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Technical Overview

Our research to date has demonstrated the ability to produce hydrogen from biomass under stable however limited number of hours.  A future of large scale renewable hydrogen production using non-oxidative technologies, generates a high percentage of co-products in the form of a solid sequestered carbon. The need was to add value to this material in order to justify large scale handling and usage. The goal was the development of a sequestering material that could increase crop yields, soil carbon content, water holding capacity, nutrient retention, cation exchange capacity and microbial activity while decreasing farm chemical runoff, nutrient leaching, and greenhouse gas emissions.  The the advantages of an adsorbent charcoal provided many of the characteristics we sought and creating a material that farmers could rely on to slowly release imbedded nutrients continuously to the crops or forest during the growing season was one of our first development goals.    

Adding nutrients to soils does not mean that the become available for plant growth, (Schleppia et.al) Nutrients can be leached from the soil, they can bind with clay materials reducing availability, or escape through atmospheric interactions.  The purpose here is to review our investigation of ways to reduce the effects of leach losses and increase microbial activity through modifying the nutrient carrier.  It appeared that charcoal was providing significant benefits (History and origin of Terra Preta soils and future perspectives

We first began our investigation by looking at how charcoal may leach out a nutrient.  We had made a number of types of char during our 100 hour run.  Our goal in the run was to produce hydrogen with a co-product.  The co-product char was highly dependant on processing conditions.  As you can see from the chart, in our start up phase we had significant variations in operation conditions.

The changes in gas flows, feed rates and heat rates eventually smoothed out to stable run conditions as we tweaked process parameters.  However, these changes in  process gave us an opportunity to examine the materials that were being made. 

 

After the run, we measured the density of each material stored in the sealed 55gallon drums. Each barrel had been labeled with a date and time so that we could match it up with the corresponding production data.    The first physical measuring of all the barrels gave us 3 distinct materials.  Most of the char was a low density, material produced during the long stabilized run conditions for the hydrogen experiment.  The high density, represented only a small portion of the total and the high variations in process conditions during that time made it difficult to pinpoint any specific set of parameters that produced that material.  

At this point we decided to see if there were any other attributes other than just density that made these three materials different.  We ground a up a 40 grams of each material and sieved it to 30 mesh, making a small grainy powder and then added two grams to 50ml water.  In both the high and medium density chars, the powders immediately sank to the bottom of the flask.  The high density floated and had to be stirred vigorously before it sank.  It appeared that the that the open structures of the higher temperature char had no resistance to water at all.  

We decided at that point to run a lab bench scale experiment to reproduce these materials under precise controls so that we could accurately determine what temperature of the process created the materials and what its effect would be on the performance of the material as a nutrient carrier. An important consideration to be considered was the energy source that could facilitate microbial involvement, but that was to come from an unexpected source.  The answer would come from an abstract reference to research on the microbial consumption of volatile fatty acids (by the USGS) and support our decision to continue exploration of lower temperature process conditions.

We produced 5 different chars, one a 900C, 600C, 500C, 450C and 400C.  Normally a nitrogen purge is used but in past experiments we have discovered little difference in chars made with a nitrogen purge and those using a much simpler system.  A metal can pint size paint style can which had been heated several times to 800C to remove an volatiles, has a 1/4 hole drilled in the top.  A 1/4" stainless tube fits snuggly in the hole.  The biomass (peanut hull pellets) are weighed and placed in the can and the top is sealed.  The can is then placed in a heated furnace at the appropriate temperature.   An external thermocouple is inserted into the furnace and operated a separate controller to give precise control of the temperature of the can in the furnace.  Within a few minutes after placing the can in the preheated furnace, the pyrolysis vapors begin to escape.  At 10 min intervals, a small 1/16 thermocouple is inserted through the 1/4 exhaust tube an a temperature of the material is taken directly.  After several experiments we were able to gage that until the high volatile gas evolution slowed, we could never get readings above 350 in the material.  So we changed our method and began taking internal sample temperatures after the gas slows had slowed to minimal amounts which would generally be about 370-380.  Once we were within 50 degrees, the thermocouple would be left in the sample. In each case we would bring the samples up to the target temperature for 1 minute.  

We would remove the can from the furnace and turn it upside down on a smooth surface metal  table .  We have found that the material still evolves some CO2 and with the small hole, no oxygen can get to it until it has cooled to a point where it will no longer oxidize.  We produce all our samples in this way. Next we ground the and sieved the materials and weighed out a 20 gram sample.  We prepared a solution of 40% NH4NO3 (ammonium nitrate).  We had measured the pH of each sample in water and discovered that by soaking the chars in the ammonium nitrate solution it would give us a substantial increase in pH of the char. Then by measuring the pH after we rinse the char through a cone filter with 100 ml of water (pH8) in succession, we could measure the leaching rates of each material. 

In these experiments there was very little difference until the last one.  After three or four rinses the materials would stabilize at the pH 8 of the rinse material. The 400C char showed very little change and it was only after the 9th rinse that it began to drop a bit faster but even after 12 rinses it still had not stabilized.   It looked like a good candidate for further testing.  

The material is what could be consider comparable to those that have been made in a smoldering forest fire.  Chars has been  found to support microbial communities (Janna Pietikainen,1999)   The breakdown of plant matter, the adsorption of these nutrients by a layer of char below and a safe haven for microbes to flourish , Janna suggest are the reason for the success of microbial communities in char in her study.  However, char exposed to high intensity fire and temperature, as we have seen above, may adsorb but may not provide an immediately available energy source (fatty acids) or offer the same levels of retention that could offer "the best" material for long term slow release of nutrients.   

If the hypothesis is that we want to adsorb, store (reduce leaching effects) and yet provide safe haven and a rich environment for microbial communities to flourish, then perhaps the science of char production may offer help.

As you can see char enters a phase from 280 to 500 that is exothermic.  Once started, it continues on its own.  It does not require oxygen at this point and will continue to heat.  If oxygen is present or if the material is left in its exothermic environment it will continue past the structural and chemical reforming zone and become normal char.  In certain temperature ranges of pyrolysis the long chain hydrocarbon molecules will form polycondensates referenced by Runkel and Wilke, which will eventually volatilize and leave the char.  The deposition of condensables in a char bed is well known and generally the issue has been how to keep these materials from building up on down stream process.  The design of our reactor was developed specifically for this reason.  However, intra-particle condensation leads to increased char mass and a modification of the surface structures.

We have review the surface structure with a scanning electron microscope (SEM).  The final structure of the material may provide clues to predicted performance.

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