This overview of the historical developments and the future of energy efficiency in Indian ammonia plants is provided by the Fertilizer Association of India (FAI).
During the initial phase of development of the fertilizer industry worldwide, energy efficiency was not a key consideration in the design of ammonia plants, mainly because of the cheap availability of natural gas, naphtha and electricity. After the oil crises of the 1970s, however, process designers started to pay more attention to energy efficiency as well as reliability and the capital cost of the plant.
In 1984-1985, a few highly energy efficient ammonia plants were commissioned around the world, with specific energy consumption values close to 29 GJ/t ammonia. The first large scale ammonia plants (1350 tpd) that use gas as feedstock were also built in India around the same time. However, the energy consumption of the Indian plants was higher than the high-efficiency plants elsewhere, as Indian project owners preferred plants with proven technology and equipment. This was due to their experience with poor performance of some of the plants commissioned in the 1970s, which experimented with technology and used locally-produced equipment.
As productivity and energy efficiency gained in importance, Indian ammonia plants started to introduce a number of new measures. During the 1980s, the main retrofit introduced was a Purge Gas Recovery (PGR) unit for recovery of hydrogen from purge gas. All the PGR units were based on cryogenic separation of hydrogen, which was recycled back into the synthesis loop. Some of the later generation plants installed hydrogen recovery units using the membrane process. Till then, the purge gas was burnt as fuel in the reformer. This measure alone reduced specific energy consumption by 0.63-1.05 GJ per tonne of ammonia.
The second most significant development was the increased availability of reformed tubes that had better metallurgy than the HK-40, or equivalent, which was used in most reformers. This allowed larger inner space for the packing of catalysts and, hence, higher throughput. It not only increased the reformer capacity but also helped to improve energy efficiency of operations. This new practice continued even into the 1990s in various plants. Other measures included the use of solvent promoters in CO2 removal sections, and better heat recovery in the convection zones of reformer furnaces.
During the 1990s, in the reformer sections, there was renewed emphasis on the recovery of additional waste heat from the off-gases of reformer furnaces. In the relatively new plants, the temperature of off-gases was brought down to as low as 150-160ºC from more than 200ºC. Along with other heat integration steps in the ammonia plant, these steps reduced energy consumption by 0.1-0.2 Gcal (0.42-0.84 GJ) per tonne of ammonia. Around the same time, CO2 removal systems were improved; new measures included using better solvents, better packing in absorption and desorption towers for higher mass transfer efficiency, and a change over from LP condensate stripper to MP condensate stripper. In one of the plants, energy consumption was reduced from 0.15 GJ /Mol CO2 to 0.105 GJ/Mol CO2 with a change of solvent from ammine to Methyl Di-ethanol Amine (MDEA).
Other efforts included changing the internals of synthesis converters in old plants from axial flow to radial-axial or radial flow, which allowed the use of more active catalyst of finer particle size without increasing the pressure drop through the reactor. This in turn resulted in an increase in conversion per pass and thus reduced the energy consumption of recycling synthesis gas. This measure alone saved 0.84-1.05 GJ per tonne of ammonia.
Almost all old ammonia plants changed from analog to digital instrumentation with screen based controls. The installation of distributed control systems (DCS) and programmable logic controllers (PLC) became the norm in the 1990s. Most ammonia plants that were commissioned in the 1980s and designed for 33.05-34.73 GJ per tonne were able to reduce their energy consumption down to 32.22-32.64 GJ per tonne of ammonia with the use of such systems.
Large ammonia capacity of about 5 million tonnes was added in India in the 1990s through nine new plants. Each of these plants had a capacity of about 0.5 million tonnes of ammonia per year. A number of these plants included new features, such as a gas turbine (GT) drive for process air compressors with heat recovery from the exhaust gas of a turbine.
During the second half of the 1990s, a number of plants undertook projects to stop bottlenecks in their operations, and energy saving measures were implemented simultaneously. These measures included:
- the installation of combustion air pre-heaters in reformer stacks to bring down the exhaust temperature to as low as 120oC whenever the sulfur content of natural gas allowed
- the introduction of low temperature shift (LTS) guards with additional heat recovery to preheat boiler feed water, which helped to reduce CO slip from 0.20 to 0.10 mole percent at the exit of the LTS convertor, resulting in energy saving of 0.84 GJ/t NH3 in some plants
- changing from single to two stage regeneration in CO2 removal sections, which allowed better heat integration
- changing single stage flash vessel systems in the regenerator section to multistage flash vessels with ejectors
- using a mechanical steam compressor
- installing a hydraulic turbine to recover energy from high pressure process fluids.
There have also been some process modifications in the synthesis loop. The installation of an S-50 converter with a medium pressure waste heat boiler is reported to have saved 0.29 GJ/t of energy. Using an additional converter increased conversion from 14 to 19 per cent, or even higher, resulting in savings in recycled energy. Similarly, one of the units installed an S-300 converter with an energy saving potential of 1.46 GJ per tonne of ammonia.
Using liquid ammonia to wash synthesis gases has helped remove CO2 and moisture impurities. Discharge from synthesis gas compressors was originally going to the first stage ammonia separator via chillers and then to the convertor. After additional purification, it now goes directly to the convertor and thus saves energy in the chillers. In some other plants, energy consumption was further reduced by chilling the makeup synthesis gas and air at suction end of process air compressor with the use of absorption refrigeration using low-grade waste heat from the plant.
Numerous other small measures have been implemented by ammonia plants in India. For example, feed gas saturation with process condensate has helped to conserve energy. Most plants use a DCS and take full advantage of automation. A number of units have installed advanced process control (APC). While energy savings are small in the range of 0.13-0.21 GJ per tonne of ammonia, the operation of the plant becomes very smooth with the control of most operating parameters within a narrow range. Using variable frequency drives for fans and pumps have also become common. A number of plants have changed steam turbine drives to motor drives for higher efficiency.
As a result of these energy conservation efforts and the addition of capacity through more efficient plants, the weighted average energy consumption of ammonia plants in the country was reduced from 52.17 GJ/t NH3 in 1987-88 to 36.9 GJ/t NH3 in 2011-12, showing an almost 30 percent improvement in the energy efficiency of ammonia production. This change is depicted in Figure 1 below.
Figure 1: Historical Trends in Energy Consumption in Indian Ammonia Plants.
Energy Efficiency Projections for Ammonia Production
The Fertiliser Association of India makes projections for the requirement of ammonia in agriculture use. It is assumed that, with the exception of a few, most of the existing plants will continue to operate in 2030. Beyond that, some of the older plants will close. An IEA (1) working paper has projected ammonia production of 19, 26 and 33 Mt by 2015, 2030 and 2050, respectively. The present capacity based on liquid hydrocarbons will be changed to gas by 2015- 16. It is envisaged that average energy consumption of the existing plants will be 34.61 GJ per tonne of ammonia by 2015-16 and will decrease to 33.69 GJ per tonne of ammonia by 2030. Efficiency improvements will come as a result of further modernization of gas-based plants and a change of feedstock from fuel oil and naphtha to natural gas by 2013-2014. These plants will retain use of coal for the generation of steam and power. Naphtha-based plants will also change to gas by 2014- 2015. Further, it is assumed that additional production will come from new plants with an average energy consumption of 29.26 GJ per tonne of ammonia by 2030.
Projections for 2050 are highly uncertain. According to FAI estimates, there will be demand of about 28 million tonnes of nitrogen by 2050. However, how much of it will be based on domestic or imported ammonia remains speculative. It is expected that the production of ammonia in India will be in excess of 30 million tonnes. A significant part of present capacity may be replaced by the plants based on the best available technology with energy consumption levels in the order of 28 GJ per tonne of ammonia beyond 2030. Given these assumptions, the projections for production and energy consumption are given in the table below.
|Year||Prodution (millon tonnes)||Energy Intensity (GJ/t NH3)||Total Energy Consumption (PJ)|
Carbon Dioxide Emissions
Carbon dioxide (CO2) is generated during ammonia production. It can be extracted and used for production of urea. In India, all ammonia plants (except two) have integrated urea plants. There is a direct relationship between energy consumption and carbon dioxide emissions.
International Fertilizer Industry Association (IFA) reported on the carbon dioxide emission factors for ammonia plants based on gas, naphtha and fuel oil feedstock, and the FAI has worked out the CO2 emissions from Indian plants using these factors. The factor for Indian ammonia plants was calculated taking into account the type of energy form used and the average energy consumed for each type of energy form (natural gas, naphtha and fuel oil). Indian plants produced 13.25 million tonnes of ammonia in 2009 and they are estimated to have emitted 29.6 million tonnes of CO2 – i.e. an average emission factor of 2.23 tonne of CO2/tonne of ammonia produced. The CO2 emissions trend from 1987-88 to 2011-12 is plotted in Figure 2 below. The graph shows that CO2 emissions per tonne of ammonia have reduced from 3.60 to 2.26, following a similar trend as energy consumption.
Figure 2: Historical Trends in CO2 emissions from Indian Ammonia Plants.
The targeted production of ammonia is estimated to increase to 18.2 Mt of ammonia by 2015, 26.4 Mt by 2013 and 30.15 Mt by 2050.These figures take into account the expected revamping and retrofitting of existing gas-based plants, the implementation of energy conservation schemes, the conversion of non-gas-based plants to gas plants, the commissioning of new plants based on more energy efficient technologies, and the occasional closure of some old plants. It was estimated that the CO2 emission factor per tonne of ammonia will have progressively improved to 1.92, 1.82 and 1.75 t/t by 2015, 2030 and 2050 respectively.