Published · Vili Power Solutions
Typical electrical loads
Every digester is different, but most have some combination of these loads:
- Mixers. Top-entry, side-entry, or submersible mixers keep the tank contents moving. They are often the largest motors on site and run for long periods.
- Pumps. Feed pumps, transfer pumps, recirculation pumps, and digestate pumps move material between reception pits, tanks, and storage.
- Feed and solids handling. Augers, choppers, conveyors, and separators for incoming feedstock and outgoing solids.
- Heating. Digesters have to stay warm. Heat exchanger circulation pumps, boilers, and their controls keep the process at temperature, often using waste heat from an engine.
- Blowers and gas handling. Gas boosters and blowers, air injection for biological sulfur removal, gas chillers to pull out moisture, and compressors where gas is upgraded or piped off site.
- Flare. The flare burns gas that cannot be used. Its igniter, flame detection, and valves need to work when the engine or upgrading equipment is down.
- Support systems. Ventilation for enclosed buildings, lighting, heat tracing on exposed piping, and the plant's control power.
Many of these motors run on variable frequency drives, which makes drive installation, cooling, and setup a big part of digester electrical work.
Controls and instrumentation
A digester is run by its control system. A PLC typically handles the sequencing of pumps, mixers, and heating, with an operator screen locally and often remote access. Instruments feed it tank levels, temperatures, gas pressure, gas flow, and gas quality.
Gas detection is part of this. Detectors for methane and hydrogen sulfide in enclosed spaces are typically tied to alarms, ventilation, and shutdown logic. Those interlocks are safety functions. They should be tested on a schedule, and nobody should bypass them to keep the plant running.
Good control wiring is labeled, documented, and kept separate from power wiring where the design calls for it. When something goes wrong at night, clear drawings and labels are what let a technician find the problem quickly. See our PLC and controls page for more on panel and control work.
Hazardous (classified) locations
Biogas is mostly methane, which is flammable. Areas around gas-holding membranes, gas piping and valves, blowers, condensate traps, and similar equipment may be hazardous (classified) locations under the National Electrical Code.
The area classification is not decided in the field. It comes from the engineer of record's drawings, which show the boundaries and the class and division (or zone) of each area. NEC Articles 500 and 501 then set the rules for equipment and wiring in those areas: equipment listed for the location, approved wiring methods, conduit seals where required, and specific grounding and bonding requirements.
For owners and operators, the practical points are:
- Keep the area classification drawings current and on site.
- Install equipment exactly as the classification requires. A general-purpose light, switch, or receptacle added later inside a classified area defeats the design.
- Check with the engineer before moving equipment, adding a building, or changing gas piping, because the classification boundaries may change.
- Keep conduit seals, enclosure covers, and fittings intact. A missing cover bolt on an explosion-proof enclosure matters.
Corrosion and moisture
Hydrogen sulfide in biogas is the main corrosion problem. Even small concentrations attack copper and silver, which means terminals, contacts, circuit boards, and connectors corrode much faster than in a normal plant. Add condensation from warm, wet gas and process areas, plus wash-down, and enclosures have a hard life.
Things that help:
- Enclosures rated for the environment, typically stainless steel or nonmetallic in corrosive areas, sealed properly and kept closed
- Corrosion-resistant conduit and fittings, such as PVC-coated steel or nonmetallic conduit where permitted
- Drives, PLCs, and MCCs located in a separate electrical room with clean, conditioned air, away from the process area where possible
- Conduit routed and sealed so gas and moisture do not travel through it into electrical rooms and panels
- Drains and breathers on enclosures where condensation collects
- Regular inspection for green or black corrosion on terminals and contacts
Maintenance and uptime planning
A digester that stops mixing or heating for long enough starts to lose its biology, and restarting it can take a long time. Electrical maintenance planning should start from that fact.
- Know which loads are critical and what happens when each one stops.
- Keep spares for critical drives, starters, and control components, or know exactly where to get them quickly.
- Record drive parameters and PLC programs, and keep backups off the plant computer.
- Schedule thermal imaging of panels and MCCs under load to find heating connections before they fail.
- Consider bypasses or redundancy for loads that cannot be down long.
- Plan shutdowns for electrical work around the process, with material on site before anything is turned off.
Generator and CHP wiring
Many digesters burn biogas in an engine generator or combined heat and power unit. On those projects, the electrical contractor typically handles equipment-level work: the engine's auxiliary motors and controls, jacket water and heat recovery pumps, gas train controls, ventilation, and the wiring between the generator, its switchgear, and the plant. The utility interconnection, including protective relaying and the agreement to run in parallel with the grid, is set by the utility and designed by the engineer of record. We work from those designs and coordinate with both.
When to call
Call if you are planning a digester, adding equipment, dealing with corrosion or recurring failures, or need a contractor who will plan work around the process. We work on digesters across Northern Colorado, many of them on or next to dairies around Greeley. See our anaerobic digester electrical and dairy electrical pages, then contact us or call (970) 685-2068.
