Electricity + Control June 2017

PRESSURE + LEVEL MEASUREMENT

trips, steam availability must stabilise before it may be restarted. This

stabilising period is some continuous time interval where sufficient

steam must be present to keep the turbine operational. The basic

layout of the engineering works under consideration which produces

off-gases, steam from residual off-gases and ultimately generate

electricity through steam turbines is given in

Figure 1

A mathematical model was formulated in [2] to address optimal

power co-generation and [3] demonstrated the applicability of this

model. It was further investigated by [3] how power co-generation

at this engineering plant could improve if the optimisation model

from [2], rather than the plant’s operational philosophy, was used.

This article investigates the additional energy potential that a power

co-generation plant may potentially utilise from residual off-gases

being flared. Since power generation is not the core business of this

engineering works, focus is mainly placed on delivering end-products

and flaring percentages were never investigated and believed by

management to be regulated at 10% of the total off-gas flow.

To demonstrate the potential influence on power co-generation

the mathematical model from [2] will be used for various scenario

simulations.

Figure 1: A basic generic layout for the engineering works.

Utilising Fluctuating Off-Gases

With Less Flaring

Philip Venter, Fanie Terblanche, Martin van Eldik, University of the North-West

For an engineering plant to stay competitive, day-to-day operations must be continuously improved. These improvements may range from

procuring more energy efficient plant equipment to addressing operational procedures [1].

n process engineering plants, various by-products are being

formed throughout the production chain… delivering a number

of end products. These by-products should be utilised fully to

generate maximum revenue. If a by-product is in a gaseous form and

possesses the ability to combust in an oxygen enriched environment,

it is known as a burnable off-gas and will be utilised by the engineer-

ing works as an energy resource. For all further reference, mention

of an off-gas will imply a burnable off-gas.

Common engineering practice is to generate steam, in boiler

houses, from off-gases that are not used in any of the works’ produc-

tion processes. These are referred to as residual off-gases. Off-gas

productions and the utilisation thereof forms part of a continuous

production process and thus all residual off-gases not utilised in the

boiler houses are flared into the atmosphere, wasting all of the energy

potential. However, a certain flow quantity or percentage of these

off-gases must always be flared to help with regulation of the off-gas

pipeline pressures, since the pipelines are open to atmosphere. Pres-

sure control is needed to prevent any air from entering the pipeline.

If air does enter and mix with the off-gases, an extremely hazardous

condition may arise.

Steam is utilised all over the engineering works for various pro-

duction and process heating purposes. The steam usage demands

must be met at all times. Only after addressing these demands may

the excess available steam be utilised by the power generating steam

turbines. High temperature and pressure from the steam allows en-

ergy to be withdrawn by a rotating turbine rotor. The turbine rotor

is coupled to a generator that converts the rotational energy into

electricity. Generating electricity, or power generation, under such

circumstances is also known as power co-generation.

A chemical process plant may experience non-uniformities in

chemical compositions of raw materials that enter the works or even

mass flow quantities that are not constant. These nonconformities

may result in production quantity or quality changes over time. One

result may be fluctuating of-gases and therefore steam flow produc-

tions. Furthermore, steam flow demands for plant usage purposes

are also not constant. This will contribute evenmore to the fluctuating

steam availability for power co-generation. Irregular steamflow to the

turbines will cause alternating power generation andmay result in low

steam availability at times that can cause turbines to trip. If a turbine

Plant processes

producing off-gases

Plant off-gas usages

Flaring

off-gases

Producing

steam in

boiler houses

Plant steam usages

Power generation through

steam turbines

Fluctuating raw material feeds to the works

Electricity+Control

June ‘17