Unlike other sugar sectors in the world, the Colombian agribusiness is characterized by operating throughout the year and, therefore, it faces different qualities of sugar cane and bagasse.
Unlike other sugar sectors in the world, the Colombian agribusiness is characterized by operating throughout the year and, therefore, it faces different qualities of sugar cane and bagasse.
This condition requires that the combustion systems of the manufacturing processes be flexible and robust enough to guarantee stability and reliability in the production of sugar, ethanol and electrical energy.
Since the interaction between thermofluid phenomena and chemical reactions in a combustion system has not yet been fully described or understood, Cenicaña has used computational simulation to propose solutions in this process and optimize its performance.
In the research, with emphasis on the boilers of the factories in the sector, it was identified that the energy losses related to combustion can be up to 40% of the total losses in the boiler.
With this diagnosis, Cenicaña began to work on the possibility of reducing the content of O2 in the gases and reduce the temperature at the outlet of the boiler. These two points are inherent to the process and can be manipulated through the amount and distribution of the air used.
The evaluation
Computationally, the combustion process was evaluated in a boiler with bagasse in two scenarios, in which the distribution of forced, fire and pneumatic air was modified (Figure 1). The distribution evaluated was:
- Scenario A: 75% forced air, 25% air over fire, and 5% pneumatic air (current typical condition).
- Scenario B: 35% forced air, 60% air over fire and 5% pneumatic air (proposed).
Results and conclusions
- Scenario A: combustion occurs at high levels in the boiler hearth and possibly affects the superheaters and even the main bank due to prolonged overheating of the tubes. This condition would favor failures due to creep and inefficiency. The effect is not evident in scenario B, of air handling with a higher proportion in the overfires, since the combustion occurs in low areas of the boiler hearth (Figure 2).
- Scenario B: a closer approximation to the complete bagasse combustion conditions was achieved, evidenced by lower residual oxygen (5.69%) and higher CO2 content (17%) in the gases leaving the home, with respect to the scenario a: residual oxygen (8.22% ) and CO2 (16.28%). (Table 1).
Therefore, the importance of the turbulence-time-temperature interaction in the performance of the process is highlighted (Figure 3, scenario B), because this condition, with the management of the air proportion, reduces the unburned fuel and the heat in gases and, therefore, the losses in the boiler.
The foregoing is inferred by the gas composition obtained, since lower residual oxygen contents and higher carbon dioxide formation are indicators of improved combustion, thanks to a greater release of energy from the carbon present in the fuel.
Figure 1. Geometry computational model. 
Figure 2. CO profiles2 in scenarios A and B. 
Figure 3. Velocity profiles in scenarios a and b. some definitions
creep: deformation process over time at elevated temperatures and a constant stress level (The National Board of Boiler and Pressure Vessel Inspectors).
Forced air: air supplied to the boiler in order to favor the suspension of the fuel in the hearth. Enter below the grill.
Air over fire: air in charge of promoting turbulence phenomena in the home to improve combustion. Enter above grill level
Pneumatic air: supplied air to distribute the fuel in the available area of the boiler.
KEEP IN MIND
For computational simulation, since 2011 Cenicaña has incorporated the ANSYS-CFX program into its tools.
Author:
JULIÁN ESTEBAN LUCUARA. Mechanical engineer. Factory Process Program - Cenicaña.
