Last weekend, I became personally acquainted with biomass. I spent Saturday and Sunday digging up 12-foot by 18-foot section (about 20 square meters, or 0.005 acres) of the lawn in my back yard to prepare the ground for a small vegetable garden.
The lawn was Augusta grass, suited for stifling Texas summers. I have not yet put down new seed. So, at the moment, the grass is fallow, still yellow and dry from winter exposure. In the process of ripping out the grass to define my growing plot, I shook most of the dirt from the roots back into the garden. I then laid the accumulated grass clumps out on the lawn to complete the drying process (to reduce the weight for disposal). This waste material was shoveled into garbage bags. By the end of the garden preparation process, I generated six 33-gallon trash bags full of dead, dried grass similar to this small pile.
Without thinking about it, I set these bags out on the curb, and they were collected as trash. Only later did it occur to me that I had just discarded six large bags of biomass energy. Granted, I’m not sure what I would have done with the stuff if I had thought to keep it. I don’t have a coal-fired power plant in my back yard to co-fire with biomass, nor do I have an anaerobic digester to extract organic digester gas. Perhaps the best use would have been to compost the material to offset the need for petroleum-based fertilizer in the garden. However, at this point I don’t have a compost pile – learning to compost is my next challenge after learning to garden.
Nonetheless, I ran some numbers to estimate how much energy I had just thrown away. According to Sustainable Energy: Choosing Among Options, the text I use in my Alternative Energy class, a dedicated energy crop can yield 1.12 x 108 BTU/acre-year. I harvested about 0.005 acres, and the last time I mowed was November (about 4 months ago). Certainly, Augusta grass is not an energy crop, and its growth was not rapid over the winter months. So, my approximation represents an upper bound that should give me a sense of the order of magnitude of energy discarded.
Using the numbers above, I calculate roughly 1.87 x 105 BTU (1.9 x 108 joules) of biomass energy was discarded. As a point of comparison, the energy content of a barrel of oil is 6.1 x 109 joules, about 32 times the energy content of my biomass-filled trash bags. So, I certainly won’t be powering my house with biomass energy generated from the lawn in my backyard, but I definitely have to come up with a better use for my future grass clippings than throwing them away.