Energy from organic waste: how to estimate the biogas potential of bio-waste

We estimate energy from bio-waste with a chain of calculations: mass of waste × dry matter share (TS) × organic matter share (VS) × methane potential (BMP) × calorific value of methane × conversion efficiency; for waste with a yield of 100 m³ CH₄ per tonne this means about 1000 kWh of chemical energy, that is about 380–420 kWh of electricity in cogeneration. This is an order of magnitude that allows a project to be assessed quickly, but it does not replace laboratory tests.

Step by step

1. Mass of waste in tonnes per year (fresh mass).
2. TS and VS: conversion to the mass of organic dry matter. Example: 25% TS and 90% VS gives 0.225 t VS per tonne of fresh mass.
3. BMP: methane yield per unit of VS; 1 NmL CH₄/g VS is 1 m³ CH₄ per tonne of VS. With a BMP of 450 NmL/g VS, about 101 m³ of CH₄ is produced from a tonne of fresh mass (0.225 t VS × 450 m³/t VS).
4. Chemical energy: the calorific value of methane is about 35.8 MJ/m³, that is about 10 kWh/m³. From a tonne: about 1000 kWh.
5. Cogeneration efficiency: electrical about 38–42%, thermal an additional several dozen per cent. From a tonne: about 380–420 kWh of electricity.

The value of about 50–55 MJ that appeared in the original text corresponds to the energy value of methane per kilogram, not per cubic metre (the density of methane is about 0.66–0.72 kg/m³).

Example for 1 million t of waste

Let us hypothetically assume 1 million t of waste with a yield of 100 m³ CH₄ per tonne. This gives 100 million m³ of methane, that is about 3.6 PJ (about 1000 GWh) of chemical energy, and with an electrical efficiency of 40% about 400 GWh of electricity a year. On a national scale, even if all organic waste (in the order of several to a dozen or so million tonnes, depending on the definition) could be processed, we would obtain at most single TWh, that is a fraction of national electricity production, and not "128 TWh". Biogas is therefore an important but limited source of energy.

Factors affecting actual yield

• Waste composition: fats yield many times more biogas than carbohydrates, but easily inhibit the process in excess.
• Moisture: mass that is too dry or too wet makes pumping and mixing difficult.
• Temperature: the thermophilic process is faster but requires more heat energy.
• Retention time and digester loading (OLR).
• Inhibitors: ammonia, heavy metals, antibiotics.
• Contaminants in municipal waste (packaging) and losses in collection.

From potential to project

Theoretical potential is the upper limit. A project requires: stabilised collection, TS and VS analysis, a BMP test, a heat balance and a digestate management plan. BMP studies from the literature are compiled in BMP studies, and the methodology in the article What is a BMP test.

Comparison with other sources

Biogas has the advantage of dispatchability: the installation can run around the clock, regardless of the weather, which distinguishes it from wind and photovoltaics. At the same time its scale is limited by the amount of available substrates. A comparison of waste processing technologies: Energy from waste: fermentation, incineration, pyrolysis and landfill gas.

How to calculate accurately in practice

For a specific installation it is better to start from your own data than from general indicators: delivery weights, TS and VS from laboratory analyses and BMP from a test. The calculations should take into account losses (for example storage), the installation's own energy consumption (mixing, pumps, digester heating) and annual availability (usually significantly below 100% of the time). This makes it possible to obtain net rather than gross energy and to realistically assess the return on investment. For example, gross electricity from cogeneration is reduced by the energy needed to run the biogas plant itself.

Summary

Estimates of the energy potential of organic waste are useful for a preliminary assessment, but only when the calculation is transparent and takes losses into account. Always compare the assumptions (TS, VS, BMP, efficiency) with the results and check the order of magnitude by comparing it with your own installation or with data from the literature. An error of one power of ten, as in the original version of this article, can completely change the investment conclusion.