Viability of using 'loose' groundnut shells as biomass feed stock for gasification

This post will be looking at an experiment published in the "International Journal of Renewable Energy Development", volume 4 issue 2.  The article is titled "Gasification of 'Loose' Groundnut Shells in a Throatless Downdraft Gasifier."  In the study, edible nut shells were used in a gasification system to determine the viability of using such feed stock in their natural 'loose' form to create a syngas for heating and for use in internal combustion engines.  Citation and link to article are at the bottom of the article.

Background:
As mentioned in the previous posts, the ongoing effort towards sustainability will take a multi-faceted approach.  Not any one technology will "save" us from the crisis we face.  It is vital that all our resources are utilized fully, and managed in a way that has a minimal environmental impact.  Biomass is a sustainable, cost effective way to use resources, especially agricultural waste.

Biomass in general is one of the most utilized energy resources in the world (third after coal and oil).  About 14% of the world's energy usage is derived from biomass, and in developing countries it is an even more significant proportion(35%).  It is a cost effective way to produce renewable energy, while reducing net greenhouse emissions (as a substitute for fossil fuels).  Unlike other fuels, failure to utilize biomass releases greenhouse gasses, and contaminates groundwater aquifers (upon their natural decay).

Groundnut shells (such as peanut shells) are a woody fibrous material which is generally discarded after the nuts are separated.  They are commonly dumped on the sides of roads, or burned to reduce waste and make management easier.  Kenya (where this study was carried out) is the fourth largest worldwide producer of groundnuts, and given the rural and economic nature of the country, consolidating agricultural waste for use in large industrial systems is not feasible.

Gasification is a thermo-chemical process by which a carbon based fuel is processed into a gaseous form.  Using high temperatures, and limiting oxidization of the reaction, the material is mostly unburned, and it is transformed into a combustible gas, known as a "syngas" or "producer gas". There are multiple steps by which the process occurs, however, the specifics of the process are worthy of their own post, and not vital to understanding the study.  By using gasification instead of direct combustion, higher thermal efficiency can be achieved, smoke can be attenuated, and the gas can be used in internal combustion engines.  The resulting gasses produced can be much more easilly "scrubbed" of contaminants pre-combustion making the process much cleaner than direct combustion.

  There are many types of gasifiers (updraft, downdraft, fluidized bed etc.) with the main variations being how the gasifier is heated, where the fuel and oxidizers are fed, as well as how the fuel interacts with the flow of oxidizer. The study we will look into used a "throatless downdraft gasifier" for a few reasons.  Although throated gasifiers are able to more efficiently generate syngas, using loose biomass (such as groundnut shells) introduces a large concentration of undesirable volatiles (tar) into the final product.  These gases precipitate after combustion and form a "gunk" that makes them unsuitable for internal combustion applications. For this reason, a downdraft gasifier was used because it is known to minimize these contaminants.

Why is this important?
This study was carried out in Nigeria which is the fourth largest groundnut producer in the world.  Utilizing this resource effectively will help reduce needs for fossil fuels, increase agricultural value, as well as reducing pollution by smoke and greenhouse gas emissions.

Throated downdraft gasifiers (which most of the research has been done with) have flow problems with loose material (such as groundnut shells).  These issues affect the quality of the syngas negatively, thus loose material is generally pelletized to maintain uniformity within the gasification chamber.  This study utillized a throatless downdraft gasifier to determine the quality and quantity of the syngas produced from such a system, thus giving insight into larger scale usage.

The Experiment

What they did
A small 5 kW laboratory sized throated downdraft gasifier was used.  A gas cleanup unit (comprised of a series of filters) was placed downstream of the gasifier, and upstream of a sample port.  The producer gas was flared (burned) when it was not being sampled.  Air was used as the oxidizer; it was inputted by use of an air compressor.  Flow was controlled using a valve, and measured using a rotameter (flowmeter).  Thermocouples were used to measure temperatures in the pyrolysis zone as well as oxidation zone.

The gasifier was started using charcoal,oil and paper to start the combustion in the oxidation zone.  Once the zone was at sufficient temperature, and excreted gasses were opaque, shells were loaded into the gasifier.  Unlike commercial gasifiers which use a continuous loading of fuel, this setup was run in batches.

What they measured
Using a gas chromatagraph, the N₂, CO, CO₂, H₂ and CH₄ composition of the syngas was measured.  The performance of the gasifier measured using independent variables of gas flow rate, air to fuel ratio, equivalence ratio and groundnut shell consumption rate.

What they found

• Content of gas was very similar to gas produced from wood.
• Ash content was low, making the gasifier well suited for continuous use with minimal cleaning.
• Gas produced had low enough volatile content to be used directly in an internal combustion engine.
• Low density of the shells was not too low to be used well in the gasifier setup.
• Producer gas was on the high end of documented values for the downdraft gasifier systems.
• The ideal air flow in this experiment was 0.0071 m³s^-1  through the reactor.  Above and below that flow rate two things occurred.  First, the composition of combustible to gas (CO, H₂, CH₄) to non-combustible gas(CO₂, N₂)was lower.  The other effect of increasing or decreasing the flowrate past 0.00071 m³s^-1 was a decrease in the heating value of the gas produced.  So, not only was there less combustible gas produced, the gas produced was less effective.

Thoughts

There were a few things that struck me from this article besides the grammatical errors, ambiguous references to gas flow rate (weather produced or oxidizer) and references to incorrect figure numbers.

The ability to use a less dense material could have helped optimize conditions so that a very large portion of the shells was transformed into combustible gas. This could be part of the reason the ash content was noted to be so low, and the heating values were on the high end of the values that are seen for this type of system.  It seems plausible this type of gasification could be used well for similar loose, low density biomass material (which is not ideal in other style gasifiers) and would not be limited to groundnut shells.  

Regardless of environmental concerns, it would seem this type of utilization of waste biomass would be particularly useful for less developed oil importing countries.  Utilizing waste products effectively would cut down on import spending, as well as putting more money into the local economy would seems like a win-win.  Since these products would otherwise be burned without being utilized to create fuels or discarded on the side of a road or burried, there is a carbon neutral effect.   By using the shells to produce energy through gasification, the greenhouse gasses that would otherwise be released from the natural decay can be avoided, excess smoke is mitigated, while simultaneously producing energy as well as increasing agricultural value of the product.

It makes a lot of sense these professors carried out this study given the production rate of groundnuts in Kenya, and the rural nature.  It was noted that commonly the shells are discarded on the sides of the roads so it would be common for people on the roads to see them as waste.  Since agricultural "waste" would commonly be seen as such, publicizing this locally would be a great way to help bring the ideas of biomass conversion to a place where it is far from mainstream.  The environment as well as the economy would see positive effects.

What do you think?

Please mention any thoughts you have relating to this article or biomass gasification in general!


Citation
Kuhe, A and Aliyu, S.J. (2015) Gasification of 'Loose' Groundnut Shells in a Throatless Downdraft Gasifier. Journal of Renewable Energy Development, 4(2), 125-130.
http://dx.doi.org/10.14710/ijred.4.2.125-130