Decarbonisation Technology – August 2021

may therefore be a prudent step before such a shift.

100

Relative Carbon footprint as % of Base

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.

HFO

LFO

RFG Min MW RFG Max MW

LNG-150

Figure 1 Comparative analysis of carbon merit

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 CO 2 . However, extraction and utilisation of hydrogen from RFG and associated life-cycle CO 2 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 H 2 S 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

LNG case- 150

LNG Case-130

LNG Case-110

Flue gas exit temperature, °C from APH

150 91.6

130 92.6

110 (*)

Fuel efficiency, %

93.2

Fired duty, Gcal/hr (MMBtu/hr)

50.8 (202)

50.2 (199)

49.8 (198)

Carbon emissions, t/d

286 7.58

283 7.96 78.5

280 8.33 55.1

Air preheater duty, GCal/hr

APH LMTD, °C

100.5

(UA) Parameter (#) 151,180 ( *) The flue gas temperature for this case was set at 110°C to take care of any incidental pipeline compatible H 2 S of the order of 4-5 ppm and to avoid any mist formation from the heater stack at cold climate conditions. (#) UA parameter refers to basic heat transfer equation; Heat Duty = U*A* (LMTD). 75,400 10,1400

Table 3 LNG enhanced heat recovery case