Aug-2021

Fuel oil to liquefied natural gas

Most furnaces in Europe and the US have already shifted to fuel gas firing, so is there any carbon reduction advantage in these furnaces shifting to LNG?

Ankur Saini, Akhil Gobind and Rupam Mukherjee
Engineers India Limited

Article Summary

A simple Google search of ‘Climate Change’ yields about 60,00,00,000 results in 0.80 seconds. Spending even a few minutes on the first page of the search brings forth articles on varied topics such as hurricanes affecting the American coastlines, raging wildfires ravaging large places of Australia, cloud bursts and heavy downpours disrupting life and business in the Asian sub-continent, extended stretches of drought over parts of Argentina, never-before-seen soaring summer temperatures in Europe, and so on. From these, one thing is certain — ‘climate change’ is a threat, and it is not local. And there is no scope for ‘Not in my backyard!’.

‘Climate change’ and ‘global warming’ are two sides of the same coin. Global warming due to greenhouse gas emissions is a major issue engaging the most prominent leaders and economies. Carbon emissions due to anthropogenic sources of fuel are constantly on the rise. The silver lining is that ambitious targets and roadmaps have been laid out on inter-governmental levels to curb this menace. In this critical transition toward sustainable operation, combustion equipment like furnaces in the oil and gas industry have their role to play. Traditionally, in many parts of the world, heavy fuel oil (HFO) is fired in oil refinery furnaces, prioritising operating cost over the carbon emission aspect. However, over time, cleaner fuel sources have evolved that provide better emission characteristics and may drive refinery operations towards a more sustainable future. Of late, liquefied natural gas (LNG) has gained much importance as an energy source due to enhanced production and availability from various fields. LNG is touted as the transition fuel leading the way towards a low carbon future.

This article examines various common fuels fired in a refinery furnace on the carbon footprint yardstick. The article will traverse the carbon emissions generated from traditional HFO firing through to LNG, with lighter fuel oil (LFO) and refinery fuel gas (RFG) as pit stops in between.

Furnace fuels and their carbon footprint
In general, furnaces utilise fuel generated within the refinery complex for their heat demand. Fuels commonly used are HFO (Type-6 fuel oil), LFO and RFG. While the fuel oils are derived primarily from the vacuum residue and other cutter stocks, the RFG is generated from the off-gas produced from various refinery units. The composition of RFG can vary depending upon various operating and unit availability scenarios. Along with the three fuel types, a fast-emerging fourth option is LNG, exported into the refinery from the gas grid or a specific LNG source. Typical characteristics of these four fuels are noted in Table 1. For a better and more accurate assessment, the entire range of fuel gas from minimum molecular weight to maximum molecular weight has been considered. Typical RFG composition does not remain constant and varies with parameters such as the diet of crude processed, a single unit or a group of units taken out for maintenance, availability of off-gas PSA unit, and so on.

The above fuels were studied on a 46.5 GCal/hr case study furnace operating in a 180,000 BPSD refinery. Excess air for HFO and LFO were considered as 20% in line with API recommendation. Similarly, being inherently cleaner and easier to burn, excess air for RFG and LNG was considered as 15%. The furnace employs an air preheat system for combustion air preheating. The flue gas exit temperature from the air preheater (APH) was set at approximately 150°C for all fuels to set the datum level for comparison. However, the flue gas exit temperature from APH for RFG may be a bit lower in some instances due to the cleaner nature of the fuel compared to oil.

Key performance parameters of the various operating scenarios based on individual fuels are noted in Table 2. From here, several significant and interesting inferences can be drawn. Considering carbon emissions from HFO as the base value, the carbon footprint merit of other fuels relative to HFO is shown in Figure 1.
For better appreciation of the results, two possible scenarios were envisaged, and the inferences of the tabulated parameters were extended to these two most commonly seen scenarios:

Scenario 1: Switching from fuel oil to LNG
- Table 2 verifies the assertion that shifting from fuel oil to LNG is undoubtedly an effective step towards decarbonisation. LFO generates substantially fewer carbon emissions than HFO. More so, a very appreciable cut in carbon emissions of the order of 29% can be realised if the furnace is shifted from HFO to LNG. Fuel oil firing is prevalent in many Asian countries, as well as in many parts of Africa. Shifting to LNG is a very effective and low-cost solution in those refineries in pursuit of carbon reductions.

- As the shift is made from fuel oil firing to LNG, the refinery operating team must be vigilant of the key performance parameters. In general, it is experienced that key furnace indications such as Bridgewall temperature and tube metal temperature may increase. This may be due to shifting the heat load from the convection section to the radiant or a variation in flame characteristics observed in the fuel gas flames. A thermal study may therefore be a prudent step before such a shift.

Scenario 2: Switching from RFG to LNG — the gas ‘conundrum’
- This area is of greater interest and complexity, as most of the furnaces in Europe and the US have already shifted to fuel gas firing. So, for these furnaces, is there any advantage in shifting to LNG? Apparently, from Table 2, there is no clear winner, at least on the carbon reduction yardstick. The defining parameter here is the molecular weight of the fuel gas. Furnaces operating on a fuel gas with a molecular weight lighter than LNG can see an increase in their carbon footprint if shifted to LNG. 

For example, the case study furnace sees an increase in its carbon footprint of 7300 t/y if LNG is preferred over the Min MW RFG. However, on the contrary, LNG has the upper hand over the Max MW weight fuel gas by 6%. On an annual basis, shifting to LNG from the Max MW RFG can lead to 7600 tonnes of carbon emission savings. Thus, LNG vis-à-vis RFG presents a conundrum that has to be decided on a case-by-case basis depending upon the most frequent RFG composition.

The possibility of extracting the hydrogen component from RFG and adding it to the hydrogen pool may help unload the hydrogen generation unit (HGU) and reduce overall CO2. However, extraction and utilisation of hydrogen from RFG and associated life-cycle CO2 is a different analysis altogether and beyond the scope of this article.

- Notwithstanding the above trend, there is one interesting aspect. The flue gas exit temperature for the LNG case was set the same as for the RFG firing case to model the actual scenario of any operating furnace where APH and associated auxiliaries are already fixed. However, LNG inherently has much less H2S content than RFG, which means that the acid dew point is very low in the case of LNG.

In fact, for the study case LNG with ‘nil’ sulphur content, the dew point is that of water and is as low as 60°C. This beckons the opportunity to extract more heat from the flue gas in the APH, thereby increasing the combustion air temperature and furnace fuel efficiency. The question now is will this lead to a clear winner between RFG and LNG?

- This exercise was taken up, and additional cases of LNG firing with enhanced heat recovery were worked out. For practical appreciation, these additional cases were basically to model a scenario where the refinery is ready to invest in augmenting the existing APH to one with a larger surface area for extracting more heat. The refinery may have to replace the existing APH with a new, larger one. The results are shown in Table 3.
- Table 3 throws open another debate. It shows that additional investment in APH surface area can lead to CO2 reduction; however, the magnitude of this additional saving of only 6 t/d is debatable, especially considering that it would call for a new APH with almost double the surface area. Notably, the carbon footprint would remain higher than the Min MW RFG.