1.4　Biogas Characteristics

1.4.1　Biogas Components

The majority of biogas or nature gas are methane and carbon dioxide. The methane content of nature gas is more than 90 percent, while the methane content of biogas is less than 70 percent. In general, besides methane and carbon dioxide, there is a small amount of nitrogen, carbon monoxide, hydrogen, sulfureted hydrogen and oxygen in biogas. The methane content of biogas is 60 percent and carbon dioxide content was 35 percent. The high methane content in biogas influences its physicochemical property.

1.4.2　Biogas Terminologies

1.4.2.1　Total Solids (TS)

Total solids is defined as the weight percentage of dry sample in gross weight of the total sample. The total biomass fresh weight of the raw material included the non-volatile solids and volatile solids.

It is usually used as an indicator of gas production potential of the biomass material.

Measurement: Put the sample of raw materials into a pre-dried crucible with a constant weight. Dry it in an oven at the temperature of 105 ℃ until its weight become stable, and then calculate.

Where　W0= (sample weight + crucible weight) – crucible weight

W0——weight of wet sample,g;

W1——weight of dry sample,g.

Example 1: If we heat a bale of straw of 5g at 105 ℃ in an oven, the stable weight of sample would be 4.2g. Then

Example 2: If we heat fresh cattle dung of 10g at 105 ℃ in an oven, the stable weight of sample would be 2.0g. Then

1.4.2.2　Volatile Solids (VS)

Volatile solids is defined as the weight percentage of organic component of the total sample, which can be degraded by microorganism to produce biogas. The value of VS equals to the total biomass fresh-weight minus non-volatile solids.

Measurement:Having finished TS test, we can put the dry sample and crucible inside a muffle, turn the temperature knob to 550 ℃ and switch on. First, let door open a little, ignite the sample, and then close the door. Burn at 550℃± 20℃ about 2～3 h until its weight is stable. When it is cooled, the ash weight can be obtained, and then calculate.

In formula　W2——ash weight ,g.

Example 3: If we burn a bale of straw to its stable weight of 4.2g at 550 ℃ in a muffle for half an hour, the ash weight will be 0.84g. Then

Example 4: If we burn cattle dung to its stable weight of 2.0g at 550 ℃ in a muffle for half an hour, the ash weight will be 0.2g. Then

1.4.2.3　Hydraulic Retention Time (HRT)

HRT is defined as the mean retention time of raw feedstock in the biogas plant, generally expressed in day or hour. Namely:

In formula　V0——effective volume of plant ,m3;

Vt——feed-in volume every day,m3.

Example 5: The daily materials loading is 60 kg in a 6m3digester, 90％ of digester volume is available for fermentation. HRT of the digester is calculated as follow:

(Assume: 1kg=1L as w/water)

Example 6: In some hoggeries, HRT of biogas engineering is 30d, with available volume of (fermentative volume) 600m3. The daily loading of materials is calculated as follow:

(Assume: 1m3water = 1000kg water, feeding = 20 t/day)

1.4.2.4　Gas Production Rate (GPS)

Gas production rate of volume (GPRV) is biogas-producing capacity of digester in unit volume and unit time, expressed in m3/(m3·d), L/(L·d), mL/(mL·d) etc. Then

In formula　Vd——daily biogas yield;

V——volume of digester.

Example 7: The daily biogas yield is 1.5m3in a 6m3digester. GPRV of the digester is calculated as follow:

Example 8: Both UASB and AF biogas technique are applied to hoggery biogas engineering in Shenzhen, China. The volume of reactor is 137m3, while the available volume is 130 m3. The daily biogas yield is 210 m3. GPRV is calculated as follow:

1.4.2.5　Organic Space Loading Rate (OLR)

The organic space loading rate is defined as the organic loading which could be carried by biogas plant per unit volume per day. It refers to the quantity of organic pollutants which can be accepted by biogas plant per cubic meter of biogas plant volume per day. If the sample is solid waste, kg TS/(m3·d) or kg VS/(m3·d) can be used as the unit of rate of organic load. If the sample is liquid waste, kg COD/(m3·d) or kg BOD/(m3·d) can be used.

The name of working unit: kg TS/(m3·d), kg VS/m3·d in biogas anaerobic digestion kg COD/(m3·d), kg BOD/(m3·d) in waste water treatment.

Example 9: The TS of feedstock is 6％, and daily loading is 10t in the biogas digester, with an available volume of 400m3. The OLR of the biogas engineering is calculated as follow:

Daily loading solids = 10t × 6％ = 0.6t = 600kg

Example 10: The TS of feedstock is 8％, and daily loading is 100 kg in the biogas digester, the biogas digester has an available volume of 6m3. The OLR of the biogas engineering is calculated as follow:

Daily loading solids = 100kg × 8％ = 8.0kg

1.4.2.6　Gas Production Rate of Materials (GPRM)

Biogas production rate of materials is the gas yield of materials under a certain condition. It is defined as biogas yields per TS feedstock, which is used as a judgment for potential biogas generation of material. It indicates what kind of materials is suitable for biogas production. Different materials have different gas production rates.

The biogas production rate of raw materials can be described by three terms. The theoretical GPRM value is based on chemical components of the material, for agricultural organic wastes the GPRM value is about 0.7m3/kg TS or 0.35m3CH4/kg COD. The experimental GPRM value is determined by experimental methods, which show the maximum gas generation of the material. The practical GPRM value is smaller than the experimental GPRM value, which depends on technique condition. In practice, the GPRM value of cow dun is 0.25m3/kg TS, GPRM value of human excrement, pig and chicken dung is 0.30m3/kg TS, and the value for straw or stalk is 0.25m3/kg TS.

The unit of GPRM is m3/kg TS. The relationship of theoretical GPRM (T-GPRM), experimental GPRM (E-GPRM) and practical GPRM (P-GPRM) is T-GPRM > E-GPRM > P-GPRM.

The formulas of GPRM are as follows:

Example 11: A five-member-family, keeping four pigs in the household, will build a 6m3or 8m3biogas digester of “three in one” type (toilet, animal house and biogas digester).

Questions:(1)HRT=?(6m3and 8m3);

　　　　(2) How many biogas yield are there everyday?

Analysis: Fermentative feedstock includs pig dung and human excrement.

Known:Fermentative concentration TS is 6％.

Then:(1)Daily pig dung=4(pigs)×6kg/pig=24kg(TS=20％)

Daily loading material (pig) = 24×20％÷6％ = 80 (kg)

Daily human excrement=5(members)×0.5kg/person=2.5kg(TS=15％)

Daily loading material (person) = 2.5×15％÷6％ = 6 (kg)

Daily loading material (mixture) = 80+6 = 86 (kg)

With an available volume of 6 m3biogas digester,HRT=6m3×90％÷86kg/d=63 (d)

With an available volume of 8 m3biogas digester,HRT=8m3×90％÷86 kg/d=84 (d)

(2)GPRM of pig dung and human excrement is 0.30m3/kg TS.

Daily pig dung TS=24×20％=4.8(kg)

Daily human excrement TS=2.5×15％=0.375(kg)

Daily loading total material TS=4.8+0.375=5.175(kg)

So GPRM=5.175 kg×0.30m3/kg TS=1.55(m3)

1.4.2.7　Biochemical Oxygen Demand (BOD)

BOD is defined as the consuming oxygen per liter waste water which is needed for degrading organic waste by microorganism under the condition of 20℃and aerating. The unit of BOD is mL/L. The more organic waste in water, the larger BOD value is. The BOD value can indicate the organic loading of the waste water indirectly.

1.4.2.8　Chemical Oxygen Demand (COD)

COD is defined as the consuming oxygen which is needed for chemical reaction between organic matter and potassium dichromate. The unit of COD is mg/L or kg/L. In general, every kg COD can generate 0.35 m3of methane. The larger chemical oxygen demand, the more organic matter there will be. Only when the waste water is too poisonous to measure the biochemical oxygen demand, can we use chemical oxygen demand.

Both COD and BOD are applied for describing the quantity of organic matter in material. BOD can indicate the quantity of organic being degraded by microorganism, while COD value is larger than BOD due to some nondegradable matter in the material. The ratio of BOD over COD ranges from 0.4 to 0.8.

Example 12: About 100 m3waste water is discharged by a wine factory per day. The COD value is 40000 mg/L. Calculated the methane production and biogas production.

The COD amounts of the discharging waste water = 100m3× 40000mg/L =4000kg

Methane yields = 4000kg × 0.35m3= 1400m3

Biogas yields = 1400m3÷ 60％ = 2333m3

1.4.2.9　The Relationship of Biogas Parameters

There are some principles for the designed parameter in biogas engineering. The higher organic long loading rate, the higher volume gas production rate. The hydraulic retention time is for high concentration and high temperature, while short for low concentration and low temperature. The hydraulic retention time for manure is 60d, while for straw is 90d. The relationship of the organic loading rate, feedstock concentration and hydraulic retention time is:

Example 13: When feedstock has a 6m3biogas digester, 5000kg fermentative material will be needed. By using the pig dung alone as material, the TS of ferment concentration is 6％. How much pig dung is needed, with a TS of 20％? How much water is needed?

Total solids of 5000kg material = 5000×6％ = 300 (kg)

Pig dung added amounts = 300÷20％ = 1500 (kg)

Water added amounts = 5000−1500 = 3500 (kg)