Landfill Manager’s Notebook: It’s a Chemical Reaction…at Your Landfill!

You bet, and an ever-increasing number of landfills are taking advantage of it. It’s a simple process really-and quite natural. Organic material-which is rich in carbon-is decomposing. And in the process, methane and carbon dioxide are created.

Even though this is a natural, commonplace process-methane is perhaps the most common organic compound on the planet-most of it has nothing to do with landfills. What’s happening in your landfill is a small example of what’s happening in forests and fields around the world.

But now, back to the issue of landfill gas. For a variety of reasons, landfill gas-and especially methane-must be controlled.

Second, it is a greenhouse gas-one that has approximately 21 times more impact (as a greenhouse gas) than CO₂.

Third, landfill gas can accumulate in confined spaces and create a dangerous (explosive or no oxygen) situation.

This can be shown by looking at normal air, which is composed of approximately 78% nitrogen (N₂) and 21% oxygen (O₂).These have molecular weights of 28 for the nitrogen (N₂) and 32 for the oxygen (O₂).This is compared with the molecular weights of 16 for the methane (CH₄) and 44 for the carbon dioxide (CO₂).

So generally-over time-methane will tend to rise, while carbon dioxide will tend to settle. But initially, the landfill gas moves as a blend.

A weighted average shows that a typical blend of air has a molecular weight of 28, while landfill gas has a blended molecular weight of 30 or higher. This explains why explosive levels of methane can accumulate in low areas or confined spaces, even though methane itself is lighter than air.

The rate at which decomposition occurs can vary dramatically from one landfill to another, depending upon the type of waste, age of waste and perhaps most importantly-the availability of moisture.

It’s common knowledge that most landfills produce methane-which must be controlled. Typically this means flaring or-much more desirably-converting it to some form of usable energy. However, conventional thinking was that a traditional landfill-gas-to-energy (LFGTE) project required a minimum of 1 million tons of waste in place. This “rule of thumb” limited the LFGTE playing field to midsize or larger landfills.

It should be noted that while a relatively small, low-tonnage landfill might accumulate 1 million tons over many years, peak methane production normally occurs within a few years after trash is landfilled-and older trash just isn’t contributing as much in terms of its potential to create methane.

In practical terms, small landfills just couldn’t make the numbers work for LFGTE.

It’s all about economics. Internal combustion (IC) engines are typically used for projects producing at least 800 kW of power. And turbines generally come into play at 3 MW or larger. These scenarios typically require a waste mass of several million tons (in place).

Small landfills produce too little gas and never come close to approaching these thresholds.

But in recent years, there have been significant advancements in smaller generators-referred to as microturbines. A single microturbine can be functional in LFGTE projects as low as 30 kW. This might be produced by a landfill with fewer than half a million tons of waste (in place).

According to a fact sheet produced by the EPA’s Landfill Methane Outreach Program (LMOP), a 30-kW microturbine could power a 40-horsepower motor or provide electricity for about 20 homes. This is more than enough electrical power to supply a small landfill.

Microturbines can also operate on landfill gas that has a methane concentration too low for other types of systems. How low? Somewhere between 30% and 35% methane.

Does a microturbine make sense in regard to tying into the power grid? Maybe…maybe not, but it can be an economic solution for a remote landfill that has an unreliable electrical power system or has to rely on a traditional generator for onsite power.

Over the years, we’ve worked at many landfills that were effectively off the grid. Now this may sound great for someone who wants to get back to nature and live off the land, but it doesn’t work when you’re running a landfill and need a scale, lights, and a functional office and shop. For most of these outback landfills, the solution has traditionally been a diesel-powered generator. Nothing wrong with that, but at $4 per gallon for diesel, a traditional generator can become very costly. This could spell opportunity for a microturbine system.

If you happen to be operating a small landfill and are wondering about microturbines as an LFGTE solution, you’ll first have to ask some questions, starting with How much methane is your landfill generating?

As (carbon-rich) organics within the landfill decompose, methane and carbon dioxide are created, in roughly a 50:50 ratio-often with the methane content being a bit higher than the carbon dioxide. Chemically, methane (CH₄) consists of 1 carbon atom combined with four hydrogen atoms, and carbon dioxide (CO₂) is 1 carbon atom combined with two oxygen atoms.

But as previously noted, moisture is key to methane production. Here’s why: During decomposition, free oxygen is quickly consumed and the process goes from aerobic to anaerobic. At that point, the only oxygen available for decomposition is water (H₂O). No water means no oxygen…which means no decomposition. You gotta have water.

Under the right conditions, your small landfill could be generating plenty of gas to justify installing a microturbine. The best first step might be to contact LMOP at:

US Environmental Protection Agency
Landfill Methane Outreach Program
Climate Protection Partnership Division
1200 Pennsylvania Ave. NW
Washington, DC 20460-7937
(888)-782-7937