A Feasibility Study of Using the Organic Rankine Cycle for Power Generation from the Flue Gases of Recovery Boilers

Abstract
Almost 415 tons/h of flue gases with a temperature of 160􀔨 is released to the atmosphere from the recovery
boiler of a pulp mill with capacity of 1000 air dried tons of pulp per day. This is a large waste heat stream
that can be used to generate power, to decrease the operating costs of a pulp mill, and to save CO2 emissions.
In this work, the feasibility of using an Organic Rankine Cycle (ORC) with ammonia as the working fluid
to generate power from the flue gases of recovery boilers is studied. CHEMCAD and Taguchi methods are
used for simulation of the process and for optimization of operating conditions, respectively. The
temperature of the ammonia and flue gases at the exit of evaporator, exit pressure of the pump & turbine,
and the degree of sub-cooling of ammonia at the exit of condenser are five operating parameters that are
manipulated to optimize the process. Three different scenarios are defined: minimizing the net power cost,
maximizing the ORC efficiency, and maximizing the net profit. Different aspects of these scenarios such
as net power generation, cost, efficiency, and CO2 emission saving are discussed, and optimum operating
conditions are reported.
Keywords: Waste heat recovery system, Recovery boiler, Power generation, Organic Rankine Cycle
(ORC), Pulp mills 

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