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Pyrolysis of biomass and coal in a bubbling fluidized-bed reactor; fate of fuel-bound nitrogen
Gepubliceerd door: Publicatie datum:
ECN BM 1-1-2004
ECN publicatienummer: Publicatie type:
ECN-C--04-008 ECN rapport
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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.

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