Sludge to X-Unlocking the full potential of sewage sludge

0. Daegu Metropolitan City

Daegu Metropolitan City is a metropolitan city in southeastern South Korea. Daegu Metropolitan City has a population of 246.5 million and is classified as one of the nation’s highest temperatures due to the high mountainous basin terrain. Daegu Metropolitan City is an area with the Nakdong River, one of the four major rivers in Korea, and the Kumho River, which flows across the center of Daegu. Seven sewage treatment plants near the river are responsible for the water quality of Daegu Metropolitan City. Currently, Daegu City has the nation’s best sewage treatment capacity by expanding environmental basic facilities to handle all sewage generation and introducing high-level treatment facilities and total treatment facilities to remove nitrogen and phosphorus that cause algae and river flow.

1. Sewage Sludge Treatment Problems in Daegu Metropolitan City

1.1 Sewage Sludge Treatment Promotion Method

The Daegu Metropolitan Government reclaimed 50% of sludge from sewage treatment plants and recycled 50% of the sludge from cement. Recently, the method of using sewage sludge as fuel for power plants has recently attracted attention. Daegu City is also pushing for 100% solid fuel for sewage sludge starting in 2021. Compared to the previous method of treating sewage sludge as landfill and cement materials, sludge and fuel-type are more efficient in terms of the environment as it has the advantage of being $70/ton cheaper and can collect 100% of sludge.

1.2 Problems and Solutions of Sewage Sludge Treatment

Currently, FeCl₃ are used to dehydrate sludge from anaerobic digestion. FeCl₃ is combined with SO₄^2- and S ions in Sludge to form FeS Fe₂S₃, which exists in solid form in the sludge.The use of FeCl₃ reduces odor by suppressing the generation of H₂S in the form of a gas. It also shows the highest dehydration efficiency when used with filter press than other dehydrants.(FeCl₃+FilterPress: moisture content 60%) However, due to FeS Fe₂S₃ formed in the sludge, the sulfur content in the sludge increases and becomes difficult to fuel. If a compound in the form of FeS exists in the sludge, it is difficult to extract only FeS in the sludge. Therefore, our team’s purpose is to propose several different dehydrants that are appropriate for use with FeCl₃ and to introduce a process that removes hydrogen sulfide and generates energy at the same time. In addition, we will introduce ways to reduce the sulfur content of dyeing industrial wastewater in accordance with the characteristics of wastewater entering the sewage treatment plant in Daegu

• Using a new dehydrants mixture with FeCl₃
• Energy recovery through H₂S/SO₄^2-Using a new dehydrants mixture with FeCl₃
• Reduction of sulfur components entering sewage treatment plant

2. Solution

2.1 Using a new dehydrants mixture with FeCl₃

2.1.1 Selection of dehydrants

In addition to FeCl₃, using the proper ratio of other dehydrants can reduce the production of FeS in sludge than using FeCl₃ alone and release some of it in the form of H₂S, which can be adjusted to the sulfur content standard as an auxiliary fuel. Several dehydrants that can be used with FeCl₃ are introduced below.

① cationic polymer coagulant-1 (C-210P)

C-210P, a product of OCI SNF company, is one of the cationic polymer coagulants, and the optimum coagulation conditions were tested when reacted with sewage sludge[2].

As a result, at the C-210P 5.5ml injection volume per 200ml of sludge, the zeta potential value converges close to 0 mV, forming an optimal injection concentration. In addition, the method of pre-reacting PAC and then injecting C-210P coagulants had a higher dehydration efficiency of at least 30% to 75% than a single injection of C-210P.

The table below shows the product specifications for cationic polymer coagulant (C-210P).

√ Conclusion

-If C-210P coagulant is injected after pre-reacting pac, it shows high dewatering efficiency when it reacts with sewage sludge.

-An on-site test is required to see how much FeS is reduced and how much H₂S is generated more than using 100% FeCl₃ by mixing FeCl₃ with PAC+C-210P in an appropriate ratio and reacting with the sludge of the Daegu city sewage treatment plant. jar-test)

② Cationic polymer coagulant-2 (Eco-friendly polymer coagulant)

Recently, from an environmental point of view, many eco-friendly polymer coagulants have been developed, and application to actual sites is required. There are basic chemicals for the treatment of domestic wastewater and industrial wastewater, but depending on the characteristics of these chemicals, secondary environmental pollution may occur when reacting with sludge. Therefore, treatment of sewage sludge using an eco-friendly polymer coagulant through verification of dewatering efficiency can be a good alternative. An eco-friendly polymer coagulant containing natural polymers is a patented technology[3], invented for the treatment of various industrial wastewater and sewage sludge. This material is made of natural polymer substances such as collagen (nonionic natural substance), gelatin (both natural substance) and chitosan (cationic natural substance) as raw materials, and the manufacturing method is detailed in the patent.

※ Jar-test

The jar-test is an experiment conducted to determine the optimum amount of coagulant to be injected and to calculate the optimum pH. It simulates the actual treatment process, that is, chemical injection, blending, floc formation, and precipitation. The coagulant treatment process affects the dehydration process according to the characteristics of the properties of the sludge (pH, viscosity, solid concentration, etc.) depending on the amount of coagulant injected. The amount of flocculant to be added varies depending on the total solids content. If the amount of coagulant is increased than the appropriate amount, the dehydration is lowered due to the excessive amount of the coagulant. It is very important to find the optimal coagulation conditions. Therefore, after selecting a coagulant, a jar test must be performed when applied to the actual site. In order to apply C-210P, eco-friendly polymer coagulant, and FeCl₃ to the actual Daegu sewage treatment plant, jar-test should be accompanied. Then, pilot scale and online testing should determine the operating conditions of the filter press, such as the optimum filtration cycle time and pressure, the required filtration area and the need for filter aids.

2.1.2 How to increase filter press dehydration efficiency

When using a mixture of FeCl₃ and a cationic polymer coagulant, the sulfur content in the sludge can be reduced, but the filter press dewatering efficiency can be reduced compared to the conventional treatment with 100% FeCl₃. (After FeCl₃100%+filter press: moisture content 60%)

In order to improve sludge dehydration efficiency, the inline high-speed mixing system developed by the company has applied the inline chemical mixing method used in water purification facilities, not the cohesive stirring method by excessive injection of existing coagulant. As a result of applying the in-line chemical mixing method, a polymer coagulant was injected into the sludge pipe flowing into the dewatering process and rapidly mixed within a short time, thereby showing excellent floc formation even in the injection of a small amount of polymer coagulant. In addition, it was found that the effect of reducing the moisture content of sludge in the dehydrator was very high.

The in-line high-speed mixing device allows the uniform mixing of polymeric coagulant and sludge for a short period of time by installing a stirrer equipped with a slit in an inline form in a sludge supply pipe and mixing at high speed (1000-1750 rpm).

√ Advantages

– It improves dehydration by forming harder floc with a smaller amount of polymer coagulant injection compared to conventional cohesive stirring methods.

-Increased sludge treatment per unit time

-Reduction of polymer coagulant usage and sludge moisture content

– Additional effects such as reducing the maintenance cost of dehydration facilities (blockage of filtration due to excessive use of polymer coagulants), reducing work load (control of polymer coagulants and frequency of polymer coagulant preparation), reducing washing water volume of dehydrator.

– 100% domestic technology reduces production cost

As a result of the on-site demonstration, the improvement of sludge moisture content was reduced by more than 3%, the use of polymer coagulant was reduced by more than 40%, and the amount of sludge treated per hour was increased by 15%. The expansion of these new technologies will greatly reduce operating costs of KRW 629 million annually and KRW 5.09 billion in 10-year effects, as well as improvement effects when expanded to sewage treatment plants and wastewater treatment facilities and wastewater treatment facilities.

2.2 Efficient S removal and energy recovery

2.2.1 Removal of H₂S and Energy Production

The combination of FeCl₃ and cationic polymer flocculants for sludge dehydration can reduce FeS in sludge than before. However, the anaerobic digestive system produces much more H₂S gas. Therefore, facilities are needed to desulfurize H₂S gas. It is expected to be more economical and efficient in the long run if energy can be recovered from H₂S as well as elimination of H₂S gas. Therefore, several processes are introduced below to recover energy from hydrogen sulfide. If one of the following processes is applied to the sewage treatment plant in Daegu, it is expected that more economical resources will be available if only initial costs are invested.

1. Application of the Microbial Process for Hydrogen Sulfide Removal and Bio-Sulfur Production

Two preprocessing facilities were installed in parallel using the THIOPAQ® method to remove high concentrations of H₂S. Each facility consists of three main unit processes. It is a sulfur separation recovery system consisting of an absorption tower that absorbs H₂S, a bioreactor that converts sulfide into elemental sulfur by microorganisms, and a precipitator and dehydrator. When gas enters the absorption tower, H₂S reacts with caustic soda (NaOH)(pH 8~9) circulation water and formula (1) sprayed from the top through the nozzle. Some of the sulfide(NaHS) produced is then changed to sulfuric acid by Equation (2). The consumption of alkalinity is mostly by Equation (1), but a small amount is consumed according to Equation (3), producing carbonate, such as Equation (4).

H₂S + NaOH → NaHS + H₂O (1)

2NaHS + 4O₂ → 2NaHSO4 + H₂SO₄ (2)

CO₂ + OH– → HCO₃ – (3)

HCO₃ – + OH– → CO₃^2- + H₂O (4)

The solution(NaHS + H₂O) produced in the absorption tower is subsequently transferred to the microbial reactant. The microbial reactant is operated at pressure similar to atmospheric pressure, and the air supply of the reactant is controlled by an automatic control system to minimize sulfate generation. In microbial reactants, NaHS components are converted to sulfur, as shown in Equation (5), by the action of Thiobacillus. In microbial reactants, a 25% NaOH aqueous solution was continuously resupplied using an instrument-attached pump, partly regenerated by the OH– pretense (5) lost in the absorption tower, but also by Equation (6). In order to prevent the accumulation of sulfate ions during the operation period, the aqueous solution and effluent of the microbial reaction tank were controlled by a certain level of automatic measurement device to release the generated gas into the atmosphere.

NaHS + 1/2O₂ → S⁰ + NaOH (5)

2NaHS + 4O₂ → NaHSO₄ ↔Na₂SO4 + H₂SO₄ (6)

The liquid leaked from the microbial reaction tank was separated from the water in the sediment tank and recycled as much as necessary to maintain the process, and the rest was recovered after dehydration. In addition, some of the supernatant water was recirculated into the absorption tower and the surplus was discharged into the wastewater. The above figure shows the process composition of the H₂S removal facility of the THIOPAQ® method used in this study. According to a balanced analysis of S-Load materials, bio-sulfur production was 92.0%, untreated H₂S was 7.4% and wastewater discharge was 0.6%. Bio-sulfur is 94.8% sulfur and 2.34% Na, with trace amounts of other ingredients and no harmful heavy metals detected. Living conditions were 94.8% and Na 2.34%. There were few other ingredients and no harmful heavy metals were detected.

√ Advantages

– Maintains a stable removal rate H₂S (average 99.4% removal as a result of experiment)

-Due to the hydrophilicity and small particle size of sulfur produced, it can be used as a disinfectant in the future

2. From stench to resource: Splitting hydrogen sulfide with solar energy

Biofuel plants, sewage treatment plants, and oil stations can emit a significant amount of hydrogen sulfide gas at high concentrations. There is a photoelectrochemical process that uses solar energy to split this by-product into sulfur and hydrogen and convert it into a source of raw materials. Hydrogen sulfide gas enters the anode of the electrochemical cell into the electrolyte. Here, the chemical reaction precipitates sulfur into an amber solid and converts hydrogen into a cation. Only hydrogen protons converted to cations can pass through the Nafion film membrane. The second reaction is photoelectrochemistry. Hydrogen protons are reduced in cathode by receiving electrons and returned to oxidation in opposite anode poles. This is due to the production of electron-hole pairs in anode poles due to light. This oxidation-reduction cycle continues to circulate, allowing hydrogen sulfide to split into sulfur and hydrogen by sunlight.

√ Advantage

-Hydrogen and sulfur of hydrogen sulfide can be recovered simultaneously by applying Photoelcetrochemical (PEC).

2.2.2 SO₄^2- Removal and Energy Generation in Sludge

1. Microbial fuel cells (MFCs)

Microbial fuel cells are devices that can convert chemicals directly into electrical energy by electrochemical reactions associated with biochemical pathways.[7]

√ Advantages

-The removal rates of sulfide and acetate linked to upper anaerobic sludge reactants are 98% and 46%, respectively.

-If the device can be successfully developed, the formed sulfur must accumulate and potentially yield a high efficiency of the cost.

-Electricity energy production

.

2.3 “S” source reduction (Removal of sulfur in wastewater from Daegu Dyeing Industrial Center)

2.3.1 Substitution of chemicals containing sulfur used in wastewater treatment in the Daegu Dyeing Industrial Center

Daegu City has seven sewage treatment plants. In order to convert all the sludges from the seven sewage treatment plants in Daegu City into fuel, the sulfur content in the final sludge must be lowered below the fuel standard at all sewage treatment plants. To achieve this, it is necessary to know the sulfur concentration in the final sludge at each sewage treatment plant in Daegu and the characteristics of the wastewater flowing into each sewage treatment plant.

As a result of the investigation, the largest amount of sulfur was detected in the Dalseocheon sewage treatment plant, which is related to the Daegu Dyeing Industrial Center, among the sewage treatment plants in Daegu city. In the case of dyeing wastewater, the reason for the high concentration of sulfur is that waste SO₄^2- is used in large quantities when treating wastewater at the wastewater treatment plant of the Daegu Dyeing Industrial Center. (Daegu Dyeing Industrial Center Wastewater Treatment Plant SO₄^2- concentration is 400mg/L of influent water, 1600mg/L of effluent water) Therefore, if the sulfur content in the wastewater of the Daegu Dyeing Industrial Center is reduced and then discharged, the amount of sulfur to be treated at the Dalseocheon Sewage Treatment Plant in Daegu City will be reduced. We propose a solution to neutralize the wastewater as another alternative to SO₄^2- in the secondary chemical treatment facility of the Daegu Dyeing Industrial Center wastewater treatment plant.

Wastewater treated in various industrial facility flows into the Daegu City Sewage Treatment Plant. Among these, the concentration of SO₄^2-, which is a form of sulfur ion that appears mainly high, was compared by displaying in the table below.

Among the various samples in the table, the concentration of SO₄^2- occupied the highest part at 1.680mg/L in the dyed wastewater treated effluent.

2.3.2 Change of chemicals in the secondary chemical treatment facility

The wastewater flowing from the Daegu Dyeing Industrial Center Management Corporation to the common wastewater treatment plant has a high load of organic pollutants, so it is treated with a three-stage treatment method, such as a primary physical treatment facility, a secondary chemical treatment facility, and a tertiary biological treatment facility. The wastewater treatment process in the existing Daegu Dyeing Industrial Center was as follows.

In the secondary chemical treatment facility during the dyeing wastewater treatment process, chemicals are added to change the physical state of dissolved and suspended substances so that they can be removed by Sedimentation. Among the drugs used in secondary chemical treatment facilities, H₂SO₄ adjusts the pH of dyeing wastewater. The technology to replace H₂SO₄ with carbon dioxide, which affects the sulfur content of the finally discharged dyeing wastewater, is shown in the figure above. In principle, carbon dioxide (CO₂) from the boiler combustion gas from the Dyeing Industrial Center in Daegu is used to neutralize dyeing wastewater. Currently, sulfuric acid (H₂SO₄) is used as a neutralizer for dyeing and leather wastewater, but it can be used as a substitute for this chemical. This process will enable efficient use of carbon dioxide in relation to the Climate Change Convention.

CO₂ should be used as a substitute for H₂SO₄ to neutralize dyeing wastewater with the technology above. We designed the Pilot Plant, a process to know the specific amount of CO₂ for this.

<Pilot Plant Design>

1. Process overview and application criteria

– Waste water currently treated at the secondary chemical treatment facility of the Dyeing Corporation in Daegu: 65000 m³/d.

– Pilot plant design that can treat strong alkaline dyeing wastewater discharged from dyeing industrial complex on a 2708m³/hr scale.

–The production gas for wastewater neutralization uses bituminous coal (bituminous coal), which is currently used in the dyeing industry, as fuel. (The bituminous coal is well used for thermal power generation, and it is not produced in Korea, so it is being imported and used)

– Waste water is discharged to pH 7 after neutralization at an average pH of 10.5 (approximately 12.64 ppm NaOH).

– The temperature of the wastewater discharged from the neutralizer tank is 40℃ and the temperature of the exhaust gas is 40℃.

2. Basic Design by Unit Process

1) gas components and flow rates

-Since bituminous coal is not produced in Korea, all of it is imported and used. Therefore, the design was performed using the average of the elemental ratios of bituminous coal in each region of the UK (Bagworth, Lea Hall, Littleton, Cresswell, Cortonwood, Cwm, Taff Merthyr, Cynheidre), which is one of the bituminous coal producing countries[9].

The exhaust gas flow rate was calculated as the CO₂required to neutralize 2708 m³/hr of 12.64 ppm NaOH wastewater to pH 7, assuming a 70% CO₂ conversion rate.

2) Calculation details

2)-1. Combustion gas composition

Bituminous coal is composed of 86.975% of carbon, 4.8125% of hydrogen, 0.75% of sulfur, 5.81% of oxygen, and 1.5125% of nitrogen by mass fraction. Assuming complete combustion, the theoretical oxygen demand required for combustion is calculated as follows.

C + O_{2 →} CO₂

Since one mole of carbon reacts with a mole of oxygen, one mole of carbon dioxide reacts with a equivalent ratio, 72.41 mol of oxygen is required to completely burn.

72.41 mol O₂ * 31.998 g O₂/1 mol O₂ = 2316.97 g O₂

S(g) + O₂ → SO₂(g)

One mole of sulfur and one mole of oxygen react with a equivalent ratio, producing one mole of sulfur dioxide (SO2), which requires 0.23 mol of O₂ to completely burn.

0.23 mol O₂ * 31.998 g O₂/1 mol O₂ = 7.39 g O₂ is required.

H₂ + 1/2O₂ → H₂O

Since 1 mole of hydrogen and 1/2 mole of oxygen react with a equivalent ratio, 1 mole of water vapor is produced, 11.935 mol of O₂ is required to fully combust 23.87 mol of hydrogen.

11.935 mol O₂ * 31.998 g O₂ / 1 mol O₂ = 381.89 g O₂

And, assuming that the O₂ contained in bituminous coal participated in the combustion, 1.815 mol (O₂) decreases, so the total amount of oxygen required for combustion can be calculated. In theory, the total number of mol O₂ required is 82.76 mol (2648.15g O₂). Assuming that the ratio of excess air flow to induce full combustion is 1:1.1, the composition of the final combustion product is shown in the table below.

Assuming the concentration of dyeing wastewater is 12.64ppm and the flow rate of wastewater is 2708m³/hr → 2708m³/hr * (3.16*10-4mol/L) * 1000L/m³ = 855.728 mol/hr.

Assuming that NaOH and CO₂ react in an equivalent-to-equivalent ratio, the required amount of CO₂ at this time is 855.728 mol/hr.

NaOH + CO₂ → NaHCO₃

Assuming an average conversion rate of 70%: 885.728 mol/hr x 100/70 = 1222.47 mol/hr.

Since the composition of CO₂ in the gas is 16.13%, the total gas flow required:

1222.47 mol/hr *(100/16.13) = 7578.86 mol/hr = 7.579 kmol/hr.

Since the amount of carbon dioxide generated by the combustion of bituminous coal in the dyeing complex is about 2400 to 4000 tons/day, it is expected that there will be no problem using 1290 ton of carbon dioxide, which takes one day, as a neutralizing agent for dyeing wastewater.

√ Advantage

-Reduced carbon emissions in connection with climate change conventions by using CO₂ for wastewater neutralization

3. Econimics

3.1 S source reduction

-Using 2000~2500 ton/month of sulfuric acid to neutralize wastewater in the dyeing industrial complex,
-Unit price of sulfuric acid: $1.00/ton
⟹ $83.3/day of sulfuric acid consumption cost
⟹ Operating cost of CO₂ neutralization facility: $530/day

3.2 FeCl₃+ cationic polymer coagulant

To put FeCl₃ and cationic polymer dehydrant together, the amount of sulfur and hydrogen sulfide generated by reducing the amount of FeCl₃ was designed.

First of all, the data surveyed and the data by advice are as follows.
FeCl₃ : 24300 kg/day
S (sulfur) : 54.03 kg/day (1.92 %)
H₂S : 20 mg/L (2%) (the initial concentration (1500 mg/L) data does not have the exact information specified, so it is assumed from the data previously received).

It was designed by calculating the mole ratio of each component.

Currently used FeCl₃ is 24300 kg, 24300 kg/day * 1/162.2 mol/g * 10³ g/kg = 149815 mol/day.
Sulfur in sludge produced per day is 54.03 kg/day (1.92 %) and 54.03 kg/day * 1/32 mol/g * 10³ g/kg = 1688 mol/kg.

It is assumed that the amount of FeCl₃ was reduced from 24300 kg/day to 20000 kg/day, and added certain amount of cationic polymer dehydrant. (When FeCl₃ is 20000 kg/day, 123304 mol/day)

149815 mol/day (currently FeCl₃): 1.92% (sulfur ratio produced with FeCl₃) = 123304 mol/L (reduced FeCl₃) : X % (expected ratio of sulfur)
X (Expected S Ratio) = 1.58 %
When FeCl₃ is reduced to 20000 kg/day, sulfur in sludge is expected to decrease by 1.92% to 1.58%. (-0.34 %)

However, as mentioned in previous texts, the smaller FeCl₃, the more H₂S can be generated.
(Additional H₂S removal and energy generation methods are also detailed above.)

The approximate amount of H₂S was also calculated.

The ideal reaction formula for FeCl₃ is as follows.

① 2FeCl₃ + 3H₂O -> 2FeS + S + 6HCl ② 2FeCl₃ + H₂S -> 2FeCl₂ +S + 2HCl

With FeCl₃ we used, the ratio to H₂S is theoretically ideal. (FeCl₃ is 24300 kg, 149815 mol/day) (when Q is 1650 m³/day, H₂S is 1500 mg/L, 72580 mol/day)
In addition, the accuracy of the design is likely to be reduced because the exact amount or concentration of FeS cannot be measured when calculated in a ② reactive manner. So we assumed by referring to the most ideal reaction formula, ①.

If FeCl₃ is reduced to 20000 kg/d, calculated in proportion,
2: 1 = 98% (H₂S removed ratio)
1.64 : 1 = Y %
Y = 80.36% (H₂S removed ratio)
As expected, the elimination rate of H₂S decreases. Calculated in concentration, it is expected to produce H₂S of 196.4 mg/L.

⟹ In conclusion, reducing FeCl₃ and injecting a certain amount of cationic polymer dehydrant reduces the amount of sulfur, but additional H₂S removal is required.

<Cost calculation>

Cationic Polymers was considered as chemical to be administered with the existing reduced FeCl₃. Cationic Polymers were previously used with FeCl₃ in wastewater treatment. As an efficient wastewater treatment ratio, Cationic Polymers were FeCl₃ : Cationic Polymer = 100:1. For the cost per ton of polymer, Cationic Polymer stands at $799 per ton.

As a result of the calculation, it was 3,727.62$ when using only FeCl₃. When using the added FeCl₃+Cationic Polymer, the total cost per day was 3,211.82$, which was cheaper than when using only FeCl₃. This result is expected to reduce the cost by $515.8 per day compared to the conventional FeCl₃ alone.

3.3 wastewater treatment plant sludge fuel conversion
-Final sludge generation: 73.67ton/day
⟹73.67ton/d*$110/ton = $8103/day
⟹73.67ton/d*$40/ton = $2946/day

• reference

[1] http://www.daemyungchem.co.kr/shop/data/upload/20160315171852_890.pdf

[2] Variation of Sedimentation & Dwaterability Characteristics of Sewage Sludge under Various Coagulants (Journal of Korean Society on Water Environment, Vol. 30, No. 3, pp.311-318)

[3] Eco-friendly polymer coagulants containing natural polymer (Patent: 10-2008-0058368)

[4] http://www.waterindustry.co.kr/tech/tech01.php?ptype=view&idx=96756

[5] Application of the Microbial Process for Hydrogen Sulfide Removal and Bio-Sulfur Production from Landfill Gas (New & Renewable Energy 16(1), 2020.3, 68-76)

[6] An Integrated Photoelectrochemical–Chemical Loop for Solar‐Driven Overall Splitting of Hydrogen Sulfide ( Angew. Chem. Int. Ed. 2014, 53, 4399 –4403)

[7] Chemical and biological technologies for hydrogen sulfide emission control in sewer systems: A review (WATER RESEARCH 42 (2008) 1–12)

[8] Daegu Environmental Corporation’s Major Business Report(2019.2)

[9] Variation of nitrogen content and functionality with rank for some UK bituminous coals (FUEL, 1989, Vol 68)

*Our team*