<- cbg-sim

Methodology · v0.1

cbg-sim is a steady-state, lumped-parameter model of a compressed biogas plant. It is calibrated against published Indian CBG sector data and is intended for educational and first-pass decision-support use.

Scope

The model computes nine outputs for any plant configuration: daily biogas volume, methane fraction and CBG mass at IS 16087:2016 spec, daily fermented organic manure (FOM), daily biochar (Phase 2+), daily food-grade CO2 (Phase 3+), capex, daily and annual revenue/opex/margin/payback/IRR, and lifecycle carbon intensity in gCO2e/MJ on well-to-tank basis.

What v1 deliberately does not do

  • No dynamic time-step simulation (steady-state only)
  • No microbial population dynamics or detailed kinetics
  • No ammonia, VFA, or H2S inhibition explicitly
  • No feedstock seasonality (assumes year-round availability at design rate)
  • No stochastic Monte Carlo (deterministic; sensitivity in §10 of spec)
  • No detailed financing (no debt, tax, or working-capital cycle)
  • No CFD or hydraulics inside the digester

Calibration status

The model is calibrated against three reference plants: Verbio Lehragaga (33 TPD CBG, ~₹180 cr capex), Reliance Kanigiri (100 TPD initial, ₹139 cr phase-1 capex), and Bagepalli Biogas (CDM cattle-manure precedent). Capex calibration matches Verbio within 3%. The TPD definitional ambiguity surfaced during testing (CBG output kg/day vs feedstock throughput t/day) has been resolved by standardising on the consumer convention.

Three open calibration items remain, each requiring named-operator review: empirical eta_AD for Indian Napier, soil-carbon credit value, and biochar credit value. All bracketed conservatively in v0.1.

Sources

Every parameter traces to a citation. The full reference list is in the spec document (§12). Primary sources include the IEA Outlook for Biogas and Biomethane (2020), Biogas Magazine Edition 31 BMP tables, the Gujarat Biomass White Paper conversion-efficiency tables, the EAC International Consulting feedstock framework, the GMI/TERI January 2025 eleven-plant study, and PPAC SATAT pricing.

Full specification

The model is documented in a 25-page peer-reviewable specification covering notation, equations, parameter tables, calibration, sensitivity, and an open reviewer cover note.

The spec lives outside the application. Send a request for the v0.1 PDF to the team.