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Optimization study on a modern regeneration boiler cold end operation and its feedwater system integration into energy system of a paper mill

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Abstract

The potential of modern regeneration boiler cold end and feedwater system integration with energy system of a paper mill is studied and measures for its better exploitation are proposed and assessed. Mathematical model has been developed, comprising material and heat balances, heat transfer equations, and steam turbine model. Its accuracy has been tested on the design and actual operation of a real system firing 70 t/h of dry solids. The simulation coped with design and measured data very well and also followed the operation changes performed in field tests excellently. Nine proposals aimed at real system efficiency improvement were assessed on the yearly benefit and simple payback period base, with yearly benefit resulting either from marginal fuel savings (scenario 1) or from condensing power production increase (scenario 2). Immediately applicable measures included deaeration at full steam pressure and heat load minimization in the first stage of the boiler feedwater regenerative heating—this novel finding contradicts the boiler design where full load regenerative heating is applied. Further measures increase the internal and external (available in paper mill) heat utilization. Their combination yielded either up to a 7 t/h internal steam consumption decrease (2.25% of nominal boiler steam output, scenario 1) or up to a 1.4 MW electricity production increase (2% of nominal steam turbine power output). Several of the proposed measures merit attention of boiler vendors and paper mills energy managers as viable tools towards low energy intensity pulp and paper mill industrial sector.

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Abbreviations

cel :

Electricity price (€ MWh−1)

cp :

Specific heat capacity at constant pressure (kJ kg−1 K−1)

cs :

Steam price (€ GJ−1)

h:

Specific enthalpy (kJ kg−1)

k:

Slope of steam turbine working characteristics (kJ kg−1; kWh t−1)

\( \dot{m} \) :

Mass flow (t h−1)

p:

Pressure (kPa, bar)

PST :

Electric output of steam turbine (kW)

\( \dot{Q} \) :

Heat flux (exchanged heat) (kW)

t:

Temperature (°C)

U.A:

Heat flux transferred in a heat exchanger per unit driving force (kW K−1)

∆his :

Isentropic enthalpy difference (kJ kg−1)

∆tLM :

Logarithmic mean temperature difference (°C)

ηmech :

(steam turbine) mechanical efficiency (-)

abs.:

Absolute (pressure)

A:

Parameter in Eqs. (7, 8)

AH:

Air heater

B:

Parameter in Eqs. (7, 8)

BFWH:

Boiler feedwater heater

BL:

Black liquor

CCT:

Condensates collection tank

CONC:

Concentrator

COND:

Condenser

DEA:

Deaerator

ECO:

Economizer

ESP:

Electrostatic precipitator

FG:

Flue gas

FGC:

Flue gas cooler

HDS:

High dry solids

HERB:

High energy regeneration boiler

HPS:

High pressure steam (around 20 bar(g))

HW:

Hot water

LPS:

Low pressure steam (around 5 bar (g))

MPS:

Middle pressure steam (around 11 bar (g))

OH:

Operating hours (per year)

P&P:

Pulp & paper

RS:

Reference state

ST:

Steam turbine

TIC:

Total investment cost

VHPS:

Very high pressure steam (above 40 bar (g))

W:

Intercept of steam turbine working characteristics in Eqs. (6, 8)

WPH:

Water preheater

bp :

Backpressure

cond :

Condensing

FG :

Flue gas

mol :

Molar

p :

Pressure

w :

Water

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Funding

This work was financially supported by the Slovak Scientific Agency, Grant No. VEGA 1/0659/18 and by the Slovak research and Development Agency, Grant No. APVV-15-0148 and the project Science and Technology Park STU Grant No. ITMS26240220084, co-financed from the European Regional Development Fund.

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Authors and Affiliations

  1. Department of Chemical and Biochemical Engineering, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, 812 37, Bratislava, Slovakia

    Miroslav Variny, Marek Blahušiak, Ján Janošovský, Michal Hruška & Otto Mierka

  2. GRUCON s.r.o., Nezábudková 24, 821 02, Bratislava, Slovakia

    Miroslav Variny & Otto Mierka

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  1. Miroslav Variny
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  2. Marek Blahušiak
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Correspondence to Miroslav Variny.

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Variny, M., Blahušiak, M., Janošovský, J. et al. Optimization study on a modern regeneration boiler cold end operation and its feedwater system integration into energy system of a paper mill. Energy Efficiency 12, 1595–1617 (2019). https://doi.org/10.1007/s12053-019-09804-z

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