Design and Operation of a 10 kWth Chemical-Looping Combustor for Solid Fuels
Doctoral thesis, 2009
Chemical-looping combustion (CLC) is a new fuel conversion technology with inherent CO2 capture. In this process, the fuel and the combustion air are never mixed and the oxygen is transferred to the fuel via an oxygen carrier in the form of a metal oxide circulating between two reactors, e.g. interconnected fluidized beds, called air and fuel reactors. The result is that the CO2 produced is not diluted by the nitrogen in the air and can be recovered by condensing and removing the water produced during the combustion. In this work, a 10 kWth chemical-looping combustor for solid fuels was designed, built and operated. Three fuels were tested: a South African coal and two petroleum cokes from Mexico. The oxygen carrier was ilmenite, an iron titanium oxide. Testing involved continuous fuel feed operation as well as discontinuous feed in the form of batches. Influence of operational conditions was investigated, including effects of oxygen carrier circulation between air and fuel reactors, fuel feed rate, fuel reactor temperature, and fluidizing velocities in various sections of the reactor system.
The aim was to find out if chemical-looping combustion with solid fuels was feasible and to establish any potential show-stopper for the process. In particular, three key performance criteria were in focus: i) the solid fuel conversion, ii) the actual carbon capture of the system, here the proportion of gaseous carbon leaving the fuel reactor to total gaseous carbon leaving the unit, and iii) the gas conversion, evaluated here by means of the oxygen demand, i.e. the fraction of oxygen lacking to achieve full gas conversion.
The results gave proof of the concept. The CLC combustor was successfully operated with continuous fuel feeding for nearly 90 h, but with a total of over 450 h with ilmenite at high temperature - above 850°C - without any agglomeration. Ilmenite appeared to be a suitable oxygen carrier for CLC with solid fuels. During testing, carbon capture efficiencies varied between 70 and 96% depending on fuel and operation. The solid fuel conversion was low, between 50 and 80%, due to low cyclone efficiency. The oxygen demand was typically in the range 30-35% based on CO, CH4, H2S and H2. It was shown that most of the oxygen demand, as much as 75-80%, was associated with the fuel volatiles which are not really in contact with the oxygen carrier, due to the reactor configuration. However, results indicate that the oxygen demand associated with char gasification products could be as low as 5% for the South African coal, showing that reasonably high syngas conversion is possible.
Also, by complementing continuous testing with batch testing, it was possible to model the system and derive a correlation between measured operational data and actual circulation mass flow, as well as a model that describes the carbon capture efficiency as a function of the residence time and the char reactivity. Moreover, the possibility to measure the circulation mass flow was also used to estimate the limit for which the circulation mass flow is sufficient to sustain the reactions in the fuel reactor.
Solid Fuel Conversion
Carbon Dioxide Capture