Grate- fired energy crop conversion. Experiences with brassica carinata and populus sp
- Autores: Maryori Coromoto Díaz Ramírez
- Directores de la Tesis: Christoffer Boman (dir. tes.), Francisco Javier Royo Herrer (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2014
- Idioma: español
- Tribunal Calificador de la Tesis: Juan Esteban Carrasco García (presid.), José Luis Sánchez Cebrián (secret.), Olli Sippula (voc.)
- Materias:
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- Resumen
Interest towards the participation of biofuels into the energy sector in Europe has gained more importance in last decades. Particularly, contribution of biomass for supplying heating demands is expected to increase considerably in the coming years. Focused on this goal, different solid biofuel feedstock streams, such as agricultural residues and dedicated energy crops, have been recently explored. Especially, dedicated energy crops are foreseen as an attractive option for improving participation of bioenergy in the European energy mix because of providing collateral benefits related to their production. Nevertheless, some current barriers attributed to their conversion into energy for heating (and power) should be overcome to guarantee a sustainable use of these novel fuels.
Although energy crops share many characteristics with other solid biofuels, such as stemwood assortments, important differences are identified with regard to chemical properties. In particular, aspects linked to inorganic matter (ash) attributes and related implications during combustion might limit potential applicability of energy crops. Besides aspects related to the fuel characteristics, they are identified some limitations concerning the conversion technology. Grate fired-combustion has been extensively used for heating applications. However, this technology is still characterized by rather bad adjustability and sensitivity to considerable variability of fuel properties, particularly, in terms of the ash attributes. Accordingly, it seems that a sustainable evolution of the sector will depend on addressing efforts towards the development of the most advanced and environmentally safe utilization of dedicated energy crops with the highest cost-effective relation.
Following this premise, the general objective of this thesis was to generate new and deeper knowledge related to the grate combustion behavior of energy crops, particularly in terms of the ash matter attributes, required to define relevant aspects to consider for an appropriate conversion of the novel biofuels.
Towards the achievement of the aforementioned general objective, two pelletized energy crops species, Brassica carinata (brassica) as one herbaceous, and Populus sp. (poplar) as one woody, were defined as representative fuels of the two core types of dedicated energy crops. With the aim to identify main differences among the species and also to gain knowledge in a broader ash composition perspective, other pelletized biofuel types were also included in this research. These fuels were; i) standardized woody fuels (as a reference), ii) a blend pelletized in the proportion of 50 weight % wet basis (wt%, w.b.) brassica and poplar raw materials, and iii) agricultural residues, Manihot esculenta, cassava stems. The cassava has not previously been evaluated as fuel so far but is a biomass resource of increasing interest among other agricultural residues. To achieve a deeper understanding of combustion behavior of the aforementioned fuels and ash chemistry aspects involved during their grate conversion, several experiments were performed at three different controlled and monitored conversion systems scales. The three combustion setups applied in this work were; i) one medium scale unit (250 kWth), ii) one smaller facility commercially used for residential heating (25 kWth), and iii) a laboratory scale reactor.
The experimental research work recognized, particularly, the relevant role of the ash matter fractionation, transformation and the operational and emissions ash-related implications during grate conversion. In general, these aspects showed to be closely dependent on an interrelationship between the biofuel attributes and conditions related to the conversion technology, (i.e., design features and operational parameters).
It has been elucidated the effect of concentration and proportion of main ash forming elements in the tested fuels on the ash fractionation and speciation behavior. Particularly, comparison of experiences among the poplar, brassica and cassava fuels provided further insight into the complex chemistry of the different fuel types. It was identified that for both the brassica and poplar fuels, ash transformation was dominated by the Si-chemistry whereas for the agricultural cassava samples, it was mostly P-dependant. These differences resulted in a very different fractionation behavior, which lead to a more significant proportion of the remaining ash fraction on the grate (bottom ash) for the Si-rich poplar and brassica fuels compared to the P-rich cassava fuels. Proportion among formed acidic and alkali metal and alkaline earth compounds, also differed among the tested fuel types and had effect on the ash chemistry behavior.
Concerning the speciation of the different ash fractions, it was recognized for both brassica and poplar that the bottom ash fractions are composed by a complex mixture mainly constituted by components with refractory elements (e.g., Si, Ca and Mg) and a minor fraction including more volatile elements (e.g., K and S). An interesting observation was that P-compounds were retained in brassica bottom ash fraction as grains embedded in a silicate-rich melt, which indicated a rather high stability for the phosphates. Deposits and fine particle emissions were mainly composed by K-chlorides, -sulfates and -carbonates. The amount of these two ash fractions has been identified to be even higher for brassica compared to poplar in agreement to differences noticed with regard to ash content and composition.
The ash transformation paths were recognized to be also dependent on local fuel conversion conditions that exists in the grate area. Particularly, the local temperature profile influenced the ash fractionation and speciation.
Results showed that at typical operational temperature in grate systems, an important fraction of the more volatile elements (e.g., K, S and Cl) was released, thus forming deposits and fine particles. For both the poplar and brassica fuels, the emission level of particles emitted was estimated to be considerable and, basically, totally dominated by the fine fraction mode (i.e., particles <1 micrometer). Furthermore, the temperature on the grate was also a critical parameter concerning slagging and sintering. Bottom ash from the brassica and poplar fuel tests exhibited a high sintering degree when temperature increased above 1000 C. For brassica, the proportion of the highly sintered ash (slag) fraction was substantial compared to poplar. Distinctions between the two fuels were not only qualitatively identified by the bottom ash sintering degree tendency but also with regard to the proportion of each sintered fraction formed and composition, as mentioned previously.
The experimental research illustrated that the fractionation and speciation of ash forming matter had implications on the overall conversion process effectiveness at typical grate firing conditions. Ash accumulation on the grate exacerbated an effective air-to-fuel contact and mixing needed for achieving high combustion effectiveness. This effect was more evident during the brassica fuel test because of its higher total ash content and sintering tendency. In order to manage the ash implications on grate operability and suitability level of technology tested, high oxidant conditions (i.e., primary lambda above 1) were applied to the fuels conversion and, consequently, the achievement of a deep air staging was limited during tests.
Characteristics of the bottom ash fraction (i.e., quantitative and qualitative aspects) had implications not only on the burnout control but also on the NOX emissions. Formation of this pollutant was related to the Fuel-NOX mechanism and, therefore, affected predominantly by the Fuel-N content but also because of oxidant conditions on grates. As a result of increments on air supply to improve burnout deteriorated by bottom ash, the NOX conversion rates increased for all the fuel tested. Consequently, the NOX emissions were also influenced by the ash matter condition on grates. Also here, the highest NOX emissions resulted for the herbaceous crop. Besides the highest Fuel-N content was noticed for the brassica pellets, the primary lambda ratio applied and tested for this fuel was rather considerable.
Overall, the research performed using the dedicated energy crops has contributed to determine that different measures should be considered to address the fuel peculiarities, particularly with regard to the ash-related implications during grate combustion. It can be concluded that a rigorous control of bed temperature is needed to increase the grate operability. To be successful on this task, strategies such as flue gas recirculation and/or a deep air-staging should be assessed. Furthermore, fuel conditioning by applying, for instance, fuel additives or blending, might be also needed. In this work, incorporating a blend of brassica and poplar (e.g., the blend pop50%-br50%) provided a reduction of NOX emissions almost similar to the blending ratio applied for them, in line with what would be expected. Finally, with regard to fine particles emissions, it seems that a combination of the primary fuel/process related measures and secondary gas/particle cleaning related measures will be a useful strategy to achieve the emission restrictions imposed by the European threshold values.
