Gasification of Energy Crops: What it is, its issues, and its potential

The demand for energy in this country is enormous, and fossil fuel prices are high and volatile. Industries in the region are looking to land-based bioproducts as replacement feedstocks for coal, gas, and petroleum. The race for the perfect biorefinery is on, and the papers are full of reports that promise "real soon now" we'll be making substantial quantities of everything from biofuels to pharmaceuticals out of lignocellulosic materials. In the meantime, there are many questions about building the supply chain to get huge quantities of biomass to those dream refineries. Getting the supply chain jump-started requires early markets for the materials, and there are three "bridge markets" in our region that have potential: fuel pellets for residential and small commercial furnaces, co-firing coal boilers with some proportion of biomass, and gasifiers. The third technology, gasification, turns dry, solid biomass into a fuel gas that can be used in a boiler or in an internal combustion engine.

Warming up the demonstration gasifier

We are displaying a small-scale gasifier for Agronomy Day to demonstrate conversion of feedstock into a combustible gas. The gasifier we're using is a "partial oxidation" type that takes in a limited amount of air as the oxidizer. The output of the gasifier, if it's designed and operated right, is a fairly clean combustible gas that can be fed directly into a steam boiler or engine. (This gas is not to be confused with the "synthesis gas" or "syngas" produced in a chemical refinery as a feedstock for making other products.) Multiple zones within the gasifier do different things to the feedstock—drying, pyrolysis, reduction— and the gas mixture that exits the partial oxidation gasifier contains particulates and tars that have to be filtered out before the gas is finally burned to produce heat. The gasifier product, referred to by several names, such as "producer gas" and "town gas" (harking back to gasification in early-20th-century city gasworks converting coal or wood), contains carbon monoxide, hydrogen, a little methane, nitrogen gas, water vapor, carbon dioxide, and some level of undesirable elements such as chlorine and potassium, depending on the feedstock. Its energy content is variable and always less than pipeline natural gas—typically 20% or less of natural gas energy content. Feedstocks such as wood or energy crops need to be processed before gasification, usually by chipping or grinding, to allow the material to feed through the system and also to reduce the particle size for efficient conversion.

Chiptec gasifier

Why is a gasifier better than a directcombustion boiler for generating steam? Gasification is attractive because it separates the initial combustion steps from the boiler and thus gives the process designer a chance to clean up the fuel stream before it contacts the boiler itself. Air emissions from the boiler stack and system maintenance are important considerations for gasifier design and fuel gas cleanup. Regulators consider the size of the installation when considering whether a gasifier needs an air permit and, if so, which level of emissions control is appropriate.

The potential for gasification in the Midwest is mainly for large-scale heat, rather than direct combustion in engines. Many public buildings, small school campuses, and small towns could be served by gasifiers for their heating needs. Medium-scale gasifiers in the range of 2 to 125 MBTU/hr (million BTU per hour) are available in the U.S. If we assume a gasifier-to-useable-heat conversion rate of 0.1 dry tons of feedstock per million BTU (0.3 kg/kWh), biomass at $75/dry ton delivered would then convert at $7.50/MBTU. Compare this to burning natural gas at an average 2010 price of $8/MBTU, and gasification of locally grown feedstocks begins to look attractive.

Ted Funk

Ted Funk
Agricultural Engineering Ext. Specialist