Biogas Production and Energy Recovery

Comprehensive biogas modeling for anaerobic digestion processes

Overview

The biogas production module calculates biogas yield, composition, and energy recovery potential from anaerobic digestion of wastewater sludge. It integrates with the GHG emissions calculator for carbon footprint assessments.

Services

Service	Purpose
BiogasProductionService	Calculate biogas yield from substrate composition
BiogasCompositionService	Determine gas composition and quality
EnergyRecoveryService	Calculate CHP electricity and heat recovery

Biogas Production

Substrate-Specific Yields

Based on the Buswell equation, different organic compounds produce different amounts of methane:

Substrate	Formula	CH4 Yield (Nm³/kg VS)	CH4 Fraction (%)
Carbohydrates	(C6H10O5)n	0.415	50%
Proteins	C5H7NO2	0.496	70%
Lipids	C51H98O6	1.014	70%

Buswell Equation

The theoretical biogas composition is calculated using:

$$C_nH_aO_bN_c + \frac{4n-a-2b+3c}{4}H_2O \rightarrow \frac{4n+a-2b-3c}{8}CH_4 + \frac{4n-a+2b+3c}{8}CO_2 + cNH_3$$

For glucose (C6H12O6): C6H12O6 → 3CH4 + 3CO2

Usage

import { calculateBiogasYield, getBiogasProductionService } from '@/flow/services/simulation/carbon';

const result = calculateBiogasYield({
  vsFeedRate: 5000, // kg VS/d
  vsDestructionEfficiency: 55, // %
  substrateComposition: {
    carbohydrates: 60, // %
    proteins: 25, // %
    lipids: 15, // %
  },
  srt: 20, // days
  temperature: 35, // °C
});

console.log(result.totalBiogas); // Nm³/d
console.log(result.methaneFlow); // Nm³ CH4/d
console.log(result.specificYield); // Nm³/kg VS destroyed

Temperature Effects

The temperature correction factor uses the Arrhenius equation:

$$k(T) = k(35) \times \theta^{(T-35)}$$

Regime	Temperature	θ	Rate Multiplier
Psychrophilic	<20°C	-	Reduced
Mesophilic	30-40°C	1.04	1.0×
Thermophilic	50-55°C	1.06	1.3-1.5×

Inhibition Effects

The service calculates inhibition from:

Factor	Threshold	Effect
Ammonia (TAN)	>3000 mg/L	Moderate inhibition
VFA	>2000 mg/L	Reduced hydrolysis
Sulfide (H2S)	>200 mg/L	Methanogen inhibition
pH	<6.5 or >7.6	Process upset

Biogas Composition

Typical Composition

Component	Range	Typical	Unit
CH4	55-70	65	vol %
CO2	25-45	35	vol %
H2S	100-10,000	1000	ppm
NH3	50-200	100	ppm
H2	<1	0.1	vol %
H2O	2-7	4	vol %

H2S Estimation

H2S is produced from:

1. Sulfate reduction: SO4²⁻ → H2S (by sulfate-reducing bacteria)
2. Protein degradation: S-amino acids → H2S

import { calculateBiogasComposition } from '@/flow/services/simulation/carbon';

const composition = calculateBiogasComposition({
  substrateComposition: { carbohydrates: 60, proteins: 25, lipids: 15 },
  temperature: 35,
  influentSulfate: 50, // mg/L SO4
  ph: 7.0,
});

console.log(composition.h2sContent); // ppm
console.log(composition.lhv); // MJ/Nm³

Heating Values

Property	Formula	Typical Value
LHV	CH4% × 35.8 MJ/Nm³	~23 MJ/Nm³
HHV	CH4% × 39.8 MJ/Nm³	~26 MJ/Nm³
Wobbe Index	HHV / √(ρ/ρ_air)	~27 MJ/Nm³

Quality Assessment

import { assessBiogasQuality } from '@/flow/services/simulation/carbon';

const quality = assessBiogasQuality(composition);
console.log(quality.suitableForCHP); // boolean
console.log(quality.warnings); // string[]
console.log(quality.recommendations); // string[]

Energy Recovery

CHP Performance

import { calculateEnergyRecovery } from '@/flow/services/simulation/carbon';

const energy = calculateEnergyRecovery({
  biogasFlow: 2000, // Nm³/d
  ch4Percentage: 65, // %
  chpElectricalEfficiency: 38, // %
  chpThermalEfficiency: 45, // %
  parasiticLoad: 50, // kW
  availabilityFactor: 95, // %
  gridCarbonIntensity: 0.4, // kg CO2/kWh
  electricityPrice: 0.10, // $/kWh
});

console.log(energy.grossElectricity); // kWh/d
console.log(energy.netElectricity); // kWh/d
console.log(energy.carbonCredit); // kg CO2/d avoided
console.log(energy.revenue); // $/d

Energy Balance

Output	Typical Efficiency	Formula
Gross Electricity	30-42%	E_input × η_elec
Net Electricity	25-38%	Gross - Parasitic
Thermal Recovery	40-50%	E_input × η_therm
Useful Heat	20-40%	Thermal × η_util
Losses	10-25%	E_input - Elec - Thermal

Carbon Credits

Carbon Credit (kg CO2/d) = Net Export (kWh/d) × Grid Intensity (kg CO2/kWh)

Typical grid carbon intensities:

Region	kg CO2/kWh
UK	0.20-0.25
EU Average	0.25-0.35
US Average	0.40-0.45
China	0.55-0.65

Integration with GHG Calculator

The biogas services integrate with the GHG emissions calculator:

import { calculateBiogasYield, calculateCH4Emissions } from '@/flow/services/simulation/carbon';

// Calculate biogas production
const biogas = calculateBiogasYield(yieldParams);

// Use in GHG calculations
const ch4Emissions = calculateCH4Emissions({
  biogasProduction: biogas.totalBiogas,
  methaneContent: 65,
  biogasCollectionEfficiency: 97,
  biogasLeakageRate: 2,
  dissolvedCH4: 15,
  digestateFlow: 500,
  flareEfficiency: 99.5,
  isFlared: false,
  isUtilized: true,
  gwpHorizon: '100-year',
}, 'advanced');

Sludge-Specific Calculations

Primary Sludge

import { calculatePrimarySludgeBiogas } from '@/flow/services/simulation/carbon';

const result = calculatePrimarySludgeBiogas(
  5000, // VS feed rate (kg/d)
  35, // temperature (°C)
  20 // SRT (days)
);

Typical composition: 60% carbs, 25% proteins, 15% lipids

Waste Activated Sludge (WAS)

import { calculateWASSludgeBiogas } from '@/flow/services/simulation/carbon';

const result = calculateWASSludgeBiogas(
  3000, // VS feed rate (kg/d)
  35, // temperature (°C)
  20 // SRT (days)
);

WAS has lower VS destruction (~85% of primary) due to microbial cell walls.

Mixed Sludge

import { calculateMixedSludgeBiogas } from '@/flow/services/simulation/carbon';

const result = calculateMixedSludgeBiogas(
  5000, // Primary VS (kg/d)
  3000, // WAS VS (kg/d)
  35, // temperature (°C)
  20 // SRT (days)
);

API Reference

BiogasProductionService

Method	Description
calculate(params)	Calculate biogas yield
getLastResult()	Get last calculation result
subscribe(listener)	Subscribe to updates

BiogasCompositionService

Method	Description
calculate(params)	Calculate gas composition
assessQuality()	Assess biogas quality for CHP
subscribe(listener)	Subscribe to updates

EnergyRecoveryService

Method	Description
calculate(params)	Calculate energy recovery
verifyBalance()	Verify energy balance
getAnnualTotals(factors)	Calculate annual totals
subscribe(listener)	Subscribe to updates

Best Practices

1. Substrate Analysis: Determine actual composition via COD fractionation
2. Temperature Control: Maintain stable temperature (±1°C)
3. SRT Selection: 15-20 days for mesophilic, 10-15 for thermophilic
4. H2S Monitoring: Install desulfurization if >500 ppm
5. H2 Monitoring: Use as early warning for process upset
6. CHP Sizing: Size for 80-90% of average biogas production

Related Topics

• GHG Emissions Calculator
• Anaerobic Digester Process Unit
• Carbon Dosing Service

References

1. Buswell, A.M. & Mueller, H.F. (1952). "Mechanism of Methane Fermentation"
2. Batstone, D.J. et al. (2002). "ADM1 - Anaerobic Digestion Model No. 1"
3. Metcalf & Eddy (2014). "Wastewater Engineering" Chapter 13