There is considerable concern over the current volume of greenhouse gas emissions and the effect that these may have on the global climate. Carbon dioxide (CO2) is the principal greenhouse gas believed to be driving anthropogenic climate change and represents around 70% of all greenhouse gases generated globally.
To achieve lasting reductions in carbon dioxide, wide scale changes in the world's patterns of energy consumption will be needed. For example, use of renewable energy will need to be promoted, as well as increased energy efficiency and the development of fuel alternatives. One alternative fuel is biofuel.
Biofuels or bio-oil(s) are renewable resources and their use has significantly fewer net emissions of greenhouse gases than fossil fuels when burned. Biofuels or bio-oil(s) can be used as an alternative to fossil fuels in vehicles, heating and the generation of electricity.
Another way to reduce carbon dioxide in the atmosphere is to capture and store some of the atmospheric carbon dioxide. The capture of carbon gases for storage is referred to as “sequestration”. Sequestration of carbon in gaseous form (as the gas is released, for example at power plants) is a technically complex and high cost solution. An alternative approach is to sequester carbon dioxide in trees by reforesting areas of land. On average between 40-50% of all material in trees is carbon. Importantly, reforestation requires large areas of land to store significant amounts of carbon dioxide. In addition, the carbon dioxide that is stored in trees can only be held if the area remains forested If the area is cleared, much of the carbon dioxide returns to the atmosphere.
A method of sequestering carbon dioxide is described in PCT specification WO2008/079029.
Activated Carbon:
Activated Carbon is a charcoal product that has a micropore structure that exhibits a significant specific internal surface area. Activated carbon has many uses, mainly involving the adsorption of unwanted materials, thus in the sugar industry it is used to remove the dark brown colour so that white sugar can be made. However it could also be of significant value in the removal of pollution, particularly if it is suitable for adsorbing larger molecules.
A further possible use for activated carbon is through the absorption of nitrogen containing species, such as urea, then applying the product to agricultural land. Activated carbon does not retain these species strongly, and hence would act as a slower-release agent for nitrogenous fertilizer while placing fixed carbon in the ground and thus, provided the carbon came from biomass, assisting the reduction of carbon dioxide from the atmosphere. Activated carbon can also absorb methane and hence act as a possible assistant in inhibiting methane emissions from certain stock. In short, it has significant potential further uses, and hence additional methods for its manufacture should be welcome in the market.
The manufacture of activated carbon is well-known. Carbon containing species are usually carbonized at about 600° C., then, when this is complete, it is activated, usually by oxidation, e.g. with steam or air, or by treating it with chemical methods, such as zinc chloride, phosphoric acid, or any number of other chemicals. Activation is usually carried out somewhere between 800° C. to 1200° C., under quite carefully controlled conditions. Generally speaking, the heat is provided externally to the carbonaceous material, which is usually in a rotating kiln of some description.
In such a kiln, there are two sources of heating for the biomass: conduction through direct contact with the walls, and black body type radiation from the walls. Biomass has a poor thermal conductivity and as the mass reaches approximately 300° C. it begins to degrade endothermically. Some of the products of such thermal degradation are highly viscous tars that flow into pores of the biomass, which is undesirable for the formation of activated carbon as by filling pores it tends to reduce the end surface area.
The carbonization is slow because as the biomass approaches about 300° C., it begins to degrade and produce degradation products. The degradation products are often liquid tars, which can flow. Anything that fills in developing pores of the carbon is undesirable.
The surface area of Activated Carbon is usually determined by low pressure Nitrogen adsorption (B.E.T method). Since this adsorption is generally attributed to a monolayer, the surface area can be calculated. The surface area of one gram of activated carbon is typically about 500 m2 and ranges from about 200 m2 to about 2500 m2.
Recarburiser Carbon:
Carbon is the most important component added to commercial steel comprising 0.15-1.5% of the mass of finished metal product. The carbon content of steel influences specific mechanical properties such as hardness, strength and thermal properties such as melting point and ‘weldability’.
During the process of steel production, it is common practice for most foundries to combine some steel scrap with primary iron concentrate in the metallic charge. The percentage of steel scrap used is a function of price, availability, alloy level and other economic factors.
The quantity of carbon introduced into the smelter by the charge materials (i.e. steel scrap and iron returns) is usually lower than the value targeted in the finished steel product, which is usually within the range of 3.0 to 4.0% C. Therefore, carbon units termed “recarburisers” are added to the batch to increase the level of elemental carbon in steel products to the specified level.
During the smelting stage of the manufacturing process, recarburisers are mixed into the molten steel bath, allowing elemental carbon to be absorbed and distributed homogeneously throughout the metallic charge.
Recarburisers for commercial steel production require a content that has a high proportion of fixed carbon, high calorific value. Crucially, recarburiser material must have a low volatile, nitrogen and sulphur contents to ensure safe, efficient progression of reduction-oxygen reactions and to minimise harmful emissions.
Additionally it is desirable that recarburisers are highly porous to provide maximum surface area contact with the molten steel batch to facilitate efficient uptake of elemental carbon.
It is common for recarburisers to be derived from high-grade bituminous or anthracite coals and premium versions being synthesised from petroleum products. Some foundry's use steel billets that have been specially manufactured with a specific carbon content and use as a recarburiser in the metallic charge.
Nut Coke:
Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Since smoke-producing constituents are driven off during the coking of coal or biomass, coke forms a desirable carbon fuel for furnaces in which conditions are not suitable for the complete burning of bituminous coal or biomass itself. Coke may be burned with little or no smoke under combustion conditions, while bituminous coal would produce significant amounts of smoke.
Nut Coke is characterised by a much larger particle size (typically 25 mm×55 mm) compared to other fuel cokes, such as injection coke or pea coke, which are typically smaller than 10×10 mm. Unlike recarburiser used to increase the level of carbon in a molten charge of steel, nut coke can contain up to twice the allowable mineral content and five times the moisture content. This aspect allows for a wider range of woody biomass feedstocks to be considered for nut coke production.
Low volatile matter content in coke is of critical importance to facilitate safe, stable energy release and to minimise the risk of explosive release during combustion. Low Sulphur and Nitrogen contents are also required to minimise the formation sulphur dioxide and NOX during the combustion process.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.
It is an object of at least preferred embodiments of the present invention to provide an apparatus and/or method for processing biomass or biomass to produce charcoal, bio-oil(s), activated carbon recarburiser carbon, and/or nut coke that overcomes one or more of the drawbacks of known methods for converting biomass to activated carbon and recarburiser carbon, and/or to at least provide the public with a useful choice.