Fast pyrolysis and nitrogenolysis of biomass and biogenic residues

: production of a sustainable slow release fertiliser

Abstract

The production of agricultural and horticultural products requires the use of nitrogenous
fertiliser that can cause pollution of surface and ground water and has a large carbon footprint
as it is mainly produced from fossil fuels. The overall objective of this research project was to
investigate fast pyrolysis and in-situ nitrogenolysis of biomass and biogenic residues as an
alternative route to produce a sustainable solid slow release fertiliser mitigating the above
stated problems.
A variety of biomasses and biogenic residues were characterized by proximate analysis,
ultimate analysis, thermogravimetric analysis (TGA) and Pyrolysis – Gas chromatography –
Mass Spectroscopy (Py–GC–MS) for their potential use as feedstocks using beech wood as a
reference material. Beech wood was virtually nitrogen free and therefore suitable as a
reference material as added nitrogen can be identified as such while Dried Distillers Grains
with Solubles (DDGS) and rape meal had a nitrogen content between 5.5wt.% and 6.1wt.%
qualifying them as high nitrogen feedstocks.
Fast pyrolysis and in-situ nitrogenolysis experiments were carried out in a continuously fed
1kg/h bubbling fluidized bed reactor at around 500°C quenching the pyrolysis vapours with isoparaffin.
In-situ nitrogenolysis experiments were performed by adding ammonia gas to the fast
pyrolysis reactor at nominal nitrogen addition rates between 5wt.%C and 20wt.%C based on
the dry feedstock’s carbon content basis. Mass balances were established for the processing
experiments. The fast pyrolysis and in-situ nitrogenolysis products were characterized by
proximate analysis, ultimate analysis and GC– MS. High liquid yields and good mass balance
closures of over 92% were obtained. The most suitable nitrogen addition rate for the in-situ
nitrogenolysis experiments was determined to be 12wt.%C on dry feedstock carbon content
basis. However, only a few nitrogen compounds that were formed during in-situ nitrogenolysis
could be identified by GC–MS.
A batch reactor process was developed to thermally solidify the fast pyrolysis and in-situ
nitrogenolysis liquids of beech wood and Barley DDGS producing a brittle solid product. This
was obtained at 150°C with an addition of 2.5wt% char (as catalyst) after a processing time of
1h. The batch reactor was also used for modifying and solidifying fast pyrolysis liquids derived
from beech wood by adding urea or ammonium phosphate as post processing nitrogenolysis.
The results showed that this type of combined approach was not suitable to produce a slow
release fertiliser, because the solid product contained up to 65wt.% of highly water soluble
nitrogen compounds that would be released instantly by rain.
To complement the processing experiments a comparative study via Py–GC–MS with inert and
reactive gas was performed with cellulose, hemicellulose, lignin and beech wood. This revealed
that the presence of ammonia gas during analytical pyrolysis did not appear to have any direct
impact on the decomposition products of the tested materials. The chromatograms obtained
showed almost no differences between inert and ammonia gas experiments indicating that the
reaction between ammonia and pyrolysis vapours does not occur instantly. A comparative
study via Fourier Transformed Infrared Spectroscopy of solidified fast pyrolysis and in-situ
nitrogenolysis products showed that there were some alterations in the spectra obtained. A
shift in frequencies indicating C=O stretches typically related to the presence of carboxylic acids to C=O stretches related to amides was observed and no double or triple bonded
nitrogen was detected. This indicates that organic acids reacted with ammonia and that no
potentially harmful or non-biodegradable triple bonded nitrogen compounds were formed.
The impact of solid slow release fertiliser (SRF) derived from pyrolysis and in-situ nitrogenolysis
products from beech wood and Barley DDGS on microbial life in soils and plant growth was
tested in cooperation with Rothamsted Research. The microbial incubation tests indicated that
microbes can thrive on the SRFs produced, although some microbial species seem to have a
reduced activity at very high concentrations of beech wood and Barley DDGS derived SRF. The
plant tests (pot trials) showed that the application of SRF derived from beech wood and barley
DDGS had no negative impact on germination or plant growth of rye grass. The fertilizing effect
was proven by the dry matter yields in three harvests after 47 days, 89 days and 131 days.
The findings of this research indicate that in general a slow release fertiliser can be produced
from biomass and biogenic residues by in-situ nitrogenolysis. Nevertheless the findings also
show that additional research is necessary to identify which compounds are formed during this
process.

Date of Award	17 Dec 2013
Original language	English
Supervisor	Tony Bridgwater (Supervisor)