Design and engineering of biogas plants - The Biogas: Science, Production and Applications - page 196

organic materials such as sheep wool, cotton, flax, coconut fibre, etc. can be
used although they are considerably more expensive. The thickness of the
insulation is designed in order to optimise the costs of the insulation in
comparison with cost savings due to reduction of heat losses. As a rule of
thumb, aimed heat transfer values are 0.3W/m 2 K for mesophilic reactors
and 0.2W/m 2 K or less for thermophilic reactors (Eder and Schulz, 2006).
The resulting insulation thickness is 10-18 cm.
For reactor heating, hot water passes through pipes in the digester.
Heating pipes used to be cast in the concrete wall and floor, but tension due
to temperature differences can cause cracks in the concrete and wear the
system. Hence, heating pipes are now commonly placed on the inside of the
digester wall.
The required heating power depends mainly on the digester temperature,
volume, form and insulation as well as the temperature of the fed substrate
and the ambient climate. To estimate the required power, the following
three components are calculated and summed.

1. Power for substrate heating (inside or outside the digester)

P ¼ RC D T ½ 8 : 3

where R is the mean feeding rate (g/s), C is the specific heat (specific
heat of water = 4.186 J/gK) and,

T is the temperature difference
between incoming substrate and digester (K).
2. Compensation of heat losses through radiation

Δ

T 0 SU

P ¼ D ½ 8 : 4

T ´ is the temperature difference between the outside and inside
of the digester (K), S is the digester surface area (m 2 ) and U is the heat
transfer value of the digester wall (W/m 2 K).
3. Compensation of heat losses through evaporation

where

Δ

￿ ￿ ￿ ￿ ￿ ￿

P ¼ FESat ½ 8 : 5

where F is the flow rate of produced biogas (m 3 /s), E is the evaporation
enthalpy (evaporation enthalpy of water 2.260 kJ/kg) and, Sat is the
water saturation of produced biogas (kg/m 3 ).

Most frequently, 'waste' heat from a combined heat and power (CHP) unit
is used as the energy source for digester heating.