618 CJ.Banks et al /Bioresource Technology 102 (2011)612-620 500 15000 400 a 12000 300 ◆input 200 9000 balance 100 6000 output 3000 stored in 100 tanks -200 100 200 300 400 Day 6000 5000 b 一input 4000 output 3000 2000 output change 1000 in storage -balance -1000 100 200 300 400 Day Fig.6.Mass balance (wet weight)during the study period:weekly(a)and cumulative(b). Fig.6a and b show the mass balance plotted on a weekly and of to measure directly the heat output associated with the CHP cumulative weekly basis.On a weekly basis (Fig.6a)there is some or the amount of this heat that used to maintain the temperature variability,a proportion of which can be attributed to the problem of the digestion plant.The calculated gross energy output of the of estimating the quantities of digestate and fibre stored on site. CHP plant was 2781 MWh which at a recovery value of 53%would From Fig.6b it can be seen that at the point when materials were provide a further 1474 MWh of energy in the form of extractable last taken off site and accurate weights recorded,the difference be- heat,in addition to the electrical energy output. tween input and output weights without consideration of storage When starting the CHP unit,a small amount of natural gas was was only 190 tonnes out of a total input of 4823 tonnes(4%). used before switching to biogas.During the study period this to- talled 1534 m(0.4%of the total methane production of the plant). No electricity was generated as a result and this component is 3.6.Gross electricity and heat outputs from the CHP unit therefore not included in Table 5,but is taken into account in the overall energy balance. Values for electricity and heat outputs during the study period At the time when the CHP unit is not generating electricity,due from the CHP unit only are shown in Table 4.There was no way to scheduled maintenance,breakdown,or gas quality below the threshold limit,the biogas is burnt in a separate boiler unit to pro- Table 2 duce hot water. Mass balance for study period (wet weight). Parameter Unit Value 3.7.Electricity and heat requirements of the process plant Food waste input kg 3936.504 Water input(washwater) kg 1490.000 Electrically-powered equipment involved in operation of the Total input kg 5426,504 plant included the raw waste shredder,macerators,feed pumps Methane kg 275,177 biogas compressor pumps,CHP and boiler water feed pumps,belt Carbon dioxide 451,473 Water vapour kg 12,526 press,air filtration and minor ancillary equipment such as convey- Digestate 3969.080 ors.The primary consumers of electricity included the heat dump- Fibrea g 39,240 ing fans,gas mixing compressors,air filtration unit for reception All waste leaving site kg 35.820 hall,air filtration biofilter for digestate hall,raw waste shredder, Total output 经 4783,315 Wet tanks 92.433 and pasteurisation heating pump.Intermediate consumers were Stored material Total storage 煙 30.000 the feed and discharge pumps.gas holder inflation fans and the 122,433 CHP water pump.There were no individual electricity meters on Balance accounted for 是 520,756 these and the power taken depends upon the equipment load,so 90.4% cannot be calculated directly from hours run and plate capacity. Any liquid digestate produc d recirculated through the cess and leaves Parasitic energy is therefore given as an overall figure representing the site as whole digestate. the total number of kWh consumed on site (Table 5).Fig. 6a and b show the mass balance plotted on a weekly and cumulative weekly basis. On a weekly basis (Fig. 6a) there is some variability, a proportion of which can be attributed to the problem of estimating the quantities of digestate and fibre stored on site. From Fig. 6b it can be seen that at the point when materials were last taken off site and accurate weights recorded, the difference be￾tween input and output weights without consideration of storage was only 190 tonnes out of a total input of 4823 tonnes (4%). 3.6. Gross electricity and heat outputs from the CHP unit Values for electricity and heat outputs during the study period from the CHP unit only are shown in Table 4. There was no way of to measure directly the heat output associated with the CHP or the amount of this heat that used to maintain the temperature of the digestion plant. The calculated gross energy output of the CHP plant was 2781 MWh which at a recovery value of 53% would provide a further 1474 MWh of energy in the form of extractable heat, in addition to the electrical energy output. When starting the CHP unit, a small amount of natural gas was used before switching to biogas. During the study period this to￾talled 1534 m3 (0.4% of the total methane production of the plant). No electricity was generated as a result and this component is therefore not included in Table 5, but is taken into account in the overall energy balance. At the time when the CHP unit is not generating electricity, due to scheduled maintenance, breakdown, or gas quality below the threshold limit, the biogas is burnt in a separate boiler unit to pro￾duce hot water. 3.7. Electricity and heat requirements of the process plant Electrically-powered equipment involved in operation of the plant included the raw waste shredder, macerators, feed pumps, biogas compressor pumps, CHP and boiler water feed pumps, belt press, air filtration and minor ancillary equipment such as convey￾ors. The primary consumers of electricity included the heat dump￾ing fans, gas mixing compressors, air filtration unit for reception hall, air filtration biofilter for digestate hall, raw waste shredder, and pasteurisation heating pump. Intermediate consumers were the feed and discharge pumps, gas holder inflation fans and the CHP water pump. There were no individual electricity meters on these and the power taken depends upon the equipment load, so cannot be calculated directly from hours run and plate capacity. Parasitic energy is therefore given as an overall figure representing the total number of kWh consumed on site (Table 5). Fig. 6. Mass balance (wet weight) during the study period: weekly (a) and cumulative (b). Table 2 Mass balance for study period (wet weight). Parameter Unit Value Food waste input kg 3936,504 Water input (washwater) kg 1490,000 Total input kg 5426,504 Methane kg 275,177 Carbon dioxide kg 451,473 Water vapour kg 12,526 Digestate a kg 3969,080 Fibre a kg 39,240 All waste leaving site kg 35,820 Total output kg 4783,315 Wet tanks kg 92,433 Stored material kg 30,000 Total storage kg 122,433 Balance accounted for kg 520,756 % 90.4% a Any liquid digestate produced is recirculated through the process and leaves the site as whole digestate. 618 C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620