Biochars produced from sewage sludge show promise for use as soil amendments that could both enhance plant growth and sequester carbon. In a previous study, we showed that moist sewage sludge obtained from a rural, residential region of Oahu, Hawaii, could be carbonized, and the heavy-metal content of its biochar did not exceed United States EPA regulations limiting its use as a soil amendment. In this paper, we show that biochars produced from the nearby residential community of Hawaii Kai cannot be used as a soil amendment in Hawaii because their contents of Zn, Mo, and Cr exceed state regulations. Likewise, their contents of Cd, Cu, Ni, and Zn exceed Belgian regulations. These heavy metals are retained in the biochar during carbonization, whereas Hg and (to a lesser extent) As, Cd, and Se are released and thereby depleted in the biochar. Biochar within the carbonizer does not adsorb the heavy metals which are released during pyrolysis; consequently, these leave the carbonizer in its exhaust stream.
In agreement with prior experimental work, thermodynamics predicts differences in the outcome of biomass pyrolysis conducted in sealed, constant-volume systems as opposed to constant-pressure systems. In particular, much higher values of the fixed-carbon yield can be expected in constant-volume systems. Avicel cellulose is known to give very low char and fixed-carbon yields; thus, char and fixed-carbon yields from cellulose have been given primary attention in this work. Our tubing bomb results reveal (i) fixed-carbon yields that realize the theoretical "limiting" values when the vessel is pre-pressurized to a modest pressure with N 2 gas, and (ii) a gas product composed of steam (water) and CO 2 , with traces of CO and virtually no tars. Above a small range of modest temperatures and pressures, the char endures a molten phase and becomes a hard coke. The ash content of the char/coke reflects the composition of the glass wool and kao wool materials used to hold the Avicel powder in place.
ABSTRACT:This paper reviews lab-scale flash carbonization experiments under elevated pressure using Norwegian wood as a feedstock. Norway's silicon and ferrosilicon industry has been urged to reduce fossil CO 2 emissions by increasing the use of charcoal as a substitute for coal and coke in the reduction process. As charcoal is not produced in Norway, large amounts of it are imported from South-Asia. Norway now intends to produce charcoal locally using optimum carbonization techniques from local biomass and forestry waste. That is where the pressurized flash carbonization experiments come in. Birch, spruce and forest residue or GROT samples (GRener Og Topper, i.e. branches and tops) were carbonized, enabling the analysis of the impact of pressure and FC canister insulation on their respective fixed carbon yields. Forest residue (FR) proved to be proper to make charcoal, as fixed carbon contents of 80% could be achieved at moderate pressures. The fixed carbon yields of spruce and birch wood reached over 90% of their theoretical values. The high charcoal yields can result in remarkable cost savings for the metallurgical industry, while at the same time making excessive deforestation unnecessary. The use of coal will soon be abandoned and charcoal 'mines' could become an obvious choice. 1.IntroductionBecause of the increasing demand for (Si-based) solar panels, for computer chips (following Moore's law) and for silicon for the massive metal production of countries like China 1 , the demand for silicon alloys and other ferroalloys has never been higher. The metal and alloy industry heavily depends on coal as a raw product. In electric arc furnaces, coal acts as a reductant and a carbon source, while in the older blast furnaces coal also acts as an energy source.These industries will not face raw silicon shortages, because over 25% of the earth's crust consists of this element, usually found in the form of silica SiO 2 . However, justifying the use of coal as a raw product in the manufacturing process of computer chips, photovoltaic panels and other metal commodities will pose a problem, especially with changing climate change policies.There are not many substitutes for coal, but charcoal is one. Charcoal has low ash and sulfur content compared to coal, while still maintaining high SiO reactivity, which makes charcoal
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