A Numerical Modeling Example – Kraft Recovery Boilers

(Excerpted from Steam/its generation and use, 42nd edition)

Situation

A kraft process recovery boiler, as its name implies, recovers energy and chemicals from black liquor, a byproduct of the papermaking process. Air and liquor delivery systems control several complex
and interacting combustion processes (black liquor spray, deposition and burning on furnace walls, char bed burning, smelt flow) that affect boiler performance (capacity, reliability, emissions, chemical recovery, and energy efficiency). Good air jet penetration and effective mixing of secondary and tertiary air are desirable for complete combustion and reduced emissions of carbon monoxide (CO) and hydrogen sulfide. Distribution of air to three or more air injection levels produces fuel-rich conditions in the lower furnace that are desirable for smelt reduction and reduced NOx emissions. Flow and temperature uniformity in the furnace minimizes carryover of inorganic salts, provides an even heat load, and minimizes deposition on convection surfaces at the furnace exit. Uniform distribution of liquor spray ensures adequate drying of liquor spray, minimum carryover, and stable char bed combustion.

Analysis

Detailed combustion models for black liquor have been developed and are used in conjunction with computational fluid dynamics (CFD) modeling. Black liquor combustion is simulated for individual droplets as they heat up and burn in suspension. Stages of combustion along a single trajectory include drying, devolatilization, char burning, smelt oxidation, and molten salt formation. The trajectories of thousands of particles determine the distribution of liquor spray in the furnace for a range of droplet sizes. (See Fig. 1) Combustion processes on the walls and char bed are also simulated with particle deposition, char burning, smelt flow and char accumulation. These capabilities are useful for evaluating the effect of air and liquor delivery systems on combustion processes in the furnace and for predicting the quantity and composition of particulate that leaves the furnace.

Fig. 1 Liquor spray distribution in the lower furnace of a recovery boiler.

Results

Fig. 2 shows iso-surfaces of gas speed in the furnace. An iso-surface is a computer-generated image in which the specified field variable (e.g., speed) has the same value. These images help visualize air jet penetration and the interaction of jets from neighboring air ports. The char bed shape is approximated so its impact on flow in the lower furnace can be evaluated with the model. In this air system design, jets of air penetrate across the furnace at each air level for effective coverage over the entire furnace cross-section. This arrangement of jets produces more uniform upward flow and effective mixing with combustion gases. Gas velocity and temperature distribution predictions, shown in Fig. 3, are used to analyze the flow distribution and heat transfer in the furnace and convection pass.

Other information such as char bed surface temperature and burning rates, gas species concentrations (i.e., O2, CO, NOx), and wall heat flux distribution are also generated. Results are used by boiler designers and operators to evaluate air system designs, liquor spraying systems, liquor firing capacity, char bed combustion instabilities, convection pass fouling, furnace wall corrosion, and CO and NOx emissions. The results shown were created by CFD software developed by B&W.

Fig. 2 Iso-surfaces of gas speed depicting jet penetration in the lower furnace of a recovery boiler.

Fig. 3 Gas velocity vectors (left) and temperature contours (right) at the center of the furnace of a recovery boiler.