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The 5 Most Critical Factors in the Design of Every Biogas Plant – Xergi Biogas

In this article Xergi Biogas (now Nature Energy) identifies the 5 most critical factors in the design of every Biogas Plant.

The highly experienced and well-respected Danish and UK based Anaerobic Digestion and Biogas Plant design and build contractor Xergi, has provided useful information on their website about achieving the efficient design of every AD Plant.

The guidance is in the form of a discussion of the 5 most vital design factors which in their experience it is essential for every AD Plant design to get right, for successful operation.

We have been given their kind permission to repeat these 5 points here, on our blog site, for the benefit of our readers. We recommend that all of our readers note each point carefully.

This is particularly important when you consider that the information comes from such an experienced company source.

Watch our video below on the Xergi Biogas Top Design Success Factors:

The original article posted in October 2014 was updated in July 2021 to include the forthcoming UK Environment Agency best practice recommendations for commercial AD plant design and operation. These are shown in green.


1. Design Must Incorporate a High Degree of Flexibility

If there is one thing for certain about biogas plants it is that the customer's requirements will continuously evolve throughout the life of any biogas plant. It is difficult, if not impossible to anticipate many of those changes, so the flexibility to alter biogas plant configuration to suit feedstocks as needed and in a manner that suits the client's particular site requirements, is essential.

Even though the set-up for each client is carefully customized for each project, the design must incorporate a level of versatility that will ensure that the configuration of the equipment can be changed to maintain efficient operation.

This must allow both for biomass inputs which continually vary, and as the marketable output products also change according to demand and new technological developments come available.

All biogas plants perform biological (organic) treatment in addition to providing renewable energy.

This is defined as: “Decomposition and stabilisation of biodegradable waste.”

In the design of every biogas plant which complies with good practice guidelines, it is done in a controlled environment, resulting in a stable, sanitised material that can be applied to the land for agricultural purposes, to improve the soil structure or add needed nutrients.

2. Accurate Dosing Equipment to Aid Balancing Feedstock Input is Essential

Image introduces Xergi Biogas and the biogas essential design features
Xergi considers this so important that they have developed a special module within their biogas plant that ensures that all the biomass constituents will be adequately pre-processed despite continually varying feedstock sources and composition.

The module has to be able to provide a high degree of control, in order to achieve a high level of process efficiency, without unnecessarily limiting the plant operator's choice of biomass source/ type.

To keep the digester performing at its best, you must understand the dosing parameters and make timely changes to the feedstock and micronutrient dosing. You will need to be able to show maximum efficiency in the reduction of volatile solids and chemical oxygen demand (COD) in the substrate

When a gas pressure alert situation is detected, you must implement an interlocked alarm device that causes reactor feeding to halt automatically.

To monitor, record, and display data for continuously monitored parameters, you must employ Supervisory Control and Data Acquisition Equipment (SCADA).

3. Ideal Process Temperature

Xergi applies a design philosophy, which involves plants running at the highest feasible process temperature. By doing so, it makes it possible to take the fullest advantage of the natural ability of the micro-oganisms to ferment the substrate within their digesters at the highest possible biogas (methane) producing potential.

You'll need to keep an eye on the temperature and gas pressure of the digester at all times. Within your management system, you must identify and establish all operating parameters and constraints.

Depending on the environment used and the system architecture, you must determine the optimal operating temperature.

The temperature of the digester must be kept within +/- 2 degrees Celsius of the working temperature.

4. Mixing Completely with the Lowest Energy Consumption

Reactor design needs to include a stirring and agitation system that gets as close as possible to providing a completely stirred reactor. Xergi has developed its own technology for this. It is optimized for low energy use to minimize the draw on the plant's own electricity generation. The importance of this lies in keeping operating costs low while producing high gas production rates for the feed material utilized must never be underestimated.

At your facility, you should design and execute an energy efficiency plan. This includes:

  • defining and calculating the site's energy consumption of the task (or tasks) you conduct and the waste stream(s) you accept, as well as
  • setting annual key performance indicators, such as specific energy consumption (expressed in kWh/tonne of waste processed) and
  • planning periodic improvement targets and actions.

As part of your facility's EMS, you'll be asked to examine and update your energy efficiency strategy on a regular basis.

You should keep track of your energy balance. This must include a breakdown of your energy usage and creation (including any energy or heat exportation) by source type (electricity, gas, conventional solid fuels, conventional liquid fuels, and waste).

5. User-Friendly Remote Control of Operating Systems to Enable Quick and Appropriate Operational Adjustments to be Made

Xergi uses a remote-controlled SCADA monitoring and operating system at all their anaerobic digestion plants.

All indications of plant status are viewable, and access to all operating system adjustments are available, via a smartphone app.

This is important to ensure that the gas production process is kept within stable (high efficiency) reactor conditions. And, can be accessed at any time to ensure the smooth running of their plants, which normally run day and night 365 days a year.

To monitor, record, and display data for continuously monitored parameters, you should deploy Supervisory Control and Data Acquisition Equipment (SCADA).

Any methane emission occurrences must be recorded using SCADA data logging. The date, time, and length of the release must all be documented. (To identify releases, a temperature sensor can be installed within the release pipe.)

Your SCADA system must be used for monitoring and interlocking.

Pressure and vacuum relief valves (PVRV) must be installed on all vessels where there is a possibility of pressurisation. The date, time, and duration of pressure relief events must all be recorded. (Release events must be identified by SCADA systems.)

For further information on the design of every Biogas Plant visit the Xergi (Nature Energy) website here: www.xergi.com

[First published October 2014. Latest update: July 2021]

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