4. ENVIRONMENTAL DATA
Environmental data for the BAT production processes are generally known with a high degree of reliability, while data for storage and transfer are more uncertain and of much less importance. Hence, data for production will be given the highest attention.
4.1 Production Input Requirements
4.1.1 >4.1.1 Feedstock
The typical feedstock requirements for modern plants are (approximately):-
| Conventional reforming: | 22.1 GJ(LHV*).t -1 NH | |
| Excess air reforming : | 23.4 GJ(LHV).t -1 NH | |
| Autothermal reforming : | 24.8 GJ(LHV).t -1 NH | |
| Partial oxidation : | 28.8 GJ(LHV).t -1 NH |
* Lower Heating Value
4.1.2 Fuel
Assuming an efficient stand-alone plant with no energy export and no other import than feed-stock and fuel, the fuel requirements are (approximately):-
| Conventional reforming: | 7.2-9.0 GJ(LHV).t-1 NH | |
| Excess air reforming : | 5.4-7.2 GJ(LHV).t -1NH | |
| Autothermal reforming : | 3.6-7.2 GJ(LHV).t-1 NH | |
| Partial oxidation : | 5.4-9.0 GJ(LHV).t-1 NH |
The uncertainty in autothermal reforming is mainly due to the type of compressor drives.
4.1.3 Water and air
In the steam reforming processes process steam is taken from the plant steam system, usually from an extraction turbine. The net consumption according to the stoichiometic conversion is 0.6-0.7kg.kg-1 NH3 , the total supply at a S/C ratio of 3.0 will be about 1.5kg.kg-1 NH3 .. In partial oxidation much less steam is fed to the gasification reactor, but additional steam is needed in shift conversion (1.2kg.kg-1 NH3 in total).
Process air supply: In conventional reforming the nitrogen supply equals the ammonia nitrogen content plus the purge losses, ie. about 0.85kg N 2 .kg-1 NH3 or about 1.1kg air.kg-1 NH3 . In the excess air reforming and gas heated reformer cases the process air requirements are about 50% and 100% higher, respectively. In the partial oxidation process the amount of air fed to the air separation unit is approximately 4kg.kg-1 NH3 , based on the oxygen requirement.
Boiler feedwater: Assuming all steam condensates are recycled, only the process steam consumption has to be replaced by outside water. This will be 0.7-1.5kg.kg-1 NH3 (see above), depending on process condensate recycle or not. Small additional losses and potential import/export have to be allowed for in practice.
Air and/or water for cooling: Will differ from one site to another.
4.1.4 Solvents and additives
The consumption of solvent in the CO2 removal unit should not normally exceed 0.02-0.04kg.t-1 NH3 , or about 2kg.h-1 for a BAT capacity plant. Solvent losses are mainly caused by leaks.
The usual treatment additives and regeneration agents are used in the boiler feedwater preparation units. The consumption figures should not differ from those of a standard steam boiler plant at the same location.
4.1.5 Catalysts
Approximate consumption figures, based on average filling volumes and normally recommended operating periods, for a gas based conventional reforming plant, are given in the table below. The consumptions refer to a capacity of 1,500t.d-1.
|
Catalyst type |
Typical replacement m3.y-1 | |
|
Hydrodesulphurisation |
1 | |
|
Sulphur removal |
5 | |
|
Primary |
5 | |
|
Secondary reforming |
4 | |
|
High temperature shift |
10 | |
|
Low temperature shift |
20 | |
|
Methanation |
2 | |
|
Synthesis |
10 |
Actual consumptions in existing plants may differ considerably from the guidance figures above.
4.1.6 Energy requirements
The total energy requirement in the reforming BAT processes is 28.8-31.5GJ (LHV).t-1 NH3 for a stand-alone plant with no energy export and no other import than feedstock and fuel. When using process waste heat in a gas heated reformer, the process itself will not produce enough steam to drive all the compressors. A part of the power needed may then be imported from a more efficient power plant outside the process plant. In such cases the total energy consumption may be lowered and approach the present practical minimum of 27 GJ(LHV).t -1 NH3.
3.
In partial oxidation plants the total energy requirement is 36.9(35.1-37.8)GJ(LHV).t -1 NH3 .. This includes imported power and/or auxiliary steam for driving the machinery.
4.2 >4.2 Production Output
4.2.1 Ammonia
Ammonia production in the typical size BAT plant is 1,000-1,500t.d-1 ,500t.d-1 (300,000-500,000t.y-1). The production not used in downstream plants on site is stored as described in Chapter 3.
Commercial anhydrous ammonia has two grades of purity:-
- Anhydrous ammonia min. 99.7 wt %, water content (about 0.2% wt)
- Anhydrous ammonia min. 99.9 wt %
4.2.2 Carbon dioxide
Carbon dioxide is produced according to the stoichiometric conversion and may be recovered for down-stream uses. The carbon dioxide production in steam/air reforming of natural gas is 1.15-1.30kg.kg-1 NH3, dependent on the degree of air reforming. A CO2 /NH3 mole ratio of 0.5 (weight ratio 1.29), the stoichiometric ratio for urea production, is obtainable in the heat exchange reformer concepts. In partial oxidation of residual oils the CO2 production is 2-2.6kg.kg-1 NH3 , dependent on feedstock C/H ratio.
Carbon dioxide in the combustion gases is not included in the above figures, but is shown in Figure 3.
4.2.3 Sulphur
In BAT partial oxidation most (87-95%) of the sulphur content of the feed to the gasifier is recovered in the Claus unit.
4.2.4 >4.2.4 Steam export
Modern steam reforming processes can be designed with no steam export or with some export of low/medium pressure steam if this can be favourably used on site. Steam export is usual in excess air reforming processes where the process air compressor is driven by a gas turbine, and in cases when electric power is used for driving one or more of the main compressors.
Processes with gas heated primary reforming may be designed for zero steam export even with some power import or gas turbine drive.
The partial oxidation process has a steam deficit if all compressors are steam driven.
4.3 Production Emissions and Wastes
4.3.1 Emissions into air from steam reforming plants
From steam reforming plants with a fired primary reformer emissions into air come from the following sources:-
- Flue-gas from the primary reformer
- Vent gas
from CO2 removal
- Breathing gas from oil buffers
(seals/compressors)
- Fugitive emissions (from flanges, stuffing boxes
etc.)
- Purge and flash gases from the synthesis section (usually added to
the
- primary reformer fuel)
- Non-continuous emissions (venting and
flaring)
4.3.1.1 Flue-gas from the primary reformer
The flue-gas volume, at 3% (dry gas base) oxygen, for a gas-based conventional steam reforming plant producing 1,500t.d -1 , is approximately 200,000Nm3 .h -1, containing about 8% CO2 (dry gas base), corresponding to 500kg CO2.t -1 NH3 .. The flue-gas volume from excess air reforming may be lower. The other pollutants in the flue-gas are:-
NOx : 200-400mg.Nm-3 , (98-195ppmv), or 0.6-1.3kg.t-1 NH3 expressed as NO2
SO 2 : 0.1-2mg.Nm-3 ,, (<0.01kg.t-1) ,, depending on fuel
CO: <10mg.Nm-3 ,, (<0.03kg.t -1)
The NOx emission depends on several factors and the following features reduce the emission:-
- Low combustion air and fuel gas preheat
-
Steam/inert injection
- Low ammonia content in injected purge-gas
- Low
excess oxygen
- Low NOx burners
- Post-combustion
measures
The SO2 emission comes from the sulphur in the fuel gas and can be calculated by a simple mass balance.
4.3.1.2 Vent gases from CO2 > removal
More or less of the CO2 product may have to be vented, depending on the CO2 requirements of other production facilities on the site. In some cases, high purity CO2 is used, while an air-CO2 mixture from a stripping column is vented. The CO2 contains small traces of synthesis gas and absorption solvent vapour.
4.3.1.3 Breathing gas from oil buffers
This contains traces of NH3 , synthesis gas and lube oil.
4.3.1.4 >4.3.1.4 Fugitive emissions
The diffuse emissions from flanges, stuffing boxes etc. should be minimised.
4.3.1.5 >4.3.1.5 Purge and flash gases
The purge and flash gases from the synthesis section are usually washed with water to remove/recover ammonia, and the purge gas may be treated in a recovery unit, before routing the off-gases to the primary reformer fuel gas system. The off-gases are thus combusted and end up as part of the flue-gas. It is important to remove the ammonia as far as possible, as it will contribute considerably to the flue-gas NOx emission.
4.3.1.6 Non-continuous emissions
Emission of NOx during flaring synthesis gas at start-up or trip situations is estimated to be 10-20kg.h -1 as NO 2 [1]. Some plants without a flare, vent to the atmosphere.
4.3.2 >4.3.2 Emissions into air from heat exchange reforming
The flue-gas volume (from auxiliary fired equipment) and thus NOxand CO2 emissions are considerably reduced, as these processes have much more internal combustion than the fired reformer processes because all the reforming process heat is generated by internal combustion.- Total C02 emissions are fixed by the energy consumption. For self-sufficient plants the flue-gas volume in heat exchange reforming is about 50% of the figure for conventional steam reforming. In plants with power import the flue-gas volumes are lower. Reduction of NOx emissions by 80% has been claimed.
4.3.3 Emissions into air from partial oxidation plants
The partial oxidation process has the same emission sources as described for the reforming- process except for the primary reformer flue-gas. A partial oxidation plant may also have auxiliary boiler(s) for power steam production, if more efficiently off-site produced power is not available. The fuel to the auxiliary boiler/superheater together with possible scrubbing equipment determines the amount of CO2 in the flue-gas. Tail gas from sulphur recovery will also contain sulphur oxides. This means that the CO2 emission from partial oxidation plants (max 1,500mg.Nm-3 ) is higher than in the reformer flue-gas. Other additional emissions may be H2S (0.3ppmv), methanol (max. 100ppmv), CO (30ppmv), and dust (traces, max. 50mg.Nm-3 ). NOx emission (max. 700mg.Nm-3 ) depends on the factors listed for the reformer flue-gas and the nitrogen content of the fuel.
Excess nitrogen is usually vented.
4.3.4 Emissions into water/P>
Pollution problems related to water, during normal operation, may occur due to process condensates or due to the scrubbing of waste gases containing ammonia. In partial oxidation, soot and ash removal may cause pollution problems, if not properly handled.
Process condensate is found primarily in the condensation section prior to the CO2- removal, of the order of 1m 3 per ton NH3produced. Without treatment this condensate can contain up to 1kg of ammonia and 1kg methanol per m3 . More than 95% of the dissolved gases can be recovered by stripping with process steam and are recycled to the process.
The stripped condensate can be re-used as boiler feedwater make-up after treatment by ion exchange. Total recycle is obtained in this way. In some cases the process condensate is used for feed-gas saturation and thus recycled to the process.
Usually the ammonia absorbed from purge and flash gases is recovered in a closed loop so that no aqueous ammonia emissions occur. Emissions into water from the production plant during normal operation can thus be fully avoided.
Soot from gasification in partial oxidation processes is usually recovered and recycled to the process. Traces of soot and slag are emitted to water.
4.3.5 Solid wastes
The BAT ammonia processes do not normally produce solid wastes. Spent catalysts and mol.sieves have to be removed and valuable metals are recovered from them. In partial oxidation plants sulphur is recovered in the Claus plant and can be used as feedstock in sulphuric acid units. The ash can be upgraded and used as an ore substitute.
4.4 Environmental Data for Ammonia Storage and Transfer
In a refrigerated storage, the cold losses are balanced by recompressing and recondensing the evaporated ammonia. During recompression, some inerts containing also traces of ammonia cannot be condensed but must be flared or scrubbed with water. Small continuous emissions may thus occur, in addition to minor non-continuous emissions during loading operations.
4.5 Environmental Hazards Associated with Emissions and Wastes
The production of ammonia is relatively clean compared to many other chemical processes. During the normal operation of a reforming plant, only the NOx and CO2- emissions have to be considered. In partial oxidation plants with oil-fired auxiliary boilers the reduction of SO2 - emissions can be achieved by using low sulphur fuel oil. Generally the emissions from modern ammonia plants have little environmental impact.
4.6 Emission Limits and Guideline Values for Ammonia Production in Some West-European Countries
Two types of emission values are of importance:-
- Legally binding emission limit values for specific
pollutants which apply for
- ammonia production
- Guideline values which
are not legally binding but provide the background for
- requirements laid
down in individual permits
Specific legally binding emission limits for ammonia production are only laid down in Germany. In the Netherlands and in Germany limits for emissions from boilers have been laid down which include chemical reactors.
Specific emission guideline values are laid down in the United Kingdom.
With regard to the other countries, no national emission limits or guidelines are fixed for ammonia production plants. Very often the values are the subject of negotiations between the operator and the authority responsible for granting licences. In some countries, these authorities are of local character, so that even within one and the same country, different air pollution requirements may apply for comparable plants. At least in the Benelux countries and Ireland, the licensing authorities take into account emission limits applied in other countries, in particular those laid down in Dutch and German law, in their negotiations with operators. In practice, these values play the role of guideline values.
Indirectly, the emission limits laid down for combustion installations play an important role because the energy consumption of an ammonia plant is relatively high. The emissions from a fired reformer or auxiliary boilers should conform with emission limits and guideline values adopted by the Council of EC in Directive 88/609/EEC or applied by the state.
More detailed information is given in Reference [1].