coal fluidised bed steam generator pid tuning...coal fluidised bed steam generator – pid tuning 1...

PID Tuning Case Study

Coal Fluidised Bed Steam Generator – PID Tuning 1

This document contains proprietary information of IPCOS NV/BV and is tendered subject to the condition that no copy or

other reproduction be made in whole or in part for use other than Client's own internal use, and that no use be made of

information herein except for the purpose for which it is transmitted, without express written permission of IPCOS NV/BV

1. Introduction

Fluidised bed combustion is a combustion technology used to burn solid fuels. In its most basic form,

fuel particles are suspended in a hot fluidised bed of ashes and other particulate materials (sand,

limestone etc.) through which air is blown to provide the oxygen required for combustion. The

resultant fast and intimate mixing of gas and solids promotes rapid heat transfer and chemical

reactions within the bed. Fluidised bed combustion allows usage of a broad variety of low-grade solid

fuels, i.e., not only most types of coal but also woody biomass at reasonable high efficiency and without

need of further complex fuel preparation. In order to keep the amount of sulphur emitted in the form

of SOx under control, limestone is used to precipitate out sulphate after combustion.

This report refers to is a coal fluidised bed steam generator located in the Lorraine region, north-east

of France. The basic schematic of process and controls of the referred coal fluidised bed boiler is shown

in Figure 1.

Figure 1 – Coal fluidised bed boiler process and control scheme

Fresh coal is fed to the boiler together with the solids in circulation retained by the cyclone. The

combined fresh coal, in addition of limestone and sand, and the circulating char mass is directed to the

fluidised bed. Due to high heat transfer promoted by the sand, fresh coal releases its volatile

compounds right after being fed to the system. These light hydrocarbons are burned in an upper

section of the combustion chamber. Solid carbon combustion occurs on the surface of the char

particles – also known as surface combustion. As combustion occurs, carbon is converted into flue






gases and the char particles reduce in size and weight. The lighter they become, the easier they will be

carried over by the lifting gases, where the cyclone separates the fine ashes from the char particles

which will be kept in circulation for further combustion.

Primary air in combination with part of the combustion flue gas – or fumes – is set into the combustion

bed from underneath, fluidising the heavier particles and carrying the fine particles over to the top.

Primary air provides oxygen to the combustion whereas the fumes recirculation ensures enough gas

for fluidisation, at the same time keeping the total amount of oxygen limited for temperature control.

In order to keep CO emission limited, part of the primary air is injected at 0.5 m height instead of

underneath the bed.

In order to burn both light hydrocarbons expelled from the coal and by-products of char incomplete

combustion, one more air stream denominated secondary air joins the combustion bed at 2 m height.

This air stream is also used for oxygen excess control.

The total stream containing the flue gases and ashes particles are suctioned by an induced fan and

pass through a bag filter in order to protect equipment downstream. Part of the ashes-free flue gases

are recirculated as lifting gas and for combustion temperature, as explained above, whereas the

remaining part exchanges heat with the boiling feed water economizer. After cooled, the flue gases

are finally directed to the stack.

2. Starting Scenario Steam pressure (PC03): always in MAN before tuning

Fumes recirculation to fluidised bed (FC03): always in MAN before tuning

Fluidised bed combustion temperature (TC01): always in MAN before tuning

Steam flow (FC06): always in MAN before tuning the steam flow controller output drives the

carbon feed conveyors speed, used to calculate the actual load

Load controller (QC01): based on the coal feed conveyors speed and coal properties, a

calculation is done to estimate the actual total amount of coal input and the total required air

for combustion. This block receives speed information from the coal feed conveyors and sends

out a signal to both primary air flow controller and secondary air flow controller. The

parameters related to primary and secondary air split have been reviewed so that the master

load controller could be tuned

3. After Tuning Results

3.1. Steam Pressure – PC03

The steam pressure controller has been found to be always in MAN mode. It has been tuned and taken

in AUTO. Figure 1 shows the results of before tuning in MAN mode and after tuning and taking the

controller in AUTO mode.






Figure 1 – Steam pressure controller before and after tuning and setting to AUTO (blue = PV, green = OP)

3.2. Steam Flow and Desuperheater – FC06, TC03 and TC02

The steam flow controller has been found to be always in MAN mode. It has been tuned as a

2x2 system with the steam pressure using IPCOS INCA Aptitune and then taken in AUTO mode. Adjusts

on the load controller calculations were also made in order to allow proper utilisation of the steam

flow controller. Figure 2 shows the results of before tuning in MAN mode and after tuning and taking

the controller in AUTO mode.

Figure 2 – Steam flow controller FC06 before and after tuning and setting to AUTO (blue = PV, green = OP)






Note that the fast frequency cycles on Figure 2 are consequence of a problem detected on the BFW

injection valve problem, which has been fixed by the client during the last plant stop. The control

schema consists of a triple cascade where the final desuperheated steam temperature drives an

intermediate desuperheating temperature, which in turns drives the boiling feed water flow controller

(Figure 3).

Figure 3 – Desuperheater temperature control schema

After changing the BFW valve, both 1st and 2nd desuperheater temperatures could be properly tuned,

as shown in Figure 4 and Figure 5.

BEFORE AFTER

Figure 4 – Intermediate desuperheater temperature control TC02 (red = set-point, blue = PV, green = output)

.

BEFORE AFTER

Figure 5 – Final desuperheater temperature control TC03 (red = set-point, blue = PV, green = output)

Superheated steam

Saturated steam

BFW injection

FC05

TC02

TC03






3.3. Fluidised Bed Temperature – TC01 The steam flow controller has been found to be always in MAN mode. Its slave controller, the fumes

recirculation flow controller, has also been found to be always in MAN. Both controllers have been

tuned and taken in AUTO. Figure 6 shows the results of before tuning in MAN mode and after tuning

and taking the full cascade in AUTO mode.

Figure 6 – Fluidised bed temperature TC01 before and after tuning and setting to AUTO (blue = PV, green = OP)

3.4. O2 Excess – AC01

After adjustments related to the split between primary and secondary air, the master load controller

could be taken in service. The tuning work revealed the possibility to reduce the O2 excess set point

from 6% to 2.5%, for operation at reduced load only (Figure 7). The marked areas on Figure 7 refer to

a special manoeuvre carried out with the coal feeder.

Figure 7 – Boiler flue gas % O2 excess control AC01 before and after PID tuning assessment






4. Conclusion Coal fluidised bed steam generators are complex systems for process control due to the combined

presence of fast and slow dynamics processes, circulation of coal and partial recycle of the flue gases.

On top of that, the different air inputs play each one a different role in the combustion process, which

means that the split between these streams is fundamental for achieving a good trade-off between

energy efficiency and control of fluidisation, combustion emission levels. As expected, such a system

consists of many interactive control loops.

The base-layer control assessment carried out by IPCOS allowed the client to operate the steam

generator more stably and efficiently. Moreover, it also revealed the malfunctioning of the

desuperheater BFW injection valve, which has been replaced. As a result, not only the steam generator

has been brought to a more stable and efficient condition but also the steam quality delivered to on-

site production plants has been increased due to both steam pressure and temperature fine control.

All PID tuning activities have been carried out with IPCOS INCA Aptitune, which offers the possibility to

obtain optimal tuning for interacting systems – as the pair steam flow and pressure, for example.

coal fluidised bed steam generator pid tuning...coal fluidised bed steam generator – pid tuning 1...

Documents