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Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 620-624
Production of a Nitrogenous Humic Fertilizer by the Oxidation-Ammoniation of Lignite Jose Coca," Ricardo Aivarez, and Antonlo B. Fuertes Department of Chemical Engineering, University of Oviedo, Oviedo, Spain
Two lignite samples were oxidized with nitric acid (20% by weight) at 75 O C and treated afterward with ammonia in a fluidized bed reactor in a range of temperatures from 100 to 375 O C . The effects of temperature, ammonia flow rate, and reaction time on the total nitrogen content of the product are reported. The ammoniation reaction can be considered to be completed after 2 h. The product contained an amount of total nitrogen ranging from 7 to 13% by weight which increased as the ammoniation temperature increased. Soil nitrification measurements of the nitrogenenriched lignites show that the maximum conversion to nitrates and rate of nitrification are exhibited by the product obtained at the lowest ammoniation temperature, Le., 100 O C . Maximum conversion of nitrogen to nitrates was 45% for the product obtained at 100 O C , which compares well with similar products such as ammoniated peat (35%) and ammonium nitrohumates (45 %), and it is lower if compared with conventional fertilizers: urea (80%) and ammonium nitrate (75%).
Introduction It has been recognized for many years that fertilizers obtained from low-range coals (i.e., lignite and peat) by reaction with ammonia combine the properties of both humus or organic manure and the chemical nitrogen fertilizers. Several studies were carried out in the 1930's in order to produce coal-based fertilizers by processes that involved the oxidation of a carbonaceous material followed by a treatment with ammonia (Car0 and Frank, 1929; Car0 and Frank, 1930; Car0 and Frank, 1932; Scholl and Davis, 1933; Davis and Scholl, 1939). Two of the most important features of nitrogenous humic fertilizers are (1) good control and conditioning of the soil properties (mechanical properties, pH, retention of nutrient ions, microorganism activity, and heat capacity of the soil) and (2) controlled release of nitrogen, avoiding losses by run-off water as it occurs with synthetic fertilizers, in which more than 5 0 4 0 % of the input nitrogen is lost. With the advent of chemical nitrogen fertilizers, such as ammonia or some of its salts and urea, nitrogen-enriched coal fertilizers became uneconomical and their potential interest declined for many years. However, since 1960 several attempts were made to improve some of the known processes, mainly by using high pressures or making the reaction with ammonia in a fluidized bed reactor. Some of the most relevant studies are summarized in Table I. As shown in Table I, the two reactions involved in the process, i.e., oxidation and ammoniation of the carbonaceous material, are carried out either as a sequence or simultaneously. Depending on the method used, the nitrogen content of the product varies, as does also the chemical nature of nitrogen groups. In the past two decades new expectations arose with a promising process for obtaining a product called NEC (nitrogen enriched coal). In this process (Mukherjee et al., 1966) a certain amount of coal was treated in a fluidized bed reactor with an ammonia-air mixture. The final product had a high nitrogen content ( 20% ) and nitrogen f i t i o n was in organic form, which would presumably give a slow nitrogen release to the soil. Unfortunately, later studies (Mukherjee, 1978; Brown and Berkowitz, 1968; Berkowitz et al., 1970) have shown the limitations of NEC as a fertilizer, as it is incapable of releasing nitrogen into the soil. This behavior is likely to be due to the stable
-
0196-432118411223-0620$01.50/0
configurations of the fixed nitrogen, which are of the amidine or isoindol type. It has been shown (Oka et al., 1974, 1980) that, if the carbonaceous material is oxidized with nitric acid followed by treatment with ammonia at 200 OC, the final product contains organic nitrogen, but with configurations other than those of the amidine or isoindol type. This process yields a product with a slow rate of nitrification and none of the limitations of NEC. Bearing these facts in mind, it can be concluded that nitrogen-enriched coal fertilizers are not nowadays an economic alternative to conventional nitrogen fertilizers. However, they may find specific applications in farming or gardening when a bifunctional fertilizer is desired (i.e., with organic matter and nitrogen content). Typical examples of potential use are horticulture, golf courses, indoor gardening, and possibly as an ingredient of topsoils for mining and industrial waste sites. This paper describes the preliminary results of a process for obtaining a carbonaceous fertilizer by using lignite as raw material. The first step is the oxidation of lignite with nitric acid. The second step is the reaction of the product with ammonia in a fluidized bed reactor, in a range of temperatures from 100 to 375 "C. The effect of several operating variables and the results of the nitrification soil tests are also presented. Experimental Section The lignite sample L-1 used in the present study was collected from the Utrillas field (Teruel, Spain) and had the following percent composition, on a dry ash-free basis: C = 78.1, H = 4.8, N = 0.8, S = 3.9, and 0 (by diff.) = 12.4. The humidity content of the raw sample was 10.2% and the ash content 11.9%. The bulk sample was ground and dried for 5 days at room temperature, and screened fractions between 0.1 and 0.4 mm were collected. A few experiments were carried out with a second lignite sample, L-2, proceeding from the Meirama field (La Coruiia, Spain) with the following percent composition, on a dry ash-free basis: C = 63.5, H = 4.0, N = 0.7, and 0 + S (by diff.) = 31.8%. The ash content of the sample on a dry basis was 31.4%. The preparation and treatment method of sample L-2 was the same as for sample L-1. The oxidation step was carried out by making a slurry with 100 g of lignite and 100 mL of distilled water in a 0 1984 American Chemical
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
621
Table I. Methods for the Production of Nitrogenous Humic Fertilizers from Low Range Coals
-
reaction steps
2. coal
reference
NH,OH
0 2
1. coal
raw material
humic acids
__f
ammonium humates
heat NH40H HN 0, +nitro-humic acids heat
nitro-ammonium humates
lignite
Mukherjee (1961)
lignite lignite lignite lignite
Higuchi and Asakawa (1960) Nigro (1964) Schwartz et al. (1965) Ramachandran et al. (1974)
lignite, peat lignite, peat lignite lignite lignite
Scholl and Davis (1933) Brazler (1963) Mukherjee et al. (1965) Prasad et al. (1974) Guruz (1980)
lignite lignite lignite
Mukherjee (1957) Mukherjee et al. (1966) Cudmore (1968) Brown and Berkovitz (1968)
0,. N H 4 0 H
3. coal
4 . coal
-A nitrogen
heat, pressure
humic fertilizer
02,NH,
heat
nitrogen-enriched coal (NEC)
T S
U
u
u
Figure 1. Schematic diagram of the ammoniation fluidized bed reactor: (A) nitrogen cylinder; (B)needle valve; (C)rotameter; (D) manometer; (E) damping tank; (F, I) preheaters; (G, G') heating elements; (H)gas flow distributor; (J) temperature measurement thermocouples;(K)temperature control thermocouples;(M) fluidized bed; (N) glass cyclone; (0) gas outlet; (R)temperature controllers; (S) fine solids collection; (T)digital thermometer.
three-neck flask provided with a thermometer, a stirrer, and a graduated dropping funnel. The slurry was heated and when a temperature of 75 "C was reached, the nitric acid (20% by weight) in the dropping funnel was added at a rate of 13 mL/min. The reaction took place for 2 h while the temperature was kept constant at 75 "C. After this period of time, the reaction mixture was filtered through a porous glass plate and the product was washed into 2 L of distilled water and dried for 4 h at 110 "C. The nitrogen content of the oxidized product increased to 3.7% for sample L-1 and 3.3% for sample L-2 as a result of nitrogen fmtion into the coal structure as nitro and nitroso groups (Chambury and Polanski, 1957). This fixation takes place along with the oxidation process, which generates carboxyl and hydroxyl groups. The ammoniation reaction was carried out in a fluidized bed stainless steel reactor illustrated in Figure 1. The reactor (27 mm i.d.1 consists of two parts, divided by two stainless steel screens, enclosing a layer of glass wool. This division supported the sample and acted as a gas distributor when ammonia was passed through the reactor. The lower part, filled with glass beads (3 mm diameter), preheats the incoming gas to the desired reaction temperature and the reaction takes place in the upper part. Both parts are externally heated by two independent l-kW electrical heaters, connected to two automatic temperature regula-
tors (Philips, Model Plastomatic 11). Temperatures were measured by 6 iron-constantan thermocouples, as shown in Figure 1, connected to a digital thermometer (Philips). In order to study the hydrodynamics of the fluidized bed a glass replica of the stainless steel reactor was constructed. Each experimental run started with the introduction of 15 g of oxidized lignite through the top of the reactor. The reactor was heated and in a period of 8 min the reaction zone reached a temperature of 100 "C. At this point ammonia was fed into the reactor, after passing through a damping tank (E) and a preheating system (F). The flow of ammonia was controlled by a needle valve (B) and measured with a rotameter (C). If the system was operated at higher temperatures, the temperature could be raised at a rate of 13.3 OC/min. Reaction times were measured from the instant at which ammonia was introduced into the system. Volatile compounds produced during the experiment and small amounts of solids entrained by the outlet gas stream were collected by a glass cyclone (N). Once the reaction was stopped by turning off the ammonia valves, the rector was cooled and the solid product analyzed for nitrogen content. The determination of total nitrogen, ammonia nitrogen and KMn04-solublenitrogen was made according to AOAC methods (AOAC, 1975). Nitrogen percentages are referred to the dry sample. Results and Discussion Taking the nitrogen content of the product as the objective function, its variation was investigated as a function of the following set of independent variables: temperature, ammonia flow rate, and reaction time. A systematic study of these variables was carried out for the lignite sample L-1. Only a scarce number of data were obtained for the lignite sample L-2. Unless specifically stated, all the data reported correspond to sample L-1. Effect of Reaction Temperature. Experiments were carried out at seven levels of temperature, 100,150,200, 250,300,325, and 375 "C, and three flow rates of ammonia, 88, 126, and 188 L/h (at standard conditions). The reaction time for all the experiments was 2 h. As shown in Figure 2, the nitrogen content of the product increases with the reaction temperature and also, although less noticeably, with increasing flow rates. The maximum nitrogen fixation was of the order of 10.2% at a temperature of 300 "C. The effect of temperature on nitrogen fixation was also studied with two samples of lignite L-2, the raw material as such and a sample previously oxidized with nitric acid. Temperatures of operation were 100, 150, 200, 250, and 300 OC; for every experiment the reaction time was 2 h and
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
Table 11. Analysis of the Different Forms of Nitrogen Content ('7%) as a Function of the Reaction Temperature. Ammonia Flow Rate: 126 L/h; Reaction Time: 2 h lignite L-2 linnite L-1 " raw material oxidized material
T,"C
Nr
100
7.9 8.9 9.8 10.1 10.3
150 200 250 300
N K
"H,
2.4 1.8 1.1 0.9 0.8
4.0 4.0 4.6 4.0 4.2
Nr 4.7 4.8 5.4 7.8
"H3
3.4 2.4 1.4 1.0
NK
N T
NNH3
NK
4.1 3.4 3.8 4.5
10.6 10.7 11.1 12.4 12.9
4.8 3.4 1.8 1.1 0.8
6.2 6.4 6.0 6.0
I
I
I
I
5 IO 15 20 AMMONIA FLOW RATE, c m / s
,u
u 100
200
300
LOO
TEMPERATURE " C
Figure 2. Effect of temperature on nitrogen fixation in an oxidized sample of lignite L-1. Ammonia flow rate: ( 0 )88 L/h; (A)126 L/h; and (0) 188 L/h; reaction time: 2 h.
Figure 4. Effect of ammonia flow rate on nitrogen fixation in an 190; (0)150 oxidized sample of lignite L-1. Temperatures, O C : (0) (A)200; ( 0 )250; ( 0 ) 300; (+) 350; um$ minimum fluidization velocity.
-.
2s
12-
L
?+-
g
2 u
8
10 -
% ax
k+-
-
/~--A--&--&-
2a
-
L
gQ 8-
2:: 61
-~
100
150
200
250
TEMPERATURE, O
300
C
Figure 3. Effect of temperature on nitrogen fmtion in two samples of lignite L-2; (A) raw material; (A) previously oxidized sample; ammonia flow rate: 126 L/h; reaction time: 2 h.
the flow rate of ammonia 126 L/h at standard conditions. A plot of the results is shown in Figure 3. As shown in Figure 2, the maximum nitrogen fixation is attained at about 300 "C, decreasing beyond this temperature. Similar resulta were obtained for the process of coal-ammonia-oxygen reaction (Mukherjee et al., 1966; Brown and Berkowtiz, 1968) and for the reaction, under pressure, of oxygen with a coal slurry in aqueous ammonia (Guruz, 1980). There is no clear evidence as to this temperature limitation, but it could be due to partial pyrolysis of the lignite sample at temperatures higher than 300 "C. Results from Figure 3 clearly show that the increase in nitrogen fmtion is higher by a factor of 2 when the sample has been previously oxidized with nitric acid. Effect of Ammonia Flow Rate. By varying the reaction temperature and the flow rate of ammonia through the bed and by keeping the reaction time constant, the effect on the nitrogen content of the product was ascertained. The results presented in Figure 4 show that there is no substantial increase in the nitrogen content of the product, as the ammonia flow rate increases at tempera-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 623
Analysis of the different nitrogen forms and the corresponding experimental conditions are reported in Table 11for the two lignite samples (L-1and L-2) studied in this work. It is apparent from Table I1 that the ammonia nitrogen content, NNHa, decreases as the temperature increases for samples L-1 and L-2. In the latter case, at high values of T, it does not make much difference in NNH,whether the sample has been previously oxidized or not. %he ammonia nitrogen content enhances the product solubility, since nitrogen can be dissolved as NH4+,but as the temperature increases, the ammonium nitrohumates decompose into the corresponding amide and water in a first step, finally evolving as ammonia if the temperature is further increased. The KMn04-soluble nitrogen, NK, has usually been considered as a criterion for the nitrogen available to the plant (Mukherjee, et al. 1966; AOAC, 1975). As shown in Table 11,NKis in the range of 4-69'0, independently of the ammoniation temperature, and the total nitrogen content NT is about 2-2.5 times the NK value. However, as has been pointed out (Hamence, 1950) the fact that an organic nitrogenous fertilizer dissolves in alkaline KMn04 solution is no guarantee that nitrogen will be converted into ammonia and nitrate in the soil. There are examples in which the NK value as the index of nitrogen availability fails, e.g., urea (Davis, 1955) and NEC, the product obtained from the coal-ammonia-oxygen process. This latter has NK values as high as 30% but it has been demonstrated that it is a poor fertilizer (Berkowitz et al., 1970). As will be shown later, for the products obtained in this work, the NK value is not a suitable proof of fertilizer efficiency. The nature of the ammoniation reactions has yet to be fully ascertained. However, a mechanism can be suggested in order to explain the variation of nitrogen as temperature increases. The proposed mechanism is 0
0
I1
II
NH3
COOH
--
COONH,
I
I1 0
0
.IIII
II
!-!
NH3_
I11
-H20
OH ~ONH,
CONH,
IV
'CONH~
V
It has been pointed out (Mazumdar et al., 1967) that after treatment of coal with nitric acid the main constituents in the coal structure are polycarboxylic acids, nitrohydroxy, and/or quinonoid acids. In the hypothetical mechanism proposed herewith, attention will be focused on two main sites for reaction: carboxylic groups and the &lactone ring. If the ammoniatidn reaction takes place below 100 "C, the reaction will be I 11; i.e., the corresponding ammonium salt is formed and the lactone ring remains unaffected. At higher temperatures reactions I1 I11 IV V will take place. In an initial step (I1
- - -
-
amides are formed. In the presence of ammonia the lactone rihg opens and a compound of type IV is formed, which has a tendency to yield a secondary amide that forms an heterocyclic ring (lactam configuration). This mechanism may explain why the amount of KMn04-soluble nitrogen remains almost constant, independent of the reaction temperature, and it corresponds to the sum of ammonia nitrogen and amidic nitrogen. As shown in Table 11, for lignite L-2, the following relations are observed for the ammoniation reaction at 100 O C NT(oxidized material) - NT(before ammoniation) = 7.3 NK/0.82 N
NT(raw material) - NT(before ammoniation) = 4.0
-
-
NK
This statement requires the support of soil tests, as will be shown next. Hence, there is no variation of NKwith change in temperature, in spite of the fact that previous oxidation and increasing the ammoniation temperature led to an increase in NT. Besides, the type of nitrogen which NK accounts for would be in such a stable form that its assimilation by microorganisms in the soil would prove to be difficult. Statistical Fitting of Data. By using a multiple regression analysis the data can be fitted to an equation that allows the calculation of the total nitrogen content as a function of two variables. From the experimental data it is rather obvious that the two variables that influence the total nitrogen content ( YN) to a higher extent are the reaction temperature (XT) and the ammonia flow rate (XF). Reaction times are kept constant at 2 h. The equation used for fitting the experimental results was the following YN
= a0
+ alXT + a2XF + a3XT2 + a4XF2 + a5XTXF
For the range of variables involved: YN(7.6-10.3%), XT (100-350 "C), and XF (5-20 cm/s), the constant values are a. = 1.782; al = 3.914 X u2 = 3.831 X u3 = 5.736 X lW5 a4 = -9.912 X lod; u5 = -3.431 X low5.The standard error estimate is 18% and the multiple correlation is 0.984. Soil Tests. As has been stated before, there is no general agreement as to which type of nitrogen content is the most reliable index of the degree of availability of nitrogen to plants. In order to answer the former question, experiments were conducted employing soil as a medium to study the conversion of the nitrogen contained in the fertilizer into nitrate. The soil tests were carried out following a simple technique described in the literature (Hamence, 1950), and in spite of the complexity of the nitrification process, the results obtained provide some evidence concerning the nitrogen availability of the f rtilizer. Each test was performed with 200 g of a culture soil with a particle size lower than 0.75 mm and washed with 500 mL of distilled water. A dosage of 0.02 g nitrogen (as NT in the fertilizer or urea) was added to the 200 g of soil sample, and the conversion of nitrogen to nitrates was followed for a period of 21 weeks. The efficiency of the fertilizers with respect to the formation of nitrates was studied with ammoniated lignite L-1 samples at the following temperature: 100 OC (A), 150 O C (B), and 250 "C (C). Also, a blank test (E) using only soil and a urea test (D) were carried out. Results for the former tests are shown in Figure 6. It can be noticed that nitrate content in the soil increases with time. As would be expected, urea gives the highest values of nitrate content and rate of nitrification. It is remarkable that the conversion of nitrogen to nitrates
-
-H20
----t
111) by heating the ammonium salts of carboxylic acids,
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Ind. Eng. Chem. Prod. Res. Dev., Vol.
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promoting an enzymatic reaction because of the compact nature of coal structure (Rogoff et al., 1962). Conclusions Two lignite samples were oxidized with nitric acid (20% by weight) and reacted with ammonia in a fluidized-bed reactor. The products had a total nitrogen content as high as 10-12%, depending on the coal rank. Nitrification tests carried out in soil show the highest percentage of nitrogen converted to nitrate and nitrification rate for the product obtained by ammoniation at 100 "C. Registry No. Nitric acid, 7697-37-2; ammonia, 7664-41-7.
Literature Cited V
I
2
L
6
'0 '2 14 16 TiME. weeks
8
18
20
22
Figure 6. N i t r i f i c a t i o n testa in soil for seven ammoniated lignite (L-1) samples; ammoniated lignites a t temperatures ("C) of: (A) 100, (B)150; (D) urea; (E)b l a n k test.
decreases when the lignite-based fertilizer was obtained at higher temperatures. The highest conversion of nitrogen to nitrate was 45% for the product obtained at 100 "C. This value is comparable with the one reported for similar products: ammoniated peat, 35% (Davis and Sholl, 1939) and ammonia nitrohumates, 45% (Chakrabarty et al., 1968) and it is lower if compared with conventional fertilizers: urea, 80% and ammonium nitrate, 75% (Chakrabarty et al., 1968). There is not much difference between the results obtained with the blank test (E) and the fertilizer produced at 250 "C (C). This behavior has also been observed for ammoniated peat (Davis and Scholl, 1939). The former tests are a good indication of the fertilizer value, as nitrate compounds are readily used by plants. They also show that there is a correlation between the ammonia nitrogen in the fertilizer and the nitrogen conversion to nitrate. This correlation does not exist with the KMn04-soluble nitrogen which has the highest value for the fertilizer obtained at 250 "C. Hence, if nitrogen is attached to the coal structure as an organic functional group its solubility will be very limited. Furthermore, it is very unlikely that microorganisms will be able to attack the active sites of coal, thereby
Berkowttz, N.; Chakrabartty, S. K.; Cook, F. D.; Fujikawa, J. I. Soil Scl. 1970, 110(3), 211-217. Bratzler, K. b i g . Patent 643 237, Nov 4, 1963. Brown, H. M.; Berkowitz, N. AIcMProgr. Symp. Ser. 1966, 64(85),89-97. Caro, N.; Frank, A. R. BrRish Patent 347641, Jan 30, 1929. Caro, N.; Frank, A. R. French Patent 689041, Jan 28, 1930. Caro, N.; Frank, A. R. German Patent 559 254. Sept 17, 1932. Chambury, H. R.; Poiansky, T. S., Serial No. 58. Coil. Miner. Ind., The Pennsylvania State Universky, Mar 1, 1957. Cudmore, J. F. Australian Coal Research Lab., Ltd., PR 68-4, 1968. Davis, R. 0. E.;Schoii, W. Ind. €ng. Chem. 1939, 31(2), 185-189. Giiriiz, K. Fuel 1960, 59, 772-776. Hamence, J. H. J . Sci. Food Agric. 1950, I , 92-96. Higuchi, K.; Asakawa, K. Japan Patent 8658 ('58), Sept 27, 1960. Mazumdar, B. K.; Chatterjee, A. K.; Lahiri, A. Fuel 1967, 46, 379-386. Mukherjeee, P. N.; Lahirl, A. Indian Patent 62337, Nov 21, 1957. Mukherjee, P. N.; Bhowmik, J. N.; Mukherjee, A. K.; Ramachandran, L. V.; Lahiri, A. P f M . Natl. Acad. Scl. Indla 1961, 31A, 124-126. Mukherjee, P. N.; Bhaumik, J. N.; Lahiri, A. Indian J. Technol. 1965, 3 , 90-92. Mukherjee, P. N.; Bakerjee, S.; Ramachandran, L. V.; Lahiri, A. Indlan J . Techno/. 1966, 4, 119-123. Mukherjee, P. N. Personal communication, Central Research Institute, Jeaigora, Dhambad, India, 1978. Nigro, C. Agfochimice 1964, 9(1), 53-74. Oka, H.; Inoue, S . ; Sasaki, M. Kitami Kogyo TankiDa@a&u Kenkyu 1974. 6, 131-138. Oka, H.; Inoue. S.: Sasaki, M. Nenryo KyOk8lShi 1960, 59, 241-249. Official Methods of Analysis"; AOAC, William Horwtiz, Ed., Washington, 1975; pp 15-19. Prasad, M.; Chowdhury, S. B.; Roy, A. K.; Banerjee, S. Indian J . Technol. 1974, 12, 172-176. Rogoff, M. H.; Wender, I.; Anderson, R. B. "Microbiology of Coal", Information Circular No. 8075, V. S. Department of Energy 1962; pp 1-38. Ramachandran, L. V.; Saran, T.; Sinugh, G.; Mukherjee, P. N.; Lahiri, A. I n dlan J. Technol. 1974, 12, 403-407. Schoii, W.; Davis, R. 0. E. Ind. Eng. Chem. 1933, 25, 1074-1078. Schwartz, D.; Asfeid, L.; Green, R. Fuel 1985, 44, 417-424.
Received for review D e c e m b e r 1, 1983 Revised manuscript received M a r c h 20, 1984 Accepted April 23, 1984
