Effect of boiler feed water composition on inorganic scaling in once-through steam generators estimated using a Monte Carlo modelling approach

Once-through steam generators (OTSGs) produce steam required to recover hydrocarbons from oil sand deposits. OTSGs generate steam at high pressure and temperature, using boiler feed water (BFW) derived from produced water, recycled condensate boiler blowdown (BBD), and small amounts of make-up water sourced from local groundwater. During the OTSG operation cycle, BFW undergoes significant physical and chemical changes, which can cause varying degrees of mineral (scale) precipitation, depending on the BFW quality. Scaling has negative impacts on OTSG performance and has in the past resulted in OTSG tube leaks. In this study, we performed thermodynamic simulations using a Monte Carlo approach with the objective of determining how the composition of the BFW and the steam quality affect scaling. We used 3 different scenarios, characterized by low, intermediate, and high iron-to-BFW ratios to represent various situations of BFW interaction with OTSG pipes. Within each scenario, BFW compositions were randomly assigned within industry-relevant variations of variables including steam quality, pH, and concentrations of SiO2(aq), Mg2+, Ca2+, Fe2+, Cl-, HCO3-, K+, Na+, SO42- and O-2 and were allowed to precipitate scales according to thermodynamically controlled solubilities of minerals as they were heated and boiled. Our results show that inorganic scale in OTSGs is composed mostly of aegirine and various Mg and Mg/Ca silicates. We show that the concentrations of dissolved Si, Mg, and Fe available for interaction with BFW are the main factors controlling the mass and mineralogy of scale, whereas the total dissolved solids (TDS) and Ca concentrations within typical chosen operating limits have negligible impact on the scale mass in OTSGs. The modelling results further indicate that efforts to minimize the concentration of Mg in BFW to very low levels (<0.01 mg/kg(BFW)) show great promise for minimizing inorganic scale formation in OTSGs. Our equilibrium modelling revealed that steam quality has little impact on the total mass of inorganic scale formed in OTSGs because most of the mineral precipitation occurs at temperatures below 250 degrees C, before boiling starts. However, this finding may not be fully valid if strong kinetic barriers prevent process waters from achieving equilibrium via scale precipitation, especially at lower temperatures. Moreover, because nucleated minerals may be transported through the OTSG without precipitating on the piping walls, increasing steam quality reduces the capacity of BBD to carry over crystallized mineral phases in suspension.