Conversion of Cottonwood and Switchgrass to Charcoal via Carbonization Process

Linda Benedict, Blazier, Michael  |  5/26/2015 11:37:38 PM

Sammy Sadaka, Hal Liechty, Matthew Pelkki and Michael Blazier

Crop biomass can be co-fired with coal to produce energy. Co-firing has the potential to reduce carbon dioxide emissions from coal-fueled plants. Research has demonstrated that when co-firing is conducted with relatively low ratios of biomass to coal, there are significant reductions in both solid waste generation and emissions. However, the nature and chemical composition of raw biomass can lead to significant increases in infrastructure costs or reactor problems if co-firing is conducted with high ratios of biomass to coal. Moreover, high biomass-tocoal ratios may increase reactor corrosion and decrease efficiency. Consequently, co-firing using high proportions of raw biomass is a challenging process for heat and power production systems.

Ameliorating raw biomass physiochemical properties and concentrating its energy density could increase acceptance of biomass for co-firing operations. Carbonization is a promising thermochemical process that can produce biomass with properties comparable to coal and make biomass feedstocks more favorable for co-firing. Carbonization, which takes place in the absence of oxygen at temperatures of 750-930 degrees F, converts raw biomass into charcoal-like feedstock. During the carbonization process, biomass chemical bonds break down, producing a charcoal-like material in addition to combustible gases and tar. Carbonization breaks down the complex substances in biomass into elemental carbon and chemical components.

Cottonwood and switchgrass from LSU AgCenter and University of Arkansas research sites were used for a carbonization experiment. Woody biomass is more suitable than many other energy sources for co-firing because it contains fewer ash and alkali components. Switchgrass can also be co-fired with coal as a cleaner-burning energy alternative to low-grade coal. Biomass samples were carbonized in a carbonization reactor and placed in a muffle furnace at 750 degrees F for two hours. The weight loss of the sample was determined after allowing the sample to cool down. Physical, chemical and thermochemical characteristics of raw and carbonized biomass were measured.

Feedstock volatile matter values were determined by heating the feedstock under controlled conditions and measuring weight loss, excluding the weight of moisture. The initial volatile matter content was 78.0 percent for cottonwood and 73.9 percent for switchgrass. Generally, carbonization significantly reduced the volatile solids content of cottonwood to 25.3 percent and of switchgrass to 25.9 percent. Ash content and fixed-carbon content increased significantly for the cottonwood and switchgrass samples. The ash content values increased from 1.8 percent to 5.8 percent for cottonwood and from 4.9 percent to 10.6 percent for switchgrass. Fixed carbon also showed similar trends, with a change in cottonwood from 20.2 percent to 68.6 percent. Switchgrass fixed carbon also increased from 21.2 percent to 63.4 percent. Carbonization drives off hemicellulose and cellulose from the biomass, leading to an overall reduction of the sample weight. This reduction led to the increases in ash and fixed-carbon contents.

Analyses were also performed on the raw and carbonized cottonwood and switchgrass samples to determine their carbon, hydrogen, oxygen and nitrogen contents. Carbon, hydrogen, oxygen and nitrogen for raw cottonwood and switchgrass were not significantly different from each other. The carbonization process increased carbon and nitrogen concentrations, but it decreased hydrogen and oxygen concentrations. Carbon concen tration increased from 49.7 percent to 74.9 percent for cottonwood and from 47.8 percent to 56.0 percent for switchgrass. The increase in carbon concentration and the decrease in oxygen concentration decreased the oxygen-to-carbon ratio for the carbonized biomass compared with the raw biomass. The oxygen- to-carbon ratio decreased from 0.65 to 0.21 for cottonwood and from 0.62 to 0.47 for switchgrass. During carbonization of feedstock, hemicellulose decomposes significantly, followed by cellulose. This was observed during the carbonization process, resulting in the decline of the oxygen-to-carbon ratio in feedstock. The reduction of the oxygen-to-carbon ratio in the carbonized fuel would enhance its combustion behavior. The values of the oxygen-to-carbon ratio of the carbonized feedstock are approaching that of lignite coal (0.35-0.45).

The average heating values were 7,738 Btu per pound for raw cottonwood and 6,964 Btu per pound for raw switchgrass. Carbonization of cottonwood and switchgrass positively affected the heating values. The heating values increased to 12,381 Btu per pound for carbonized cottonwood and increased to 11,779 Btu per pound for carbonized switchgrass. These increases in energy concentrations may be attributed to the release of the non-combustible vapors and gases from the biomass during the carbonization process. In other words, the heating value of the feedstock increased due to the increased concentration of the combustible components in the produced charcoal. The mechanisms of carbonization process described earlier would provide an explanation of the higher-density energy of carbonized material.

Carbonization was effective at converting cottonwood and switchgrass into charcoal that had some properties similar to coal. As such, carbonized biomass from these feedstocks could be viably co-fired with coal to enhance use of renewable materials in largescale heat and power production systems. Heating values were 1.7 times greater for carbonized biomass relative to raw biomass for both feedstocks. Heating values for cottonwood and switchgrass were similar, but cottonwood had ash contents nearly half those of switchgrass.

This research was funded by Sun Grant and USDA Sustainable Agriculture and Research Education programs until 2013 and currently is supported by the Agriculture and Food Research Initiative of the National Institute of Food and Agriculture.

Sammy Sadaka is an assistant professor and extension engineer in the Department of Biological & Agricultural Engineering, University of Arkansas Division of Agriculture. Hal O. Liechty is George R. Brown Endowed Professor, and Matthew H. Pelkki is a professor and Clippert Chair in the School of Forest Resources, University of Arkansas at Monticello. Michael Blazier is an associate professor at the LSU AgCenter Hill Farm Research Station, Homer.  

(This article was published in the spring 2015 issue of Louisiana Agriculture.)

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