TAPPI PEERS 2019

TAPPI PEERS 2019 Improved Burner Designs for Optimized Lime Recovery Kiln Performance on Natural Gas Fuel Martin Beddows, Richard Manning, and Leo Newell, KFS-Metso, United Kingdom ABSTRACT Increasing production and reducing operating costs is a constant battle for the operators of lime recovery kilns in pulp mills. Operating costs are mainly driven by the underlying cost of fuel which vary significantly over long periods of time. In the past 20 years oil, petcoke, and more recently natural gas have been the most economic fuels. Each fuel however has an effect on the kiln production due to the nature of flame generated by the chemical reaction. This is most noticeable with the conversion to natural gas which produces a flame which is less radiant than petcoke and oil. KFS has developed a kiln burner design to minimize the impact on the reduced radiance from the natural gas flame. This paper compares kiln performance between traditional kiln burner designs and the new burner development using site data collected following conversion projects. HISTORY Kiln Flame Systems was formed in 1999 after a successful management buyout of the original UK-based company which had been trading from the same premises since 1984. During the period 1984 to 1999, the previous company had focused on rotary kiln combustion, primarily in the cement industry, using the physical modeling skills developed at the University of Surrey. Moving from a university to an industrial basis rapidly improved the development and accuracy of the physical modeling techniques [1]. At the outset of the company, the old model laboratory was upgraded and the engineers responsible for the previous modeling put in place a system to share the knowledge within the entire engineering team. Programs have been developed to enhance the understanding and accuracy of transfer of site data from site to model data as inputs which led to vastly improved results. While we had been the lead supplier of petcoke burners to the US pulp and paper industry between 2003 and 2008 (75% of new installs), when the natural gas prices collapsed in 2008, there was a drive to improve our OPTIMIXTM G gas burner technology. A lengthy design and modeling program was completed in 2011 with the first install of the patented OPTIMIXTM G-X at a mill in Florida, USA that year. Since then there have been over 40 successful G-X burner installations around the world. KFS were acquired by METSO in December 2018 to provide in-house combustion expertise. TRADITIONAL FUELS The combustion process involves conversion of fuel to oxidized components releasing energy primarily in the form of heat and light. For traditional fuels used in the Kraft recovery process, the core components of the fuel are carbon and hydrogen in the form of hydrocarbons. Using the simplest form of hydrocarbon, methane, the basic combustion process is [2]:

CH4 + 2 O2 + 7.7 N2 CO2 + 2 H2O + 7.7 N2 fuel air combustion products

The actual heat released in the kiln is the ‘net’ heat reflecting the conversion of fuel to CO2 (gas) and water vapor. In the US heat release values are quoted in ‘gross’ terms reflecting higher energy release to produce CO2 (gas) and liquid water. The most common fuels used in lime kilns are natural gas and No. 6 (heavy) fuel oil (HFO) as they are readily available. HFO is more efficient in terms of heat transfer and results in higher kiln capacity. Natural gas easier to handle and maintain, and is currently cheaper. However, natural gas burns with significantly less luminosity resulting poorer radiation and flatter heat flux profile along the kiln starting further from the kiln discharge as shown below (Figure 1).

FIGURE 1 : HEAT TRANSFER PROFILE: HFO VS. NATURAL GAS

As a result in the differences in heat transfer and kiln operation, the combustion air requirements are notably higher for natural gas (Table 1). Table 1: Changes in Air Requirements - HFO vs. Natural Gas [3]

Parameter HFO Natural Gas

Heat Release – Net 17,410 Btu/lb 40.5 MJ/kg 935 Btu/scf 36.7 MJ/m3

Heat Release – Gross 18,440 Btu/lb 42.9 MJ/kg 1,037 Btu/scf 40.7 MJ/m3

C:H mass ratio 8 : 1 8 : 1 3 : 1 3 : 1

Combustion air per fuel 14 lb/lb 14 kg/kg 17 lb/lb 17 kg/kg

Combustion air per thermal load 823 lb/MMBtu 354 kg/GJ 810 lb/MMBtu 348 kg/GJ

Flue gas per fuel 15 lb/lb 15 kg/kg 18 lb/lb 18 kg/kg

The overall impact on kiln operation is to produce different positions for the calcination zone within the kiln. This in turn leads to very different temperature profiles in the kiln for the operator between oil and natural gas firing (Figure 2). The zones for drying, heating and calcination change also which impacts on the kiln dynamics, responsiveness and capacity.

FIGURE 2 : TEMPERATURE PROFILE: HFO VS. NATURAL GAS

Economic drivers have been the prime reason for the industry move towards use of natural gas. With the discoveries of large quantities of shale gas derived natural gas, the price difference between HFO and natural gas has been maintained sufficiently high since 2009 (Figure 3).

FIGURE 3 : ECONOMIC DRIVERS : HFO VS. NATURAL GAS

CASE STUDY - INTRODUCTION For the case study, a European-based lime kiln with recently installed KFS combustion equipment is evaluated. The kiln is of modern configuration, 100 m by 3.8 m (328 ft by 12.5 ft) with latest technology product coolers and external dryer. Kiln dust is collected in a electrostatic precipitator and returned directly to the feed-end of the kiln. Nominal design production rate is around 350 te/d CaO (385 st/d) with a burner design rated for heat release of around 28-35 MW Net (106-132 MMBtu/h Gross).

As with all our projects, a baseline for current operation is measured and analyzed for future design and engineering work. The baseline for the base case is summarized in Figure 4 below. This data is used to establish the combustion characteristics and input parameters for subsequent modeling and burner design (Figure 4).

FIGURE 4 : CASE STUDY - PLANT PROCESS ANALYSIS

Issues in operation reported by the mill were: repeated cooler plugging leading to kiln downtime (Figure 5) excessive shell temperatures primarily caused by the flame envelope almost completely filling the kiln

cross-section (Figure 6); the flame appears loose without shape excessive dust in the kiln hood region leading to nuisance flame out trips

FIGURE 5 : CASE STUDY - RESTRICTION OF COOLERS DUE TO PLUGGING

FIGURE 6 : CASE STUDY - FLAME PROFILE UNDER EXISTING OPERATION

Additionally, the mill wished to reduce NOx emissions and improve kiln efficiency. CASE STUDY - PHYSICAL MODELING This process has been described previously [4]. The model of the case study kiln is shown in design and as fabricated (Figure 7).

FIGURE 7 : CASE STUDY - PHYSICAL MODEL

Aerodynamic Modeling The technique allows dynamic visualization of the system aerodynamics by use of neutral buoyancy tracers (or beads) to “label” the flows. Figure 8 shows the model run for the case study under current conditions. Relatively straight flow lines along the axis of the kiln indicate very limited swirl and good aerodynamics for combustion. Therefore, no corrective work was required to straighten airflows generated from the cooler.

FIGURE 8 : CASE STUDY - AERODYNAMIC MODELING

Combustion Modeling This is used to define the stoichiometric mixing envelope between fuel and oxygen and ultimately define flame shape within the kiln. This allows interpretation of the point where combustion is complete and if there is any flame impingement on refractory. Figure 9 shows operation with the current burner and the replacement burner. The existing burner was required to operate with excessive momentum to keep anchored to the burner and central to the kiln. This resulted in flame impingent at the walls and lack of flame stability at the tip. The upgraded burner showed significant improvement. The new design allowed for operation at lower momentum resulting in a more stable flame profile (no longer puffing as in current operation) with the flame mixing envelope improved. The flame was more centralized away from refractory.

Original Burner

New Burner

FIGURE 9 : CASE STUDY - COMBUSTION MODELING

CASE STUDY - RESULTS Installation The new burner was installed in 2018 (Figure 10). Optimization was completed at the end of the year, and a tuning visit completed in Mar-19.

FIGURE 10 : CASE STUDY - NEW BURNER INSTALLATION

Plugging and Kiln Availability The overall operation of the kiln improved immediately with the plugging issue primarily resolved leading to increased kiln up-time in operation (Figure 11). The increased kiln availability was further improved through significantly less dusting minimizing downtime due to nuisance flame out trips.

Original Burner New Burner

FIGURE 11 : CASE STUDY – PLUGGING ISSUE RESOLVED

Shell Temperatures Prior to the burner upgrade the mill were concerned with high shell temperatures particularly in the burning zone. The flame profile and heat transfer characteristics previously describe were directly responsible. The new burner provided an significant improvement in flame profile and heat transfer resulting in the lower shell temperatures (Figure 12).

FIGURE 12 : CASE STUDY – SHELL TEMPERATURES

Tuning - Kiln Temperature Profile The new burner design is very quick and easy to tune. During the tuning visit in Mar-18, the kiln operation was dramatically improved within a shift period as shown in Figure 13 below.

FIGURE 13 : CASE STUDY – BURNER TUNING - TEMPERATURE

Tuning - Emissions As the kiln settled during the shift, the kiln was operated at lower feed-end O2 levels with consistently low CO and reduced NOx as measured in mg/Nm3 (Figure 14). [Note: Values emitted for confidentiality reasons].

FIGURE 14 : CASE STUDY – BURNER TUNING - EMISSIONS

SUMMARY Natural gas has historically performed less well in lime recovery kilns leading to production limitations and product quality issues primarily due to its lower radiative heat transfer. New burner designs have provided the opportunity to get gas burners operating much closer to the traditional oil burners in terms of kiln temperature profile and production. A case study has been provided to demonstrate the impact of latest technology on overall kiln performance, including the ability to tune the burner operation on the run. The original kiln performance in the case study was limited due to burner design and operation. Replacement with latest patented technology resulted in eliminated the long-term plugging problems leading to increasing kiln availability and annual production. The improved product quality reduced dust generation from coolers into the kiln hood region leading to less flame-out nuisance trips further increasing kiln availability and annual production. The lower dust generation led to lower dust leaving the kiln and reducing load on the dust handling systems resulting in improved opacity in the stack. The better heat transfer also improved operating kiln efficiency through lower carbonates, lower shell temperatures (also reducing concern on mechanical wear and potential outages). Burner tuning was able to demonstrate rapid changes and improvement to lower NOx assisting in meeting new environmental targets. REFERENCES 1. Jenkins, B.G. and Mullinger, P.J., “Industrial and Process Furnaces: Principles, Design and Operation”,

Chapter 6, 2008, Hardcover ISBN: 9780750686921 2. “Lime Kiln Combustion, Heat Transfer and Optimization”, Section 2.6, TAPPI Kraft Recovery Course

(KROS), Florida, USA, January 2019

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Target NOx

3. Manning, R.P., Tran, T.N., “Impact of Co-firing Biofuels and Fossil Fuels on Lime Kiln Operation”, International Chemical Recovery Conference, Tampere, Finland, June 2014

4. Beddows, M.L., Champion, J., Manning, R.P., Sansom, C.P. and Xu, Z.G., “From Physical Modeling to

CFD: - 20 Years of Development in Kiln Modeling Techniques”, TAPPI PEERS, Portland, OR, October 2018

5. Manning, R.P., Sansom, C.P. and Xu, Z.G., “Modeling Techniques Used to Examine Fuel-Air Mixing in

Lime Kilns”, TAPPI PEERS, Jacksonville, FL, 2016

Improved Burner Designs for Optimized Lime Recovery Kiln Performance On Natural Gas Fuel

by

Martin Beddows, Richard Manning and Leo Newell

combustion by design

Improved Burner Designs for Optimized Kiln Performance on Natural Gas

page 2© KFS

104

303

2

41

Metso KFS

◼ Established 1999 to provide complete combustion systems for rotary kilns– detailed engineering evaluation to produce custom design for each kiln

– use in-house developed software and validated physical modeling techniques combined with CFD

◼ Developed patented proprietary burner design technology

◼ Unique database of over 230 lime recovery kilns worldwide allowing empirical and parametric modelling to analyze opportunities for improvement

◼ Applications worldwide in Pulp & Paper, Lime, Alumina and Minerals industries

◼ Acquired by Metso Dec-18Worldwide Burner Install Base


page 3© KFS

Abstract

◼ Increasing production and reducing operating costs is a constant battle for the operators of lime recovery kilns in pulp mills– operating costs are mainly driven by the underlying cost of fuel

◼ In the past 10 years oil and natural gas have been the most economic fuels– each kiln fuel has a differing effect on the kiln production and operation

– this is most noticeable with the conversion to natural gas which produces a flame which is less radiant than oil or petcoke

◼ KFS has developed a kiln burner design to minimize the impact on the reduced radiance from the natural gas flame

◼ This paper compares kiln performance between a traditional kiln burner and the new burner development using site data collected following a recent installation


page 4© KFS

TRADITIONAL FUELS....

NATURAL GAS AND OIL


page 5© KFS

CH4 + 2O2 + 7.7N2

combustion products

CO2 + 2H2O + 7.7N2

Basic Combustion


page 6© KFS

Heat Transfer In Lime Kilns - HFO vs. Gas

Distance from Burner

Natural Gas

Heavy Fuel Oil

Fo

rce o

f H

eat

to W

all

s a

nd

Bed

Ma

teri

al


page 7© KFS

Air Requirements - HFO vs. Gas

Parameter

Heat Release – Net 17,410 Btu/lb 40.5 MJ/kg 935 Btu/scf 36.7 MJ/m3

Heat Release – Gross 18,440 Btu/lb 42.9 MJ/kg 1,037 Btu/scf 40.7 MJ/m3

C:H mass ratio 8:1 8:1 3:1 3:1

Combustion air per fuel 14 lb/lb 14 kg/kg 17 lb/lb 17 kg/kg

Combustion air per thermal load 823 lb/MMBtu 354 kg/GJ 810 lb/MMBtu 348 kg/GJ

Flue gas per fuel 15 lb/lb 15 kg/kg 18 lb/lb 18 kg/kg

HFO Natural Gas


page 8© KFS

Temperature Profile In Lime Kilns - HFO vs. GasTem

pera

ture

(°F

)

Product Feed-end

Natural Gas

Heavy Fuel Oil

Tem

pera

ture

(°C

)

500

600

1800

2000

250

350

1000

1100


page 9© KFS

Economic Drivers - HFO vs. Gas

Fuel Price References:[1] Natural Gas: https://www.eia.gov/dnav/ng/hist/rngwhhdm.htm[2] Heavy Fuel Oil: https://www.macrotrends.net/2479/heating-oil-prices-historical-chart-data

https://www.eia.gov/dnav/ng/hist/rngwhhdm.htm

https://www.macrotrends.net/2479/heating-oil-prices-historical-chart-data


page 10© KFS

Environmental Drivers

◼ CO2

– generation from lime kiln combustion only

– 155 kmol/h from oil vs. 126 kmol/h natural gas

– 19% reduction

◼ NOx

– can be lower with natural gas


page 11© KFS

CASE STUDY…


page 12© KFS

Case Study - Introduction

◼ European-based lime kiln with recent KFS installed combustion equipment

◼ Modern lime recovery kiln configuration

– 100 m by 3.8 m (328 ft by 12.5 ft) with modern product coolers and external dryer

– ESP dust handling system

◼ Production rate of ~350 te/d CaO (385 st/d)

◼ Heat release of ~28-35 MW Net (106-132 MMBtu/h Gross)


page 13© KFS

Case Study - 2018 Performance


page 14© KFS

Case Study - Process and Combustion Audit Observations

◼ Operational issues

– repeated cooler plugging

– excessive shell temperatures

◼ Operational needs

– NOx reduction

– efficiency improvement

Restriction of coolers due to plugging


page 15© KFS

Case Study - Existing Burner Observations

◼ Flame filling almost complete cross-sectional area

◼ Sub-optimal flame ↔ bed interaction

◼ Lack of flame shape (“loose”)

◼ Excessive dust in kiln hood region leading to nuisance flame out trips

page 15


page 16© KFS

Case Study - Scale Model (37:1)

◼ Secondary air passes through cooler inlet and cooler grates

Burner opening

Cooler grates

Cooler inlet

Actual modelModel drawing


page 17© KFS

Case Study - Aerodynamic Modeling

◼ Existing conditions

◼ Burner not modeled

◼ Good kiln aerodynamics – no indication of excessive swirl motion

◼ No indication for need to modify secondary air openings

◼ No indication for need to change burner insertion distance


page 18© KFS

Case Study - Combustion Modeling Existing vs KFS Burner

Existing burner

KFS burner

◼ Flame profile more central with much less disturbance or wall impingement with KFS burner


page 19© KFS

Case Study - Combustion Modeling KFS Burner

◼ Poor burner design required very high momentum to main flame profile

◼ Significant improvement with KFS burner

◼ Lower momentum from KFS burner as a result of improved design

◼ KFS produces steady flame (no longer puffing)

◼ Flame mixing envelope improved - flame centralized away from refractory


page 20© KFS

Case Study - Installation and Commissioning

Natural gas inlet

Primary air inlet

Fiber optic flame scanner


page 21© KFS

Case Study – Plugging Resolved

ORIGINAL NEW BURNER


page 22© KFS

Case Study - Flame Shape Comparison Before and After Install

ORIGINAL NEW BURNER


page 23© KFS

Case Study – Shell Temperatures


page 24© KFS

Case Study – Performance Enhancement Through Tuning


page 25© KFS

Case Study - Emissions Tuning


page 26© KFS

Case Study – NOx Emissions vs Site Target Value

◼ NOx values 6% less than target value (average) across test period


page 27© KFS

Case Study – NOx Emissions vs Site Target Value

◼ Tabulated values highlight the excellent performance of the KFS burner after installation and commissioning

Parameter KFS vs Previous Burner (%)

Total Production 15% Increase

CaO Production 15% Increase

Maximum Shell Temperature 15% Reduction

Average NOx 11% Reduction

SFC Total Production 3% Reduction

SFC CaO 3% Reduction


page 28© KFS

Summary

◼ Kiln performance was limited due to burner design and operation

◼ Replacement with latest patented technology resulted in:

– eliminated plugging, increasing kiln availability and annual production

– reduced dust generation from coolers leading

– to less flame out nuisance trips increasing kiln availability and annual production

– to less loading on dust handling systems and improved opacity in the stack

– lower shell temperatures reducing concern on mechanical wear and potential outages

– overall improved kiln efficiency

– better heat transfer, lower shell losses and increased kiln up-time

– lower NOx assisting in meeting new environmental targets

Documents

TAPPI PEERS 2019