Controlling excess combustion air and unburned carbon losses is the key to finding the sweet spot in this biomass boiler approach.

Bagasse, a renewable biomass fuel, is the fibrous residue remaining after sugarcane stalks are crushed to extract their juice. Bagasse typically generates 3,000 to 8,000 Btuh/lb of fuel, depending on moisture content. It has become a vital element of government policies for biomass-based renewable energy systems that have a goal of zero net greenhouse gas emissions.

Sugar cane is grown and locally processed in many developing countries in subtropical zones throughout the globe. The highest producing sugar cane nations are: Brazil (33%), India (23%), and China (7%). In the United States, sugar cane is commercially harvested and processed in Florida, Louisiana, Texas, Puerto Rico, and Hawaii.

Sugar extraction from the harvested cane starts with washing, chopping, and shredding of the cane prior to repeated crushing between rollers to express the sugary juice. Sugar cane crushing yields about 30% by mass bagasse. Five million tons of bagasse burned equates to one million tons of fuel oil saved. So bagasse is a great source for renewable biomass energy with zero net greenhouse gas emissions (CO₂ emissions from bagasse combustion equals CO₂ absorbed from the air during sugar cane growth).

BAGASSE STEAM BOILERS

Bagasse is typically burned in stationary grate boilers, moving grate boilers, or fluidized bed combustors. Combustion depends on bagasse feed rate, air (oxygen) trim flow rates, furnace temperature, and bagasse moisture content. The variability of moisture content makes it challenging to hold constant heat generation.

Newer bagasse boiler designs take into account more automation in drying and dispersing bagasse, yielding up to 5,000 to 8,000 Btuh/lb of bagasse. Combustion of low-moisture bagasse facilitates constant boiler heat control.

Bagasse is commonly dispersed onto combustion grates to maintain constant furnace temperature and water pressure. Operators either mechanically or pneumatically spread bagasse across the boiler grate in spreader-stoker furnaces. This action speeds drying and promotes rapid combustion. Continuous-feed bagasse boilers require dry, moisture-controlled bagasse. Outside air piped between the hot bed and the boiler grate helps to ensure proper oxidizing conditions.

STEAM AND ELECTRICITY

Steam plays a large role in the sugar extraction and refining processes. In newer boilers, bagasse supplies more energy than required to produce and refine sugar. Excess electricity can be sold to the local power grid or third parties.

Because selling excess power is relatively new to the sugar industry, plants are placing more emphasis on modernization, discipline, and energy conservation when operating a bagasse cogeneration plant. Fine tuning of the combustion process requires controlling excess combustion air and unburned carbon losses.

THE ABB SOLUTION: SMA MONITORING OF BAGASSE COMBUSTION

of the oxygen and unburned combustibles in the flue gases. The ABB SMA2 with O₂ and combustible COe sensors is designed expressly for this purpose.

Monitoring excess oxygen alone can provide a way to control combustion efficiency. However, leaks in the furnace wall along with wet bagasse can skew the measured values of excess oxygen. Wet bagasse contributes to the vapor content of the flue gas which in turn reduces the partial pressure of oxygen. This phenomenon yields a lower measured value of excess oxygen in the process.

The SMA2 contains a second sensor to monitor combustible emissions (COe). This sensor accurately monitors levels of partially burned fuel in the flue gases, which mainly consist of carbon monoxide. The additional COe sensor provides operators and engineers a trim control measurement so the plant can optimize the production of process steam and electricity while minimizing fuel waste. TB