Gas Measurement in Pulp & Paper Mills

Several steps within the pulp and paper making process create emissions that must be monitored and reported. High temperatures during the washing and screening processes generate exhaust gases. Any bleaching process includes chlorine or peroxide that must be vented.
Creating paper pulp relies on a careful balance of low velocity air flows among the various processes. For example, the recovery boiler following the digester must be modulated to follow changes in the digester load.

 Read Brief



Pulp & Paper Mill Gas Metering Applications

Primary Air
Primary Air in paper mills is used to fire boilers: primary, secondary and tertiary (over fire) air is sent to the combustion boiler, stoker boiler and recovery boilers. Fuel is also pumped to the many combustion chambers and mixed with air. Careful measurement is crucial to ensure a proper fuel/air ratio for a clean and efficient burn.

Gas Characteristics:
High Volume, High Flow Rate
Secondary Air
Oxygen is pumped into the furnace above the fire ball to help completely burn noxious gases.

Gas Characteristics:
High Volume, High Flow Rate.
Tertiary Air
Oxygen is pumped into the furnace above the fire ball to help completely burn noxious gases.

Gas Characteristics:
High Volume, High Flow Rate
Recirculated Gas
Hot gas is exhausted from the combustion chamber and recirculated back to the bottom of the burner to be more completely burned, used to pre-heat boiler water, diverted to cogeneration stations or to heat exchangers. Once it has cycled through enough to remove maximum impurities it is filtered and sent to the stack to be released.

Gas Characteristics:
Hot, Particulates, Condensing, High Volume, High Flow Rate.
Exhaust Gas
Hot gases that have been recirculated are sent through a series of filters to remove particles and harmful compounds. Flow measurement before and after filtering is important to gauge efficiency of filters and leaks.

Gas Characteristics:
Hot, Particulates, Condensing, High Volume, High Flow Rate
Stack Gas
After filtration remaining gases are exhausted through the stack. Emitting harmful gas is prohibited by environmental regulations so continous monitoring and measuring is required.

Gas Characteristics:
Hot, Particulates, Condensing, High Volume, High Flow Rate



Paper Mill Gas Metering Instruments


Primary Air

Primary Air in paper mills is used to fire boilers: primary, secondary and tertiary (over fire) air is sent to the combustion boiler, stoker boiler and recovery boiler to be mixed with fuel at the bottom of the boiler (primary) to control burn size and thoroughness (secondary) and to prevent the fire from travelling too high (tertiary). Aeration air is bubbled through fermentation vats. Cool air is heated with recirculated exhaust gases and piped into the pulp beds to dry the sludge. Pulp mills are complex facilities with many recirculating systems.

Combustion Gas

Hot gases exhausted from the combustion boiler, stoker boiler and recovery boiler are collected and cycled back into the boilers to be burned again, then recirculated to heat exchangers and steam generators to produce power for the plant. The hot combustion gases are cycled through a series of applications each requiring less energy than the last. First through the combustion chamber, then the hood make up air, the building make up air and finally used to heat the process water. This system ensures maximum energy transfer based on a processes needs. Each application should be measured precisely for maximum mill efficiency.

Stack Gas

Excess gases from the digesters and boilers are released as steam after impurities are removed. A recovery furnace or kiln may be used to reclaim spent chemicals. The remaining gas contains a high volume of C02 which is sent to the stack to be released. The greatest amount of stack emissions come from combustion: furnaces, kilns, boilers and turbines which may burn fossil fuels or biomass. All of these sources send heated gases to the stacks to be released so pulp mills are required to report emissions continouisly.

Digester Gas

Pulp digesters pressure cook wood chips and introduce chemicals to break down the fibers and seperate out impurities. Chemical pulping utilizes a combination of heat and liquor (recycled digester liquid and chemicals). Additional gases are released from the chemical reaction between the wood chips and liquors. Extreme heat from the liquor recycling process produces steam which is used throughout the mill. Excess digester gas is laden with chemicals and water vapor, it is hot and may flow at a high rate when the digesters are first vented.

Condensing Gas

Most processes in a paper mill are moisture laden; condensing gas flow. The recovered combustion gases used to heat the mills many processes contain a high degree of moisture. Water vapor will condense out of a gas for multiple reasons: as temperature drops the gas’ ability to hold water is diminished and water droplets form. A reduction in pressure will cause condensation to form in the stream. The temperature differential between hot gas and a cold pipe will cause moisture to condense out. As water condenses in a pipe it collects on the walls, eventually running along the bottom.