 |
| Register | Login | |||||
 |
Main
| Memberlist
| Active users
| Calendar
| Chat
| Online users Ranks | FAQ | ACS | Stats | Color Chart | Search | Photo album |
|
| | ||
|
| Register | Login | |||||
|
Main
| Memberlist
| Active users
| Calendar
| Chat
| Online users Ranks | FAQ | ACS | Stats | Color Chart | Search | Photo album |
|
| | ||
| 0 users currently in The Pit of Despair. |
| Acmlm's Board - I3 Archive - The Pit of Despair - . |
New poll |  | Thread closed |
| Add to favorites | Next newer thread | Next older thread |
| User | Post | ||
|
Deleted User Banned                Since: 05-08-06 Last post: None Last view: 6296 days |
| ||
| Table 4.
Temperatures in an unheated farm-scale reactor during the course of a year Temperature (¡ÆC) Soil (at 1 m depth)a Manure tankb Assumed operational temperature January 4 7–9 February 4 n.m. March 5 n.m. April 3 n.m. May 9 n.m. June 11 n.m. 11 July 15 17–20 17 August 17 17–20 18 September 14 15–17 15 October 13 12–14 13 November 8 n.m. December 4 n.m. a Monthly average as calculated from Nimmermark [23]. b Measured data from an unheated digester tank for cow manure (Önnestad, Sweden). 3. Results 3.1. Operation of the pilot scale reactors Fig. 2 shows the methane yield obtained for the reactors at different temperatures during months 5–14 at OLRs of 0.5–0.6 kg VS m−3 day−1. Results of between 11 ¡ÆC and 18 ¡ÆC were obtained with reactor 1 and of between 20 ¡ÆC and 37 ¡ÆC with reactor 2. Results from 11 ¡ÆC, 12 ¡ÆC and 32–35 ¡ÆC were obtained with the substrate from the year-2 harvest, whereas the other results was obtained with the substrate from the year-1 harvest. The yield increased with the temperature and the highest yield obtained was 0.40 m3 CH4 kg VS−1 at 30 ¡ÆC. At 11 ¡ÆC, the yield was only one-third of that obtained at 30 ¡ÆC. At temperatures above 30 ¡ÆC the yield decreased. During months 5–14, the operation of both reactors was stable, although some accumulation of VFAs occurred. In reactor 2, the pH increased with increasing temperature from pH 7.3 at 24 ¡ÆC to 7.6 at 37 ¡ÆC. At temperatures below 30 ¡ÆC, the acetic acid concentration was generally below 0.05 g l−1, but it occasionally increased up to a level of 0.20 g l−1. The concentration of propionic acid varied between 0.03 and 0.22 g l−1. At 30 ¡ÆC and higher, acetic acid and propionic acid were rarely detected, and then usually at concentrations of less than 0.10 g l−1. In reactor 1, the pH remained between 7.2 and 7.5 at temperatures of 17–11 ¡ÆC. At temperatures of 16 ¡ÆC and above, the concentrations of acetic acid and of propionic acid were below 0.10 g l−1 and 0.13 g l−1, respectively. At temperatures of 15 ¡ÆC and below, the acetic acid and the propionic acid concentrations were in the range of 0.04–0.35 g l−1 and 0.10–0.38 g l−1, respectively. (11K) Fig. 2. Methane yield obtained at different temperatures at an OLR of 0.5–0.6 kg VS m−3 day−1 and an HRT of 90 days. Results for 11–18 ¡ÆC were obtained with reactor 1 and results for 20–37 ¡ÆC with reactor 2. Results for 11 ¡ÆC, 12 ¡ÆC and 32–27 ¡ÆC were obtained with the substrate from harvest year 2. The other results were obtained with the substrate from harvest year 1. The error bars show the standard deviations; n = 2 for 18 ¡ÆC, 22 ¡ÆC, 24 ¡ÆC, 26 ¡ÆC, 27 ¡ÆC, 32 ¡ÆC, 35 ¡ÆC and 37 ¡ÆC; n = 3 for 12 ¡ÆC and 28 ¡ÆC; n = 4 for 13 ¡ÆC, 25 ¡ÆC, and 30 ¡ÆC; n = 5 for 14 ¡ÆC; n = 6 for 20 ¡ÆC; n = 7 for 11 ¡ÆC and n = 8 for 17 ¡ÆC. Table 5 shows the results of increasing the OLR at operational temperatures of 15 ¡ÆC and 30 ¡ÆC, respectively, obtained during months 16–24. The reasons for higher values being obtained in reactor 1 for acetic acid and ¥á at an OLR of 1.6 kg VS m−3 day−1 than for an OLR of 2.1 kg VS m−3 day−1 was that the reactor was run first at the higher loading rate. The optimal gas production rates were obtained at the highest loading rates tested which were 4.1 kg VS m−3 day−1 for reactor 2 (30 ¡ÆC) and 2.1 kg VS m−3 day−1 for reactor 1 (15 ¡ÆC). However, there were signs of instability at these loading rates since acetic acid and propionic acid accumulated, resulting in a decrease in pH and an increase in ¥á (Table 5). The optimal loading rates were therefore found to be 1.6 kg VS m−3 day−1 for reactor 1 and 3.3 kg VS m−3 day−1 for reactor 2. For reactor 1, the methane yield decreased with increasing OLR. For reactor 2, the methane yield decreased slightly for OLRs higher than 0.5 kg VS m−3 day−1, and also when the reactor was overloaded at 4.1 kg VS m−3 day−1, but at loading rates of 1–3.3 kg VS m−3 day−1 the yield was fairly constant. Table 5. Monitored parameters of the biological process OLR (kg VS m−3 day−1) pH ¥á Acetic acid (g l−1) Propionic acid (g l−1) Methane yield (m3 CH4 kg VS−1) Methane production rate (m3 CH4 m−3 day−1) Minimum Maximum Minimum Maximum Minimum Maximum Minimum Maximum Reactor 1 0.5 7.4 7.5 0.27 0.32 0.07 0.28 0.09 0.20 0.23 0.11 1.0 7.4 7.5 0.31 0.39 0.37 0.72 0.25 0.77 0.17 0.17 1.6 7.1 7.2 0.59 1.01 2.30 3.11 1.01 2.28 0.14 0.22 2.1 7.2 7.5b 0.46 0.91a 1.10 2.74 1.97 3.03 0.11 0.24 Reactor 2 0.5 7.4 7.4 0.24 0.32 0 0.08 0 0.04 0.39 0.18 1.0 7.4 7.5 0.22 0.37 0 0.08 0 0.01 0.32 0.33 2.1 7.4 7.6 0.23 0.32 0.02 0.78 0.01 0.25 0.32 0.66 3.3 7.5 7.6 0.27 0.32 0.04 0.60 0.02 0.11 0.29 0.98 4.1 7.0 7.2b 0.32 1.04a 1.44 3.16a 0.31 0.71a 0.24 0.97 a Increasing concentration during the period. b Decreasing concentration during the period. 3.2. Energy balances The energy input and the energy output for the pilot scale reactors are shown in Fig. 3. The energy input is divided into the energy needed for feeding, heating and stirring, respectively. The energy needed for heating the substrate and the digester tank was estimated from the monitored values and compared to the calculated values. The monitored values, however, were found to be unreasonably high, since the energy needed for heating the substrate was 1.44 times as high as the calculated value and the energy needed for heating the tank was 5.5 times as high as the calculated value. Because, as taken up in the discussion, the heat losses from the pilot-scale reactors appeared unnecessarily high, and the calculated values were considered to be more realistic, the latter values were used. The energy for feeding includes preparation of the substrate slurry, pumping of the substrate and the effluent, and recirculation of the reactor slurry before and after feeding. Since for OLRs at 1–4 kg VS m−3 day−1 feeding occurred once a day, the energy input for preparing and feeding the substrate only decreased slightly for an increase in OLR. The energy needed for heating and for stirring per tonne of VS increased with a decrease in OLR since HRT increased with a decrease in OLR. Because of the high energy consumption that stirring and heating required, the energy balance at an operational temperature of 30 ¡ÆC is negative for OLRs of 1 and 0.5 kg VS m−3 day−1, whereas at an operational temperature of 15 ¡ÆC it is negative at all OLRs. (39K) Fig. 3. The measured energy production and the calculated energy consumption for the pilot-scale reactors at different OLRs at 15 ¡ÆC (A) and 30 ¡ÆC (B) shown as the energy production () and the energy input for pretreatment and feeding of the substrate (¡á), for heating () and for stirring (). Fig. 4 shows the energy balance calculated for a theoretical farm-scale system in which the reactors are similar to those used in the pilot-scale experiment but are scaled up and have improved insulation. For this scenario all the energy balances are positive with energy inputs ranging from 19–26% of the energy produced. |
| Add to favorites | Next newer thread | Next older thread |
| Acmlm's Board - I3 Archive - The Pit of Despair - . |
| Thread closed |
