Biogas digesters: the critical role for regenerative blowers 

The simple, compact, and non-contacting design of regenerative blowers provides significant advantages in safety, performance, low maintenance, longevity, and noise reduction for biogas applications, compared to positive displacement rotary lobe blowers and other blower technologies. Neil Lynch reports

Regenerative blowers, a well-established technology commonly used for handling corrosive and highly combustible gases in applications such as landfills and chemical processing, are gaining wider adoption in the expanding biogas digester sector. 

The simple, compact, and non-contacting design of regenerative blowers provides significant advantages in safety, performance, low maintenance, longevity, and noise reduction for biogas applications, compared to positive displacement rotary lobe blowers and other blower technologies.

Biogas is becoming increasingly prevalent on farms. By leveraging biogas digesters, agricultural businesses can achieve considerable cost savings, enhance environmental stewardship, and unlock new revenue streams from renewable energy and byproducts. Through anaerobic digestion, farms transform livestock manure and organic waste into biogas, a valuable renewable energy source. 

This process significantly curtails methane emissions, aiding in climate change mitigation. Furthermore, digesters produce nutrient-rich digestate that serves as a natural fertiliser, boosting soil health and reducing dependence on chemical alternatives. These systems also enhance odour control and pathogen reduction, thereby improving farm hygiene and fostering better relations with neighbours.

Digesters, which come in the form of covered lagoons or hard-walled tanks, contain and circulate manure or other waste slurry. From these digesters, a blower system transfers biogas to a compressor. This compressor either feeds into a collection pipeline system or directly supplies an RNG (Renewable Natural Gas) plant. In a cluster system, multiple farms or dairies connect to a shared pipeline leading to an RNG plant, where the gas is refined and then injected into a natural gas pipeline or transported via trucks to end users.

The RNG system comprises subsystems for dehydration, H2S removal, CO2 separation/CH4 recovery, and removal of N2 and O2. In some cases, a final compression system is necessary to achieve the required utility pipeline pressure. Blowers, compressors, and heat exchangers are integral within and between these subsystems to maintain the necessary pressures, flows, and temperatures. 

Demands on blowers in biogas applications

When selecting blowers for agricultural biogas systems, several special considerations must be addressed. First and foremost, the blowers should be specifically designed to handle explosive and hazardous gases safely. 

Maintaining a consistent and optimal gas flow is essential for the stable operation of anaerobic digesters, which in turn ensures efficient biogas production and system sustainability. 

Since biogas systems are frequently situated outdoors and in remote areas, the reliability and durability of blowers are of utmost importance. They must withstand corrosive environments and efficiently manage particulates and moisture without incurring blockages or damage. This capability is crucial for minimising maintenance costs and efforts, as well as preventing downtime.

The regenerative blower features a simple yet sophisticated design that imparts its remarkable qualities: safety, consistent and reliable performance, low maintenance, durability, and noise reduction. With fewer components, it is more robust and easier to maintain than other blower designs.

The blower operates through the interaction of two precisely engineered components: an impeller surrounded by an annular (ring-shaped) housing. Typically mounted directly on the motor shaft, the impeller includes numerous outward-facing vanes. The housing is designed with inlet and outlet ports to ensure seamless airflow into and out of the system. 

Operating principle of regenerative blowers 

The rotating impeller generates centrifugal force, creating a vortex of air, which is fundamental to the regenerative principle. The housing guides and channels the air, enhancing the regenerative process. 

The impeller blades passing the inlet port draw air or other gases into the blower. The impeller blades then, by centrifugal action, accelerate the air outward and forward. Here, the regenerative principle takes effect as the annular-shaped housing turns back the air to the base of the following blades, where it is again hurled outward. Each regeneration imparts more pressure to the air. 

As the air reaches the stripper section at the outlet, it encounters the part of the blower where the annulus size is narrowed to more closely fit the sides and tips of the impeller blades. Here, the air is ‘stripped’ from the impeller and redirected out of the blower.

This compact, efficient design enables each rotation to generate pressures or vacuums comparable to those produced by significantly larger multi-stage or positive displacement blowers.

Benefits of non-contacting design

The essential advantage of regenerative blower design lies in its strategic spaces. Not only are the gaps between impeller blades effective at efficiently and consistently generating air pressure, but equally important is the gap between the impeller and its surrounding housing. 

The non-contacting design of the impeller and housing offers several significant advantages for biogas applications:

Safety: By eliminating metal-to-metal contact, the risk of sparking is minimised, which is crucial when handling highly flammable biogas, thus preventing combustion. 

Durability and Longevity: This design enhances resilience against particulates commonly found in biogas, which can cause wear in other blower designs. Such durability is vital for maintaining efficiency and extending the equipment’s lifespan.

Regenerative blowers versus PD rotary lobe blowers

The benefits of the non-contacting design are clear when comparing regenerative blowers to other blower designs used in biogas applications, such as positive displacement rotary lobe blowers.

Rotary lobe blowers utilise two symmetrical, figure-eight-shaped impellers that rotate in fixed relation to each other and in opposite directions within an elongated cylinder (Figure 2). Each time a lobe of an impeller passes the blower inlet, it captures an air volume equivalent to one-fourth of the blower’s displacement. 

This design demands exceptionally tight clearances between the impellers and between the impeller and housing to prevent slippage. These close tolerances pose a risk of sparking and explosion. Since the rotary lobe components are made of steel, they are particularly susceptible to sparking, especially at high rotational speeds. 

Furthermore, narrow clearances present additional challenges for biogas applications. High concentrations of , particulates can cause clogging and abrasion, accelerating wear and tear and impairing performance. To address these issues, most biogas applications require rotary lobe blowers to be equipped with filters, significantly increasing maintenance costs and effort.

Beyond the safety and durability benefits of the non-contacting design, regenerative blowers provide significant operational advantages that make them ideal for biogas applications. These include consistent performance, high efficiency, and reduced noise levels.

Regenerative blowers excel at maintaining steady flow rates despite fluctuating system conditions, a critical feature for anaerobic digesters where stable pressure is essential for optimal microbial activity. In contrast, positive displacement (PD) blowers may suffer from flow variations due to pressure changes, potentially impacting biogas production efficiency.

Regenerative blowers operate efficiently at moderate pressures, making them ideal for many biogas systems. Although PD blowers perform well at high pressures, they tend to be less efficient at the lower pressures typical in biogas applications. This efficiency at the desired pressure range results in reduced energy costs and enhanced operational performance. 

Regenerative blowers are notably quieter than PD blowers. Their reduced noise levels contribute to a more pleasant working environment and minimise noise pollution, operating at 85 decibels for regenerative blowers compared to 100 decibels for typical rotary lobe blowers. This eliminates the need for expensive and bulky acoustic steel sound enclosures specifically designed for noise reduction in PD Blowers.

Virtually maintenance-free

Regenerative blowers are engineered to be virtually maintenance-free, significantly reducing the total cost of ownership. Without the need for oil or filter replacements, these blowers streamline operations. Their minimal maintenance demands ensure a longer lifespan and uninterrupted performance. 

This low-maintenance, hassle-free operation is especially beneficial for dispersed biogas digesters on farms and dairies spread over large areas, where frequent servicing can be both challenging and expensive. Reliable equipment ensures consistent energy production, thereby supporting sustainable farming practices.

Regenerative blowers are particularly well-suited to the demands of biogas applications due to their ability to safely provide high airflow rates at a relatively low noise level. Their oil-free operation minimises contamination risks, essential for maintaining sustainable biogas production. These blowers offer durability and resilience against harsh environments, which is vital for outdoor installations. Additionally, their capability to handle varying air and gas mixtures efficiently ensures consistent performance. This makes regenerative blowers an ideal choice for supporting the rigorous demands of biogas systems, contributing to reduced maintenance and increased operational reliability.

Neil Lynch, Senior Applications Engineer for Bison, An AMETEK Business.