & Sons, Ltd | Biofuels, Bioprod. Bioref. 4:310–325 (2010); DOI: 10.1002/bbb

Modeling and Analysis: Is grass biomethane a sustainable transport biofuel?

Table 10. Typical values for greenhouse gas savings for biofuel systems from the renewable energy directive4 Biofuel system

% savings in greenhouse gas compared to fuel replaced

Wheat ethanol


Rapeseed biodiesel


Sunflower biodiesel


Sugarbeet ethanol


Palm oil biodiesel


Biogas from MSW


NE Korres et al

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Tilman D, Hill J and Lehman C, Carbon-negative biofuels from low-input high-diversity grassland biomass. Science 314:1598–1600 (2006).

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Conclusions With reference to Fig. 3, it may be seen that the sustainability of grass biomethane is subject to a range of possible parameters. These may be compared with typical values from the Renewable Energy Directive4 as outlined in Table 10. The analysis in this paper suggests that an essential element of a grass biomethane facility is the reduction in emissions associated with parasitic energy demands. Thus the purchase of green electricity and the minimization of thermal energy input are essential. As supported by others21 the vehicle must be optimized for biomethane; bi-fuel vehicles may not meet this criterion. Thus through process optimization it may be said that a reduction in emissions of 54% may be effected. Grassland sequesters carbon; a value of 0.6 t C ha-1 yr-1 is deemed conservative. This would lead to a reduction in emissions of 75% which would suggest that grass biomethane is one of the most sustainable indigenous non residue European transport biofuels.

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Acknowledgements Beatrice Smyth and Thanasit Thansiriroj for innovative discussions.

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Dr Anoop Singh Dr Anoop Singh is a research fellow in bioenergy and biofuels in the ERI. He has an MSc and a PhD in Environmental Sciences. Dr Singh previously worked at a number of research institutes in India (TERI, IARI, BHU and VBSPU). He has published widely in the field of the waste management, and biofuels and their life cycle analysis.

ecosystems and the influence of management, climate and elevated CO2. New Phytologist 164:423–439 (2004). 56. Kiely G, Leahy P, Sottocornola M, Laine A, Mishurov M, Aldertson J and Carton O, Celticflux: Measurement and modelling of greenhouse gas fluxes from grasslands and peatland in Ireland. EPA Strive Programme 2007–2013. 2001-CD-C2-M1 (2008). [February 1, 2010].

Dr Nicholas E. Korres Dr Nicholas E. Korres is a research fellow in bioenergy and biofuels in the Environmental Research Institute (ERI), University College Cork (UCC). He has a BSc in Agronomy, an MSc in Crop Physiology, a PhD in Weed Science and an HDip in Operational Research & Applied Statistics. Dr Korres has extensive work experience in the UK and Greece and has published widely in agronomy and related energy issues.

Abdul-Sattar Nizami Abdul-Sattar Nizami is a PhD researcher investigating grass biomethane as a transport fuel in the ERI. He has an MSc in Environmental Engineering from Chalmers University of Technology, Sweden and a BSc (Hons) Degree in Environmental Sciences from the Punjab University, Pakistan.

Dr Jerry D. Murphy Dr Jerry D. Murphy is the lead PI in bioenergy and biofuels research in the ERI and has represented Ireland on the International Energy Agency Bioenergy Tasks for biofuels and biogas. He has a BE in Civil Engineering, an MEngSc in Anaerobic Digestion and a PhD in energy from wastes.

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