When aiming to utilize LCMW biomass as feedstock for an energy supply chain a number of technical, environmental, socio-economic and legal factors have to be taken into account. During task 4.2 of the greenGain project key success criteria were identified to match the LCMW feedstock types with the suitable conversion technologies and define LCMW pathways.
The key success criteria are the potential and availability of feedstock type, suitability for conversion technology, costs of the pathway, and the environmental and socio-economic performance. Based on these criteria the LCMW pathways were assessed and their performance evaluated at local scale.
Below we take a closer look at the potential conversion route from biomass harvesting to the energetic consumption on the example of feedstock type CZ-LCMW 4 (Grass-urban) and CZ-LCMW 5 (Grass-road) in Kněžice and Týn nad Vltavou of Czech Republic (refer to the table 1: Potential and properties of landscape conservation and maintenance work (LCMW) biomass in the greenGain model regions; greenGain deliverable D4.2).
Herbaceous biomass in biogas plants
The number of biogas plants in Europe is growing, especially in eastern European countries e.g. Hungary, the Czech Republic, Slovakia and Poland where an increase of 18 % was observed in 2013 (EBA, 2015). Special incentives are paid to encourage the biogas production due to which many farmers started to grow energy crops e.g. maize, for its profitability. But the increase in maize cultivation areas led to the shortening of crop rotation which in turn has an impact on landscape characteristics, species diversity and soil and water quality (Phelken et al., 2016). In addition, energy crops as feedstocks for anaerobic digestion are posing a competition with food production. According to 2014 EBA Biogas report, there are > 14500 anaerobic digestion plants in Europe and 80 % are using agricultural feedstocks. Therefore, a focus is put on finding alternative biomass sources e.g. biomass resulted from the landscape maintenance and conservation work (LCMW) which is currently an underutilized energy resource. To facilitate the utilization of LCMW biomass, Germany amended its legal framework (EEG 2012) according to which the biomass from cultural landscape management excluding grass from roadsides, is no longer required for further processing and is recognized as a renewable resource. In the greenGain project, biomass from LCMW in four countries and seven model regions has already been recognized as promising alternative feedstock for gaining energy. As a part of the project, respective sustainable potential of various LCMW biomasses was calculated after subtracting technical, economical, implementation and social constraints from theoretical potential (see greenGain deliverable D5.1 and D5.2). In general, three types of LCMW feedstock i.e. woody, herbaceous and a mix of both woody and herbaceous biomasses are available from the study regions.
Territorial origin and type of biomass
In model regions Kněžice and Týn nad Vltavou of Czech Republic, biomass originated from the maintenance of urban areas and roadsides are of particular interest. This biomass is further divided according to its properties into woody and herbaceous biomass i.e. trees and grass. The grassy biomass available from urban and roadside maintenance of both Kněžice and Týn nad Vltavou is classified as CZ-LCMW 4 and CZ-LCMW 5, respectively (refer to the table 1: Potential and properties of landscape conservation and maintenance work (LCMW) biomass in the greenGain model regions; greenGain deliverable D4.2).
Kněžice is a municipality with an area of 19 km2 located in Nymburk District of Central Bohemia. The public green spaces in this region constitute an area of 12 ha from where the grass is obtained during landscape maintenance work (CZ-LCMW 4). Cleaning of roadsides incorporates only 1 % of municipality area and the total grass area is estimated to be 4 ha along 10 km stretch of roadways (CZ-LCMW 5).
Týn nad Vltavou is located in South Bohemia and has an area of 262.43 km2. Urban spaces e.g. parks, stadiums, squares etc. in this region have a grass area of over 80 ha (CZ-LCMW 4), whereas grass from its roadsides is available from an area of 85 ha spread along 720 km long roads in the region (CZ-LCMW 5).
Seasonality and potentials of the feedstock type
The grass is cut in both regions from April to October. The number of treatments for landscape maintenance varies between localities. In small municipality like Kněžice, at an average 3 treatments are carried out annually for both CZ-LCMW 4 and CZ-LCMW 5 grass. Whereas, in the region of Týn nad Vltavou urban area grass CZ-LCMW 4 is treated 3-5 times and roadsides grass CZ-LCMW 5 is treated 2-3 times per year.
In Kněžice, the sustainable potential for CZ-LCMW 4 (Grass-urban) is 60-75 % of 292-366 t/yr of theoretical potential and for CZ-LCMW 5 (Grass-road) sustainable potential is only 30 % of 120 t/yr of theoretical potential (refer to the table 1: Potential and properties of landscape conservation and maintenance work (LCMW) biomass in the greenGain model regions; greenGain deliverable D4.2), which is mainly because of technical and economical limitations (see Deliverable 5.2 of greenGain for details). Similarly, a significantly reduced sustainable potential is observed for CZ-LCMW 4 and CZ-LCMW 5 grass from Týn nad Vltavou region. About 90 % of grass from roadsides is left on the ground as mulch and in urban areas only 50 % grass recovery is possible due to technical and economic reasons. With the present sustainable potentials, a total of 255 t fresh matter/yr grass is available in Kněžice and a total of 812 to 2248 t fresh matter/yr grass is available in Týn nad Vltavou from LCMW activities.
Process chain and logistics
The steps involved in production of biogas from grass include:
- Mowing: It can be done with tractors mounted with different mowing units assembled according to the type of operation e.g. mowing of roadsides or of parks.
- Harvesting and collection of grass: The mowed grass can be collected with field choppers, absorbed with exhauster fitted on the mower, trailer wagons or balers.
- Cleaning of grass: This step is only required if grass contains impurities e.g. sand or plastic bags, bottles, metal cans etc. especially in urban areas. Sieves, hydraulic stirrers or gravitational separation are generally used.
- Transport: After optional cleaning, the comminuted harvested grass may be first collected in a container or baled or could be directly transported with a tractor, trailer wagon or a truck.
- Storage in silos (or direct use): If the grass is intended to be preserved for later use then it is compacted and stored in airtight silos otherwise there is a danger of spoiling. Ensiling also serves as bio-chemical pre-treatment.
- Anaerobic digestion: Grass or grass silage is supplied to anaerobic reactor mostly as a co-substrate. For wet-fermentation, pumps are required to feed the slurry in reactors.
- Supply to CHP: The biogas produced is supplied to biogas CHP for the recovery of heat and electricity.
Economical grass recovery technologies and logistics are main factors while deciding energy recovery from LCMW grass. The optimum logistic system depends upon the distance to anaerobic digestion plant as well as upon mowing and harvesting methods. Loose grass requires larger transport volumes than comminuted and baled grass. Further, drying of grass after cutting may reduce volume but that may lead to a reduction in energy value. The density of loose grass typically varies from 50 to 70 kg/m³ DM and when pressed in bales the density increases up to 100-150 kg/m³ DM. Taking average densities of loose and baled grass, volumes of loose or baled grass available from both model regions can be calculated (table 1).
Table 1: Estimation of average grass volumes available from LCMW in both model regions of Czech Republic. Average treatments/yr = 3
|Volume of loose grass (m3/yr)||Volume of baled grass
|Volume of loose grass (m3/treatment)||Volume of baled grass (m3/treatment)|
|Týn nad Vltavou||349.16 to 966.64||5819.33 to 16110.67||2793.28 to 7733.12||1939.78 to 5370.22||931.09 to 2577.71|
Proximity of storage or digestion plant to the harvesting sites is a primary economic issue and operational costs increase linearly with the distance. Beside mowing and harvesting costs, the final economic impact of the baling needs to be considered before transportation. Boscaro et al. (2016) investigated three different logistic scenarios for transporting grass to anaerobic digestion plant located at a distance from 5 to 30 km. They reported that under economic aspects, direct transport of grass after harvesting was most convenient for short distances (< 5 km), for longer distances (30 km) an interrupted transport chain consisting of temporary storage was better while transport of grass in round bales was less advantageous than interrupted transport chain.
Anaerobic digestion: a promising pathway for the utilization of grass
Thermochemical conversion technologies are suitable for dry biomass as feedstock whereas, for conversion of wet biomass at lower temperatures, biochemical conversion pathways are more efficient. Studies describing the use of plant biomass for anaerobic digestion are available in the literature (Weiland 2010). Principally, the biomass obtained from LCMW activities with the exception of woody biomass could be converted to biogas via anaerobic digestion. Grass can be co-digested with different agricultural feedstocks without the requirement of major process change. The energy from biogas is recoverable and used for producing heat, electricity or biofuels whereas, the digestate can be used as manure. Grassy biomass as a raw material for biogas production is also considered within the concept of Green-Biorefinery which is a promising future pathway for energy generation (Prochnow et al., 2009).
Herbaceous biomass from LCMW or other agricultural residues confirms substantially to the known anaerobic degradation scheme involving four process steps namely hydrolysis, acidogenesis, acetogenesis and methanogenesis. The structurally poor carbohydrates e.g. glucose can easily be attacked by microorganisms and are rapidly degradable whereas, substrates having a high percentage of structural carbohydrates e.g. lignocellulosic biomass require longer digestion periods as hydrolysis is a rate limiting step. The main components of biogas are methane (CH4, 60-70 %) and carbon dioxide (CO2, 30-40 %). Variable amounts of water (H2O), hydrogen sulphide (H2S) and some traces of ammonia (NH3), hydrogen (H2), nitrogen (N2), and carbon monoxide (CO) may be present in biogas depending upon the properties of feedstock. Dry matter (DM) content of the substrate plays an important role in biogas production.