Three Waste-to-Energy Solutions

What is this stuff we call waste?

We all add to the waste stream every day - probably without thinking much about it. According to Webster, waste is “damaged, defective, or superfluous material,” also considered, “an unwanted by-product,” or refuse, which is defined as “a worthless or useless part of something.” In other words, it is something that was created that no longer has any use for anyone. In a given day an individual's waste stream might include packaging from a new purchase, discarded coffee cups, take-out food containers or used up toothpaste tubes - just to name a few once-valuable items that become waste every day.

So if we have a waste problem, which we do, (Americans generate some 200 million tons of waste per year) it appears that there are two ways to address it. First, stop creating the waste in the first place, or, second, find a way to to turn that waste back into something useful. In other words, turn it into a raw material for some other process.

Janine Benyus, author of Biomimicry, explains that nature merges waste and raw materials - fallen leaves turn into fertilizer for new saplings in the forest, for example. Benyus calls this “waste equals food.” If we are to become sustainable, she suggests we need to follow nature’s lead.

I don't expect we'll ever find ourselves eating municipal waste, but since we also have an energy problem, converting that waste into energy would certainly be the next best thing. Besides, what is food after all, if not a source of energy?

Waste-to-energy is already happening today and there are many technologies to make the process cleaner and more efficient. Let’s have a look.

1) Waste incineration

The simplest approach is to just burn the stuff directly. These basic incinerators are in operation in over 1000 plants around the world, mostly in Europe and Asia. After some presorting, the municipal solid waste (MSW) is dumped into a bunker where it is burned. The heat is used to create steam, which generates electricity, and the exhaust is processed by an extensive air pollution control system (here's a video).

Many waste items contain mixed materials (e.g. metal and plastic), which are difficult to separate and can’t be economically recycled. So the only disposal options for these items are incineration or landfill. At least with incineration, some value is extracted in the form of energy. However, there are questions about the resulting emissions from incineration plants. Greenpeace and other groups have long fought these plants, raising concerns over heavy metals, as well as dioxin and furans, and new pollutants formed during the incineration process. They also give off CO2 and other greenhouse gases that contribute to climate change - so emissions capture systems must be very robust.

Waste incineration also produces a waste product of its own - ash - which must be treated as hazardous waste. However, as the capture technology improves, these risks are being mitigated and managed. In super-green Denmark, incineration plants are very popular and effective - only 4 percent of all trash ends up in landfills, compared with 54 percent in the U.S.

Should waste incineration be considered a renewable energy source? While the generation of MSW seems endless, incineration is first and foremost an attempt to manage the waste problem and reduce landfills, rather than a truly renewable fuel, since it does create some waste of its own. And while these plants produce cheap energy and reduce the volume of waste sent to landfills, both good things, ultimately it would be better for society to produce less trash in the first place.

2) Landfill gas

There are low carbon alternatives to waste incineration. The first is relatively common landfill gas collection. As organic matter in landfills decomposes, it gives off several gases, primarily methane. Methane is the principal component of natural gas. It is also a potent greenhouse gas, twenty times stronger than CO2. When this gas is collected it can be used as a power source, rather than a contribution to global warming.

When methane is burned to generate electricity, the byproduct is CO2. Although CO2 has global warming impacts of its own, the conversion is actually much better than you might think. First of all, CO2 is preferable to methane because of its lower heat-trapping effect. Beyond that, if the volume of trash in the landfill is similar to the volume of food and other products that are being grown above ground (capturing carbon as they grow), the process can essentially be considered carbon neutral.

Some landfill gas projects simply siphon off the methane, others heat the waste in a pyrolysis process giving rise to syngas. This process can spin off useful byproducts and does not produce the hazardous waste associated with incinerator ash. North Carolina’s ReVenture Park is an example of such a project.  One problem facing landfill gas projects right now is that natural gas prices are at historic lows which make the financial return on investment much less attractive than it was a few years back.

3) Waste to ethanol

So far, all of these waste-to-energy concepts have led to electric generation. But there are lots of ways to make electricity renewably. What about liquid fuels to power transportation? Waste can provide an alternative to gasoline, diesel fuel and biofuels.

But wait, couldn’t waste be a biofuel? Well, it turns out that municipal solid waste contains roughly 18 percent food and 40 percent organic matter. So maybe that’s not such a crazy idea. Craig Stuart-Paul, CEO of Fiberight, located in Catonsville, Maryland, certainly doesn’t think so. Several years back they developed a method to separate organic from inorganic waste.  Working with Novozymes, they came up with a way to break down organic pulp from packaging materials such as cardboard into cellulosic ethanol. Considering that cardboard will sit untouched in a landfill for many years without breaking down, the fact that Fiberight can convert it into cellulosic ethanol in just a few days is quite impressive.

Once they receive the municipal waste, they separate the organic from the inorganic waste. Then they remove the recyclables. Finally, they extract the organic pulp from the liquid. The liquid is used to produce compressed natural gas (CNG), which many garbage trucks are beginning to use as fuel. This means the trucks can fuel up when they come to dump the trash. In the future, the CNG will also be used to power the plant itself. But the real novelty is in the final step where the remaining pulp is converted, using their proprietary process, into cellulosic ethanol. All told, 80 to 85 percent of everything that comes in is put to some use and thus avoids the landfill. The process is quite unique. According to Stuart-Paul, "We can take a [disposable] diaper, strip off the plastic, and recover the cellulose and the poo that’s in it," converting the one to ethanol and the other to biogas.

This is already happening in a "pre-commercial reference plant" in Lawrenceville, Virgina, with an annual capacity of one million gallons of ethanol. This process will move to a full-scale plant in Blairstown, Iowa, next year, which, serving 225,000 people living in the three surrounding communities, can produce 6 million gallons of ethanol plus an additional 4.5 million (diesel equivalent) gallons of CNG.

Suddenly, our municipal material recovery facilities (MRF), which used to handle only recyclables, can begin to handle all waste, in what is called a "dirty MRF,"  and our waste stream begins to resemble something that you might expect to find on a forest floor where trash equals food and nothing is wasted.

Images Courtesy of Fiberight

RP Siegel, PE, is an inventor, consultant and author. He co-wrote the eco-thriller Vapor Trails, the first in a series covering the human side of various sustainability issues including energy, food, and water in an exciting and entertaining format. Now available on Kindle.

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