How to Estimate Potential of Energy Generated from Biogas from POME (Palm Oil Mill Effluent)

In this post, I’ll explain how to estimate the energy potential of biogas generated from Palm Oil Mill Effluent (POME). To convert POME into energy, it undergoes anaerobic digestion to produce methane-rich biogas. If you’re new to these concepts, feel free to check out my previous post on biogas and biorefineries.

To ensure a POME-to-energy project is financially viable, estimating its potential energy yield is a crucial first step. This calculation relies on five key operational parameters:

Operating hours: the average number of hours the palm oil mill is in operation in a day (hours/day)
Operating days: the average number of days the mill is in operation in a year (days/year)
Annual fresh fruit bunch (FFB): the amount of fresh fruit bunches (FFBs) processed in a year (ton FFB/year)
POME to FFB ratio: the ratio of POME volume produced per FFB processed (m³ POME / ton FFB)
Typical COD: the COD of the wastewater analysed by spectrophotometer (mg/L)

Please also check previous post about the difference between COD and BOD.

To calculate potential power generation, we use the following standard assumptions:

CH4 to COD ratio (CH4/COD), defined as theoretical volume of methane produced per kg of COD removed from the wastewater. The value is assumed 0.35 Nm³ CH₄/kg COD removed
COD removal efficiency (COD_eff), the percentage of COD that will be converted to methane. It is assumed 80%-95%
Methane energy value, which is the energy content of methane (35.7 MJ/Nm³)
Average electrical efficiency (Gen_eff), which is efficiency of gas engine in converting energy value of methane to electrical energy. The value is between 38%-42%

Based on POME characteristics and assumption above we can estimate potential power using the following steps:

Daily throughput (ton FFB, Fresh Fruit Bunch/day) = annual FFB / operating days
Daily wastewater flow rate (m³/day) = daily throughput × POME to FFB ratio
COD loading (kg COD/day) = typical COD × daily wastewater flow rate × (kg / 1000,000 mg) × 1000 L/m³
CH₄ production (Nm³/day) = COD loading × COD_eff × CH₄ to COD ratio
Generated power capacity (MWe) = CH₄ production × CH₄ energy value × Gen_eff/ (24 × 60 × 60)

The calculated power capacity represents the gross output generated by the gas engine. If you plan to sell this electricity to the grid, you will need to multiply this number by an availability factor (typically 90% to 98%) to account for routine maintenance downtime and distribution line losses.

Example

We will estimate the potential of energy using steps above. The data and assumptions are below.

Data:

Operating hour = 5000 hours/year
Operating days = 300 days/year
Annual FFB = 250,000 ton/year
Daily FFB = annual FFB / operating hour = 250,000 / 5000 = 50 ton FFB/hours
Ratio POME to FFB = 0.8 m³ POME per ton FFB (usually 0.7-1 m³ POME per ton FFB)
COD = 60,000 mg/L

Assumptions:

CH₄ to COD ratio = 0.35 Nm³ CH⁴/kg COD removed
COD removal efficiency = 90%
Average electrical efficiency = 38%

Calculation:

Daily wastewater flow = annual FFB / operating hour × FFB to POME ratio = 250,000 ton/year x 0.8 m³ POME per ton FFB / (300 days/year) = 666.67 m³/day
COD loading = COD × daily wastewater flow × (1000 L/m³) × (kg/1,000,000 mg) = 60,000 × 666.67 × 1000/1,000,000 = 40,000 kg COD/day
CH₄ production = COD loading × CH₄ to COD ratio × COD removal efficiency = 40,000 × 0.35 × 90% = 12,600 Nm³ CH₄/day
Generated power capacity = CH₄ production × methane energy value × average electrical efficiency / (24 × 60 × 60) = 12,600 × 35.7 × 38% / (24 × 60 × 60) = 1.98 MWe

The potential power generated ads function of COD (Chemical Oxygen Demand) for different POME to FFB ratio are shown below. The POME to FFB ratio is ranging from 0.7 to 1 m³ POME per ton FFB.