The differences between fluidized bed agglomeration and fixed bed agglomeration are primarily related to the characteristic time of contact between particles. In the fluidized bed, particles move with significant inertia, and in the fixed bed, the particles weakly migrate, whereas the packing weight contributes to better contact, so agglomeration can occur under milder conditions. In addition, with fixed bed conversion, temperature stratification is sharper.
In some cases, controlling the composition of the mineral matter can be a way to control ash agglomeration. For example, it was experimentally found in [ 104 ] that with a fraction of wood in a mixture with straw above 50%, it is possible to avoid ash softening and thus prevent agglomeration. In [ 105 ], mineral additives were used to increase the ash melting point during peat gasification. Other methods are also possible, such as washing raw biomass in water or acid solutions to remove the minerals [ 106 ]. Mineral additives make it possible to change the reaction pathways of decomposition of the organic mass, preventing polymerisation and agglomeration [ 107 ].
During grate combustion, the fuel decomposes with the influence of the freeboard. Heat flows are organized in such a way that fresh fuel is heated from above, whereas air enters, as in updraft gasification, through the bottom. Therefore, the temperature zones can alternate as the fuel moves along the grate.
Diagrams of the most widely used combustion and gasification processes are shown in Figure 3 . The updraft (counter-current) gasification process is characterised by a high temperature in the lower region of the reactor (combustion core). If the ash melting proceeds there, then liquid slag removal is realised. In the downdraft (co-current) gasification process, the ash melting can also occur in the combustion core, which is now located in the upper region of the bed. Below the temperature decreases, therefore, the formation of strong ash agglomerates becomes possible. High temperatures can be achieved even for low-calorific fuels, for example, in filtration combustion [ 102 ], due to heat recovery. Under special conditions, even the existence of several combustion fronts (volatile combustion and char combustion [ 103 ]) is possible. Downdraft gasifiers typically have a waist in the region of air supply: agglomeration in these throat regions can be critical to the operation of the gasifier.
One of the main problems in the fixed bed conversion of waste is the non-uniform airflow distribution over the cross-section of the furnace or gasifier. The distribution of the particles in the packing is random, so the local gas permeability in the packing is also a random variable. Non-uniformity of filtration flow in random packings has been the subject of chemical engineering for a long time [ 95 ]. Due to the conversion of the particles, the packing structure changes: burnout affects the size and mechanical strength of the particles, and they undergo splitting, abrasion, aggregating, etc. Small particle fractions can clog the porous space and reduce the local permeability of the packing [ 96 97 ]. As a result, non-uniformities may grow up to the size of a reactor. Thus, for example, areas with reduced porosity connecting the upper and lower parts of the bed are formed (burnouts and channels). It was shown in [ 98 ] that the channelling during biomass gasification leads to an increase in the tar yield. Burnouts and even cavities within the packing can form due to the stable configurations of the particles, especially during the gasification of low-density fuels (e.g., straw) [ 99 100 ]. The authors of [ 101 ] observed the “fire jump” phenomenon, that is, the predominant propagation of the combustion front through regions with reduced porosity (they attribute this process to differences in the characteristic times for the devolatilisation and the ignition of volatiles in pores).
Fixed-bed agglomeration due to ash sintering can occur, for example, during oxygen gasification of wet fuel: in [ 108 ] bed agglomeration was observed at a biomass moisture content of 45% or more. Under agglomeration conditions, COand Obecome the main components of the producer gas (as well as under channelling conditions [ 96 ]). The authors of [ 109 ] reported significant fluctuations in gas composition, which affected the flare stability. To decrease these fluctuations, the mechanical impact was used. In [ 110 111 ], the grate rotation speed was optimised to reduce the yield of sintered ash.
The scheme of channelling during agglomeration is shown in Figure 4 . If a clinker appears in the bed, it becomes an obstacle to the filtering gas. The gaseous reagent flows around the agglomerate along the path of least resistance, and the particles on the path undergo deeper conversion, increasing permeability.
Published data show that plastic conversion leads to agglomeration. Filtration combustion of charcoal with polyethylene mixtures was studied experimentally in [ 112 ]. It was shown that at a plastic content of more than 20%, the combustion front becomes unstable due to the flow of the molten polymer and its interaction with the carbon surface. At the polyethylene fraction of 40%, combustion becomes impossible. Tarry products of plastic decomposition can be carried over the bed and deposited in colder regions. Similar effects were observed during the combustion of charcoal with polyurethane [ 113 ], however, the critical fraction of plastic, in this case, is higher (up to 40%), possibly due to the higher viscosity of the melt: quite a complete conversion of the mixture is possible even with an unstable combustion front. Filtration combustion with a non-uniform distribution of plastic (in the form of a “charge”) was studied theoretically and experimentally in [ 114 115 ], where the efficiency of carbon combustion heat used to decompose plastic was evaluated: in this case, however, it is necessary to have a sufficient gap in the packing for providing filtration. The decomposition of polyethylene during the filtration combustion of gaseous fuel was studied in [ 116 ], and in [ 117 ] with wood additives.
The paper [ 118 ] describes experiments on a downdraft gasifier (capacity of 100 kg/h) fed by pellets made of waste and straw. It was shown that with a waste fraction of more than 60%, the stationary gasification process becomes impossible due to intensive agglomeration. With an average plastic content in waste of 13%, a critical fraction of plastics in pellets can be estimated at 8%. However, it should be clarified that the waste contained a significant proportion of non-combustible matter (15–25%).
An experimental study of the sawdust and polyethylene co-gasification in a laboratory fixed-bed reactor (with heating up to 700 °C) was carried out in [ 119 ]. The authors obtained loose agglomerates with intense burnouts near the reactor walls. Examination of the reactor shows that most of the air passed through the near-wall region, resulting in partial entrainment of particles. With a high fraction of polyethylene, the self-sustaining process of combustion and gasification of the mixture becomes impossible, even with an external heat supply, whereas tar deposits in the gas cleaning system.
125,The paper [ 120 ] investigated the gasification of wood with polyethylene. The authors provide data for mixtures with a plastic mass fraction of less than 17%. In the study of the co-gasification of wood with sewage sludge [ 121 ], agglomeration was observed with a sludge fraction of more than 33%. The authors of [ 122 ] studied the co-gasification of wood with different types of waste (rubber, plastic, sewage sludge) with a waste fraction of 20%, polystyrene fraction of up to 30% was used in [ 123 ]. Charring polymers, on the one hand, may increase bed stability, but on the other hand, agglomeration worsens if the molten polymer has time to fill the porous space. Plastics are characterised by different melting temperature ranges and melt viscosity (including different grades of the same polymer). The agglomeration conditions are probably related to the ratio between the characteristic times of melt flow and its thermal decomposition. The emphasis on such details is made in works on the fire safety of polymeric materials [ 124 126 ].
Agglomeration of mixtures of garden waste with polyethylene (up to 25%) in a downdraft gasifier was observed in [ 127 ]. With an increase in the plastic content, the heating value of the mixture increases, as well as the heating value of the producer gas and the maximum temperature. Due to this, among other things, the tar content in the producer gas decreases, but the ash melting leads to clinker formation. Increasing the polyethylene content leads to a decrease in ash content, so the agglomerates become smaller. The authors of [ 128 ] also reported on the clinker formation during the gasification of plastic-containing waste (paper industry waste pellets). An increase in the plastic fraction also leads to an increase in the temperature in the combustion core (and, at the same time, to a decrease in temperature at the reduction zone). The size of the clinkers reached 10 cm, and their analysis showed that they contain 15–30% carbon (mainly these are encapsulates that are observed, for example, in the fixed-bed conversion of other fuels [ 129 ]). In similar conditions, the authors of [ 130 ] tried to stabilise the waste pellets’ gasification process (plastic content up to 30%) by adding wood chips, and they obtained clinkers up to 15 cm in size. The authors of [ 131 ], carried out fixed-bed combustion experiments using plastic-containing wastes (plastic fraction 30–35%), and they observed different ash behaviour depending on the airflow, which largely determines the thermal regime of the conversion, and hence, the temperature of the combustion front. The updraft gasification of waste was studied in [ 132 ] (the proportion of plastic is not indicated), which pointed out the problem of burnouts forming the adhesion of particles to the walls.
2-gasification process of an equal coal-polyethylene mixture. Oxygen gasification of waste without sintering was reported in [In some cases, agglomeration does not occur even with a high fraction of plastic. For example, the authors of [ 133 ] did not report any problems with bed agglomeration (even with a plastic fraction of 80%) during the fixed-bed co-gasification of sawdust and polypropylene (instead of air, a gas mixture with an oxygen concentration of 10% was used). The authors of [ 134 ] used a horizontal reactor for steam gasification of wood chips and polyethylene mixtures, which allowed for avoiding bed blockage. The authors of [ 135 ] did not observe agglomeration during steam gasification of biomass and plastic (up to 60%) mixtures, as well as the authors of [ 136 ] when studying the allothermal CO-gasification process of an equal coal-polyethylene mixture. Oxygen gasification of waste without sintering was reported in [ 137 ]. It is possible, however, that this is a consequence of the “survival bias”: scientific articles mostly contain the results of successful experiments, reporting on achieved stationary and efficient conversion regimes; then many experiments with agglomeration simply do not reach publication. In this regard, the boundaries of the agglomeration conditions are estimated from very limited experimental material (as was done, for example, in [ 130 ]). Table 1 gives a comparison of the experimental conditions under which agglomeration was observed. Depending on the fuel composition and the reaction zone dimensions, different agglomeration scenarios are observed (accumulation of ash or fines, ash melting at high temperatures, or plastics melting at low temperatures).
An example of an inefficient conversion of plastic-containing wastes is the experiments reported in [ 138 ] (producer gas contained 5–7% oxygen). The combustion of sea plastic waste mixed with wood pellets was experimentally studied in [ 139 ], where the authors found the instability of the combustion front associated with the mechanical instability of the packing, which is observed at a sufficiently large proportion of plastic (combustion becomes impossible at a plastic fraction of 80%). The authors of [ 140 ] studied fixed-bed combustion of polyurethane foam, and stationary conversion modes were observed (the authors of [ 141 ] used catalysts).
Table 1. Comparison of experiments with fixed-bed agglomeration.
Table 1. Comparison of experiments with fixed-bed agglomeration.
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