It is desired to reduce the concentration of C02 in the

Chapter 15, Problem 15.3

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QUESTION:

It is desired to reduce the concentration of \(CO_2\) in the flue gas from a coal-fired power plant, in order to reduce greenhouse gas emissions. The effluent flue gas is sent to an ammonia scrubber, where the most of the \(CO_2\) is absorbed in a liquid ammonia solution, as shown in Fig. E15.3. A feed forward control system will be used to control the \(CO_2\) concentration in the flue gas stream leaving the scrubber, \(C_{co_2}\), which cannot be measured on-line. The flow rate of the ammonia solution entering the scrubber, \(Q_A\), can be manipulated via a control valve. The inlet flue gas flow rate, \(Q_F\), is a measured disturbance variable.

Using the available information, do the following:

(a) Draw a block diagram of the feedforward control system. (It is not necessary to derive transfer functions.)

(b) Design a feedforward control system to reduce \(CO_2\) emissions based on a steady state analysis.

Available Information:

(i) The flow sensor-transmitter and the control valve have negligible dynamics.

(ii) The flow sensor-transmitter has a steady-state gain of 0.08 mA/(L/min).

(iii) The control valve has a steady-state gain of 4 (gal/min)/ mA.

(iv) The following steady-state data are available for a series of changes in \(Q_A\):

\(\begin{array}{cc} Q_{A}(\mathrm{gal} / \mathrm{min}) & C_{\mathrm{CO}_{2}}(\mathrm{ppm}) \\ 30 & 125 \\ 60 & 90 \\ 90 & 62\end{array}\)

Questions & Answers

QUESTION:

It is desired to reduce the concentration of \(CO_2\) in the flue gas from a coal-fired power plant, in order to reduce greenhouse gas emissions. The effluent flue gas is sent to an ammonia scrubber, where the most of the \(CO_2\) is absorbed in a liquid ammonia solution, as shown in Fig. E15.3. A feed forward control system will be used to control the \(CO_2\) concentration in the flue gas stream leaving the scrubber, \(C_{co_2}\), which cannot be measured on-line. The flow rate of the ammonia solution entering the scrubber, \(Q_A\), can be manipulated via a control valve. The inlet flue gas flow rate, \(Q_F\), is a measured disturbance variable.

Using the available information, do the following:

(a) Draw a block diagram of the feedforward control system. (It is not necessary to derive transfer functions.)

(b) Design a feedforward control system to reduce \(CO_2\) emissions based on a steady state analysis.

Available Information:

(i) The flow sensor-transmitter and the control valve have negligible dynamics.

(ii) The flow sensor-transmitter has a steady-state gain of 0.08 mA/(L/min).

(iii) The control valve has a steady-state gain of 4 (gal/min)/ mA.

(iv) The following steady-state data are available for a series of changes in \(Q_A\):

\(\begin{array}{cc} Q_{A}(\mathrm{gal} / \mathrm{min}) & C_{\mathrm{CO}_{2}}(\mathrm{ppm}) \\ 30 & 125 \\ 60 & 90 \\ 90 & 62\end{array}\)

ANSWER:

Step 1 of 3

(a) The feed forward control system shown below is used to manage the concentration of \(\mathrm{CO}_{2}\) in the flue gas stream.

Consider the below terms:

Controller variable is denoted by \(Y\)

Manipulated variable is denoted by \(Q_{A}\)

Disturbance variable is denoted by \(Q_{F}\) 

Controller output is denoted by \(P\)

Error signal is denoted by \(E\)

Measured value of Y is denoted by \(Y_{m}\)

Set point is \(Y_{sp}\) 

The measure value of Y is denoted by \(\bar{Y}_{s p}\)

Change in \(Y\) due to \(Q_{A}\) is denoted by \(Y_{Q}\) 

Change in \(Y\) due to \(Q_{F}\) is denoted by \(Y_{QF}\)  

Transfer function for the final control element is represented by \(G_{v}\)

Process transfer function is denoted by \(G_{p}\)

Disturbance transfer function is denoted by \(G_{QF}\)

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