Energy-Tech Magazine

Bio-oil is often compared to crude oil, but it is instead better thought of as liquid biomass from the perspective of its elemental composition. Bio-oil has many advantages when compared to raw biomass. This energy dense liquid simplifies transportation and processing. When compared to raw biomass it is relatively homogeneous, making for easier upgrading. Although bio-oil is an ideal product when compared to biomass, crude oil bests bio-oil in terms of energy content and ease of upgrading. Bio-oil is comprised of hundreds of oxygenated compounds, which makes the upgrading processing difficult for refiners. Because of this, the traditional end use of bio-oil is combustion in a boiler for steam and power production. However, the high water content and acidic nature of bio-oil make it a less desirable boiler fuel when compared to coal or heavy fuel oil.  

The less desirable characteristics of bio-oil for combustion could be eliminated, according to new research at Iowa State. Researchers at the Center for Sustainable Environmental Technologies (CSET) are working on a product

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that overcomes the disadvantages of firing either bio-oil or biomass. Known as “co-fire pellets,” these energy dense capsules are made by mixing bio-oil and coal together and compressing the resulting mixture to form a pellet. The coal forms the bulk of the pellet, while the bio-oil acts as a binder, occupying up to 35 percent of the pellet’s mass. The bio-oil binder can be either the heavy, viscous ends of conventional bio-oil, or a fraction of bio-oil called “clean phenolic oligomers,” or CPO. Developed at Iowa State, CPO is produced by a simple water washing process in which bio-oil is mixed with water in order to recover the sugars present in bio-oil. The resulting “sugar water” is decanted from the mixture, leaving behind a fraction of bio-oil known as CPO. The advantage of using CPO as a binder is that the high value sugars present in bio-oil are recovered separately from the CPO and are then upgraded to fuels or commodity chemicals.

Co-fire pellets exhibit many advantages when compared to traditional biomass co-firing. The remainder of this article will explore these advantages in detail. As mentioned previously, woody biomass usually contains 2/3 the heating value of bituminous coal. Co-fire pellets have approximately the same or greater heating value than the coal used in the pellets. The high heating value of the co-fire pellets allows power plants running at full nameplate capacity to co-fire the pellets without a reduction in the electrical output of the power plant.  

During fast pyrolysis, most of the alkali and alkaline earth metals present in the raw biomass are removed and collected with the biochar. Therefore, the resulting bio-oil contains little to no alkali metals. The co-fire pellets that utilize the bio-oil as a binder also contain very low amounts of alkali metals, thereby eliminating the possibility of boiler corrosion and fouling or catalyst deactivation.

Another implication of the low amount of alkali metals is that a wide range of biomasses can be used in the production of the co-fire pellets. Traditionally, only perennial woody biomasses are used for co-firing due to the higher heating value and low amounts of alkali metals present in woody biomass. Annual herbaceous crop wastes such as straw and corn stover are rarely considered for co-firing because they contain high amounts of alkali metals.  Co-fire pellets utilize herbaceous crop waste while leaving the alkali metals in the biochar, contributing to its fertilizer value. Power plants can utilize a wide range of biomasses through co-fire pellets: perennial or annual grasses or woody biomass.  

Like raw biomass, the co-fire pellets have low amounts of elemental sulfur, nitrogen and mercury, which translate into reduced SO_X, NO_X and heavy metal emissions during co-firing.