The Vermont Grass Energy Partnership recently released a report titled “Technical Assessment of Grass Pellets as Boiler Fuel in Vermont”. A more fitting title would be “Yet another demonstration that grass biomass can be pelletized and combusted but is a more problematic fuel than wood”. The study does demonstrate the process, but it offers little if any new technical information, and I question why this work was even considered for Federal funding unless its proposed scope was more ambitious than what is represented in the final report. The topics covered in the report have all been addressed, and in much greater detail, by previous research. Indeed, the references cited in the report’s own bibliography, in particular the one by Jenkins, et al., contain far more useful technical information on slagging, corrosion, and emissions than this study. I have included a list of articles below. The length of the list is limited by the blog word count, not by the number of relevant references: there are dozens in addition to those listed and they can easily be found via web search. This is not to say there remains no worthwhile research to be done.
The problematic characteristics of grass fuel remain mostly unaddressed and call for work on innovative solutions. The mechanical problems related to ash fusion and clinkering have been largely dealt with already in multifuel boilers. Fuel leaching by natural or artificial means can also be employed to remove much of the low fusion temperature alkali chloride ash components. This also reduces the corrosive effects of chlorine in the flue gas. Another option is microground grass fuel burned as an explosible air entrained powder. In this case, the ash particles cool before they have a chance to agglomerate. I do not believe that the clinkering issue presents much of a technical challenge, except for residential scale pellet stoves which, for the most part, are not yet designed to have multifuel capability.
The corrosion and emissions issues posed by grass fuels are more difficult. Obviously, reducing the nitrogen, sulfur, and chlorine content of growing grass is the most straightforward approach. Grass type, soil chemistry, and fertilization practices all impact the chemical content of the growing grass and harvesting practices further impact the final chemical composition of the feedstock. I am not an agronomist and am ill informed about ongoing research in this area. I think it’s safe to say that standard best practices have yet to be established. Properly designed multifuel boilers use corrosion resistant materials and can reliably combust grass fuels. As mentioned above, chlorine content can be reduced by leaching and NOx and PM emissions can be reduced to some extent by carefully controlled staged combustion and exhaust gas recirculation. Finally, there are well established methods for flue gas scrubbing that can be employed if necessary. Cost effective small scale flue gas treatment methods need further development work.
I was surprised that the report contained no information regarding important details of grass pelleting, in particular how the process must be modified with grass and grass/wood mixtures. Were different dies used? What other process parameters were modified?
Finally, I think we should be looking at other grass fuel forms aside from pellets. Just as with woody biomass, which is typically burned as chips in non-residential boilers, baled grass is the form of choice for lowest cost local use. Efficiently stacked or containerized bales can have an effective bulk density of 15-20lb/cuft, comparable to wood chips. In my next blog, I will discuss the merits of containerized bulk biomass solutions.
PS – I apologize in advance for my negative reaction the Vermont study. My opinion is just that, and is not intended to impugn other efforts by the members of the Vermont Grass Energy Partnership.
References – available on the internet for free
Combustion and Co-combustion of Biomass: Fundamentals, Technologies, and Primary Measures for
Emission Reduction ,
Aerosols from Biomass Combustion
B.M. Jenkins et. al.
Combustion properties of biomass
E. Bjorkman and B. Stromberg
Release of Chlorine from Biomass at Pyrolysis and Gasification Conditions
Hidden interactions—Trace species governing combustion and emissions
References – available on the internet for a fee
P. A. Jensen et.al.
Transformation and Release to the Gas Phase of Cl, K, and S during Combustion of Annual Biomass,
Pretreatment of straw for power production by pyrolysis and char wash,
Removal of K and Cl by leaching of straw char,
Experimental Investigation of the Transformation and Release to Gas Phase of Potassium and Chlorine during Straw Pyrolysis
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