138 Biology and Chemistry of Jerusalem Artichoke: Helianthus tuberosus L. Biomass and Biofuel

that can be stored as syrup until needed (Bajpai and Bajpai, 1989). It is also important that appropriate cultivars are grown for biomass.

Ethanol and spirits made from Jerusalem artichoke have many applications in the food industry and as an industrial chemical feedstock. Ethanol, for instance, is a key raw material in the manu­facture of plastics, lacquers, and many other industrial products. Therefore, alternative markets exist for bioethanol should surpluses occur as a liquid fuel. Ethanol fermentation from Jerusalem artichoke also results in a range of valuable by-products, including a pulp (7% protein) utilizable as animal feed, protein concentrates, and a liquid crop fertilizer (Guiraud et aI., 1982). The economic value of animal feed by-products, in particular, is a key factor in the economic viability of using Jerusalem artichoke as an energy crop. The tuber residue remaining after ethanol distillation can be used as a feedstock for biogas production (see below). The crop has also been proposed as a dual source of ethanol and single-cell proteins, simultaneously produced using different groups of yeasts to ferment tuber extracts (Apaire et aI., 1983; Bajpai and Bajpai, 1989). The cultivation of biomass for energy on underutilized land can furthermore create new livelihoods for farmers, contribute to farm diversification, and create jobs in rural areas with high unemployment.

7.3.2 BIOGAS (METHANE)

Biogas is a fuel produced by the anaerobic decomposition of wet organic matter (biomass), through the action of bacteria. As with natural gas, the main fuel component is methane. Biogas occurs naturally, for example, in swamps and marshes, where a layer of water gives rise to the necessary anaerobic conditions, and biogas from such sources can be harnessed as a fuel. Important feedstocks for the production of biogas in commercial. anaerobic digester systems include industrial and domestic biowastes, slaughterhouse waste, and livestock manures. Any biomass can theoretically be used for biogas or methane production, and many plant species are potential candidate feedstocks, especially those rich in easily biodegradable carbohydrates (EI Bassan, 1998). However, plants with a high lignin content have a low biodegradability and are less suitable (Klass, 1998).

Biomass crops are harvested, transported, and subject to pretreatments prior to fermentation (Figure 7.2). Pretreatments may include chopping to reduce the unit size of biomass, ensilage, acid hydrolysis, the addition of cellulase enzymes, or the use of solvents to remove lignin (Gritzali et aI., 1988; Hayes et aI., 1988). Pretreatments help to break down polysaccharides and other com­pounds into fermentable sugars before digestion. Anaerobic digestion is a multistage process involving a diversity of microorganisms that digest particular plant components (e.g., carbohydrates,

Biomass (e.g., Jerusalem artichoke tops)

l Pre-treatments

(e.g., chopping, ensilage, acid or enzymatic hydrolysis)

l Anaerobic digestion

(with methanogenic bacteria)

l Biogas

l Purified methane

FIGURE 7.2 Stages in biogas and methane production from Jerusalem artichoke.

cellulose, and hemicellulose). The most important microorganisms involved are methanogt bacteria, in the kingdom Archaebacteria. Three genera groupings of methanogenic bacteria art particular importance: (1) Methanobacterium and Methanobrevibacter (e.g., Methanobacteri formicum); (2) Methanococcus (e.g., Methanococcus vannielii); and (3) Methanomicrobium, Me anogenium, Methanospirillum, and several other genera (e.g., Methanomicrobium mobile). j

methanogenic bacteria are strictly anaerobic (Klass, 1998). Fermentation generally proceeds at J: 6.5 to 8.0 and usually within one of two optimal temperature ranges: a lower range (10 to 42°( favorable to mesophilic bacteria and a higher range (50 to 70°C) favorable to thermophilic bacter. (White and Plaskett, 1981).

The biogas produced by anaerobic digestion mainly comprises methane (CH4) and carbOJ dioxide (C02), For instance, the composition of a typical biogas from a pig manure is 65% methant and 35% carbon dioxide, with a thermal content of 26 MJ·m-3 (White and Plaskett, 1981). Methant content can range from 40 to 70% in biogas. Thermal values for biogas typically range from 15.7 to 29.5 MJ·m-3. In comparison, the energy values of dry natural gas and pure methane are 39.3 MJ·m-3 (Klass, 1998). For local use (e.g., on-farm heating of greenhouses or poultry sheds) biogas can be used without further processing. However, to obtain methane, for a more valuable and efficient biofuel, the carbon dioxide and minor components can be removed using several gas clean­up processes, such as alkali scrubbing or scrubbing with pressurized water. As the scale of energy­generating operations increases, so does the importance of removing carbon dioxide, which can be utilized as a by-product. The composition of minor components present in biogas depends on the type of feedstock deployed. Hydrogen sulfides, for instance. can occur at low levels (e.g., 0.1 %) and lead to problems of corrosion and toxicity if not removed (White and Plaskett, 1981). Plant­derived feedstocks will generally give rise to fewer troublesome minor biogas' components than biowastes and other types of feedstock. The nutrient-rich digestate remaining after biogas production can be fed back into the reactor, or it can be sold as a by-product, either as an animal feed or a crop fertilizer. The cleaned-up methane can be compressed and containerized, or distributed via pipelines. Methane is mainly used to generate heat and electricity.

Jerusalem artichoke is a suitable candidate energy crop for biogas production, and it amply fulfills many of the selection criteria. It has rapid growth and is easy to cultivate, has high energy and biomass yields with low-input requirements, a tolerance of a wide range of climatic conditions, resistance to pests and diseases, good overwintering ability, and an efficient ,onvertibility to methane. The aerial parts of the crop are usually used, although tubers are also suitable. Nonstruc­tural carbohydrates in the aerial parts are readily fermented, and the stems can contain large amounts of these carbohydrates. Converting Jerusalem artichoke tops to ,ilage is an effective way of conserving aerial biomass, and enables it to be stored until required on a year-round basis. A number of studies, comparing the production of biogas (and methane purified from biogas) from fresh and ensiled tops, have demonstrated that the two types of feedstock give a similar productivity (Gun­narson et aI., 1985; Lehtomaki, 2005; Mathisen and Thyselius, 1985; Zubr, 1986). It is better to harvest fresh green material for silage than to use material harvested during the stages of senescence (Zubr, 1985). Biogas and methane yields from Jerusalem artichoke, reported in the literature, are summarized in Table 7.2. For instance, the biogas yield from Jerualem artichoke tops after 20 days' retention time in a digester was 480 to 590 m3 biogas·,l volatile solids (YS),* which was superior to six other crops (e.g., alfalfa, maize, and sugar beet) tested (EI Bassan, 1998, quoting unpublished data from Weiland, 1997).

Feedstock chemical composition is important for biogas production. The low molecular weight sugars, inulin (fructans), hemicellulose, and cellulose present in Jerusalem artichoke are all digested in biogas fermentation. The highest proportions of most digestible components for the tops are found in the stem, and the inclusion of a high proportion of stems is therefore desirable for biogas production (Malmberg and Theander, 1986). Moreover, high levels of nitrogen in the substrate

• The quantity of solids in a sample Ihm i, losl by igniliun uf lhl' UI") ,uJiu, Jl WO°c.

141 ulOIOgy ana Lhemlstry ot Jerusalem Artichoke: HelFanth·us tuberosus l.

TABLE 7.2 . Biogas and Methane Yields from Jerusalem Artichoke

Methane Biogas Methane Biogas per Wet Wt. per Area per Area

Plant Part (I·kg-' VS)' (I·kg-' VS)' (m'·ha-') (m'·ha-') Reference

Tops' - 93 m'·t - 3, J00-5,400 Lehtomaki, 2005

Tops 480-680 - 5,500 2,800 Gunnarson et aI., 1985

Ensiled tops 468 315 - - Zubr, 1985

Tubers 595 4J 1 - - Zubr, 1988

Tops 296 189 - - Zubr, 1988

Ensiled tops 331 229 - Zubr, J988

Tops 480-600 - - - Mathisen and Thyselius, J985

Ensiled tops 500-680 250-320 - - Mathisen and Thyselius, 1985

Tops 480-590 m'· t - - - EI Bassan, J998

, Unless otherwise noted.

h Freshly harvested aerial parts (stem and leaves) unless otherwise stated.

are detrimental to biogas produclion, with nitrogen levels highest in the leaves of Jerusalem arti­choke (Malmberg and Theander, 1986; Mathisen and Thyselius, 1985; Somda et al., 1999). Inulin hydrolysis is the rate-limiting step when using Jerusalem artichoke as a substrate in anaerobic digestion reactors.' :.

Farm-scale biogas plants have been operaiing in Germany since 2000, and pilot plants are now being established worldwide. Energy crops are often used as a co-substrate with industrial and household biowas'tes and animal manures. Mixtures of ensiled plant biomass with cow or pig manures have been shown to produce high biogas yields (e.g., 350 to 540 ].kg-I VS), with a mixture containing 70% silage giving the best biogas yields in one study (Mathisen and Thyselius, 1985). The anaerobic digestion of energy crops represents a renewable domestic energy source, which is amenable to decentralized farm-scale energy production in areas close to crop production. Plant biomass can be produced on set-,aside land in crop rotations, or it can utilize crop overproduction and crop residues, creating new business opportunities in rural areas (Lehtomak.i., 2005).

In a pilot anaerobic digestion system in Finland, the potential methane yields from Jerusalem artichoke were among the highest of a range of energy crops assessed, including timothy grass (Phleum pratense L.), lupine (Lupinus polyphyllus Lind!.), reed canary grass (Phalaris arundinacea L.), and nettle (Urtica dioica L.). Jerusalem artichoke had a methane production potential of 0.37 m3 CH4·kg organic matter and 93 m3 CH4·t wet weight. In trials, Jerusalem artichoke yielded an annual 9 to 16 t dm·ha" and 3,100 to 5,400 m3 CH4·ha-' , equivalent to an annual gross energy potential of 30 to 50 MWh·ha-1 energy or 38,000 to 68,000 km·ha-! of passenger car transport (Lehtomaki, 2005, 2006; Lehtomiiki and Bj¢msson, 2006). For the crops with the highest methane poteritials per hectare in Finland (Jerusalem artichoke, timothy grass, and reed canary grass) a hectare could potentially fuel one to three passenger cars (traveling an average distance of 20,000 to 30,000 km) for a year. Therefore, if the 2004 area of agricultural set-aside land in Finland had been used to produce biogas from energy crops, the methane could have potentially fueled 8 to 25% of the country's passenger cars (Lehtomak.i., 2006).

Jerusalem artichoke had a similar methane production potential for repeated cuttings in the Finnish trials, whereas other leafy energy crops assessed, such as giant knotweed (ReynoUfria sachalinensis F. Schmidt ex Maxim.) and sugar beet tops (Bela vulgaris L.), had methane potentials that increased at later harvests. Lignin levels were also unusually constant for the tops of Jerusalem artichoke, regardless ofmaturity, while nonstructural carbohydrates (fructans) increased in the stems

Biomass and Biofuel

until mid-October. Therefore, the crop can be harvested late into the season without jeopardizing the efficiency of anaerobic digestion (Lehtomiiki, 2006).

Zubr (1985) investigated the potential of 33 different raw materials, including silage from Jerusalem artichoke tops, for biogas production in Denmark. Anaerobic digestion was carried out under mesophilic' conditions (35°C), using a series of batch system fermentation reactors. The production of biogas was registered daily. Retention times ranged from 26 to 82 days, depending on the material, with optimal retention time for Jerusalem artichoke being around 33 days. Jerusalem artichoke silage comprised 17.1 % total solids (TS) and 15.4% volatile solids (VS), with 81.7% of VS being .microbiologically decomposable. In general, the ratio vsrrs is a measure of organic matter content, which was found to be relatively high in Jerusalem artichoke silage. The silage contained 31.9% crude fiber, an indication of the presence of lignin. High levels of lignin lower the ability of microbes to decompose 'raw materials, with crude fiber varying between 44.2% (wheat straw) and 12.3% (sugar beet tops) for the material tested. Carbon and nitrogen in the silage were 42.0 and 2.48% of TS, with a CIN ratio of 17. Sulfur comprised 0.13% of TS, indicating that pollution of biogas by sulfur-containing contaminants is .relatively unlikely using Jerusalem arti­choke silage. The silage yielded 421 I·kg- l TS, 468 I biogas·kg- l VS, and 315 1 methane·kg- I VS, with the biogas comprising on average 67.4% methane (Zubr, 1985).

In Sweden, research to assess the potential of the aerial parts (stems and foliage) of Jerusalem artichoke as a feedstock for biogas production has been conducted at the Swedish University of Agriculture. Yields of aerial parts up to 20 t·ha'! dry matter can be obtained in Sweden, where plants do not flower and instead produce strong vegetative growth (Wiinsche, 1985). Gunnarson et ai. (1985) found that the aerial dry matter yields for three clones ('Topinanca,' 'Variety No. 1927: and 'Variety No. 1168: the iatter a hybrid of Jerusalem artichoke and sunflower) varied between 7 and 16 t·ha'l. 'Topinanca' had the highest dry matter content. Biomass digestion experiments were performed at mesophilic (37°C) conditions on a laboratory scale (Gunnarson et aI., 1985; Mathisen and Thyselius, 1985). Biogas production was approximately equal with fresh and ensiled biomass, with a pH of around 7.5 in the digester in both cases. Fresh and ensiled material yielded 480 to 680 I biogas·kg- ' of organic material; the methane content of the biogas obtained was between 52 and 55%. The chemical composition of silage was determined before and after digestion for the production of biogas. The lignin part remained mostly unchanged, although cellulose, hemicellulose carbohydrates, and other extractable substances were much reduced after anaerobic digestion, with nonstructural carbohydrates (fructans) completely digested (Gunnarson et aI., 1985). The authors concluded that under Swedish conditions it should be possible to obtain yields of around 5,500 m3 biogas·ha" for Jerusalem artichoke. Given a methane concentration of 52%, the yield would be 2,800 m3 methane·ha". The economics of biogas production are considered in Chapter 14.1.

Using the Swedish laboratory digester, Mathisen and Thyselius (1985) reported biogas yields from batch and semicontinuous digestion of fresh and ensiled Jerusalem artichoke tops. Batch digestion experiments were used to determine the highest possible yields from a range of energy crops, with substrate added in daily portions over a 3- to 4-week period. In batch digestion experiments, freshly.chopped tops yielded 540 I·kg" VS after 1 week, 90% of the final value of 600 I·kg- ' VS. In two trials, ensiled tops yielded 470 and 510 I·kg- I VS after 1 week, 72 and 76% of the final values of 660 and 680 I·kg- ' VS, respectively. The digestion rate for Jerusalem artichoke was a little slower than for fresh alfalfa (Medicago saliva L.), kale (Brassica oleracea L.), and grass (Poa spp.), but faster than for the woody biomass used: birch (Betula sp.) and sallow (Salix sp.). In the semicontinuous digestion experiments, chopped fresh material yielded 480 to 590 l·kg- I

VS (80 to 98% of final yield in batch experiments), while ensiled tops yielded 510 to 560 I·kg" VS (78 to 86% of final yield in batch experiments). The methane content of the biogas obtained from fresh and ensiled tops of Jerusalem artichoke under semicontinuous digestion was 50 to 55%, with organic loads of between 2.2 and 3.0 g VS·I-'·day-I and a hydraulic retention time of bctwccn 43 and 59 days. The digestion of fresh material and silage was found to be a stable process, without

142 143 Biology and Chemistry of Jerusalem Artichoke: Helianthus tuberosus l.

the need for pH adjustment. Fresh and ensiled material gave similar biogas yields with both batch

and continuous digestion, which were among the highest of a range of crops and woody biomass tested. Although biogas production can proceed effectively via batch, sernicontinuous, or continuous digestion, El Bassan (1998) suggested that production on a continuous long-term basis, using a' homogeneous substrate, was the most cost-effective option. .' "..

The residue or sludge left after the pulp of Jerusalem artichoke tubers has b~~p ferrnentedand� distilled to produce ethanol can be used to make biogas (methane). Tuber residue gave higher yfelds� ofbiogas (595 ].kg-I volatile solids) than fresh tops (2961·kg-1) or ensiled tops (3311·kg-l ) (Zubr,� 1988). Zubr concluded thai using the tuber residue for further energy-production was feasible, with�

yields ofbiogas being relatively high in comparison to other plant biomass materials. The combined_.� exploitation of Jerusalem artichoke for bioethanol and methane yielded a gross energy of 159� GJ·ha-1, corresponding to 4,500 I oil equivalents (EO)·ha-l·year' (Zubr, 1988). However, the tops� from Jerusalem artichoke did not appear particularly suitable for biogas production in this study,�

.mairiJ.y because of tlie long retention time needed and the larg~ amount of solid fe~entationresidue . produced (Zubr, 1988). J.

Zubr (1988) analyzed the costs of producing Jerusalem artichoke for biofuel (ethanol and biogas) in Denmark. The total production costs, which included soil preparation, fertilizer,' seed tuber purchase, cultivation, harvesting, and transport, were calculated as 8,843 DKr·ha- ' . Seed

tubers and fertilizer were the main expenses: with the total cost of raw materials accounting for around 50% of the production costs of ethanol. The study concluded that biofuels from Jerusalem

artichoke were not competitive with fossil fuels in the market of 1988. However, the circumstances at the start of the 21st century are changing. A growing awareness of environmental problems linked to the buriiing of fossil fuels has, resulted in subsidies, tax incentives, and regulations that

favor renewable energy sources. Meanw.hile, any steep rises in the price of gasoline will make plant biomass sources, such as Jerusalem artichoke, more competitive in relation to gasoline.

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