Oxygen carrier aided combustion of solid biomass in small-scale bubbling fluidized beds

Viable small-scale solutions for combined heat and power (CHP) production using solid biomass as fuel are rare. Most concepts suffer well-known problems related to the application of biomass, particularly slagging and fouling of heat exchanger surfaces. This results in reduced electrical and combustor efficiency as well as increased maintenance issues. To overcome these prevailing challenges, this thesis identified right in the beginning small-scale bubbling fluidized beds (BFBs) combined with in-bed heat exchanger surfaces as a promising concept. As mixing issues and in-bed fuel conversion still highly affect the combustion emissions and electrical efficiency of the proposed concept, this thesis aims for further optimization by the application of the oxygen carrier (OC) ilmenite as an alternative bed material in small-scale BFBs - so-called oxygen carrier aided combustion. The focus on small-scale BFBs in this work, instead of the focus on large-scale circulating fluidized beds (CFBs) in previous research, shifts the prevailing operation and reaction conditions for the oxygen carrier. The differences result from various characteristic parameters of small-scale BFBs: higher bed material particle size, limited bed expansion, i.e. dense particle beds, and potentially higher overall excess air ratios due to none-staged combustion. This provokes an increasing local overlap of the OC oxidation and reduction and affects the oxygen transport and buffering ability of ilmenite. Therefore, in a first step, this thesis determines the local and temporal buffering ability of ilmenite in BFBs by determining the specific provided surplus in-bed oxygen compared to conventional silica sand. In line with that, this thesis presents the developed measurement methodology in laboratory environment and comprehensive experiments with different gaseous and solid fuels. In the next step, these results serve as basis for the evaluation of the enhancing effect of ilmenite on the overall plant performance. This includes the (time-dependent) reduction potential of CO emissions as well as the increase of in-bed heat release and consequently enhanced plant efficiency. A CHP plant especially designed for this purpose serves to conduct long-term experiments in field test environment and to compare 50 wt% ilmenite and pure silica sand as bed material. This applied pilot plant integrates the heat-exchanger surfaces of a 5 kWel Stirling engine directly into the fluidized bed of a 45 kWth BFB reactor. Parallel scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and particle size distribution (PSD) analysis of bed material samples support the experiments and allow the investigation of time-dependent morphological particle changes as well as ilmenite-biomass ash interactions.The experimental results confirm that ilmenite provides surplus in-bed oxygen, which helps to overcome prevailing mixing issues and thereby increases the in-bed fuel conversion. The local shift is complemented by an additional temporal supply of in-bed oxygen during transient combustion conditions. Consequently, the surplus in-bed fuel conversion achieves up to 60 % using gaseous fuels. The long-term operation (530 h) with 50 wt% ilmenite in field test environment revealed a significant reduction of the emerging CO emissions (up to 75 %). Lower combustion temperatures and lower excess air ratios favor the reduction. Simultaneously, ilmenite mitigates the time-dependent increase of CO emissions. Slightly increased electrical and combustor efficiency (+0.5-1.0 %-points, respectively +3.0-5.0 %-points) indicate a higher in-bed heat release. Accompanying SEM/EDS analysis of used bed material samples reveal the increasing incorporation of potassium into the particle core and the accumulation of calcium in an inner particle layer below an iron enriched shell. This behavior differs fundamentally from the characteristic sticky outer particle layer formation when pure silica sand is used as bed material. Thereby, ilmenite reduces the risk of ash-induced agglomeration issues. Simultaneously, aged and porous ilmenite particles involve a higher risk of attrition than silica sand, which increases the entrainment of fines and their accumulation in the ash collector. However, this loss of aged bed material can be used for bed material regeneration and, therefore, is integrated into the proposed bed material management of small-scale fluidized beds.