Electricity + Control June 2017

PRESSURE + LEVEL MEASUREMENT

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].

I 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 High temperature and pressure from the steam allows en- ergy to be withdrawn by a rotating turbine rotor.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

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.

Fluctuating raw material feeds to the works

Plant processes producing off-gases

Plant processes

Plant off-gas usages

Producing steam in boiler houses

Flaring off-gases

Power generation through steam turbines

Plant steam usages

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

Electricity+Control June ‘17

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