Biomethane as an energy carrier

 

Even though most of our natural gas is now fossil fuel, a doubling of efficiency would be just as effective as achieving 50% renewable power as far as global warming is concerned. We can simultaneously work on greening our gas supply by feeding more and more biomethane into the pipeline.  In Germany 22 billion kWh of biogas were produced in 2007. That’s a six-fold increase from 1999, driven partly by feed-in tariffs. About half of that biomethane was from landfill and sewage gas and the other half was from commercial and agricultural biomass plants. Renewable biogas is produced by natural processes of anaerobic digestion or gasification then cleaned up for sale to the gas pipeline. Sweden already gets 25% of their energy from biogas.

Energy storage is another big advantage of gas. Both the gas and the electricity grids need energy storage to take up the slack between production and consumption. Gas storage is cheap because it can simply be pumped into depleted gas wells and salt caverns. We are already storing 4.1 Tcf of gas in the US. At 85% efficiency that gas could produce 1,180 gigawatt-hours of useful power on demand. A very cheap battery!  The smart electrical grid is all about making supply match demand because electrical storage is so expensive.

Though the U.S. power grid uses significant hydro power and other renewables, CO2 emissions are still almost twice as much per kilowatt-hour as a 60% efficient natural gas fuel cell. In 2007 the U.S. power grid emitted 605 grams/kWh. The fuel cell emits only 340 grams. EIA data makes it easy to track the effects of our attempts to green the electric grid: In 1996 we emitted 627 grams of CO2 per kWh and by 2007 this was reduced to 605 grams. That’s a 2-gram per year decrease. If we continue at that rate, it will take 139 years to equal what we can do now with a fuel cell. Recent years show even less progress. There was no improvement between 2006 and 2007. Plugging into the grid is, unfortunately, a bit like plugging into a lump of coal.

People have already begun selling renewable gas into the pipeline.  Landfills, manure piles and sewage plants that used to release significant amounts of methane into the atmosphere are now selling it as green gas. Biomass and garbage can also be gasified to add to the supply. The energy balance of Grass Biomethane production is 50% better than annual crops now used. When biogas is captured instead of releasing it to the atmosphere we get a double bonus. Methane is 72 times worse than CO2 as a cause of global warming in a 20-year time frame. You may have heard 25 times, but that’s based on a 100-year time frame. Methane only persists about 8 years. Also, when manure piles are covered, N²O, which is 289 times worse than CO², can also be captured. Coal mines emit almost a trillion cubic feet of methane into the atmosphere every year.

In Cincinnati, Ohio, the 230-acre Rumpke landfill has been capped and the gas is cleaned and delivered to the pipeline to provide enough gas for 25,000 Duke Energy customers. China has an estimated 31 million biogas digesters mostly on small farms. They produce in total about 9 Gigawatts of renewable energy which is mostly used locally. Germany, Denmark, Sweden, Finland and now Ontario, Canada have feed-in tarrifs to encourage production of biogas. In Germany small farms can receive up to 25 cents per kWh for biopower. In the US, bills like SB306 that support biogas production, are still stuck in committee.

Increased system efficiency means we will need less of these renewable sources to do the job. If we’re going to gasify biomass, it is more efficient to upgrade the gas and send it through the gas grid to customer CHP units than to generate electricity less efficiently and send it over less efficient, more expensive power lines to the customer. Until we get more efficient electrical generators, generation should always be done where the waste heat can be put to good use.

Electric cars would be twice as efficient if they fueled up with natural gas and used a fuel cell to recharge a small battery. Like a hybrid with a natural gas fuel cell range extender. The expense and weight of a large battery is eliminated and the energy can be stored in a much lighter and cheaper tank. Refuelling can be much faster and could even be done at home from your natural gas connection. New, low pressure, adsorption tanks make this easy because they only require 500 psi of pressure. Recharging is a problem with batteries.  A 110v, 20A household plug can only supply 2.2 kW, which means that 10 hours of home charging will only take you 10 x 2.2 x 4 mi/kW = 88 miles. Natural gas refueling infrastructure is in place in much of the world to refuel five million vehicles worldwide.

We already have prototype hydrogen cars that work on a similar principle but hydrogen has virtually no refueling infrastructure. Hydrogen is very expensive to produce, store and transport. Its tiny molecules find the smallest leaks and fly into space. They embrittle pipeline metals by nestling into the metal matrix. Storage is extremely inefficient, requiring extremely high pressure tanks or cryonic vessels. One giant hydrogen delivery truck can service about ten customers.  Methane has one carbon atom that holds four hydrogen atoms in a tight formation making containment and dense storage easy. A gallon of liquid methane actually holds 2.5 times as much hydrogen as a gallon of liquid hydrogen!

“No carbon emissions” sounded like a great idea but 95% of our hydrogen is made from natural gas and that process emits about 30% more CO² than if we simply burned the methane. Yes, you can make hydrogen from water with electricity (at about 70% efficiency.) But you can also make carbon-negative methane from CO² and hydrogen. When you burn it, the net result is carbon neutral. The “carbon-free” cleanness of hydrogen is an illusion. Building a hydrogen infrastructure now would be folly. Biomethane can do the job now and will be cleaner and cheaper.

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