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The effect of particle size and thermal pre-treatment on the methane yield of
four agricultural by-products
S. Menardoa, G. Airoldi, P. Balsari
Bioresource Technology
Volume 104, January 2012, Pages 708–714
version is available at:
La versione definitiva è disponibile alla URL:
The effect of particle size and thermal pre-treatment on the
methane yield of four agricultural by-products
S. Menardo
a,*, G. Airoldi
b, P. Balsari
aa Department of Agriculture, Forestry, Environmental Engineering and Land Based
Economics (DEIAFA) Mechanics Section - Torino University, via Leonardo da Vinci 44, 10095 Grugliasco (TO) Italy
b Department of Agriculture, Forestry, Environmental Engineering and Land Based
Economics (DEIAFA) Topography Section - Torino University, via Leonardo da Vinci 44, 10095 Grugliasco (TO) Italy
*Corresponding author. Tel. +39 0116708610; E-mail address: simona.menardo@unito.it
Keywords: anaerobic digestion; methane yield; agricultural byproducts; pre-treatment; energy balance.
One way to optimize methane production in anaerobic digestion plants is to substitute ligno-cellulosic by-products for crops traditionally used as energy sources. However, using these by-products requires introduction of a pre-treatment system tominimize energy input and maximize energy output for an improved net energy equation. In this study, four agricultural byproducts (wheat, barley, rice straw and maize stalks) underwent various mechanical and thermal treatments prior to anaerobic digestion including particle size reduction to 5.0, 2.0, 0.5, and 0.2 cm and heat application to 90°C and 120°C.
Mechanical pre-treatment increased byproduct methane yields more than 80%; thermal pre-treatment improved yields more than 60% for wheat and barley straw. Pre-treating wheat straw improved methane yields most, regardless of whether the method was thermal or mechanical. An electric net energy balance was also completed to analyze the feasibility of the pre-treatments according to input and output of energy.
1. Introduction
Energy crops, in addition to animal effluents, are largely used to feed biogas plants as they contain high amounts of easily-digested organic matter; however, their use as energy sources is neither ethically ideal acceptable nor economically sustainable. Continued and/or increased use of these crops as biomass energy sources rather than as food sources might cause serious problems to not only agriculture, but also the world economy
(Gerbens-Leenes et al., 2009). First, high quality energy crops (like maize, wheat, and barley) are fundamental to the human diet, and they are more expensive in comparison with other agricultural by products and organic wastes. Second, as Wong et al. (2011) has
discussed quite recently, their use for energy production could negatively impact the environment due to their high levels of CO2eq emissions and their changes to agricultural
land priorities. Third, the production of these crops requires substantial input of limited resources: arable land, energy (irrigation, machines, transport), and environmental reserves (ground and surface water). Finally, these crops require intense use of
agrochemicals (pesticides and fertilizers), relative to byproduct crop, which could also negatively impact groundwater and soil quality (Schievano et al., 2009).
Fueling more research into biomass alternatives for biogas production from anaerobic digestion has been increased environmental protectionism and the growing number of biogas plants. Agro-industrial systems produce a wide variety of organic matter byproducts: ligno-cellulosic material, crop residue, protein-rich waste, pre-digested wastewater sludge, animal slurry and manure, waste paper, and household waste (Schievano et al., 2009). Unfortunately, the market for many of these byproducts is currently severely limited, if it exists at all. In the instance of cereal straws, they often remain in the field after harvest (ITABIA, 2003) and are used only when incorporated into the soil during the next seedbed preparation.
Agro-industrial byproducts and residues are often used as the broadcast source of the substrate for anaerobic biotransformation to biogas (Schievano et al., 2009), which suggests that byproduct substrates could be alternatives to energy crops in anaerobic digestion plants. Initial analysis indicates that the biogas and methane yields of such byproducts are limited due to their high content of ligno-cellulosic compounds, which make digestion by hydrolytic bacteria difficult. Pre-treatment can increase the potential
biogas yield of this hard-to-digest biomass by enhancing its degradability and promoting anaerobic microorganism action (González-Fernández et al., 2008).
Previous pre-treatment studies have focused on improving the digestibility of ligno-cellulosic material by many different means: mechanical, thermal, chemical, and through combined measures (Sharma et al., 1988; Fernandes et al., 2009). In the thermal arena, high temperatures (above 170°C) studies (Fernandes et al., 2009) have shown that ligno-cellulosic bonds are destroyed at such temperatures which makes the cellulose available to the anaerobic bacteria, albeit at considerable cost to the operation in large energy inputs and machine and safety measure investment. Alternatively, thermal pre-treatment could be performed at lower temperatures by exploiting the energy typically dissipated to the atmosphere (Balsari et al., 2011) at an agro-biomass plant—the Combined Heat and Power (CHP) station-produced thermal energy and its recovered exhaust gas heat. Similarly, mechanical pre-treatment is generally considered a high electric energy consumption process (Hendriks and Zeeman, 2009), but it also could be performed easily in any agro-biogas plant by a shredder or mill. Moreover, mechanical and thermal pre-treatments produce no inhibitory or toxic substances nor do they promote hard-to-digest complex molecule formation (Hendriks and Zeeman, 2009).
The aim of the present study was to determine the methane potential of four ligno-cellulosic agricultural byproducts—wheat, barley, rice straw and maize stalks—untreated and thermally and mechanically treated. A simple electricity balance of these pre-treatments was also made to evaluate the energetic feasibility of each treatment.
Byproducts-wheat, barley and rice straw and maize stalks-were sampled from agricultural farms in the northwest of Italy, Piedmont region. The rice straw and maize stalks were collected from the field post-harvest while the other biomasses were directly picked up from harvest bales. Samples were stored in a refrigerated and ventilated area before pre-treatment.
2.1 Biomass pre-treatment
Mechanical pre-treatment of the straws was performed by several means to create specific particle sizes: by scissors (5.0 cm particles), by food mixer-cutter (2.0 cm cut lengths), and by knife mill using different grids sizes to achieve the two smallest particle sizes. In the case of maize stalk mechanical pre-treatment, grass shears were used to obtain 2.0 cm cut lengths while the knife mill was utilized for 0.2 cm particles. Table 1 shows the various pre-treatments performed by biomass byproduct.
Untreated samples underwent two different thermal pre-treatments in an autoclave (2 liter Zipperclave, Autoclave Engineers, Division of Snap-tite Inc.) set at 90°C and 120°C for 30 minutes. Deionized water in an amount equal to about 20% of total solids was added to each sample prior to treatment to avoid burning the biomass during pre-treatment. In the case of rice straw, no water was added due its already sufficiently high water content. Approximately 15 and 30 minutes elapsed to reach autoclave vessel temperatures of 90°C and 120°C, respectively. A cooling time of about one minute followed while the
temperature of the high heat treated biomass fell from 120°C to 95°C through pressure valve release. Ten to 15 more minutes passed as the autoclave vessel was opened and emptied. In the case of the 90°C pre-treatment, only the second cooling time was
necessary before opening the vessel as the lower temperature treatment did not require the pressure valve to be opened after pre-treatment.
Throughout pre-treatment, the biomass temperature was controlled through a thermo-element inserted into the vessel. The temperature setting varied according to the water content of the sample and the biomass volume under treatment. After thermal pre-treatment, each sample was cooled to the ambient temperature and refrigerated at 4°C until the start of anaerobic digestion. For samples pretreated at 120°C, the steam released by the autoclave at the end of thermal pre-treatment was collected in a vessel so as not to lose volatile organic compounds, and then later added to the treated sample from which it was produced (Menardo et al., 2011).
2.2 Batch experiments
Batch measurements to assess gas yields were carried out in the laboratory according to Standard Methods VDI 4630 (2006). Glass reactors of a 2-liter inner volume were connected to tedlar® gas bags by means of tygon tubing (6.4 mm inner diameter) to
collect the produced biogas. The digesters were placed into a 40°C thermostatted room. Each trial was conducted for about 60 days during which the samples were manually stirred twice a day. Each reactor received a mixture of biomass sample and inoculum (digestate collected from an anaerobic digestion plant fed with animal effluents and energy crops), in a 2:1 ratio calculated on the basis of volatile solids content. All samples were digested in triplicate. A control sample, represented by inoculum, was also digested in batches in triplicate. The biogas volume produced by the inoculum was subtracted afterwards from the organic fraction sample yield.
2.3 Chemical analysis
Untreated and all thermally-treated samples were analyzed to determine their measure of pH, dry matter (DM), volatile solids (VS), total nitrogen (Ntot), total carbon (Ctot),
hemicelluloses (H-CEL), cellulose (CEL) and lignin (ADL) and lipids. The inoculums were analyzed for DM, VS, pH and Kjeldahl nitrogen (TKN) (AOAC, 2006). A Hanna HI 9026 portable pH meter fitted with a glass electrode combined with a thermal automatic compensation system was used to measure pH. Total solids were determined by drying samples 24 hours at 105°C while volatile solids were measured after 4 hours at 550°C in a muffle furnace (AOAC, 2006). Nitrogen content was measured by elementary analyzer (CHN, Carlo Erba Instruments). Fibers and lignin were determined by the Van Soest method (1991) while lipids were determined with the Soxlet (AOAC, 2006). The protein amount was calculated by the FAO coefficient method (1970) (Ntot per 6.25).
Biogas and methane yields were monitored daily throughout the experimental period. Biogas composition was determined using a Draeger XAM 7000 analyzer with infrared sensors whereas biogas volume was determined using a Ritter Drum-type Gas Meter type TG05/5 volume meter. Daily biogas and methane yields were normalized to 0°C and 1013hPa. The specific yields of biogas and methane were subsequently expressed as normal liters per kg of volatile solids. Headspace volume was taken into account in the calculations of percent methane by a correction factor as reported in VDI 4630 (2006). Biogas and methane yields data were statistically analyzed through one-way ANOVA after confirmation that the variances were homogeneous by the Levene test. Samples that showed significant differences via the ANOVA test were further analyzed using the Tukey test (α=0.05).
2.4 Electric energy balance
To further assess the feasibility of these pre-treatments, a balance of their direct electric energy consumption was also completed. The electric energy balance was performed considering only those biomasses significantly affected by pre-treatments.
The additional electric energy obtained by the increased methane yield of the pre-treated biomasses was compared to the electric energy demand of the various pre-treatments. The methane yields of the untreated biomasses, expressed as m3 of methane per ton of
fresh matter, were subtracted from the yields associated with each pre-treated biomass to obtain the net methane yields. If a methane HHV (Higher Heating Value) of 38 MJ/Nm3
and an electric efficiency of 38% are assumed, then 1 Nm3 of drawn methane produces
about 4 kWhel. This value was then multiplied by the change in methane yield to calculate
the increased electric energy expressed as kWhel per ton of fresh matter.
To calculate the electric energy consumption of the mechanical and thermal pre-treatments, we assumed that pre-treatment operations were carried out with the machinery usually available on a farm or at a biogas plant.
For the mechanical treatment, we assumed utilization of the following machines: - straw bale breaker,
- straw shredder, and
- straw hummer grinder with a 0.5 cm screen size.
For the thermal treatment we assumed the following equipment to be available: - straw bale breaker,
- 300 liter pressure steam oven.
The pressure steam oven listed above is considered small in volume to use in a biogas plant, but for the purposes of the electric energy balance calculation, we believe it is representative of thermal pre-treatment energy demands. We believe the energy demand calculated value is conservative given the higher efficiency associated with the larger pressure ovens more appropriate for biogas plants.
Table 2 displays the electric energy demand by specific machine. The electric energy consumption of the pressure steam oven could be reduced from 100 to 40 kWhel/t d.m.
(Tingshuang et al., 2002) if the thermal energy produced by the CHP were employed to raise the water temperature. Consequently the electric energy is used exclusively for steam delivery and materials handling. In such a scenario, the thermal energy produced the methane-fed CHP would be capable of raising the water to a temperature to about 120°C without ever tapping the remaining 400-450°C exhaust gases produced (Katta et al., 2008) and available to increase the water temperature.
3. Results and discussion
3.1 Effect of thermal pre-treatment on chemical composition
The samples thermally treated did not show relevant variation in chemical composition compared to those not treated (Table 3). The dry matter content (DM) ranged between 28.7% (rice straw) and 90.5% (barley straw). Also wheat straw and maize stalks showed high content of DM, respectively 86.6% and 87.7%. The DM was lower in thermally-pretreated samples, due to the water addition before the pre-treatment. Water was not added to rice straw as it was already adequately wet. The volatile solids (VS) content of untreated biomasses was included between 89.0% (maize stalks) and 95.7% (wheat
straw). The effect of thermal-pretreatment on the volatile solid (VS) content of biomasses was not significant. Properly to avoid VS losses, the steam released during the cooling procedure was collected after each treatment and compounded with the biomass.
The thermally pretreated biomasses, in some cases, showed a slight decrease in protein and fat percentage when compared to untreated ones. Only the wheat straw showed an increase of protein content, from 3.2% to 3.8% and 3.9% after the thermal pre-treatment. Fiber composition was not significantly affected by thermal pre-treatment. Only the hemicellulose fibers showed a slight decrease when wheat straw and maize stalks were treated at 120°C, which is likely due to some solubilization. This same phenomenon did not appear in rice and barley straw. It is reported that the hemicelluloses start to solubilize at about 150°C (Garrote et al., 1999), but partial solubilization can start at lower
temperatures as the potential for thermal reactivity depends mostly on biomass
composition (Hon and Shiraishi, 1991). Specifically, low pre-treatment temperatures do not break ligno-cellulosic bonds, but promote organic matter hydrolysis and breakdown during anaerobic digestion (Wang et al., 1997). This effect can advantage organic matter degradation by anaerobic bacteria, and consequently, improve the methane production of thermally-pretreated samples (Wang et al., 1997).
The pH of all thermal-pretreated biomasses, both at 90°C and 120°C, were lower than untreated ones. As reported in other studies (Han et al., 2010; Pan et al., 2008), a slight acidification of the biomass can occur after thermal pre-treatment.
3.2 Methane yield
The untreated samples resulted in the following methane yields according to biomass: 246 lN·kg-1VS for maize stalks, 240 lN·kg-1VS for barley straw, 197 lN·kg-1VS for rice
straw, and 182 lN·kg-1VS for wheat straw (Figure 1). These productions agreed with
results published in other studies (Sharma et al., 1988; KTBL, 2005; Dinuccio et al., 2010; Chen et al, 2010; Zhu et al., 2010).
3.2.1 The effect of particle size on methane yield of agricultural by-products
Methane yields were significantly higher after pre-treatment for only the barley and wheat straws. For these two biomasses, each pre-treatment significantly (α=0.05) increased the methane production when compared to untreated samples.
Mechanical treatment of barley straw significantly improved methane yields according to the particle size reduction as shown in a previous study (Sharma et al., 1988). We found barley straw methane yields increased between 19.2% and 54.2% from mechanical treatment through particle size manipulation. Table 4 displays the increased methane yields as particle size was reduced from 5.0 cm to 2.0 cm to 0.5 cm.
Increasing the surface available to the bacteria, the digestibility of the biomasses was improved and the hydrolytic phase of anaerobic process was promoted.
Wheat straw size was reduced to 5.0 cm and 0.2 cm, and both pre-treatments experienced significantly increased (α=0.05) methane yields as compared to the untreated sample. Any statistical difference failed to appear between the two mechanical treatments. The results obtained from wheat straw trial agreed with those obtained in similar studies
(Sharma et al., 1988; Bauer et al., 2009). For wheat straw, the mechanical pre-treatment improved the methane yield from 56.6% to 83.5% for particles of 0.2 cm in size. Rice straw and maize stalks did not showed any significant methane yield improvement after mechanical pre-treatment. Rice straw was reduced to only 5.0 cm due to its high water content, which would have made difficult and demanded much more energy for further size reductions. This simple and basic pre-treatment, obtainable at a real plant through a shredder resulted in no methane yield improvement. In the case of maize stalks, neither the 2.0 cm particle size (254 lN·kg-1VS) nor the 0.2 cm particle size (272 lN·kg -1VS) significantly (α=0.05) improved the final methane yield when compared to the
untreated samples (246 lN·kg-1VS). Pre-treatment to 2.0 cm was also not strong enough to
increase organic matter degradation. On the contrary, only the 0.2 cm pre-treatment affected the maize stalk organic matter degradation at the early stage of anaerobic
digestion by speeding the start of the hydrolytic phase (Figure 2). After this initial peak in methane yield at 15 days, methane production decreased rapidly to daily methane
production at lower levels than the untreated sample. Consequently the cumulative methane production of maize stalks 0.2 cm particle size after about 60 days appeared not significantly different from untreated maize stalks. The main effect of the 0.2 cm particle size pre-treatment was reduced organic matter degradation time. The possibility to obtain for a ligno-cellulosic biomass the reduction of organic matter degradation time is also very important for operative management of a biogas plant, since might allow for a shorter hydraulic retention time and for a smaller digester volume.
Thermally-pretreated maize stalks and rice straw showed no significant improvement in the methane yields compared to untreated samples. The thermal pre-treatment at both 90°C and 120°C did not affect the methane yields of these samples in terms of total volume or increased speed of organic matter degradation. The maize stalks are normally considered to have a very low digestibility due to their highly lignificated tissues. Zhu et al. (2010) highlighted that maize stalks treated for 24 hours with a 1.0% or 2.5% NaOH solution resulted in no increased methane production. In fact, to obtain a relevant effect a substantial increase in the percent NaOH in the pre-treatment solution was required. For the thermal pre-treatment of maize stalks the employment of higher temperatures could be necessary to obtain an increase of methane production. But pre-treatment at higher temperatures requires more energy and could produce compounds toxic for bacteria. Hendriks and Zeeman (2009) reported that thermal pre-treatments at temperatures of 160°C and higher causes the solubilization of lignin, resulting in production of phenolic compounds that in many cases have an inhibitory or toxic effect on bacteria, yeast and methanogens/archae. To increase the effect of thermal pre-treatment at lower
temperature, close to 100°C, the time of the pre-treatment could be also increased, but as reported in Carrère et al. (2008) the thermal treatment time has very little impact on anaerobic biodegradability. But very long pre-treatment time has also very high energy consumption.
Barley straw, on the other hand, was both positively and significantly influenced by thermal pre-treatment. The methane yields of pre-treated barley straw samples increased about 40% compared to untreated one. Barley straw treated at 90°C delivered a methane yield of 340 lN·kg-1VS while the 120°C treatment failed to provide any additional
methane yield with a measured production of 338 lN·kg-1VS. Wheat straw methane yields
were similarly improved by thermal pre-treatment at both 90°C and 120°C and followed the same pattern as barley straw by demonstrating little difference in methane yields between the pre-treatments. Finally, it is of note that methane yields whether thermally or mechanically pre-treated wheat straw were undifferentiated.
Fiber content and methane yields showed no significant correlation since the
pre-treatment temperature was low and, therefore, limited in its ability to solubilize the fibers (Klimiuk et al., 2010). Specifically, we found the hemi-cellulose to only slightly
solubilize in just some of the samples. The low temperature pre-treatments, at nearly 100°C, has been recommended as a pre-digestion step to enhance the biological activity of some hydrolytic bacteria (Nielsen et al., 2004).
3.3 Electric energy balance
The amount of electric energy required for the thermal and mechanical pre-treatments under consideration has been calculated and compared to the increased electric energy obtained from the pre-treated samples. Table 5 presents only those biomasses for which pre-treatment significantly affected methane production (barley and wheat straw). The calculation of electric energy balance for the thermal pre-treatments considered only the 90°C pre-treatment because significant differences were absent at 120°C for both barley and wheat straws. Among the mechanical pre-treatments, we considered only particles of 0.5 cm as the energy demand of the straw hummer grinder was determined for this size particle only.
Thermal pre-treatment required electric energy of 55 kWh/t of dry matter. The electric energy demand of the pressure steam oven was reduced from 100 kWh/t d.m. to 40 kWh/t d.m. based on an assumed thermal energy production by the CHP as previously explained.
For mechanical pre-treatment, the total electric energy demand was 30 kWh/t d.m. to reduce the size to 0.5 cm for both the barley and wheat straws.
The thermally-pretreated barley straw produced a methane increase of 85 Nm3/t d.m.,
which corresponded to an output of 340 kWhel/t d.m. The mechanical pre-treatment made
a larger improvement to the barley straw methane yield (110 Nm3/t d.m.) which equated
to a final energy value of 442 kWhel/t d.m. The increased electric energy yields of wheat
straw pre-treatments were 433 kWhel/t d.m and 541 kWhel/t d.m for thermal and
mechanical pre-treatments, respectively.
While both straw types had a positive electric energy balance, it was more positive for the wheat straw. Even only electric energy, not thermal energy produced by the CHP, is employed to increase the temperature of the steam pressure oven, the energy balance was positive. However, the increased energy produced by the thermal and mechanical pre-treated barley and wheat straws would be enough to meet the electric energy demands of the pre-treatments. For both barley and wheat straw, the higher electric energy balance value was obtained through mechanical pre-treatment. It was also in the case of barley straw that the mechanical pre-treatment produced the best increase in methane yield. For wheat straw, the two pre-treatments failed to show any significant difference in increased methane yield.
The mechanical pre-treatment can be reproduced easily in a full-scale working biogas plant due to the machines usually available on an agricultural farm. The thermal pre-treatment is more difficult to replicate in a biogas plant due to its reliance on high temperatures and a heating system, but this pre-treatment does allow the operation to employ the thermal energy produced by the CHP that is usually lost to the atmosphere and causing further negative environmental effects.
Conclusions
Mechanical and thermal pre-treatment of agricultural byproducts analyzed in this paper demonstrated positive results by improved methane yields and electric energy balances. Barley straw appeared a more useful biomass pre-treatment candidate as it showed the largest differences across the various treatment. After undergoing mechanical pre-treatment to a cut length of 0.5 cm, it resulted in both large methane yield increases and electric energy balance increases. On the contrary, rice straw and corn stalks
pre-treatments yielded no significant improvement in methane production. Further research work was planned to deeply evaluate the feasibility of these pre-treatments in a real-scale biogas plant.
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Table captions
Table 1. Mechanical and thermal pre-treatments of the biomasses.
Table 2. Electric energy demand of representative machines used for the energy balance of biomass mechanical and thermal pre-treatments.
Table 3. Main chemical characteristics of the tested biomasses.
Table 4. Methane yield increases of the treated samples compared to the untreated ones (expressed in percent). The increase is shown only when differences were statistically significant (α=0.5).
Table 5. Comparison of the increased electric energy produced by the pre-treated
biomasses relative to those untreated and the electric energy required to complete the pre-treatments. Only those pre-treatments with significant (α=0.05) methane yield differences are displayed.
Pre-treatment Barley straw Wheat straw Rice straw Maize stalks Mechanical 5.0 cm O O O 2.0 cm O O 0.5 cm O 0.2 cm O O Thermal 90°C O O O 120°C O O O O Table 1.
Mechanical pre-treatment Thermal pre-treatment
Machine Electric energy demand (kWh/t d.m.)
Machine Electric energy demand (kWh/t d.m.)
Bale breaker 1 3 Bale breaker 1 3
Shredder 1 12 Shredder 1 12
Hummer grinder 1 15 Pressure steam oven2 40 (100) 3 1 Data reported by Cormall machine technical sheets (www.cormall.dk).
2 Technical data of Pressure Steam Sterilizer LX-B, Huatai Autoclave Sterilizer, capacity
300f.
3 Real electric energy demand.
Biomass DM VS Protein Lipid H-CEL CEL ADL Ctot Ntot (TKN1) C/N pH (% FM) (%DM) (%DM) (%DM) (%DM) (%DM) (%DM) (%DM) (%DM) Inoculum 6.2 75.8 6.51 7.9 Barley straw 90.5 94.3 6.2 0.8 30.0 46.8 9.6 46.3 1.0 46 7.9 Barley straw 5.0cm Barley straw 2.0cm Barley straw 0.5cm Barley straw 90°C 16.9 94.6 5.2 0.5 27.2 47.3 10.0 45.5 0.8 57 7.5 Barley straw 120°C 17.4 95.0 5.1 0.6 30.2 49.1 11.0 45.9 0.8 57 7.5 Wheat straw 86.6 95.7 3.2 1.7 29.7 49.8 9.4 45.8 0.5 92 6.9 Wheat straw 5.0cm Wheat straw 0.2cm Wheat straw 90°C 27.7 94.9 3.9 0.9 24.6 46.4 10.9 50.2 0.6 84 6.8 Wheat straw 120°C 27.9 95.7 3.8 0.9 22.9 46.8 11.1 49.9 0.6 83 6.8 Rice straw 28.7 91.9 5.5 1.8 35.1 33.5 8.3 43.2 0.9 48 8.1 Rice straw 5.0cm Rice straw 90°C 28.1 91.0 5.2 1.8 35.3 33.4 7.1 43.4 0.8 54 7.8 Rice straw 120°C 28.2 91.7 5.2 1.8 31.8 38.8 7.5 43.4 0.8 54 7.7 Maize stalks 87.7 89.0 7.2 1.1 26.7 43.9 5.6 42.5 1.2 35 7.3 Maize stalks 2.0cm Maize stalks 0.2cm Maize stalks 120°C 50.5 89.6 6.8 1.0 22.1 44.1 5.1 42.4 1.1 38 7.1
1 The nitrogen amount in the Inoculum has been determined by Kjeldahl analysis (TKN).
Biomass Increased methane yield* (Nm3/t d.m.) Increased electric energy yield (kWhel/t d.m.) Electric energy consumption of pre-treatment (kWhel/t d.m.) Δ kWhel/t d.m. Barley straw, 90oC 85 340 55 285 Barley straw, 0.5 cm 110 442 30 412 Wheat straw, 90oC 108 433 55 378 Wheat straw, 0.5 cm 135 541 30 511
* compared to untreated sample Table 5.
Figure captions
Figure 1. Biomass-specific methane yields. The bars indicate standard deviation. Figure 2. The specific daily methane production of untreated and all pre-treated maize stalk samples.
