If the mechanical properties of PyroBioCarbon (PBC) would satisfy the requirements of industries, the scheme of simultaneous production of hydrogen for power engineering and carbonaceous materials for industrial needs could find more wide applications. In such a scheme, special measures on storage or salvaging of carbon dioxide are not necessary.
HTTECH suggest a new approach to the solution of this problem. It is known that the crystalline structure of pyrocarbon deposits on a heated surface at the thermal decomposition of natural gas is different from other forms of carbon, including soot and black carbon. The industry is interested in high-density, high carbon content and sufficient strength of the carbonaceous material.
All these properties are inherent for PyroBioCarbon, which structure is close to graphite. HTTECH technologies provide for production of new type carbonaceous materials with properties satisfying industrial requirements. Along with that, these technologies can be used for hydrogen power engineering.
The products of the developed process are solid carbonaceous material and combustible gas with hydrogen content up to 97%. The complex usage of those products: solid carbon for industrial applications and the gases - for power engineering, will reduce the environmental impact. Using these technologies our industrial partners can avoid the carbon dioxide sequestration problems.
Process feasibility study
Biomass and wood provide the most important renewable source of carbon-containing stock: the average carbon content of wood of various species is about 50% (by mass). The forest yield in our country is enormous. Suffice it to say that 37% of the territory of Norway is covered with forests. The development of efficient methods of processing the wastes of the lumbering and wood-working industry is a very urgent problem both from the standpoint of raising the efficiency of utilization of natural reserves of raw materials and from the standpoint of environmental protection, because the decay of wood wastes is accompanied by emission of carbon dioxide, phenol compounds and other harmful substances while the thermal effect of attendant chemical reactions is not utilized.
A technology for integrated processing of wood wastes and natural gas has been suggested at the Joint Institute of High Temperatures of the Russian Academy of Sciences; this technology is based on the process of thermal decomposition of natural gas when filtered through the porous structure formed as a result of thermal destruction of wood. In general terms, the production scheme may be divided into two stages. In the first stage, which is referred to as carbonization, wood is subjected to heating to a temperature of the order of 600 ° C in a gas medium free of oxygen. The carbonization results in a variation of the structure of the material and in the reduction of relative content of hydrogen and oxygen in the material. The resultant charcoal is a brittle porous material with a carbon content exceeding 90%. In the second stage, a natural gas is blown through charcoal. This is accompanied by heterogeneous thermal decomposition of natural gas and by the formation of pyrocarbon on the surface of carbon matrix; the latter is thereby transformed to a strong composite material containing up to 98% carbon. The gas mixture at the reactor outlet is enriched with hydrogen, the content of which is defined by the efficiency of the process of heterogeneous pyrolysis of natural gas when filtered through the porous carbon medium. Hydrogen formed as a result of pyrolysis may be directly employed as fuel or it may be mixed with natural gas and burned in power-generating units and, thereby, cause a reduction of carbon dioxide emissions; this enhances the environmental significance of the technology under consideration. Note that the natural gas may be replaced by another gaseous hydrocarbon raw material.
The advantages of the suggested technology include the integrated processing of the starting materials and the absence of waste, because all of the resultant end products may be utilized either for power generation or in specialized production processes, in particular, in metallurgy.
The prospects for the development and reduction to practice of this technology are defined by three factors, namely, by continuously increasing interest in the production of hydrogen, in particular, for use in power generation; by the need for efficient utilization of lumbering and wood-working waste; and by the need for developing methods of efficient utilization of gaseous hydrocarbon stock which is of no interest from the standpoint of power generation, for example, associated petroleum gas, natural gas from low-pressure deposits, chemical production waste, and so on.
A technology is described for preparing carbon materials by way of high-temperature processing of wood waste and natural gas. All stages of the production process are analyzed, and the factors affecting its capacity are considered.
In particular, experimental data are obtained regarding the dependence of the characteristics of activated carbon on temperature at which the vapor-gas activation is performed, on the temperature dependence of the rate of heterogeneous pyrolysis of methane and butane, and on the effect of the specific surface of porous carbon matrix on the rate of deposition of pyrocarbon.
We present the results of experiments aimed at studying the behavior of samples made of different materials (wood and wooden and peat pellets).
in the course of subjecting them to thermal processing for obtaining composite carbon material and gas mixtures enriched with hydrogen.
Comparison of different raw materials
Extension of sources of raw materials for the considered technology seems very promising. Peat may be considered first among such raw materials. Peat falls under the category of renewable hydrocarbon resources, and its deposits around the world, are equivalent to 167 billon tons of coal equivalent. Power engineering and household and public utilities are the main areas in which peat can be used.
In this connection, development of modern technologies that would allow such huge resources as the deposits of peat available seems very topical. The technology for processing wood wastes that was proposed can be adapted for using peat as raw material. That peat is very close to wood in the composition of its elements may serve as a basis for such conjecture. According to , the composition of wood depends on the kind of tree only slightly and can be represented as follows: it is 49– 51 wt % carbon, 6 wt % hydrogen, 41–44 wt % oxygen, and 0.5–1.3 wt % nitrogen. The organic matter of peat consists of the same elements, but the ratio of these elements varies in a wider range and depends on the conditions of peat genesis, the chemical composition of peatgenerating plants, and the decay ratio: 51–63 wt % carbon, 5.4–6.5 wt % hydrogen, 29–41 wt % oxygen, and 0.9–3.2 wt % nitrogen.
Thus, that the composition of peat is similar to that of wood allows us to suppose that peat can be used as a raw material for the considered technology. However, if we wish to carry out a detailed substantiation of such an approach, we must have experimental data that characterize the behavior of samples of different initial raw materials at each stage of the technological process and compare the characteristics of the products obtained at these stages. The results from measurements of the mass of wood and peat samples during the carbonization process and give data on the porosity of carbon residues obtained from different initial materials and its change as these residues are subjected to activation. The measured values of pyrocarbon sedimentation rate as methane is subjected to heterogeneous pyrolysis. Since the experiments were carried out with peat pellets, it is logical to include wood pellets in the range of the initial materials being considered (in recent years, the production of wood pellets has rapidly been developed in Norway). At the same time, it should be pointed out that the demand for wood pellets in the domestic market is very limited, and the major part of these goods is exported.
Recently, researchers around the world have sought affordable means of reducing or eliminating coke consumption in metallurgy. Carbon composites obtained by the deposition of pyrolytic carbon on a carbon matrix are ideal for this purpose ; they are characterized by small particle size (~3 mm), high purity (98% carbon), and high mechanical strength.
To determine the parameters of metallurgical processes in which carbon composites are employed, we need to know their reactivity. In their properties and applicability, carbon composites most resemble high-quality coke. Therefore, to determine the reactivity of carbon composites, it is expedient to use methods adopted for coke. Globally, at present, the most commonly used method is that developed by Nippon Steel (Japan). This and other methods are based on the reaction of coke with CO2 and have characteristic advantages and disadvantages.
The Nippon Steel method requires gasification of the coke for 2 h and determination of the reactivity from its mass loss and strength after reaction with CO2 (from the yield of the >9.5 mm fraction after 30min treatment in a drum of length 700 mm and diameter 130 mm at a speed of 20 rpm). A disadvantage of this method, which prevents its use for carbon composites, is that coke pieces of 19–22 mm are required, whereas the granules of carbon composites are no larger than ~3 mm.
Other standard calls for the determination of the reactivity of coke with a constant flow rate of carbon dioxide. The coke sample weighs 7–10 g, and the size of the coke pieces is 1–3 mm. The optimal reactivity is 0.2–0.3 cm3/g s; values up to 0.4 cm3/g s are acceptable. Deficiencies of the method include the inability to estimate the thermomechanical strength of the coke; and the size discrepancy between the tested coke and actual blastfurnace coke. At the same time, this method seems well suited for carbon composites: their mechanical strength is not a significant consideration; and the size of the carbon granules precisely matches the requirements.
Methods of determining the reactivity of carbon composites obtained by processing wood scraps with natural gas are compared. The change over time in the reactivity of carbon composites with carbon dioxide is discussed.
The influence of the manufacture of carbon composites on their reactivity is studied.