A Perspective On Feedstock

I can provide some useful perspective on “Chasing Cheap Feedstocks” (C&EN, Aug. 12, page 11).

The maps of growth rates for various energy plants can be represented approximately by one number—10 metric tons of dry matter per acre per year. The high heat content of dry matter from all sources is about 15 MJ/kg. One year is 31.6 million seconds, and 1 acre is 4,047 m². So we have 10⁴ kg/acre/year ∂ 1.5 × 10⁷ J/kg ∂ 1 year/(3.16 × 10⁷ seconds) ∂ 1 acre/4,047 m² = 1.17 J/(second m²) = 1.17 W/m². In other words, a square meter can store a little over 1 joule of thermal energy per second (1 watt) on a year-round basis.

Of course, anything done to that fuel—energy cane, switchgrass, etc.—to convert it to something other than a combustible fuel of a very low volume energy density basis will necessarily reduce that figure, and even direct combustion in an electrical-generating plant designed for the purpose would produce only about 0.4 electrical watts per square meter of land.

In all cases, fuel inputs from farming, transportation, and handling operations are not included.

By comparison, a city of 700,000 people uses about a billion watts of electricity on a year-round average basis. So if the dry matter were burned directly in an electrical-generating plant, it would require some 2,500 km² (1,000 sq miles) of such farmland to provide that power.

That’s environmental impact for you!

Howard (Cork) Hayden
Pueblo West, Colo.