abe 482—environmental engineering in biosystems 482 lecture 8.pdf · volume of biogas = 5-15%...
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ABE 482—Environmental Engineering in Biosystems
September 22
Lecture 8
Compost Recipe Example 3
� How much water should we add to our piles to reach 60% wet basis moisture content in each one?
Summary of Composting
� Composting is “controlled” decomposition
� Weight and volume reduction…but requires effort!
� Nitrogen: what you want to get rid of
� Carbon: amendment materials to mix with nitrogen
� Must balance C:N, moisture, oxygen
� Monitor temperature, airspace, density
� Compost is “done” when temperature doesn’t rise after turning
Today (Anaerobic digestors)
� Quick definition
� Applications and types of anaerobic digesters
� Types of anaerobic digesters
� Biogas production
� Sizing of digesters
� Mass and energy balance for anaerobic digesters
What is anaerobic digestion?
� Methanogenic and acidogenic breakdown of
complex organics
� Oxidation of organic or inorganic compounds in the absence of oxygen
� Produces biogas and less biomass than aerobic digestion
Applications of anaerobic digestion
� Any wet organic material
� Waste paper, grass clippings, foodwaste,
sewage, animal wastes
� Energy or silage crops
� Wood waste?
� Livestock production facilities
� Wastewater treatment facilities
Anaerobic Digester for Livestock Production Facilities
Production Building Digester
Lagoon
Biogas
Flare
Generator
HotWater
Manure Bypass
Composting orland application
DigesterProduction buildingGreenhouse
DigesterProduction buildingPower grid
Liquid fertilizer
Solid/LiquidSeparator
Anaerobic Digester for Wastewater Treatment Facilities
Types of Digesters
Types of digesters
� Low rate conventional digester� Digestion time = 30-60 days� Intermittent mixing, feeding and sludge
withdrawal� Floating cover or fixed cover
� High rate digester� Digestion time = 10-20 days� Continuous mixing, continuous or periodic
feeding and sludge withdrawal� Fixed cover
� 2 stage digester� High rate followed by a low rate
Biogas production
� Composition: 65-70% CH4, 30-35% CO2, water vapour, trace amounts of H2S, H2, N2
� Quantity: 810-1120 L of digester gas/kg of volatile or organic solids digested
� Chemical balance (carbs�sugars�methane and carbon dioxide)
C6H12O6 � 3(CO2) + 3(CH4)
180 g/mol 46 g/mol 16 g/mol
Factors controlling the conversion of waste to gas
� The type of waste being digested,� Its concentration,� Its temperature,� The presence of toxic materials,� The pH and alkalinity,� The hydraulic retention time,� The solids retention time,� The ratio of food to microorganisms,� The rate of digester loading,� And the rate at which toxic end products of digestion are removed.
Sizing of digesters
� For CSTR digesters, required volume = flow rate * hydraulic retention time
� Volume of biogas = 5-15% tank volume
Example 1Sizing of digesters
� Determine the volume of CSTR digester required to digest pig slurry from a farm with 2,000 pigs. The hydraulic retention time is 15 days.
Mass and Energy Balance
� Mass balance
� Total mass of inputs = total mass of
outputs
� Inputs = feedstock
� Outputs = digestate and biogas
� Feedstock
� % dry matter (%DM)
� % organic dry matter (%ODM)
� % organic matter reduction (%OMR)
Feedstock
� % dry matter (%DM)� %DM = dry mass/wet mass� %DM = 1 – MCwb� Dry material at 105°C for 24 hours
� % organic matter (%ODM)� %ODM = 100 – Ash%� Ash% = non-volatile organics/dry mass� Cook at 550°C to drive off volatiles
� % organic matter reduction (%OMR)� %OMR = (%ODMinput-%ODMoutput)/%ODMinput
� 30% for anaerobic digestion of cattle manure� 70% for some energy crops
Simple Feedstock Calculation
� Feedstock� Mass (rate) = 50 tonnes/day
� %DM = 30%
� %ODM = 90%
� %OMR = 66.7%
� Calculate:� Mass (rate) of dry matter and organic dry matter in input
� Mass (rate) of organic matter reduction
Simple Mass Balance
Input -OMR
ODM
Same as input
OMRSame as input
Same as input
TotalBiogasAshWater
Input
Output
Water + ash + organic
OrganicDry -organic
Total – dry
TotalODMAshWater
Digestate
Simple Mass Balance
Mass of feedstock = 50 tonnes/day
Dry matter of feedstock = 15 tonnes/day
Organic dry matter of feedstock = 13.5 tonnes/day
Organic matter reduction = 9 tonnes/day
Mass of digestate = 41 tonnes/day
Calculate:
%DM and %ODM of digestate
DM and ODM rates
4.5
ODM
5091.535
TotalBiogasAshWater
Input (tonnes/day)
Output (tonnes/day)
5013.51.535
TotalODMAshWater
Anaerobic Digestion Parameters
� Consider the Continuous Stirred Tank Reactor (CSTR)
� Hydraulic retention time
� Specific loading rate
� Specific biogas production
� Specific methane production
Next Day
� Anaerobic digester parameters
� Biogas production
� Digester heat balance