Since the availability of grass from LCMW activities is of discontinuous nature, the ensilage of grass may be required to ensure continuous availability of substrate. Use of grass as co-substrate in biogas plants running with agricultural substrates is recommendable.

Possible biogas yield

The biogas yield depends upon substrate properties e.g. dry matter (DM), volatile solids (VS), handling and storage methods of the substrate as well as upon environmental conditions. Since the availability of grass from LCMW activities is of discontinuous nature, the ensilage of grass may be required to ensure continuous availability of substrate. Use of grass as co-substrate in biogas plants running with agricultural substrates is recommendable.

For correct estimations of biogas yields, biomethane potential (BMP) assays need to be carried out with the desired feedstock. In a study, the biogas production potential of non-dried grass originated from LCMW in Italy with DM content of 39.4 % to 46.8 % and VS content of 89 % to 92 % was reported to be in the range of 526 to 576 m3/t VS. For grass ensilaged for 30 days, the biogas yield was in the range of 618 to 659 m3/t VS. The DM and VS percentage of ensilaged grass were almost similar to non-ensilaged grass (Mattioli et al., 2016). Considering an average biogas yield of 551 m3/t VS from grass and an average biogas yield of 638.5 m3/t VS from ensilaged grass with an average of 43 % DM content of which 90 % is volatile, the possible biogas yields from fresh and ensilaged grass available from urban and roadside LCMW in the model regions could be calculated (table 1). Thus, a ton of grass (fresh matter) from CZ-LCMW 4 + CZ-LCMW 5 has a potential to produce 213.2 m3 biogas or 117.26 m3 methane, assuming average methane content of biogas to be 55 %.

Table 1: Possible specific biogas yields from LCMW biomass of model regions in the Czech Republic
Region Sustainable potential (FM/yr)

CZ-LCMW 4 + CZ-LCMW 5 (Grass)

Dry matter (t/yr) Volatile solids (t/yr) Biogas yield-Grass (dam3/yr) Biogas yield-Grass Silage (dam3/yr)
Kněžice 255 109.65 98.69 54.38 63.01
Týn nad Vltavou 812 to 2248 349.16 to 966.64 314.24 to 869.98 173.15 to 479.36 200.64 to 555.48

1 dam3 = 1000 m3

Biogas production of grass from feedstock CZ-LCMW 4 and CZ-LCMW 5

About 90 % energy of the fermented substrate is converted to methane in the biogas which is produced during anaerobic digestion.

Assuming methane share of biogas produced from grass is 55 %, the potential energy yield of biogas gained after anaerobic digestion of grass or grass silage obtainable from CZ-LCMW 4 + CZ-LCMW 5 in both model regions could be calculated (table 2).

Table 2: Possible energy yields from LCMW biomass (sustainable potential) of model regions in the Czech Republic
Region Methane yield-Grass (dam3/yr) Methane yield-Grass Silage (dam3/yr) Energy yield-Grass (TJ/yr) Energy yield-Grass Silage (TJ/yr)
Kněžice 29.91 34.66 1.19 1.38
Týn nad Vltavou 95.23 to 263.65 110.35 to 305.51 3.79 to 10.49 4.39 to 12.16

However, the above energy values are subjected to deviation due to operational conditions and actual methane content of the produced biogas. Also, values are calculated at STP conditions and in praxis bioreactors operate at 37 0C which affects the gas volumes.

For energy recovery, methane is usually converted into heat energy via the boiler or into electric and heat energy via combined heat and power (CHP) plant. Most of the internal combustion engine generators have methane to electricity conversion efficiency between 25 to 40 % (Electrigaz, 2016). Considering, 1 kWh energy is equivalent to 3.6 MJ and average conversion efficiency of CHP is 33 % for electricity, 50 % for heat with 17 % loses, the electricity and heat generation from the sustainable potential of biomass CZ-LCMW 4 + CZ-LCMW 5 of the model regions can be calculated see table 3.

Table 3: Annual potential of energy and electricity generation from LCMW biomass in biogas-combined heat and power plant (CHP)
Region Potential energy-Grass (MWh) Potential energy-

Grass Silage (MWh)

CHP Electricity-Grass

(MWh)

CHP Electricity-Grass Silage (MWh) CHP Heat- Grass (MWh) CHP Heat- Grass Silage (MWh)
Kněžice 330.63 383.14 109.11 126.44 165.32 191.57
Týn nad Vltavou 1052.84 to 2914.76 1220 to 3377.63 347.44 to 961.87 402.61 to 1114.62 526.42 to 1457.38 610 to 1688.81

The average electricity consumption in the Czech Republic is estimated to be 3500 kWh/dwelling (Lapillonne et al., 2015). Thus, the energy generated from CZ-LCMW 4 + CZ-LCMW 5 biomass of respective model regions can fulfil electricity demand of up to 36 houses in Kněžice and 318 houses in Týn nad Vltavou see table 4.

Table 4: Fulfilment of electricity demand from LCMW biomass in model regions of Czech Republic
Region Electricity for households-

Grass

Electricity for households-

Grass Silage

Kněžice 31 36
Týn nad Vltavou 99 to 275 115 to 318

Suitability of anaerobic digestion pathway for CZ-LCMW 4 and CZ-LCMW 5 in the Czech model regions

The municipality of Kněžice already has a biogas plant combined with heat and power unit that has an installed electric capacity of 330 KW. The plant uses manure, sewage, straw, and food waste as feedstocks originated from local farms, slaughter houses and households. The Grass-urban (CZ-LCMW 4) is already utilized in this biogas plant as co-substrate but the Grass-road (CZ-LCMW 5) is partially utilized and rest is left on the roadside. Lack of necessary equipment e.g. for chopping, cleaning, collection and transport of the grass is the main hindrance for the full utilization of LCMW grass despite the close vicinity to the anaerobic digester and reduced transport distances in the region.

Týn nad Vltavou has two biogas plants of the company BPS Jarošovice in the region. The first plant was built in 2010 and has an installed electric power and heat capacity of 1263 kW and 1242 kW, respectively. The second plant linked with a composting unit was built in 2013 and has an installed electric capacity of 550 kW and heat capacity of 569 kW (CBA, 2016). The plant is built to serve 2000 t/yr pig slurry, 4000 t/yr grass silage, 14,600 t/yr of corn silage and 3200 t/yr of process water (Envitec biogas, 2016). In spite of its relatively larger energy potential comparing to Kněžice (table 3), grass generated from CZ-LCMW 4 and CZ-LCMW 5 in the region Týn nad Vltavou is not utilized as feedstock in biogas plants. At present, a composting plant Kompostarna Jarošovice acquires CZ-LCMW 4 grass @ 5 €/t for compost and in general CZ-LCMW 5 grass is underutilized. Because of the presence of biogas plants in the region, there is a feasibility of short distance transport of grass to plants with trucks. In the Czech Republic, the approximate price of transportation per kilometre with a truck having a capacity of 90 m3 is about 30 CZK (CZ Biom, 2011) that corresponds to 1.11 €/km according to current exchange rates, however for short distances as in this case prices are variable and fixed by operators. In a life cycle assessment (LCA) study of residual grass, it was found that the conversion of residual grass to biogas was environmentally beneficial than using it for composting as biogas allowed an 80 % reduction of the global warming potential compare to composting (GR3, 2016). The grass is commonly used as 10-20 % co-substrate with slurry or other crop feedstock. The addition of grass may also help in to increase the DM content in digester if main feedstock has a low DM content. In the case of feedstock with high nitrogen content e.g. pig slurry or manure, co-feeding of grass helps in bringing down the nitrogen levels mitigating ammonia toxicity and possible upsetting of the reactor. Since the LCMW grass is seasonal and harvesting is in small batches, this biomass might be a suitable co-substrate without storage requirement. However, some problems may arise while using grass in digesters which includes increased energy requirements for mixing grass with slurry if grass tends to float on the surface, impure grass needs cleaning to avoid abrasion as well as wrapping of long grass around moving devices may lead to dysfunction. Therefore, for wet digestion, usage of grass only as co-substrate is recommended. Besides technical issues, there is insufficient awareness among the stakeholders in the region regarding the LCMW grass to biogas value chain. The economic feasibility of digesting LCMW grass can be increased if the digestate is sold as manure especially for organic farming. An estimation of energy balance by determining annual net energy gain (NEG) and the energy return on energy invested (EROEI) can favour the recovery of untapped LCMW grass in Týn nad Vltavou as feedstock for biogas production.

Conclusion

The potential energy yield per annum via anaerobic digestion of Grass-urban (CZ-LCMW 4) and Grass-road (CZ-LCMW 5) is up to 383.14 MWh in Kněžice and 1220 to 3377.63 MWh in Týn nad Vltavou. This could meet electricity demand of 36 houses in Kněžice and 115 to 318 in Týn nad Vltavou beside supplying heat. Economical grass recovery technologies and logistics are the factors to be considered for energy recovery from LCMW grass. The optimum logistic system depends upon the distance to anaerobic digestion plant as well as upon harvesting methods. The municipality of Kněžice has a biogas plant combined with heat and power with an installed electric capacity of 330 KW, where Grass-urban is successfully utilized as co-substrate. Due to lack of transportation facilities, Grass-urban is mostly left on roads which may be utilized for energy recovery, a) if transportation could be improved and b) if Czech regulatory framework is favourable for use of roadside grass for energy production. The energy potential of LCMW grass in Týn nad Vltavou is untapped despite the presence of two biogas plants in the region which also utilize grass silage as co-feedstock. Anaerobic digestion has clear advantages in term of energy production and reduction of greenhouse gas emissions by over 80 % as compared to composting of grass, further digested sludge can be used as manure and thus, it is recommended pathway for LCMW grass generated in both model regions of Czech Republic.

Contact

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Christiane Volkmann
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CZ Biom - Czech Biomass Association
www.czbiom.cz
Jan Doležal
dolezal@biom.cz

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