OxyFuel combustion of Coal and Biomass

The power and heat producing sector is facing a continuously increasing demand to reduce its emissions of CO2. Oxyfuel combustion combined with CO2 storage is suggested as one of the possible, promising technologies which will enable the continuous use of the existing fleet of suspension-fired power plants burning coal or other fuels during the period of transition to renewable energy sources.

The oxyfuel combustion process introduces several changes to the power plant configuration. Most important, the main part of the flue gas is recirculated to the boiler and mixed with pure oxygen. The oxidant thus contains little or no nitrogen and a near-pure CO2 stream can be produced by cooling the flue gas to remove water.

The change to the oxidant composition compared to combustion in air will induce significant changes to the combustion process. This Ph.D. thesis presents experimental investigations on the combustion of coal, biomass (straw), and blends of coal and straw in air and O2/CO2 mixtures. The experiments have been performed in semi-technical scale in a once-through 30 kWth swirl-stabilized flame.

The work has focused on improving the fundamental knowledge on oxyfuel combustion of coal and straw at conditions relevant to suspension-fired boilers by clarifying the effect of the change in combustion atmosphere on fuel burnout, flame temperatures, emissions of polluting species (NO, SO2, and CO), fly ash quality, and deposit formation.

This work is one of the first to investigate the important aspects of ash and deposit formation during co-firing of coal and biomass and combustion of pure biomass in oxyfuel atmospheres in semi-technical scale. The presented work has lead to the identification of reference operating conditions which enables a direct comparison of combustion in air and oxyfuel atmospheres.

Apart from slightly improved burnout and reduced emissions of NO during oxyfuel combustion these operating conditions yield similar combustion characteristics in both environments. Co-firing coal and biomass or combustion of pure biomass in an oxyfuel power plant could yield a significant, additional CO2 reduction, or even lead to below-zero emissions of CO2 from power production.

This work has shown that no significant changes occur to the fundamental combustion characteristics for straw when burned in the O2/CO2 atmosphere. Additionally, the combustion of a coal/straw blend with a straw share of 50 wt% has added valuable understanding to the trends in ash and deposits chemistry for coal/straw co-firing.

Recirculation of untreated flue gas in oxyfuel plants will increase the in-boiler levels of NO and SO2 significantly. Experiments with simulated recirculation of NO and SO2 have provided insight into the mechanisms of the significant reduction in NO emission rates from the boiler and the increased uptake of sulphur in fly ash and deposits which were observed.

The single-largest penalty to the electrical efficiency and operating expenses of an oxyfuel power plant is the production of near-pure oxygen by cryogenic distillation. This thesis presents a possible strategy for reducing the oxygen demand and hence the penalty to the process. The strategy exploits the fact that the oxygen excess level during oxyfuel combustion is not directly linked to the flow of oxidant but can be adjusted independently.

By increasing the concentration of oxygen in the oxidant, i.e. by reducing the flue gas recirculation ratio, it is possible to achieve similar burnout at lower oxygen excess levels. Further work on implications of this strategy are necessary in order to fully clarify its potential for improving the process economics of oxyfuel combustion.

Generally, no characteristics of the oxyfuel combustion process have been identified in this work which would be detrimental to its implementation as a carbon capture technology in full-scale power plants.