Until now, I’m still dealing with LNG regasification system. I knew now that I don’t quite understand how to choose regasification method. And I understand that technology is getting better, so you have many options which type of regasification to choose, which one is the best.
I’ve studied several type of vaporizers, such as ambient air vaporizer, both natural draft and forced draft, open rack vaporizer, and intermediate fluid vaporizer, using glycol-water or propane. I had a correspondence with several vendors regarding that vaporizers. However, I haven’t encountered with submerged combustion vaporizer or intermediate fluid vaporizer with Rankine cycle. So, I can’t imagine how those vaporizers really work.
In the past, I remembered I encountered with Indirect Fired Water Bath Heater (personally, this term is really long, I can’t easily remember :D). But the application is not to regasify LNG, but to increase temperature of natural gas. In other word, it is gas post treatment after leaving vaporizer.
In this post, I want to share you summary of everything I learned until now about how to choose correct vaporizer for your condition. And I also want to share you some type of unique vaporizers (for me it is unique).
Ambient Air Vaporizer: The Simple and Best one?
In my previous post, I wrote brief explanation when is suitable condition to choose ambient air vaporizer. The key is you have warm climate condition. So, I can say AAV is almost always suitable in Indonesia due to the climate. However, is that so?
When we have to regasify 40 MMSCFD of LNG and the capacity of one AAV is 2 MMSCFD, then we need to install 2 x 20 unit of AAV. Can you imagine how much area we need? And how much fog will be generated?
But, I googled and it seems “fine” to install so many AAV’s like this picture. Although I think the fog is very awful that you barely can see anything.
One day I discussed with my friend who had experience in AAV operation. He witnessed how AAV worked. He said that the fog was very serious. But, thankfully the AAV works only during plant start up.
Main disadvantages of using AAV are:
- Need a lot of space, especially when regasification capacity is high
- Fog problem. Fog formation will reduce visibility
- It can be applied in offshore facility due to seawater current
- Forced draft AAV can be used for high LNG regasification capacity, but it required high power consumption (one vendor told me so)
When to use AAV:
- Regasification capacity is relatively small
- Peak shaving power plant. Because the AAV will operate only for about 5 hours a day, so the fog will not be generated all the time.
Open Rack Vaporizer: Check your Outfall Water Temperature & Many Treatments
I read in this paper that 70% of base load power plant used Open Rack Vaporizer type. Compared to AAV, this type of LNG regasification required less space for the same regasification capacity. But, how if your terminal is not closed to sea? I think open rack vaporizer is not a good option. How if there is river water? Technically it can be used.
I think main issue with Open Rack Vaporizer is environmental issue. When you take seawater, you need to ensure that you won’t take reef to your system. And also, when you discharge seawater, you need to ensure that water temperature is not cold enough that make fish die. There are several environment regulation to meet. The other issue about ORV is supporting systems. We need them a lot to make ORV works.
Several systems we need to provide are:
- Seawater intake system. It is a system to take water from sea.
- Seawater treatment system. Function of the system is to remove fish and debris
- Seawater pump
- Seawater outfall system
Last time, I prepared a simple presentation about prerequisite to apply open rack vaporizer.
This is summary of intake structure criteria for seawater:
- Distance of seawater intake should be sufficient from subtidal reef to minimize entrainment of reef
- Intake velocity not exceed 0.15 m/s under any operating condition
- When you need to add chemical dosing to the water, ensure that dosing location is strategic enough so that it doesn’t contaminate marine environment
Before entering our system, seawater need to be treated to remove any fish, coral, or debris. We need to install screen to prevent seawater pump and vaporizer from clogging. How to choose correct screen? Figure below shows rule of thumb to select suitable screen.
Beside debris and fish, heavy metal contained in seawater should be monitored. To excessive heavy metal concentration may attack aluminum aloy coating that leads to life shortening.
About seawater pump, we need to select suitable material that match with nature of seawater.
Seawater outfall is a system that allow seawater to be discharged to sea after used for LNG vaporizer. Location of seawater intake and seawater outfall must be studied to avoid cold water recirculation.
We also need to pay attention to cold water temperature. There is a regulation that approach temperature of seawater is maximum 5oC. For example, inlet water temperature is 25oC, so minimum allowable water temperature to be discharged is 20oC. So, the regulation is actually pretty strict.
In one source, I read that cold seawater should be discharged at subsea depth where ambient temperature of cold water is equal seawater discharge temperature. So, can you imagine if you have shallow sea, then you need long water pipeline to take and discharge seawater? In that situation, it is not economical to use ORV.
So, when one is talking about ORV, it is actually not an ORV, but it is a whole systems.
Intermediate Fluid Vaporizer: Still using Seawater
In previous project, we considered to use IFV but we declined because it actually still used seawater. We thought seawater pretreatment and post treatment will be quite the same as ORV. On the other hand, we also need to provide glycol-water or propane as intermediate fluid.
Intermediate Fluid Vaporizer is mainly used in cold climate application. If ORV is used during winter, the performance might not be as good as during summer.
I think mostly of you already hear and read about those three LNG regasification system. However, I want to share you more system, which I think pretty unique, because I haven’t heard nor read about them before. These are some of them.
Steam Heated Shell and Tube Vaporizer
Until now, I’ve never thought regasifying LNG by generating heat other than from air or water. But, actually there is an option of using steam to regasify LNG.
Because temperature of steam is high compared to water or air, we need a small LNG regasification unit. So, I think this type of vaporizer is suitable for relatively high regasification capacity and when space is an issue.
Plate LNG Vaporizer
In this technology, plate heat exchanger is used as part of intermediate fluid vaporizer system. Instead of using a single exchanger which actually consists of three exchangers, this technology use separate heat exchanger to regasify LNG. Plate heat exchanger is used to preheat propane and evaporate propane to gas state. Then the propane is used to regasify LNG at different heat exchanger.
Mobile Vaporizing Unit
Mobile vaporizing unit is basically an ambient air vaporizing which can be transported easily from one place to another. I found this is very practical to be used in remote area where regasification demand is relatively low.