Circulating fluidised bed combustors (CFBC?s) offer many advantages
over conventional firing equipment at industrial scale, the most important
being fuel tolerance and the little NOx that is emitted. However, the
future of the CFBC technology is threatened by possible release of significant
quantities nitrous oxide (N2O). Nitrous oxide is a 320 times more powerful
greenhouse gas than CO2.
The ?reburn? concept might prove to be a solution for this issue. Reburn,
i.e. the addition of secondary fuel above the bed, has proven to be
very successful in reducing NOx emissions from conventional combustion
plants. Application for the reduction of nitrous oxide emissions has
equal potential. The objective of the project ?Nitrous Oxide Reduction
in Circulating Fluidised Beds through Reburn? (REBED) was to determine
the manner by which reburn chemistry can be applied to reduce N2O emissions
from circulating fluidised bed combustion without incurring increased
NOx concentrations.
Four fuels were investigated in REBED, two coals (Puertollano coal and
Carbocol) and two wood materials (Eucalyptus and Pine). During pyrolysis
of the materials the main nitrogen-containing products are hydrogen
cyanide (HCN), ammonia (NH3), molecular nitrogen (N2), tar-bound nitrogen,
and char-bound nitrogen. HCN and NH3 play an important role acting as
gas-phase precursors for both N2O and NO in the combustion process,
while the majority of the N2O is formed by (in)direct reactions of char.
The ECN task in REBED was to perform pyrolysis experiments with the
fuels to determine the fate of nitrogen in the fuel (fuel-bound nitrogen)
as a function of four temperatures (600, 700, 775, and 885°C).
The pyrolysis experiments were performed in the ECN lab-scale bubbling
fluidised bed reactor ?WOB?. In experiments with both Carbocol and Pine
severe agglomeration occurred. Experiments with Puertollano coal and
Eucalyptus proceeded smoothly. Overall mass balances of the experiments
were prepared based concerning mass flows (feed, cyclone ash, soot,
and bed build-up), gas analyses (CO, H2, CO2, CH4, N2, Ar, C2H4, C2H6,
benzene, toluene, H2S, and COS), Solid Phase Adsorption (tars), fuel
and char composition (ultimate analysis), and wet-chemical analysis
(NH3, HCl, and HCN).
Based on the mass balances, the nitrogen distribution could be determined.
The general trend is that at 600°C the majority of the nitrogen is bound
to the char, while at increasing temperature more nitrogen is released
as N2. The other nitrogen containing compounds are formed in more-or-less
constant fractions over the temperature range investigated. For Puertollano
coal with a low volatile content, 50% of the nitrogen remains in the
char and 41% is released as N2 upon pyrolysis at 853°C. For Eucalyptus,
44% of the nitrogen remains in the char at 590°C decreasing to 6% at
845°C. At this high temperature, N2 is the main nitrogen-containing
compound (50%). Noticeable, compared to the coal, is the significant
amount of tar-nitrogen (19%).
The results presented in this report can be used to optimise a system
of CFBC operation with reburn to minimise N2O formation without increasing
the NOx production.
The nitrogen distribution data were used to determine ?rules-of-thumb?
for the prediction of the formation of N2O (precursors). The fuels are
compared based on their expected N2O-emission when utilised as primary
fuel (with a thermal input of 10 MJ) in a FBC at 850°C. Both coal fuels
with relatively high nitrogen content produce a lot of NH3, HCN, and
especially char, all of which are notorious N2O precursors. For Eucalyptus
the production of NH3 and HCN is comparable but the char?N production
is much lower due to the high volatile content. Pine produces the lowest
amount of nitrogen compounds.
However, it should be realised that the reburn (!) chemistry of the
nitrogen compounds in the fluidised bed combustion determines the final
N2O and NOx concentrations. The investigation of these issues was not
included in the present study.