Mass and Energy Balances in High Solids Digestion Following Thermal Hydrolysis Pre-Treatment
Panter, K., Ebcor Ltd
(free)
Experience of digestion following thermal hydrolysis goes back to 1995 when the first Cambi
Thermal Hydrolysis Plant (THP) plant was commissioned at Hamar Norway. This paper
addresses two areas where experience has been gained in knowledge areas: Reconciling
biogas production with mass balances across the digester; optimising energy balance in order
maximise CHP production.
Typically digester performance is gauged by measuring solids in and out of digesters and
calculating Volatile Solids Reduction (VSR) as a percentage of VS feed. This method has
always had one flaw – it does not allow for deposition or wash out of non- volatiles (ash). The
Van Kleek formula corrects for this by assuming constant ash in and out of the digester – but
this can only apply to digesters that are in a steady state over a long period of time.
Experience with THP treated sludge shows that there are anomalies when mass balance and
Van Kleek formulas are applied. There are a number of factors that have been postulated
and/or demonstrated that can cause the mass balance or Van Kleek formula to be
inappropriate. These are:
1. Underestimate of DS% feed due to inaccurate measurement of THP feed sludge
caused by high content of soluble volatile materials that are effectively measured as
water.
2. Overestimate of ash content of digested sludge due to struvite and dittmarite in BioP
plants that can give false high ash readings in digested sludge due to fixation of
ammonia and phosphate.
3. Overestimate of ash content of digested sludge due to high level of alkalinity due
bicarbonate and carbonate where CO2 has been fixed in the solids.
The solution to this is to make energy balances based on the raw sludge feed VS energy
content and reconcile this with the biogas methane produced and it energy content. This
method is described in the paper using Kapuciska plant in Bydgoszcz in Poland as an
example.
The second area discussed in the paper is the relationship between biogas produced and
how this can be used for CHP and THP heating. The paper shows that with improving energy
efficiency with the Cambi process that the heat for steam production can be provided mainly
by the CHP. Modern projects have the potential to be autothermic on CHP – depending on
the VSR and energy content of the sludge, so that the net CHP produced can maximise
ROCs. An example is based on the performance of the Cotton Valley THP plant.
KEYWORDS
Anaerobic Digestion, Thermal Hydrolysis, Mass Balance, Energy Balance Mass and Energy
Balances
