Nov-2021

Don’t let furnace emissions derail your expansion project

Predicting emissions prior to actual furnace design is not just a mathematical calculation

SHILPA SINGH and ANKUR JAIN
Engineers India Limited

Article Summary

India’s national Ministry of Environment, Forest and Climate Change (MoEFCC) promulgated an environmental impact assessment (EIA) notification, making environmental clearance mandatory under the Environmental (Protection) Act 1986 for expansion or modernisation of an existing industrial set-up or for setting up new projects. Hence, to set up a new oil refinery or to revamp an existing refinery for capacity expansion, and so on, environmental clearance is required to ensure that emissions from the industrial installation remain within permissible limits. For the energy intensive hydrocarbon processing industry, fuel firing is a necessary evil which cannot be done away with completely. Hence, every effort is made to maximise the heating potential of all fuels being burnt. If allowed to go uncontrolled, their emissions can create havoc for flora, fauna and habitation in and around the refinery complex. Over the years, emissions norms have been made more and more stringent in order to minimise impact on the environment and climate. This article explores various means for optimising gaseous emissions from fuel fired refinery furnaces. The article addresses the topic from an Indian perspective, although the strategies can be extended to any other scenario’s or country’s specific guidelines.

Regulations restricting furnace stack gaseous emissions fall into two categories. The first is the maximum concentration limit of pollutants like SOx, NOx, and CO in the flue gas discharged from individual stacks. Limiting values for these pollutants as per the MoEFCC notification are shown in Table 1.

The second regulation limits the total allowable quantity of each pollutant emitted from a complex undergoing revamp or installation. The total allowable value is provided by the authority concerned on a case to case basis after due review of multiple factors including the effects of pollution on people living in the vicinity. Accordingly, the maximum allowable limit may be lower near densely populated cities, whereas in a sparsely populated coastal area, where there is less risk to humans, higher values may be allowed. Within the same location or plant, the current allowable limit is much lower than might have been allowed 10 years ago. Meeting the total allowable limit requirement for a complex is critical. Even if the concentrations of pollutants in individual stacks as per Table 1 are met, it may be that the total quantity or flow rate of pollutants will exceed the allowable limit allocated for the overall complex on account of significant augmentation of capacity.

Predicting emissions prior to actual furnace design is not just a mathematical calculation. A consultant’s past experience brings a lot of value through optimisation and revisiting critical design aspects at the unit conceptualisation stage, helping to achieve the desired emissions as well as optimum engineering. This article discusses a number of strategies including process duty optimisation, heat recovery, alternative fuels, burner replacement, and so on, in an expansion revamp or in a grassroot refinery project. Through these, a reduction in emissions can be realised for environment clearance and compliance with statutory requirements. The special point about the strategies discussed here is the ‘least investment’ nature of the ideas compared with capital intensive routes such as DeNOx or DeSOx systems. In other words, these ‘low hanging fruit’ strategies may be explored first before opting for capital intensive equipment.

For a fair understanding of the strategies shown in Figure 1, a step-wise approach is taken with the aid of a case study of a typical refinery, operating for 15 years, with the multiple heaters described in Table 2.

An expansion is planned for the refinery to add 100% additional capacity with a similar configuration of units/heaters. Hence after revamp pollution is expected to double: total SOx will be 262.6 kg/h and NOx will be 189.4 kg/h.

But in view of stringent pollution norms, similar emission limits may not be permitted by the regulatory authority.

Typically, the allowable emissions cap for a new refinery is approximately half of what was allowed a few decades back. Therefore it is assumed for the case study that the total allowed value of SOx for the combined revamp capacity (100% original capcity + 100% new capacity) is 200 kg/h and the allowable NOx value is 140 kg/h. To arrive at a limiting value of 200 kg/hr, SOx emissions from the new capacity must be limited to 68.7 kg/h and NOx is required to be limited to 45.3 kg/h to meet a 140 kg/h total limit.

To comply with these targets, it is evident that replicating the existing configuration will not suffice. Therefore thought has to be put into reducing emissions significantly. With this aim in mind, the following avenues were explored for the additional capacity.

Can you further optimise furnace absorbed duty?
The first critical step in estimating emissions is finalisation of the fired duty of the new heaters. Fired duty depends on the process requirements and efficiency of the system. Exact process data may not be available at such an early stage of project conceptualisation. At this point, past experience comes in handy in making initial estimates of process loads. Due corrections become necessary as soon as the licensor’s/designer’s data is made available. A fundamental step is to check carefully the process duty requirement as unnecessary margins for an infrequent operating scenario may add up in firing duty, with a corresponding increase in emissions. This is quite common in hydroprocessing units where the furnace’s actual operating duty is considerably less compared to the furnace design duty. Thus emissions worked out for furnace design duty will also project an inflated figure.

For example, it is quite often observed that the heater design duty quoted by the unit licensor is twice that required for all normal operating scenarios. Because of this variation in normal operating and design duty, the firebox temperature for normal duty sometimes falls below the API recommended value for safe operation since, at lower temperatures, emissions like CO tend to increase beyond allowable limits. At times, it emerges that the substantial duty of an upstream exchanger is added as an over-design margin in heater duty, taking into account non-availability of the exchanger. For such instances, after detailed deliberation of furnace optimisation and safety, the licensor may be required to prune additional over-design built into the furnace design duty, bringing it closer to normal requirements. This results in optimisation of fired duty along with a welcome reduction in overall installation cost and improvements in operating parameters like bridge wall temperature and tube metal temperature. Corresponding emissions are also reduced significantly. Thus,  selection of furnace duty and its operating range can play a crucial role.

Applying a similar strategy for the case study, reducing unnecessary margins after due evaluation of process requirements, heater duties are revised according to Table 3. For ease of understanding, only new revamp duties corresponding to a 100% capacity increment are shown here.

Thus, with the use of a duty optimisation exercise, there is a 10.3% reduction in fired duty for new capacity furnaces with respect to existing duty, with a corresponding reduction in SOx and NOx values.