Fertilizer International 501 Mar- Apr 2021
31 March 2021
Emissions scrubbing technology
UREA PLANT SCRUBBING
Emissions scrubbing technology
We highlight recent advances in ammonia and dust scrubbing systems for urea plants from Stamicarbon, thyssenkrupp Industrial Solutions and Toyo Engineering Corporation.
TOYO ENGINEERING CORPORATION
High efficiency urea plant scrubbing systems
Japan’s Toyo Engineering Corporation (Toyo) has developed technologies to abate urea and ammonia in emissions and effluents from urea plants since its establishment as a leading urea process licensor in 1961.
The abatement of ammonia emissions is a particularly critical issue, as the recent introduction of stricter regulatory requirements are not achievable by conventional water scrubbing systems alone. Typical emissions guidelines for urea plant finishing section are 50 mg/Nm3 for urea dust and 50 mg/Nm3 for ammonia.
Toyo’s latest reduction technologies for cutting ammonia and dust emissions at urea plants are described below
Single-stage water scrubber
Toyo offers a water scrubbing system for removing urea dust at the finishing stage in prilling and granulation plants. The system has the ability to reduce the dust in exhaust air from urea finishing sections to less than 30 mg/Nm3 . The system has been adopted at more than 40 prilling towers and granulation plants worldwide and benefits from the following features:
- Low power consumption thanks to its low pressure drop of only 50-150 mm H2 O
- Efficient recovery of urea as a 45 wt-% solution
- Lower construction cost due to a simple structure and the low loading weight of the polypropylene packed bed.
The system is a packed bed type tower with demister (Figure 1). Exhaust air enters the bottom of the scrubber and rises upwards through a packed bed. Urea dust is removed when this exhaust gas comes into contact with a descending stream of water moving downwards in counter-current. Scrubbed air is finally vented to the atmosphere from the top of the scrubber after mist carry-over has been eliminated.
Toyo also offers three acid scrubbing options for removing ammonia emissions:
- A single-stage acid scrubbing system
- A double-stage acid scrubbing system
- An acid scrubbing system without by-product.
Single-stage acid scrubber
This system has the same configuration as the water scrubbing system (Figure 1). However, by injecting acid into the circulation water, ammonia emissions in the exhaust air are reduced to below 20 mg/ Nm3 . At the same time – similar to the water scrubbing system – urea dust emissions are also scrubbed to less than 30 mg/Nm3 .
The system removes and recovers ammonia gas as either urea ammonium sulphate (UAS) or urea ammonium nitrate (UAN) solution, depending on the acid injected. With this system, because it combines to form an ammonium salt instead, it is not possible to return captured urea to the urea plant.
Double-stage acid scrubber
This system combines water scrubbing and acid scrubbing in two separate stages (Figure 2). Urea dust is firstly captured by a packed bed in the lower water scrubbing stage and recovered as 45 wt-% urea solution. Acid scrubbing in the packed bed of the upper stage is then used to absorb and remove ammonia gas. This system option does allow urea to be recovered as a separate product. This scheme is suitable for plants which need to minimise by-product, or instead produce ammonium nitrate (AN) or ammonium sulphate (AS).
Eliminating by-products
Toyo acid scrubbing systems can be configured to eliminate by-products. In this system option, ammonia present in exhaust air is absorbed by sulphuric acid during an acid scrubbing stage and recovered, together with urea dust, as UAS. A small independent evaporator then concentrates the recovered UAS solution. This is sent to the urea finishing section where it is added to the urea feed.
This setup eliminates ammonium sulphate (AS) as a by-product by incorporating it within the urea production process. The final urea product obtained, which contains 0.2-0.3 wt-% AS, contains sulphur as an additional crop nutrient. This system option is suitable for plants producing urea for the agricultural market, particularly when there is no scope for producing by-products other than the main urea product.
World-class scrubbing projects
Toyo was awarded a number of major urea granulation plant contracts in the 2010s. Two notable examples that incorporated Toyo scrubbing systems are the large-scale Kaltim No 5 project (3,500 t/d) in Indonesia for PT Pupuk Kalimantan Timur (Kaltim), and a world-class project (4,000 t/d) for Indorama Eleme Fertilizer and Chemicals Limited (IEFCL) in Nigeria. Brief case studies for both projects are provided below.
Toyo case study 1: Kaltim No 5 project, Bontang, Indonesia
In 2011, Toyo was awarded a contract by PT Pupuk Kalimantan Timur (Kaltim) to construct a 2,500 t/d ammonia plant and 3,500 t/d urea plant (Table 1). Kaltim is a subsidiary of the state-owned PT Pupuk Indonesia Holding Company. The urea plant was designed using Toyo’s ACES21® urea process and spout-fluid bed granulation. It is one of the largest single-train ammonia/urea complex in Southeast Asia.
Toyo provided engineering, procurement and construction (EPC) for the whole complex on a turn-key, lump sum basis in collaboration with PT Inti Karya Persada Tehnik (IKPT), a Toyo subsidiary company. Urea production at the plant began in early 2015.
A Toyo water-scrubbing system for urea dust was also installed as part of this project. This was acceptable as domestic Indonesian regulations did not specify the need for an acid scrubbing system. A performance test was carried out at the ammonia/urea complex during the first year of production with excellent results (Table 2). Urea dust emissions at the plant were reduced to just 21 mg/Nm3 . n
Toyo case study 2: Indorama train 1 project, Port Harcourt, Nigeria
In 2013, Indorama Eleme Fertilizer & Chemicals Limited (IEFCL) awarded a contract to Toyo and its consortium partner Daewoo Nigeria Limited to jointly build the world’s largest single-train ammonia-urea complex at Port Harcourt in Nigeria. The complex, which has a design capacity of 2,300 t/d for ammonia and 4,000 t/d for granulated urea, became operative in mid-2016 (Table 3). It consumes locally-available natural gas as a feedstock and uses licensed KBR technology for its ammonia process and Toyo’s ACES21® technology for urea production.
The urea finishing section produces high quality urea granules (Table 4) and incorporates a highly-effective Toyo double-stage acid scrubbing system (Figure 2). This successfully mitigates ammonia emissions, as well as reducing urea dust emissions to the atmosphere to less than 10 mg/Nm3 (Table 4). n
Conclusions
Urea dust scrubbing: Toyo’s water scrubbing system is a proven technology able to reduce urea in exhaust air from the urea finishing section to less than 30 mg/Nm3 . By applying a double stage acid scrubbing system, urea dust emissions in exhaust air finishing section can be reduced even further – to less than 10 mg/Nm3 .
Ammonia scrubbing: Toyo’s highly effective acid scrubbing systems can reduce ammonia emission to less than 20 mg/ Nm3 . These systems are flexible, being offered in three options: single-stage acid scrubbing, double-stage acid scrubbing, and acid scrubbing without by-product.
THYSSENKRUPP INDUSTRIAL SOLUTIONS
Optimising emissions control at urea plants
Low gaseous emission limits, especially for ammonia, require an acidic scrubbing system at urea plants. Besides the urea dust and ammonia vapour emissions typically associated with the granulation unit, gaseous ammonia emitted by the plant’s synthesis unit needs to be treated as well.
When required, the ammonia in the off-gas of the synthesis unit is removed by a standalone acidic scrubbing system. These scrubbing systems typically consists of a scrubber installed within the steel structure of the synthesis unit, together with pumps located at ground level plus associated instrumentation and piping. However, this separate dedicated scrubbing system is not necessarily required, if the plant already has a scrubbing system installed at the granulation unit, as explained below.
During the engineering for two urea plants in the United States, thyssenkrupp Industrial Solutions (tkIS) engineered an efficient method to catch the ammonia emissions of the synthesis unit. This employed a more integrated engineering approach.
Traditionally, the synthesis unit and granulation unit of a urea plant are viewed as individual and separate sections. They are built using the licensor’s knowhow with each unit constrained by their respective battery limits. Yet, when viewed together, a contractor can overcome these constraints by combining both units in one interconnected ‘smart’ plant.
A single integrated scrubbing system
The characteristics of the off-gases of the synthesis unit and granulation unit do differ from one another. The flow rate of the synthesis unit off-gas is much lower than that of the granulation unit while its ammonia concentration is much higher.
Nevertheless, these two off-gas streams can both be routed to the scrubbing system of the granulation unit and be jointly treated in the granulator scrubber. In principle, this also provides the option to treat other ammonia-containing off-gases from the synthesis unit in the granulator scrubber as well, e.g. from the ammonia water tank or the urea solution tank (Figure 1).
Treating the off-gases of both units in the granulation scrubber results in lower operating costs and reduces the amount of electrical equipment in operation. At the same time, emissions are minimised, thereby reducing product losses as well. Less equipment also translates into lower investment costs.
The scrubbing systems built by tkIS for the two US urea plants were of a horizontal cross flow type design with structured internals from Kimre (Fertilizer International 485, p25). This can remove dust and ammonia to very low levels, as shown in Tables 1 and 2. With this design, the visibility of the plume can also be reduced by capturing submicron particles with an AEROSEP® stage, if required.
A simple, lower-cost alternative?
An alternative to the horizontal cross flow type design are vertical tray type scrubbers. These consist of at least one dust removal stage and one acidic stage. A major advantage of the vertical tray type design is that the distribution and removal of condensate and washing agent are less elaborate. Such scrubbers, because of their round or square shape, do however need to be carefully configured within the plant layout.
A common feature of vertical tray and horizontal cross flow type designs is that both scrubbing systems require either sulphuric acid or nitric acid as a washing agent – which then reacts with ammonia in the scrubber to form an ammonium salt solution.
If ammonium nitrate (AN), calcium ammonium nitrate (CAN) or urea ammonium nitrate (UAN) are also produced on site, the resulting small amount of weak ammonium nitrate solution can be exported to these units and converted into a marketable product.
If the scrubbing is done using sulphuric acid, the ammonium sulphate (AS) generated can either be discharged to battery limits, or returned to the urea granulator feed after treatment – using proprietary Ammonia Convert Technology (ACT) offered by thyssenkrupp Fertilizer Technology (Fertilizer International 469, p25). The resulting urea product still fulfils fertilizer-grade urea specifications while, advantageously, containing a minor amount of sulphur as well (0.1% maximum), a beneficial secondary crop nutrient. With sulphur depletion an increasingly common problem in many soils, this is a welcome urea product enhancement. Crops which are sulphur-deficient also pick up less nitrogen. Consequently, sulphur can support plant nitrogen uptake, an important property given the increasing focus on avoiding nitrogen losses.
Tailor-made scrubber design
In general, there are three different sources of emissions a scrubbing system has to deal with:
- Fine dust – Small quantities, good visibility in plume, difficult to remove
- Coarse dust – Large quantities, fair visibility in plume, easy to remove
- Ammonia – Gaseous, easy to remove.
For fine dust removal, the installation of sophisticated internals, or even an AERO-SEP® stage, is necessary to remove sub-micron particles. In this regard, horizontal cross flow scrubbers are very effective at delivering the low emission requirements associated with submicron particle removal, while also achieving a very low pressure drop.
Yet many regions of the world do not yet specify extremely low emission levels. The most commonly applied upper emissions limit for dust and for ammonia is still the World Bank standard of 50 mg/Nm³. In these circumstances, the removal of fine dust is generally not necessary.
This allows simplified scrubbing systems to be developed and installed at many locations. This has operational and cost benefits, given that fine dust removal requires much more effort and is also the main contributor to overall pressure drop. With this in mind, thyssenkrupp Fertilizer Technologies (tkFT) nowadays offers a simplified horizontal scrubbing system (compared to the BACT standard) for less stringent dust and ammonia emission requirements (e.g 50 mg/ Nm³). This simpler scrubbing system offers the following operational and cost benefits:
- Reduced number of stages/internals
- Less energy consumption due to lower pressure drop
- Lower equipment cost due to the scrubber’s more compact design.
A brief comparison of a conventional tray-type scrubbing system with this new cost-efficient horizontal tkFT scrubber concept, designed for dust and ammonia emission limits of 50 mg/Nm³, revealed the following:
Equipment size: For a 3,600 t/d plant, tkFT’s cost efficient horizontal scrubber concept can reduce the cross-sectional area of the cooler scrubber by more than 50 percent, relative to the cross-sectional area of a standard tray-type scrubber for the same size plant. This is possible by adapting the internal configuration and the mechanical design to actual needs. The smaller and simplified design also lowers the overall equipment cost. The pressure drop is far lower than for a comparable tray-type scrubber, even with the resulting higher gas velocity.
Energy Consumption: The energy consumption of a urea granulation plant is mainly determined by the amount of air used by the scrubber, and the pressure drop associated with both the scrubber and its off-gas ducting (Figure 2). In a horizontal cross-flow type scrubbing system, off-gas routing is straightforward. The off-gas travels from the outlet of the granulator to the scrubber inlet and from the scrubber outlet to the suction side of the scrubber exhaust fan, without any bend or change in direction. For a tray-type scrubber, in contrast, the off-gas needs to be routed down to enter the bottom of the scrubber and then, after leaving the top of scrubber, travels down again to the inlet of the scrubber exhaust fan, causing extra pressure drop. Consequently, the pressure drop within the horizontal cross-flow design is much lower than in a tray-type scrubber: overall, the pressure drop for a horizontal granulator scrubber is around 200 mmWC, versus the considerably higher pressure drop of 400-500 mmWC for a tray-type scrubber. The relationship between achievable emission limits and pressure drop (and therefore power consumption) for different scrubbing systems is shown in Figure 3.
Summary
The range and variety of emission requirements in new-build projects today is greater than in the past. At the same time, even existing operations are being faced with the need to adopt more and more stringent environmental regulations. Fortunately, cost-efficient revamp options are now available to meet these regulatory changes.
Permissible levels of dust and ammonia emissions at urea plants are typically governed by either local regulations or customer-specific limits. The installation of a single integrated scrubbing system for both the synthesis unit and granulation unit – whether for standalone plants or a larger complex – is a more efficient plant concept.
Together, tkIS and tkFT offer high-performance, tailor-made scrubbing systems to urea plant operators. These comply with emission limit requirements while minimising investment and operating costs. These systems are suitable for new plants as well as the revamping of existing production sites.
STAMICARBON
Experience with MicroMist™ and Jet Venturi Scrubbing systems
It is a well-known fact that granulation plants and prilling towers generate substantial submicron dust in their off-gas. The resulting negative environment effects are equally well-known.
Consequently, governments are introducing increasingly strict fine particulate emissions regulations. Stamicarbon, the innovation and license company of Maire Tecnimont, together with its partner EnviroCare International have, however, found a solution.
The two companies have co-developed the MicroMist™ Venturi (MMV) Scrubber for granulation plants and the Jet Venturi Scrubber for prilling towers. Both these innovative, high-performance scrubbers can remove submicron urea dust particles at extremely high efficiencies.
First MicroMist™ Venturi (MMV) Scrubber installation in a granulation plant
The first urea granulation plant equipped with innovative MicroMist™ Venturi (MMV) scrubbing technology was licensed in 2013. Four more installations have followed since then; two of these are currently in operation and three are under construction. MMV scrubbing technology forms one part of Stamicarbon’s overall granulation plant design (Figure 1).
The first ever MMV scrubber was installed as part of a large-scale single-line Stamicarbon urea granulation plant with acidic scrubbing. Ammonium sulphate (AS) is generated by the scrubber from the reaction between the ammonia present in the off-gas and injected sulphuric acid. This is recycled back into the granulator – eliminating any disposal streams.
The water content of the AS salt generated by the scrubbing system (about 55 wt-%) means it cannot be mixed directly with the main urea melt fed to the plant’s granulator via nozzles (water content 1.5 wt-%). Instead, a dedicated evaporation step is used to regulate the water content of the liquid urea ammonium sulphate (UAS) recycled to the granulator.
Original MMV design
The original MMV scrubber design is shown in Figure 2. This incorporates a preliminary quench scrubbing stage. The majority of the dust is separated from the air flow in this quench vessel. The scrubbing process is described below.
Exhaust air from the granulator enters the quench vessel where circulating UAS solution is sprayed into this air flow. The temperature difference between the air flow and the UAS solution promotes the collection of urea dust – with water in the gas flow naturally condensing on top of the dust particles. Within the quench, lean urea solution drawn from the sump of the MMV column is used as make-up water to compensate for evaporation losses.
On leaving the quench vessel, cooled and partly saturated air enters the bottom section of the MMV scrubber, where it is further conditioned by introducing a mist of droplets. These are created by the process condensate make-up sprayer. Lean urea solution is sprayed as a fine mist into the airflow as it enters the MMV stage. This mist collides with the sub-micron dust particles as they first enter and then exit the MicroMist ™ Venturi. This separates them from the air flow.
On leaving the MMV, the air is introduced into the middle acidic scrubbing section where ammonia is removed. The ammonia reacts with sulphuric acid solution circulating over the acidic scrubber trays to form ammonium sulphate. The resulting ammonium sulphate solution discharges from the trays into the acidic tank. The overflow from this tank flows continuously to the quench scrubber.
After leaving the acidic section, the air flow passes through a mist eliminator (demister). This prevents entrained acidic mist from entering the wet electrostatic precipitator (WESP) and the granulator scrubber fan. The air flow then enters the WESP, the final treatment step. Within the WESP, cooled steam condensate supplied to wash the walls becomes a second source of make-up.
This MMV design incorporates two overflow lines:
- A continuous overflow line: going from the MicroMist ™ Venturi column into the quench vessel.
- An emergency line to prevent overfilling: going from the quench column into the dissolving tank of the granulation plant.
Problem solving during commissioning
Unsurprisingly, some teething issues and design deviations occurred when the scrubbing system initially entered operation. These were resolved as follows:
An air flow with high concentration of entrained urea entered the MMV scrubber system from the quench. This resulted in a significantly concentrated urea solution in the MMV sump instead of a lean solution. As a consequence, no lean urea solution was available to compensate for the evaporation losses in the quench column. This was addressed by maximising the level in the MMV column and then using the overflow as make-up to the quench column. A trench was also installed in the cross-over duct to act as drain catch. The solution from this trench was then used to decrease the urea concentration in the MMV sump down to an acceptable operational level of 30 percent.
At the same time, an upset of the total water balance was avoided by returning urea entrainment to the quench vessel by gravity.
Within the MMV column, urea entrainment went from the MMV section into the acidic section. This happened as a direct consequence of the above deviation. The urea solution level in the MMV sump fell more rapidly than could be matched with fresh make-up water, risking cavitation in the Venturi tubes.
This issue was solved by closing the overflow line from the acidic tank to the quench vessel. This allowed the extra water generated to fall back into the MMV sump and restore its level.
Landmark dust emissions reduction
Once the above commissioning difficulties were overcome, official testing reported urea dust emission levels of less than half the (already stringent) target of <5 mg/Nm3 and a plume opacity of zero percent. These results – by confirming that Stamicarbon’s urea granulation plants were able to exceed even the most stringent dust emissions targets – were viewed as a milestone achievement for the urea industry globally.
New MMV scrubber Design
An updated design has now been introduced for all future projects, incorporating the lessons learned to date (Figure 3).
The main upgrades include:
- An optimised quench sump. A new reduced capex design optimises the water balance.
- Using the scrubber as small UAN production unit, where nitric acid is used as the acidic scrubbing medium. This allows producers to avoid waste generation by directly converting the acid into a UAN-32 final product within the scrubber. This solves two operational issues with one solution – as the scrubber uses acid to reduce ammonia emissions, while producing a finished final product instead of a waste salt.
- Training. Operators can now learn to operate the MMV scrubber using Stamicarbon’s Operator Training Simulator, as this now includes a specific scrubber training package.
The Jet Venturi Scrubber
The exhaust gas from urea prilling towers contains a very fine dust with an extremely large surface area. Indeed, up to 70 weight-percent of the total dust load can consist of submicron particles. This fine dust creates a highly visible, persistent plume and does not dissipate easily. Consequently, environmental regulations for prilling tower emissions are becoming stricter. Permissible dust emissions levels within Europe are currently 50 mg/Nm3 maximum. Limits are even more stringent in some other regions.
In response to this, Stamicarbon and EnviroCare International have been developing a Jet Venturi Scrubber designed for prilling towers that discharge plumes with a high submicron particle content (Figure 4, right). This scrubbing technology is designed to reach very low emissions levels.
Jet Venturi design
This dust scrubbing technology is suitable for both natural and forced draft prilling towers. The scrubbing unit can be placed at ground level or on top of the prilling tower. A tower top location is generally preferred, though, as this needs much less ducting – so reducing capex and avoiding the associated pressure drop. Advantageously, the Jet Venturi Scrubber does not require the installation of additional fan capacity, as its unique jet effect moves the off-gas without the aid of a fan.
Jet Venturi Scrubbing units can be conveniently mounted on the air exhaust stacks of forced draft prilling towers, with each stack having its own dedicated unit. Each scrubbing unit comprises of three compact stages that progressively treat and clean the off-gas until it is free of dust and ammonia contamination (Figure 5). The three stages are as follows:
1. Quench and primary Jet Venturi scrubbing stage. Coarse urea particulates are removed from the gas stream in the quench section by a spray of recirculated urea solution. This is followed by a downstream Jet Venturi section spraying a scrubbing solution.
2. Secondary Jet Venturi scrubbing stage. This uses dilute urea solution as a scrubbing liquid.
3. Optional acidic scrubbing stage. This is used to reduce ammonia emissions, if necessary.
Experience on prilling towers
Initially, a Jet Venturi Scrubber pilot unit, fabricated by Envirocare International at its US workshop, was transported to Europe for prilling tower tests carried out in cooperation with a Stamicarbon client. During the testing, all measurements were conducted by a certified and independent third party laboratory from the Netherlands.
The pilot unit consisted of two Jet Venturi elements arranged in series which function as primary and secondary scrubbing stages, respectively (see Figure 6). Four boxes were placed at the inlet and outlet of both Jet Venturi elements (numbered 1-4, Figure 6). These boxes contained mist eliminators, atomising nozzles and hold-up reservoirs to collect scrubbing liquid before it is pumped back to the nozzles.
Treated gas extracted from one of the prilling tower stacks entered through the bottom left duct and was then fed through boxes 1 to 4, and in-between the installed Jet Venturi elements, then discharged to the atmosphere via the top left duct. The sampling ports are visible in the middle of the inlet and outlet ducts (see Figure 6).
Pilot test results demonstrated that the Jet Venturi Scrubber can handle the air flow specified in its base design, yet still drastically reduce the dust content of the exhaust gas.
The expected dust emissions from a prilling tower with an installed Jet Venturi Scrubber are less than 15 mg/Nm3 . This is way below the current European emissions limit of 50 mg/Nm3 . Valuably, this scrubber technology can be installed during the debottlenecking of existing prilling towers.
Conclusions
Stamicarbon’s innovative scrubbing technologies allows urea plant owners to meet and go beyond the emissions limits of even the most stringent environmental regulations, while simultaneously reducing the associated investment costs. These are rigorous conclusions based on real-plant experiences with Micro-Mist™ Venturi Scrubbers and on-site tests conducted with the Jet Venturi Scrubber.
