CEREAL GRAINS AS SOURCE FOR SYNGAS

Wojciech Ciesielski
Institute of Chemistry, Environmental Protection and Biotechnology, Jan Długosz Academy, Częstochowa, Poland

ABSTRACT

Attention has been paid to the interactions of selected transition metal salts [Co (II), Cr(III), Cr(VI), Cu (II), Fe(III), Mn (II), Ni(II), Zn] as catalysts of the thermal degradation of barley, oat, rye, triticale and wheat grains to volatile products. Superior degradation of the barley grains occurred in the presence of Cu(II) ions. Degradation of oat starch proceeded best with the Co(II) ions. The Ni(II) ions were superior catalyst for the degradation of triticale and rye grains. Degradation of the wheat grains proceeded most efficiently with the Mn(II) ions.

Key words: biofuels, carbonizate, non-nutritional crop utilization, polysaccharides,.

INTRODUCTION

Our former study [3,4,5,6,7,8] indicated that starches of various botanical origin coordinated metal salts to form Werner type complexes with metal central ion. Structure of the inner coordination spheres of those complexes depended on cations and anions of used salts but coordination capacity of starches only in random cases depended on botanical origin of starch. The cations and anions of salts applied had also essential effect upon thermal stability of complexes, course and rate of that decomposition and amount of gaseous products and carbonizate. In some cases temperature of decomposition could be considerably lowered pointing to a possibility of energy savings once starch would be used for production of carbonizate and/or syngas. The carbonizate could potentially serve instead of coal and supplement biomass and wood as the source for manufacturing synthetic gas for the Fischer-Tropsch synthesis of fuels [1,9,10,11,12]. Because versatile, renewable sources for its manufacture would be used, such fuels might be classified as a biofuels. Additional benefit from the use of crops for that purpose would come from the fact that the crop quality and its potential contaminations bioaccumulated on their growth would not be essential for that type of consumption. Designing crops for production of biofuels not only from biomass and wood would keep agricultural production going in spite some limits put by European Union and Government of the United States for production of crops for nutritional purposes.

It has been reported [2] that already at 200°C CO₂ appeared practically as apart from steam the sole volatile product of the starch degradation. Its evolution declined to 5 wt% as temperature elevated to 500°C. The yield of CO₂ was only slightly dependent on the botanical origin of starch. On degradation, rice starch produced most, abort 1 wt%, CO₂ and the field of that gas from decomposition of cornstarch was the lowest. Carbon monoxide began to evolve from about 250°C and it reached maximum yield, 35–40%, of the total volatile fraction around 350°C in order to decline to 20–25% at 500°C. Potato starch is the best source of CO (40% at 350°C) and corn starch is the worse source for CO (about 35%) (Fig. 1).

Methane appears in the volatile fraction already at 250°C but it constituted hardly 1% of the total. Its deal increases to 40–50% of the total at 450°C and then declines to 25–40% already at 500°C. Potato is a superior source also for methane whereas cornstarch is the worst source for that gas. At 250°C also 1–1.5% alkenes form in order to reach maximum yield around 375°C. Acetic acid, aldehydes, ketones and tars constitute a minor fraction of organics. At elevated temperature tars degrade to low molecular products listed above.

In this paper catalytic effect of CoCl₂, Cr₂(SO₄)₂, K₂Cr₂O_7, CuCl₂, FeCl₃, MnCl₂, NiCl₂, and ZnCl₂ upon the thermal decomposition of barley (B), oat (O), rye (R) triticale (T), and wheat (W) grains was studied in order to select metal salts providing the pathway of decomposition most suitable for gasification of particular cereal grains.

MATERIALS AND METHODS

Materials
Cereals: barley (B), oat (O), rye (R), triticale (T), and wheat (W) were purchased in the Silesian Grains Enterprise in Czestochowa, Poland. The moisture and nitrogen content (in w%) of the ceral grains was as follow: barley – 14.8 and 1.492, rye – 13.4 and 1.947, oat – 15.3 and 2.189, triticale – 14.2 and 1.838, and wheat – 15.4 and 2.003, respectively. Moisture in the grains was determined with Rail dryers MAX 50/1 Company, RADWAG, Poland. The nitrogen content was determined with an elemental analyser Vario EL III, Elementar (formerly Heraeus).

Metal salts: CoCl₂.6H₂O, Cr₂(SO₄)₂, K₂Cr₂O_7, CuCl₂.2H₂O, FeCl₃.H₂O, MnCl₂.4H₂O, NiCl₂.6H₂O, ZnCl₂ all of analytical grade, and ethanol, 96%, of analytical grade were purchased from POCh Gliwice, Poland.

Methods
Sample preparation. 0.1 M aqueous solutions of a metal salt (10 mL) was poured into cereal grains (0.3 g) and the whole was 24 h agitated followed by drying in an oven at 50°C to constant weight. Dry samples were stored in desiccator.

Scanning Electron Microscopy (SEM) and estimation of metal deposition in grain microareas. A Nova Nano SEM 200 microscope of up to 2 nm resolution and 70–500 000 × magnification equipped with a field FEG Schotky emitter (FEJ Europe Company) was used. An EDS adapter provided estimation of the metal deposition in grain microareas.
Thermogravimetry (TG), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC). Thermal DSC-TG-DTG analysis was carried out with the NETZSCH STA-409 simultaneous thermal analyser calibrated with standard indium, tin, zinc, and aluminium of 99.99% purity. Samples of approximately 0.020 g were heated in corundum crucibles with non-hermetic lids. Corundum was the standard. The heating was performed under the static conditions in the air in the range of 20–400°C with the 5 K min^-1 temperature rate increase. Measurements were duplicated. They provided the ±0.5°C precision in the temperature reading.

RESULTS AND DISCUSSION

Thermal stability and pathway of decomposition of cereal grains was different for particular varieties. Fig. 2 presents the thermal decomposition of barley grains in terms of their thermogravimetric analysis (TG), derivatized thermogravimetric analysis (DTG) and differential scanning calorimetry (DSC).

The TG, DTG and DSC curves informed that the decomposition proceeded in several steps. The first step involved evaporation of 4.16 w% adsorbed water. A degradation of organic components of the grains began around 125°C and came to its end around 200°C to provide further 15.63 w% mass loss. The degradation passed then into the principal, the fastest step associated with a subsequent almost 50 w% weight loss. This step was over at 300°C and after that in a slow process almost monotonous weight loss up to 500°C was noted. At latter temperature, about 2 w% residue, could be likely a mineral contamination of the grain. The DSC curve informed that processes in particular stages were complex. Some effects, especially in first stages were endothermic and the other, mainly in conclusive stages also exothermic effects occurred.

Oat grains were more thermally stable (Fig. 3). Their slow decomposition began from about 100°C manifested by a shoulder on the DTG peak centered at 239.4°C. The degradation slowed down around 250°C and turned into a monotonous process leaving at 500°C 82.4 w% of original grain weight.

Triticale grains were more susceptible to thermal degradation (Fig. 4). After loss of moisture, degradation of the grains began at 177.2°C. Initially slow process accelerated from 250°C and already at 300°C original weight of the sample was reduced to 30%. At 500°C the sample completely vanished.

Grains of rye appeared to be more stable than grains of triticale (Fig. 5). Their thermal degradation started around 300°C and proceeded very fast up to 350°C leaving carbonizate wiht the 46% yield. That carbonizate could be completely gasified up to 500°C.

Pathway of the thermal degradation of wheat grains resembled that for rye (Fig. 6).

A fast single decomposition step began after 275°C and was over around 340°C leaving a stable carbonizate with 55.4% yield. The weight of the carbonizate did not decrease up to 500°C.

Comparison of the thermal stability and pathway of degradation one might conclude that for generation of CO and CO₂ from cereals triticale and barley would be superior, rye grains performed worse whereas full gasification of the oat and wheat grains up to 500°C was impossible.

After soaking cereal grains in aqueous solutions of metal salts, the grains changed their thermal characteristics. Salts could simply adsorb on the surface of grains as well as in their interior as well as coordinate to the grain components e.g. proteins and carbohydrates. The oat grains were the richest and the barley grains were the poorest in protein (2.19 and 1.15% N, respectively). Table 1 shows that amount of salts trapped by grains of particular origin was in no relation to their protein content. There was also a selectivity between grains in efficient trapping metal ions. Triticale grains were the most efficient traps for the Cu(II) ions, oat grains trapped the highest amount of the Co(II) ions and barley grains most efficiently trapped the Fe(III) ions.

Scanning Electron Microscopy of the surface of the grains after their treatment with salt solutions (Figs. 7–11) showed, that irrespectively the botanical origin of the grains, Co(II) salts formed a glazy layer on the surface and Fe(III) salt covered the surface of the grains with a net-like layer. Cu(II) salts deposited on the surface of the grains in form of small granules.

The complete gasification of the metal salt treated grains at possibly the lowest temperature would be essential from the practical point of view. Decrease in onset temperature of treated grain and rate of particular stages of degradation would also be a factor, as faster degradation in particular steps meant lower amount of material for heating.

Figures 12–19 present thermograms of barley grains treated with Co(II), Cr(III), Cr(VI), Cu(II), Fe(III), Mn(II), Ni(II) and Zn ions, respectively.

The Co(II) and Cu(II) ions similarly changed the decomposition pattern of the barley grains. Both ions prolonged the first decomposition step and shortened the second step after which there was a fast reduction of the amount of carbonizate. Ni(II) ion was that catalyst which changed the decomposition pattern of barley grain to a least extent. Also parameters of the decomposition steps, that is, onset and conclusion temperatures, loss of weight and rate of decomposition (tg α) remained almost the same (Table 1). The Fe(III) and Mn(II) ions closely resembled Co(II) and Cu(II) ions in its catalytic activity. In thermograms run for the barley grain in the presence of the latter three ions, the decomposition in the range of 150-250°C looked as a single step although DSC curve informed that there were two overlapping steps. The Mn(II) ion provided separation of those two effects. The decomposition pathway in the presence of either Cr(III), Cr(VI) or Zn proceeded entirely differently. A slow degradation of the grains soaked in solution of the Cr(III) ions started around 200°C that is, by about 50°C higher than in case of grains soaked in solutions of the Co(II), Cu(II), Fe(III), Mn(II) and Ni(II) ions. After the degradation began it proceeded slowly and almost steadily up to 500°C at which temperature the grains still left 30 w% residue.. Zinc ions very well stabilized the material of the grains. A noticeable degradation of the grains begun at, approximately, 325°C and proceeded rapidly to 450°C leaving practically no carbonizate. A slow and steady degradation of the grains in the presence of Cr(VI) initiated at 100°C and lasted up to about 350°C. At that temperature degradation accelerated leaving no carbonizate at 500°C. This stage of degradation likely had an oxidative character as suggested by exothermic DSC peak centered at 368.2°C.

Although oat grains were thermally stable to a such extent that they could not be fully gasified up to 500°C, (Fig. 3), their treatment with some salts provided complete gasification to that temperature. Fig. 20 illustrates such case. The Co(II) increased the rate of all stages of degradation and gasification was practically completed already at 450°C.

Such result provided also the Cr(III) (Fig. 21) and Mn(II) (Fig. 22) ions, however, inspection of the degradation pathways in these three cases pointed to gasification in the presence of Cr(III) ions as the most energetically feasible. The considerably fast weight loss began already at 100°C reducing the weight of heated sample by over half of its original value. In contrast to that case, gasification in the presence of the Mn(II) ions required heating the sample without any weight loss to about 260°C. The Fe(III) ions also provided a total vanishing of the sample already around 450°C but a weight loss of the sample, although very fast, started just above 375°C (Fig. 23). In the presence of Cr(VI) ions oat grains lost up to 500°C hardly 35 w% of their original weight (Fig. 24).

Also neither Cu(II) (Fig. 25) nor Ni(II) (Fig. 26) provided a full gasification of oat grains to 500°C. At that temperature remained 30 and 50% mass of samples in the first and second case, respectively.

The Fe(III) ions offered the most beneficial conditions for full gasification of triticale starch (Fig. 27). At 178.6°C very fast degradation began. Up to 280°C it reduced the weight of sample by 80%. As a matter of fact the 20% residue required heating to 500°C until it was completely gasified. For the same reasons as discussed above, promising results were collected on heating triticale starch with Zn (Fig. 28), Ni(II) (Fig. 29), and Cu(II) (Fig. 30) ions. The degradation in the presence of the Co(II) (Fig. 31) and Mn(II) ions (Fig. 32) left about 20% residue up to 500°C.

Neither Cr(VI) (Fig. 33) nor Cr(III) (Fig. 34) catalyzed gasification of triticale starch to 500°C.

The rye grains appeared to be moderately stable to thermolysis (see Fig. 5) and their stability was additionally increased by treatment with the Cr(VI) ions (Fig. 35). They induced early degradation already below 100°C but after fast degradation which ceased after 150°C decomposition turned into steady slow process leaving 40% residue at 500°C. Also degradation in the presence of the Zn (Fig. 36) and Fe(III) (Fig. 37) ions left 35 and 30% % residue on heating to 500°C, respectively. When grains were thermolysed in the presence of either Cu(II) (Fig. 38) or Mn(II) (Fig. 39) ions, at 500°C 10% residue was left. Practically only the Ni(II) ion offered complete gasification of rye starch (Fig. 40) and the course of decomposition was fairly promising.

First stage of decomposition began at 133.8 C and the sample lost 60% of its weight to 250°C. At 450°C whole sample practically volatilized.

Wheat grains when degraded up to 500°C left 55% (Fig. 6) but the Zn (Fig. 41) and Mn(II) (Fig. 42) ions catalyzed degradation to the bottom.

Because of the decomposition pattern, the Mn(II) ions seemed to be better catalyst than Zn. The Cu(II) (Fig. 43), Cr(III) (Fig. 44) and Cr(VI) (Fig. 45) ions did not provide any complete gasification of the grains. In every case 10% residue was left at 500°C.

As the catalyst, Fe(III) (Fig. 46), Co(II) (Fig. 47) and Ni(II) (Fig. 48) performed poorly. Thermolysis to 500°C left 30, 40 and 60% residue, respectively.

Except two cases e.g. Fe(III) salt on the oat and triticale grains, productivity of carbonizate was inversely proportional to a concentration of the salt on the surface of the grain.

Based on inspection of the catalyzing performance of the metal salts in respect to particular cereal grains the following considerations provided final conclusions. Degradation of the barley grains proceeded best in the presence of Co(II), Cu(II), Fe(III), Mn(II) and Ni(II) ions. The Fe(III) and Mn(II) ions provided the lowest degradation onset temperature but up to 300°C Cu(II) and Ni(II) provided the deepest degradation, 61 and 60% weight loss, respectively. Because on decomposition of the barley grain with Cu(II) between onset temperature and 300°C provided earlier weight loss that ion seemed to be superior catalyst. Also lower amount of carbonizate left at 400°C, 8 and 25%, respectively, spoke in favour of that catalyst.

Only the Co(II) and Cr(III) ions provided a complete gasification of oat starch up to 500°C. Among both, the Co(II) ion appeared to be better catalyst. Although degradation onset temperature with that ion was higher, 160°C, it provided 52% weight loss to 300°C. Onset temperature with Cr(III) ion was much lower, 100°C, but the corresponding weight loss up to 300°C reached hardly 46%. Both catalysts offered the same 7% residue at 400°C.

Triticale grains could be completely degraded up to 500°C solely in the presence of Ni(II) and Cu(II) ions. Both catalysts offered comparable effects but advantage in using the Ni(II) ions resulted from lower degradation onset temperature, 117 and 134°C, respectively. The Ni(II) ion was the sole catalyst providing complete degradation of the rye grains up to 500°C. The wheat grains efficiently degraded in the presence of Zn and Mn(II) ions. Degradation onset temperature was 150 and 178°C, respectively, but Mn(II) provided 85% weight loss already up to 275°C whereas with the zinc ions up to 300°C the weight loss reached hardly 55%.

CONCLUSIONS

Selection of the proper metal salt as the catalyst of the thermal degradation can fully gasify cereal grains.

REFERENCES

1. Boerrigter H., Greek diesel production with Fischer-Tropsch synthesis, http://www.senternovem.nl/mmfiles/26674_tcm23-279868.pdf

2. Cerniani A., 1951. Thermal decomposition of starch, Ann. Chim. (Roma), 41, 293-298.

3. Ciesielski W., Kozioł J. J., Tomasik P., 2003. Complexes of amaranthus starch with selected metal salts and their thermolysis, Thermochim. Acta, 403, 161-173.

4. Ciesielski W., Tomasik P., 2003. Coordination of cassava starch to metal ions and thermolysis of resulting complexes. Bull. Chem. Soc. Ethiopia, 17, 155-165.

5. Ciesielski W., Tomasik P., 2004. Complexes of amylose and amylopectins with transition metal salts and their thermal properties. J. Inorg. Biochem. 98, 2039.

6. Ciesielski W., Tomasik P., 2004. Werner – type metal complexes of potato starch, Int. J. Food Sci. Technol., 39, 691-698.

7. Ciesielski W., Tomasik P., Lii, C.Y., Yen, M.T., 2003. Interactions of starch with salts of metals from the transition groups. Carbohydr. Polym., 51, 47-56.

8. Lii C.Y., Tomasik P., Yen M.T., Lai V.M.-F., 2001. Re-examination of the interactions between starch and salts of metals from the non-transition groups. Int. J. Food. Sci. Technol. 36, 321-330.

9. Pimentel D., Patzek T.W., 2005. Ethanol production using corn, switchgrass, and wood. Biodiesel production using soybean and sunflower. Nat. Resour. Res. 14(1), 65-76.

10. Schanke D., Hansen R., Sogge J., Hoftad K.H., Wesenberg M.H., Rytter E., Optimum integration of Fischer-Tropsch synthesis and syngas production. US Patent 6696501.

11. Srinivas S., Malik R.K., Mahajani S.H., 2006. Fischer-Tropsch synthesis using biosyngas and CO₂. Adv. Energy Res., 317-322.

12. Zwart, R., Large scale Fischer-Tropsch diesel production. http://www.tu-freiberg.de/~wwwiec/conference/conf07/pdf/7.1.pdf

Accepted for print: 18.05.2009

Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.