Let There Be Light

Methane from grass clippings has been happening on my farm for the last 9 years. You can see an earlier post of the experimental system. Interest is finally picking up, even though few believe the hot, blue flame on the stove is methane when they see it.

I thought it might be helpful to run some numbers passed the unwashed on this subject. Costs per kilowatt of electricity in Texas are fixin’ to rise. There’s no real reason just now to hike the cost since they’ve been pumping out methyl-mercury, cadmium, lead, and arsenic, to say nothing of CO₂, from our coal fired power plants with impunity for decades in spite of EPA “regulations” and threats and even fines! Well, actually, maybe there is ONE reason. It’s an election year; and hurting the masses is how to make sure the little people understand the “costs” of supporting the wrong politicians! (see Gaza today for a global example of this “civilized” policy! What Geneva conventions?!)

All that to say, I imagine alternative energy is about to become a serious business. Which will, ironically, make the price of a kilowatt go even higher for the poor! I’ll punt here on the equality subject and move on to methane in hopes we can avoid re-inventing the wheel on alternative energy from biomass. Methane from grass has been done! Experimental, table top scale digesters are a dime a dozen.

Here’s what I know, and feel confident printing for the serious methane producer. You’ve got to figure two things—supply and demand. The math for both must be done first, then the technology for how to realize the math follows—not the other way around, guys!

Calculations for demand and how to supply that demand are as follows:

Obviously we have to start with what volume of Methane (CH₄) we can reasonably expect to make given our various resources and labor. Then we must cut demand down as much as possible to either meet the limits of supply or have a surplus--if we’re lucky. We (in this county) cannot sell surplus kilowatts back to our cooperative. Nowhere is it worth it economically (if you care about life-cycle costs) to deliberately plan to make a surplus. It just leads to waste.

Potential supply:

Methane production is a function of the right ratio of feedstock (cow dung or finely cut fertilized grass at around 25:1 C:N ratio) to water and heat on a daily basis. Think a cow stomach and flatulence, and you get the idea.

In East Texas, we have massive amounts of daily sunlight that produce equally massive amounts of fresh, high protein (where the N comes from) grass. This grass, when finely cut, is ideal for methane production. It has about a 25:1 carbon-nitrogen ratio and that is actually better than cow-dung. There is much less work (energy) involved in mowing the grass and vacuuming up the mulched material than collecting dung in buckets, or even in troughs under feeding stations and pumping it to a digester with slurry pumps.

I won’t launch into the “embodied energy” debate over the energy “mass balance” of our grass conversion, but, trust me, it is in my favor to use gasoline to cut the grass to produce methane to produce electricity. It’s almost a free lunch! Far better conversion efficiency than bio-diesel! Wait till I run my mower on compressed methane!

Given our rainfall and sunlight, we can easily collect 100-300 pounds of finely cut grass every day for about 200 days a year. The other165 we will need to have a supply of this same grass made into silage and stored for the digester and a little extra for the cows during winter.

Let’s say we cut 100 pounds of rich green grass each day for the 200 days of growing grass for the digester. How much methane gas (scrubbed of sulfur and carbon dioxide to burn @ 900 btus’ a cubic foot) can we make from 100 pounds of fresh, wet, fertilized Coastal Bermuda?

The rule of thumb in this business (for the 40 years I’ve played around with this gas) is that 2000 pounds (1 ton) of fresh grass can produce 630 cubic meters (that’s meters) of gas over a 30-day period if the temperature stays around 100⁰F. We can convert meters to cubic feet @ a ratio of 35ft³ of methane for every cubic meter produced. Translation: For every ton of grass, we can expect 22,000 ft³ of methane (CH₄) over a 30-day digestion period with stable heat. That’s 733ft³ per day of useful methane. (22,000/30=733)

That means that for every pound of grass we insert in the digester, we can expect about 10ft³ of methane. (22,000/2000). If we cut and deposit 100 pounds of grass a day, we can thus expect 1000 ft³ of CH⁴ in 24 hours time—all other things being proper.

Demand side:

How much gas will we need, and how much grass must we then cut and haul to the digester each day to make the gas we need?

In the gas business, a rule of thumb has been that one cubic meter of methane (900-1000 btus) will power a 1 horse-power engine under load for 2 hours. Put another way, for every horsepower you use, you must supply 17ft³ of scrubbed CH₄ per hour.

Our 16kW propane/NG stand-by generator requires a 24 hp engine, so we must have 275ft³ of methane each hour of run time. Remember, 100 pounds of grass will give us 1000ft³ of CH₄ a day, so we can run our generator each day for 3.5 hours on 100 pounds of grass. That’s enough to power the peak hours demand and charge the batteries. If we got all the gas we theoretically could get, we would produce 58 kilowatts a day. De-rate that by 15% for lots of reasons and you still have more than you need—or should use. Logically you would size the digester and grass collection to meet your actual demand--less than 30 kW a day, I should hope. This desktop design gets you close to that with some surplus for cooking.

Sizing the Digester

We will plan for a digester (cow stomach) that can hold 100 pounds of cut grass (and enough water to make a slurry so it flushes down the inlet pipe like a toilet) for 30 days total volume. We call this digester design a plug-flow digester with a 30-day detention time. 100 pounds of grass will need between 80 to100 gallons of water a day, so the digester must hold about 3000 gallons of slurry to start.

Every day 100 gallons of extremely valuable fertilizer and water will be excreted from the digester to a holding tank as it is “displaced” by the incoming fresh slug of material. This must go back on the grass production area to make the system truly regenerative!

An “intake” mixing basin with a water hose nearby and an easy plug or valve will allow the grass and water to be mixed as the grass is dumped into the mixer at ground level from the lawn vacuum thus requiring no human labor to load the biomass. When the grass is covered with water, the plug is pulled and the entire slurry slug swirls down the inlet pipe with a giant sucking sound into the bowels of the digester. This flush stirs the rest of the rotting mess and keeps the surface from matting over with tiny bits of material that could end up forming a paper-like crust and closing down gas production. The larger the surface area of the digester, the less likely this crust will form—so short and fat is better than deep and skinny for the tanks. The opposite is true for gas holding and pressure tanks.

Stirring beyond the daily intake disturbance is not usually needed, but it can easily be done by mounting a series of 1 inch diameter galvanized pipes with 1/8^th in. holes on the under side so that gas under pressure from the holding tank can be introduced into the digester tank to stir the slurry and break up surface scum. This gas immediately returns to the gasholder from whence it came. A separate gasholder set of tanks is required for large methane systems, but more of that later. The diagram included here shows the basic design of my 3000 gallon system.

The digester needs both an inlet and an outlet of 10- 12 inches in diameter (I use a culvert with rough ridges outside and smooth liner inside) to make flushing easy and quick.

The entire system water level is set by the invert of the outlet pipe, so that must be done carefully. I like my tanks underground and insulated to stay warm and at even temperatures year round. The grass around both tanks looks neat and green, and it is easy to keep it trimmed.

These are some of the bare minimum details and basic calculations to give you the idea that methane in areas of good sunlight and rainfall is certainly a viable way of producing useful gas for cooking and electricity in place of propane or natural gas.

A few diagrams and calculations will follow with another narrative later so you can see how to size the various components of the entire system, which includes safe methane delivery to the engine and a whole lot of other safety stuff.

If your life and landscaping is not purpose driven yet, now it can be--mow to turn grass into methane gas!

Fiat lux!