The Combustion Process from the Perspective of Energy Optimization

There is no single solution for all production needs encountered at a mill; broad-range solutions tailored to specific needs are best.

LUCA LINARI, ROSSANO PUCCI

Toscotec’s Energy and Environment Department presents the procedures used to optimize combustion, including the latest technological improvements. Toscotec’s research is focused on achieving a reduction in gas consumption as well as reducing emissions into the atmosphere.

We will examine various methods of combustion control with particular attention to the air involved in combustion and the mixing of the air used for the process, and make observations in the area of fluid dynamics. And we will focus on the burner remote service (BRS) and the optimization of the burners in order to control the combustion parameters, as well as the burner management system (BMS).

This article will illustrate the key factors involved in this project: consumption, emissions, and service. It also covers the combustion systems installed in the drying plant, i.e., in the yankee hoods for tissue machines.

The classification we are proposing largely stems from a practical and functional perspective. In doing so, of course, we referred to the principal manufacturers of burners in the tissue sector.

We will talk about combustion systems and the state-of-the-art grid- and corner-type burners, about the classification of combustion systems and the methods of controlling them, and about service and technological improvements. And we will present some examples of results found on-site.

A combustion system is defined as the set of devices that generate the combustion process (i.e., burner and gas train), those that confine it physically (the combustion chamber), and those that control it, the BMS.


Fig. 1: Corner burner

Fig. 2: Grid burner (air vein or in-line burner)

Based on the specific needs of the plant, a correct decision on these devices, i.e., correct design, construction, and commissioning of the combustion system, allows attaining high energy efficiency and a reduction in CO and NOX emissions.

BURNER TYPES

Corner burners and grid burners (also known as air vein or in-line burners):

These will be analyzed according to three aspects that characterize them:

  • The regulation of the air/gas ratio
  • The effects of excess air
  • CO and NOX emissions

Regarding differences between the corner and grid types, it is important to note that:

  • Air/gas ratio regulation in corner-type burners is of the proportional type, while in grid burners it is usually of the fixed-air type (not proportional to the gas), except for special configurations (low-emission burners).
  • Excess air in the corner-type burners ranges between 8 percent and 40 percent, depending on the operating load relative to the design point, while in the grid-type burners excess air varies between 40 percent and 55 percent (except for low-emission burners that operate with 20 percent excess).
  • CO and NOX emissions in commonly used burners vary over a wide range; in particular the CO value strongly distinguishes corner-types from grid-types (corner burners: 40-50 to 70-80 mg/Nm3 @ 17 percent O2 and grid burners: 100-130 to 40-60 mg/Nm3 @ 17 percent O2).
  • The temperatures of the combustion air for the corner-types have maximum values of 250-300°C, while the grid-types usually 200°C maximum (some special burners can operate at a maximum temperature of 300°C).

Classifications – SWIRL-type Corner – CRZ (central recirculation zone)

A hypothetical classification can be made by types:

Corner burner:

  • based on the type of mixing: swirl – the flow of combustion air and gas mix tangentially in the burner body; partial-swirl and non-swirl – mixing between the flow of combustion air and the gas is predominantly axial
  • based on the air/gas ratio control system, which can be mechanical or electronic
  • based on NOX emissions: standard, low or ultra-low

Grid burner:

  • based on the air/gas ratio control system, which can be mechanical or electronic
  • based on NOX emissions: standard, low or ultra-low

A short note on the swirl-type corner, which will probably dominate the market in the future: It can be defined by the swirl number, which, in simple terms, is defined as the ratio between the angular and axial momentum of the combustion air flow.

Swirl burners generate a defined “central recirculation” zone that acts by stabilizing the flame and reducing it in size (shortening and shrinking it), thus allowing a reduction in emissions and allowing it to operate with an air/gas ratio close to optimal.

All combustion systems installed by Toscotec are equipped with a BMS. The BMS is a management system that allows, in safety conditions, starting, operation, and stopping of the burner.

The system consists primarily of valve seal control, flame control, and all the safety devices installed on the burner and the gas valve train: for example, the UV flame detector, the gas shut-off valve and gas safety shut-off valve, and the pressure switches for minimum and maximum air and gas pressure (all devices are certified for gas applications).

The BMS also provides for handling the air/gas ratio control, which can be mechanical or electronic.

The mechanical air/gas ratio control involves the use of an actuator installed on the gas valve, while the combustion air valve is operated simultaneously by mechanical linkage.

The electronic air/gas ratio control instead involves two actuators, one installed on the gas valve and the other on the combustion air valve.

It also involves the use of an electronic device, for example the Siemens LMV51/52 or the Smartlink (Maxon), where it is possible to configure a software curve internally by setting its intermediate points so as to optimize emissions and consumption.

The use of a PLC allows configuring multiple software curves of the air/gas ratio so as to meet certain specific production needs, optimizing consumption and emissions.

With both devices (Siemens LMV – PLC) it is possible to read the process signal coming from the O2 sensor.

The air/gas ratio control is of great importance for thermal efficiency in terms of cost, for the control of the main pollutants, and for safety of operation.

In particular:

  • Excess combustion air entails a reduction in the temperature of the flame, resulting in an increase in consumption and CO, and a reduction of NOX.
  • The lack of combustion air causes an increase in the temperature of the flame, resulting in an increase of NOX and an apparent decrease in consumption (apparent because in the medium-long-term the burner body could overheat, generating an increase in consumption and maintenance).
  • The high temperature of the combustion air involves an increase in the temperature of the flame, resulting in a decrease in consumption and an increase of NOX.

The right compromise between air and gas allows optimizing emissions and decreasing consumption.

Figure 6 shows the relationship between the typical curve of pollutant emissions, lambda (λ), and oxygen (O2). Lambda represents the ratio between the mass of the combustion air and that of the fuel.

Lambda is equal to 1 when combustion is stoichiometric, i.e., the perfect ratio between the mass of the combustion air and that of the fuel so that combustion is complete.

  • λ = 1, when combustion is stoichiometric;
  • λ < 1, when there is an excess of fuel;
  • λ > 1, when there is an excess of air.

In the graph, we can see that when the oxygen increases above the optimal range of lambda (λ), the efficiency diminishes. However, when the oxygen decreases below the optimum range of lambda, combustion is incomplete with high CO.

The symbol CO identifies carbon monoxide (or carbon oxide), whereas the symbol NOX identifies generically the oxides of nitrogen.

The NOX can be classified according to the following ranges:

  • Up to 7 – 10 as ultra-low
  • Up to 15 – 20 as low
  • Up to 50 – 80 and beyond as standard; Expressed in mg/Nm3 @ 17 percent O2

SERVICE

All combustion systems installed by Toscotec can be equipped with a remote assistance system, the BRS, which allows for the supervision of all the operations of the burner, the display of flow rates, pressure and operating parameters, and recording the history of alarms and statuses.

The benefits of this system are constant monitoring of performance, reduction in the use of the on-site assistance service, and predictive maintenance.

For example, if we remotely verify that the burner is working with too low a difference in pressure of the combustion air, we can intervene promptly to prevent the burner body from deforming due to lack of air.

The BRS can be implemented in two ways. The first solution is to install a PLC, an operator panel, and a modem in the local electrical control panel of the burner.

The second solution involves installing, again inside the burner’s electrical control panel, a PLC and an operator panel. A PC equipped with TeamViewer is also used.

The first solution is used by Toscotec in stand-alone systems, while the second is used when the air system plant is managed by the machine’s DCS.

TECHNOLOGICAL IMPROVEMENTS

As far as technological improvements are concerned, as usual there is a distinction between corner and grid burners. For the corner type, swirl burners have been discussed, but to maximize its benefits, it is important to create combustion chambers that favor a progressive mixing of the air used in the process. Toscotec has developed patented chambers that can be defined as multi-staging, that are also applied to non-swirl burners because they yield useful benefits in that case, too.

These chambers are equipped with integrated mixing devices that allow reducing the average temperature of the flame, thus reducing NOX emissions, making the distribution of the output pressures and temperatures uniform. The use of this mixing device has a significant combined effect on reducing emissions and consumption.

Still on the subject of technology improvements for corner burners, we can add the use of the oxygen sensor, used in our read-only applications, which allows optimizing the combustion process.

Regarding technological improvements for grid burners, noteworthy are the progressive, pre- and post-combustion mixing devices, which make the flows uniform in terms of pressure and temperature.

In conclusion, it has been observed that emissions and consumption depend both on the type of burner and on the combustion chamber and the characteristics of the control systems, but there is no single solution for all the production needs encountered on-site.

INTEGRATED MIXERS

  • reduction of the average flame temperature
  • exponential decrease in NOx with thermal mechanism (thermal NOx)
  • more uniform distribution of pressures and temperatures

ON-SITE RESULTS

Table 2 presents examples of results in terms of emissions detected on-site with standard grid, corner, and corner at high temperature burners. The table gives a brief summary of the operating and consumption conditions for a better context to the applications.

As can be seen from Table 3, emissions (expressed in mg/Nm3 at 17 percent O2) are relatively low in terms of CO for the corner-types, and quite well aligned in terms of NOX. Note the performance of the grid burner at low emissions, especially in terms of NOX.

Figure 9 has been created by using one of the instruments that our on-site service team is equipped with to perform analyses on both the plant’s drying capacity and the emissions.

In conclusion, it has been observed that emissions and consumption depend both on the type of burner and on the combustion chamber and the characteristics of the control systems, but there is no single solution for all the production needs encountered on-site. At Toscotec we can design broad-range solutions tailored to specific needs, including taking into account all the various factors involved, such as, for example, the specific characteristics of the burner.

Luca Linari, head of Energy & Environment Division, Toscotec; Rossano Pucci, process application engineer, Energy & Environment Division, Toscotec.

 

  Weak Medium High
Swirl <<1 0.6 – 1 >1
Pressure gradient insufficient high very high
Recirculation zone not present reduced wide

 Table 1: Classifications: Swirl-type corner – CRZ
(central recirculation zone)

Fig. 3: Swirl burner

Fig. 4: Burner management system (BMS)

 

Fig. 5: Mechanical air/gas ratio

Fig. 6
Emissions – Typical O2 diagram example

Fig. 7: CFD Gas – Vectorial velocity of distribution

Fig. 8: CFD flame – gases temperature distribution

Table 2: Operating and consumption conditions

  PM GRID PM CORNER PM CORNER MS – HT  
Production
(100% effic.)
175 94.5 104  T/24h
Type of Pulp/Product Toilet Toilet KT  
Machine configuration 2 Presses 2 Presses 1 Press  
Dryness at pope 95 94 94  %
Yankee dryer diam. 18 15 15  Ft
Operating steam pressure 7 6 6.5  Bar
Blowing temperature 345 360 610  °C
TOTAL water evaporated 8610 4360 6417  Kg/h
  CONSUMPTION DATA – GROSS PRODUCTION  
Gas 53.7 48.2 95.10  Nm3/T
Steam 1.36 1.25 1.19  T/T
Total GAS YD+H 131.66 119.86 163.32  Nm3/T

 

Table 3: Emissions

  PM GRID

(70%)

PM CORNER

(65%)

PM CORNER MS – HT (84%) PM GRID (LE)

(71%)

 
CO – 17% O2 66.00 19.00 11.00 39.7 mg/Nm3
NOx – 17% O2 47.00 63.00 56.00 13.9 mg/Nm3

Fig. 9: On-site examples that can be provided by the service team