Ammonium Sulfate: The Underrated Nitrogen-Sulfur Fertilizer Creating New Value for FGD Systems

For decades, nitrogen, phosphorus, and potassium have dominated discussions about crop nutrition. Sulfur, although essential for plant growth, has often received less attention.

That is changing as agricultural producers pay closer attention to soil nutrient balance and crop quality. Sulfur is required for protein synthesis and other important plant processes. When sulfur availability is insufficient, crops may experience impaired growth, reduced yields, or lower product quality.

This creates an opportunity for ammonium sulfate. As a fertilizer that supplies both nitrogen and sulfur, it connects agricultural nutrient demand with an industrial product that can be recovered through ammonia-based flue gas desulfurization.

For industrial plants evaluating ammonia desulfurization, this connection matters. The value of the process is not limited to sulfur dioxide removal. It can also include the recovery of a product with an established place in the fertilizer industry.

1. Why Sulfur Matters in Crop Nutrition

Sulfur is an essential plant nutrient involved in protein formation and other metabolic processes. It is particularly important for crops with relatively high sulfur requirements, including rapeseed, legumes, onions, garlic, and some other vegetables.

When soils cannot supply sufficient sulfur, fertilizer application may help correct the deficiency. The appropriate product and application rate depend on soil conditions, crop requirements, climate, and the existing nutrient management plan.

Ammonium sulfate offers two nutrients in one product:

• Nitrogen: supports vegetative growth and contributes to plant protein formation.

• Sulfur: supplies sulfur in sulfate form, which plants can absorb directly.

This combination can be useful where both nutrients are required. However, fertilizer selection should always be based on soil testing and crop-specific recommendations rather than assuming that every field needs additional sulfur.

2. What Makes Ammonium Sulfate Different?

Ammonium sulfate is a water-soluble fertilizer containing approximately 20.5% nitrogen and more than 24% sulfur in the composition described in the source material.

Its sulfur is present as sulfate. Unlike elemental sulfur, which generally needs to undergo microbial oxidation before becoming available to plants, sulfate can be absorbed directly by plant roots.

This makes ammonium sulfate a practical sulfur source when crops need readily available nutrients during the growing season.

It can also contribute to nutrient management in alkaline soils, although the overall effect depends on soil chemistry, application rate, and long-term management practices.

The key point is that ammonium sulfate should not be viewed simply as another nitrogen fertilizer. Its combined nitrogen and sulfur content can make it a useful option for addressing two nutrient requirements at the same time.

3. Agricultural Demand Gives the Product a Market Foundation

The market for ammonium sulfate is supported by fertilizer demand, not solely by environmental regulations.

According to the figures provided in the source material, China's ammonium sulfate production reached approximately 23.6 million tonnes in 2025, with agricultural uses accounting for more than 70% of consumption. The source also reports that China's exports increased from 10.65 million tonnes in 2021 to 21.36 million tonnes in 2025.

These figures point to an established supply chain involving fertilizer manufacturers, distributors, and international buyers. Brazil, Vietnam, Myanmar, Indonesia, and Türkiye are identified in the source as important export destinations.

For industrial producers, an established market can improve the prospects for recovering value from ammonium sulfate generated during flue gas treatment.

Nevertheless, market access is not automatic. Product specifications, impurities, moisture content, transportation costs, local fertilizer regulations, and buyer requirements all influence commercial value.

A technically successful recovery process must therefore be supported by consistent product quality and a viable route to market.

4. How Ammonia Desulfurization Produces Ammonium Sulfate

In an ammonia-based flue gas desulfurization (FGD) system, ammonia is used to absorb sulfur dioxide from industrial flue gas. The resulting chemical process produces ammonium sulfate, which can be recovered for further use.

This creates a different by-product pathway from conventional limestone-gypsum desulfurization.

With a limestone-gypsum FGD system, the main by-product is gypsum. Depending on its quality and local market conditions, it may be sold, reused, or require disposal.

With ammonia desulfurization, sulfur removed from the flue gas can instead become part of a fertilizer product.

The source material estimates that approximately two tonnes of ammonium sulfate can be produced for each tonne of sulfur dioxide removed. Actual yields depend on process chemistry, operating conditions, product recovery, and the system's overall material balance.

The economic significance is straightforward: when the recovered ammonium sulfate meets the required specifications and can be sold or used, a by-product stream may become a source of revenue rather than only a waste-management responsibility.

5. Product Quality Determines Whether Recovery Creates Value

Not every ammonium sulfate stream is automatically suitable for agricultural use. Product quality must be controlled throughout the desulfurization and recovery process.

Important parameters may include nitrogen content, moisture, free acid, impurities, particle characteristics, and compliance with applicable fertilizer standards.

The source material cites GB/T 535-2020, the Chinese standard for ammonium sulfate, and describes an industrial product used as a raw material in compound fertilizer production.

For an ammonia desulfurization project, the objective should be to design the recovery process around the intended product specification. This includes evaluating crystallization, separation, drying, storage, and handling requirements.

Stable quality is essential for building long-term relationships with fertilizer manufacturers and other downstream users.

6. Calcium-to-Ammonia Conversion Connects Emissions Control with Resource Recovery

For plants operating limestone-based FGD systems, calcium-to-ammonia conversion may offer a route to a different by-product strategy.

The feasibility of conversion depends on the existing tower, process configuration, flue gas conditions, available space, and required modifications. A detailed engineering assessment is necessary to determine which equipment can be reused and what new process sections are needed.

MirShine Group's calcium-to-ammonia conversion technology is intended to combine sulfur dioxide removal with ammonium sulfate recovery. The company reports that its process can achieve sulfur dioxide removal efficiency above 99.5% and ammonia recovery of at least 99% under its stated conditions.

These figures should be assessed against the design basis and verified through project-specific performance testing. For plant operators, the wider evaluation should also include energy consumption, reagent costs, maintenance, product quality, wastewater management, and the economics of downstream sales.

The goal is not merely to change the desulfurization absorbent. It is to evaluate whether the whole system can deliver reliable emissions control while recovering a useful product.

7. From Pollution Control to a Circular Resource Model

Environmental equipment has traditionally been evaluated by its ability to meet emission limits. Today, operators are also looking at how treatment processes affect resource use, operating costs, and by-product management.

Ammonia desulfurization creates an opportunity to connect these priorities.

Sulfur dioxide is removed from industrial flue gas, and the recovered sulfur can enter the fertilizer supply chain in the form of ammonium sulfate. When the process is well designed and the product has a viable market, this approach can support a more resource-oriented model of industrial pollution control.

For industrial companies considering an FGD upgrade, the relevant question is not only how efficiently sulfur dioxide can be removed. It is also whether the recovered material can meet market requirements and generate value over the system's operating life.

Ammonium sulfate is more than a desulfurization by-product. With reliable recovery and consistent quality, it can connect industrial emissions control with the practical needs of agriculture.