Composting is an aerobic process that relies on microbial bio-oxidation, combining complex organic molecules with oxygen to form CO2, water and heat. The first step in designing an aeration system requires that we understand how much total oxygen is needed. The reaction below shows the simplified process.
Some systems, such as unaerated windrows, use passive infiltration to provide this oxygen via semi-anoxic conditions. However, this approach delivers minimal oxygen to the material, severely limiting the rate at which bio-oxidation can occur. Much like a fire, blowing more air on the compost reaction can increase the reaction rate if there is sufficient fuel. We have commonly seen forced aeration systems achieve in 10-15 days stability improvements (measured in change in Solvita or CO2 respiration) that require 90 days or more in an unaerated system.
Aerobic compost aeration design must consider both metabolic air (how much air is required by the biology) and cooling air (how much air is needed to prevent a pile overheating). While this varies by application, the cooling air requirements almost always exceed the metabolic requirements and drive the design. We see this in peer reviewed journals, field measurements, and our in-house aeration demand test system. The CO2 PPM curve below is typical for composting organic feedstocks, where CO2 (an indicator biological activity) shows a tremendous initial spike as microbes consume the readily available goo. This spike in exothermic activity heats up the pile. As organic matter is converted to CO2, the reaction rate begins to slow.
The total airflow required is a function of the O2 demand over the total retention time of the compost process. Therefore, the total energy (kWh) is a function of this total air required, and how efficiently the system delivers that air. The delivery efficiency can vary based on a design selections.
In our experience, this tends to be ~3-6 kWh per ton processed with some caveats discussed below.
Centralized and dedicated fan designs offer different advantages. Below are some of the key considerations:
Compost systems with distributed fans use a single large fan and distribution duct to send air to multiple zones in a system. Commonly, automated control systems modulate airflow using individual zone dampers based on a temperature feedback control sequence.
Benefits:
Challenges:
Compost systems with dedicated fans use individual fans for each compost zone. While these are often timer controlled, they can also be integrated with an automated control system to control VFD speed or Timer based on zone temperature.
Benefits:
Challenges:
For a fixed rate of delivered air, the fan efficiency and system loss determine any energy consumption differences. So what are the differences between positive and negative applications?
Compost systems require pressure to overcome distribution loss (friction in duct, corners, aeration pipe surface) and the static pressure of the compost pile material (a function of porosity, density, and velocity). For both positive and negative applications, the pile + distribution systems are mostly similar. However, the negative fan has additional ductwork and biofilter media to overcome, which requires a slightly higher total pressure than a positive only system.
While moving warm saturated air requires a stainless rather than mild steel to avoid corrosion, the impact on mass flow rate is small since their is minimal difference in density. For example:
70°F at 50% RH => 0.074 lb/CF
120°F at 100% RH => 0.065 lb/CF
Although the fan efficiency will be slightly different between a positive vs negative application, this is very low relative to the effect of pressure.
Overall, systems with negative aeration tend to require 20-30% more energy to move the equivalent volume of air as a positive system.
Since the composting microbes will require varying rates of air over the process retention time, we recommend matching the fan output to this biological demand. This can be done through cycling the air on a duty cycle (either with a damper, or on/off control of a fan), or by modulating airflow (either using a modulating damper or fan VFD).
Two common factors:
Maintaining consistent process conditions (and oxygen levels) helps the biological process. In general, continuous duty aeration or very short duty cycles, on the order of minutes rather than hours, help to maintain favorable conditions.
Fan efficiencies tend to decrease as they approach zero flow. While modern VFD’s help minimize these system efficiency losses fan performance tends to drop off rapidly below 20% speed. This can tend to favor duty cycles for applications that would result lower fan speed.
Overall, fans play a critical role in forced aeration composting. Perhaps unsurprisingly, selecting a fan configuration depends upon a projects specific goals. At ECS, we find that most applications achieve best efficiencies and project economics by using distributed fans and automated controls. However, there are some large scale systems with very short retention times where the dedicated fan approach offers greater overall value.
We’d love to hear from you and discuss what fan configuration is right for your project!
