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McCain Foods’ Ballarat manufacturing facility have started work on a massive 8.2 megawatt renewable system for its Ballarat facility that consists of a ground mounted solar array, solar car park shading and a cogeneration anaerobic digester that turns food scraps into biogas to generate energy.

Cogeneration, or combined heat and power (CHP), is valuable for food processing because it generates electricity and heat simultaneously, recovering a high amount of useable energy from the electrical and thermal (heat) energy.

Combined, the solar and CHP system will reduce the facility’s energy consumption by 39 per cent, with the CHP system also reducing the site’s reliance on natural gas by 16 per cent.

What is biogas?

Biogas is a predominately a mix of methane (CH₄), the main constituent of natural gas) and carbon dioxide (CO₂), which is produced in anaerobic digestors when specialised bacteria consume organic matter in the absence of oxygen.

In McCain Food’s case, the biogas is generated from two sources of waste, from a covered anaerobic lagoon (or CAL) which treats the wastewater, as well as solid potato waste from the new purpose built anaerobic digester. This biogas can be turned into process heat using a biogas fuelled boiler, or converted to power through a generator such as an engine or turbine. As heat is a by-product of combustion, heat recovery systems can be fitted to power generation equipment and converted to hot water or steam. In the case of turbines, the clean, oxygen rich exhaust can be also used as combustion air for hot water or steam boilers, maximising the total efficiency of the total system.

Both the quantity and quality of biogas can vary due to variation in ambient temperatures, changes in volume and/or mix of food waste and other factors. Where the quality of the biogas (i.e., the amount of methane) falls outside the fuel requirements for the generator, the generator will shutoff. In addition, many generators require to operate at a minimum load, which can be as high as 75% of the maximum power output of the generator. This means that, if the volume of biogas available falls below the gas required to operate at this minimum load, or the site electrical load falls below the minimum generator load, then the generator cannot operate.

As biogas cannot usually be stored for extended periods, these instances will result in the biogas being flared, reducing the financial return to the customer.

Power generation technology

To maximise both the amount of energy recovered from the biogas, and increase the opportunity to utilise the biogas through changes in gas production quality and quantity, McCain’s selected a modular Capstone Microturbine system for this project. The system being delivered is based on two (2) x Capstone C600S packages. Each package includes three (3) fully independent 200 kW Microturbines.

At a combined 1,200 kW of power generation, the combined packages are sized to utilise the maximum biogas production available. Capstone Microturbines can tolerate a large variation in biogas composition, making them an ideal choice for the changes in gas quality which occur with anaerobic digestion. In addition, each turbine can turn down (i.e., part load) or turn off based in response to gas volumes or power demand. This system can produce between 100 kW and 1,200 kW. Each of the 6 x 200 kW modules can be serviced while the remaining units continue to operate, providing power continuity at all times.

Heat recovery technology

Unlike other combustion based power generation systems, Capstone’s Microturbine technology utilises air bearing technology, which completely avoids the need for cooling and lubrication systems. For engine based power generation systems, the cooling and lubrication systems are sources of low grade heat, and the exhaust a source of hi grade heat. Converting all of the sources of heat is complex, as well as limiting the total amount of useful heat energy which can be recovered and used as process heat energy. Low grade heat sources are often not useful to food processing facilities which often required steam or high temperature hot water.

For McCain’s, the solution was developed to maximise the total energy efficiency of the biogas. To achieve this, Optimal developed a heat recovery solution using Saacke‘s range of cofired burner technologies. Normally, a hot water or steam boiler takes ambient air in to the burner, adds and ignites fuel to lift the temperature of this air to the required temperature to produce steam or hot water. Saacke’s burner technology allows the burner to use both turbine exhaust and fresh air as combustion air. As the turbine exhaust is much hotter than ambient air (>280°C), the burner needs to add less fuel (natural gas in this case) to produce the same amount of steam. With 18% oxygen, the turbine exhaust can be used directly for combustion, or mixed with fresh air where not all turbines are operating.

In the case of Capstone Microturbines, there is no cooling or lubrication circuits, so almost of the energy not converted to electricity is available as exhaust heat. Using the exhaust heat directly as combustion air harnesses 100% of the available thermal energy, which results in total system fuel efficiency above 90%.

The high efficiency CHP installation reduces the CO₂ emissions to an equal level as the 7 MW of PV being developed, demonstrating the unique potential of biogas to reduces emissions for the food industry.