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A comprehensive techno-economic review of the technical options currently available for the handling and preparation of sewage sludge was carried out by STC. This review encompassed all major types of preparation systems and included an assessment of sludge transportation systems, mechanical drying equipment (belt filter presses, decanter centrifuges and filter presses) as well as composting systems. In each category, systems commercially available were noted and their main characteristics compared.
It was concluded that the assessment system adopted in the study would allow for an objective overview of the different systems currently available. As sludge properties can vary widely, it is crucial that the appropriate type of handling and preparation system is selected for a commercial application. Thus, the present work has produced a decision-making table of the various preparation systems, allowing for the selection of the most appropriate technology for a particular type of sludge. This will help minimise technical risk associated with the selection and deployment of a suitable system as part of a sludge gasification-CHP scheme, as well as in other potential applications.
The fluidised bed test programme demonstrated that circulating fluidised bed gasification of thermally dried sewage sludge with hot gas cleaning is a technically feasible option and no serious problems are expected. The thermally dried sludge was shown to be very reactive, the product gas quality was reasonable and bed sintering and deposit formation could be avoided.
The high nitrogen content of sewage sludge resulted in high levels of NH3 and HCN in the product gas and may require specific attention in product gas cleaning or flue gas cleaning after gas combustion. The gas cleaning requirements can also vary depending on the sludge source, as the presence of harmful trace contaminants in sewage sludge will be dependent on local industry.
Significant formation of heavy hydrocarbons (tars) was avoided, which enabled gas cooling and filtration without tar condensation problems. As the thermal drying process incurs an energy and cost penalty on the process, the option of using composted sludge with an additive (such as wood chips) as a feedstock was also investigated and shown to generate a reasonable quality product gas. Further improvements to gas quality may be possible, if a better quality, composting additive were used.
The combustion performance of the sewage sludge derived product gas generated in the PDU-scale circulating fluidised bed gasifier (CFBG) was good and demonstrated the technical feasibility of co-combusting with higher calorific value fuels, such as natural gas. The results indicated that high levels of NH3 present in the product gas do not necessarily lead to high NOx emissions and, in optimised conditions, can lead to the reduction of NOx compounds to levels which will meet emission limits of the Waste Incineration Directive (WID) for co-combustion.
The sulphur content of the product gas will in part be dependent on the quality/composition of the sewage sludge and may lead to high emissions of SOx. Although a significant proportion of the fuel sulphur is removed during gasification by the addition of limestone to the bed, additional flue gas cleaning systems may be required to meet European emission legislation. Controlling SOx emissions in flue gas, however, is now achievable without extensive investments.
Elements, such as Ba, Cu, Hg, Pb and Zn, can be present in sewage sludge at levels significant to the disposal of the residual streams and environmental emissions from a gasifier. The distribution of these elements to the coarser bed residue and the elutriated fine material will have implications for the need for additional gas cleaning systems and ash disposal.
At gasification temperatures below 900 degrees Celsius the majority of the trace metals present in the sludge feedstock were retained in the ash residues from the fluidised bed gasifier. At temperatures above 900 degrees Celsius, virtually all of the Hg, a substantial proportion of the Pb and rather less of the Zn were released was released to atmosphere; Ba was also released with specific sludges. The high Pb enrichment in the fly ash was shown to have a strong dependency on gasifier temperature indicating capture by chemical reaction and thermodynamic equilibrium modelling was used to provide a qualitative explanation for the volatility of Pb.
The extent to which this constitutes an emission problem will then depend on the effectiveness of the gas cleaning system at removing each volatile species from the fuel gas. The most important gas cleaning parameter is likely to be temperature. A wet scrubbing system should be effective for removing all of these elements from the gas, with the possible exception of Hg.
The experience of Hg removal by wet flue gas desulphurisation in coal-fired power stations is of variable performance dependent on coal chlorine content, poor removal generally being associated with low chlorine coals whose chlorine content is still higher than those typically found in sewage sludges. In a high chlorine system, much of the Hg will be oxidised and readily soluble Hg2+, whilst in a low chlorine system the less soluble Hgo will predominate. An activated carbon system for Hgo removal from the fuel gas may therefore be needed to supplement the inherent Hgo capacity provided by the residual carbon in the fly ash. The speciation of the elements found to be enriched in the fly ash (especially Pb) will be important in determining their leachability on disposal to landfill.
The fixed bed downdraft gasifier technology was shown to be technically feasible option for producing a low quality, combustible gas from thermally dried sewage sludge for small-scale applications. To minimise potential operational problems, co-firing sewage sludge in blends with other fuels, such as a lingo-cellulosic wastes, was recommended. Optimum gasification conditions for undigested sewage sludge were obtained at an equivalence ratio of 0.47 and 21% of sewage sludge in the blend. At these conditions the lower heating value of the product gas was 5MJ/Nm3 and the cold gas efficiency (CGE) reached 88%.
For digested sewage sludge, optimum conditions were achieved at an equivalence ratio of around 0.64 and 57% of sewage sludge in the blend, where a CGE of 83% was attained. During gasifier operation, some slag formation was observed on the grate and around the throat, which hindered continuous solid flow through the gasifier and led to a decrease in the product gas quality.
The slag formation was attributed to high temperatures (>1100 degrees Celsius) attained in the oxidation zone of the gasifier bed. Decreasing the airflow rate to the gasifier will effectively reduce the temperature in the oxidising zone; however, this impacts adversely on gas quality, solid to gas conversion and tar production. A comprise for the equivalence ratio value was identified for minimising slag formation on the grate whilst achieving good fuel gas quality and solid to gas conversions. A wet gas cleaning system, comprising cyclone + water scrubbing + gas drying, was shown to be efficient for product gas cleaning prior to use in an internal combustion (IC) engine but may lead to disposal problems for the waste water.
The use of product gas, generated from a fixed bed downdraft gasifier firing blends of sewage sludge-woody wastes, as a partial replacement fuel for a diesel engine was shown to be technically feasible option. Operation in dual fuel mode allowed a reduction in diesel fuel of up to 87%, however, useful power output decreased as the proportion of sludge/wood derived product gas was increased.
A high proportion of the energy was lost to the coolant, although customising the combustion chamber design for duel fuel operation should lead to improvements in overall efficiency. The exhaust gas temperatures are also higher for dual fuel operation than for pure diesel operation, so a higher level of thermal stress may be expected in the engine. The gasifier product gas is cleaned in a series of cyclones, wet scrubbers and cooling systems prior to the use in the diesel engine and contains relatively low levels of tars, moisture, alkali metals and soluble nitrogen and sulphur compounds. Subsequently NOx and SO2 levels in the exhaust gas were low when the engine was operated in dual fuel mode. CO levels, however, increased and for a commercial plant installation of an oxidation catalyst may be required for operation at low loads.
An overall efficiency of the system (gasifier and engine-generator) of 19% was achieved when the engine was run at 23kWe, the sewage sludge content of the gasifier feedstock was 43% and the energy content of the engine fuel was supplied by 90% of the sludge/wood product gas.
EMC's gasification test programme examined the use of spouted fluidised bed gasification. A programme of ten gasification trials was carried out in the EMC 350 kW spouted fluidised bed gasifier using four sources of sewage sludge samples as the feedstock. The impact of parameters, such as fuel conditioning, co-firing with coal and gasifier bed temperature on factors was investigated.
The spouted bed gasification test programme confirmed that the thermally dried sewage sludge pellets were very reactive and significant operational problems were encountered when the dried sludge pellets were fired as a single fuel. This was postulated to be attributable to the vigorous spouted bed environment. Pre-treating the sludge pellets with water and co-firing with a higher calorific value support fuel, however, improved the gasifier operability significantly.
Improvements to fuel gas quality were also attainable by increasing the gasifier temperature. The results have also demonstrated the importance of selecting co-firing fuels, which are compatible in terms of reactivity and ash melting characteristics. For co-firing applications, the ideal scenario would be to operate under optimal gasification conditions for the primary fuel and select a secondary fuel, which has a similar or higher reactivity, low levels of contaminants, complimentary calorific value and comparable or higher ash melting characteristics.
The work carried out in IChPW comprises two comprehensive parts. The first part included experimental investigation of the pyrolysis process (as the first stage of gasification) of sewage sludge and/or mixture sewage sludge/coal leading to the enhanced fundamental understanding of the process. The second part was focused on the development of mathematical model of sewage sludge gasification in the circulating fluidised bed reactor.
A bench scale investigation of pyrolysis process was carried out. The experiments were performed using 5 different types of feedstock (coal, raw and dry sewage sludge and sewage sludge/coal mixtures) at 5 temperatures of the process (520-950 degrees Celsius). The results of the test were discussed and used for the determination influence of process temperature and composition of the feedstock on pyrolysis products yields (including char, gas and tar), chemical structure transformation of organic substances during thermal processing of the selected feedstock, changes of the char structure, particular internal surface development.
Experimental data were used for modelling of sewage sludge gasification. The products distribution of the pyrolysis and its chemical composition and structure strongly depend on process parameters, particularly on temperature, and physico-chemical properties of feedstock. Generally, temperature increase affects the decrease of char and increases the gas yield. Moreover at higher temperatures the ash content in a char increases and volatile matter decreases.
The pyrolysis with the temperature increase of dry and wet sewage sludge leads to higher yields of tars; water chemically formed in comparison to coal pyrolysis. But the tars yields from wet sludge pyrolysis are higher than from dry sewage sludge. It is due to different chemical structure of dry and wet sewage sludge; pre-drying process effected decomposition of peptidoglicane - main building material of bacteria cellular walls. Pyrolysis temperature of the order of 700 degrees Celsius is critical for decomposition of sewage sludge both wet and dry and its mixture with coal, particularly on char structural parameters and chemical composition.
Considering structure of resulting sludge char it can be stated that the properties of it strongly depend on pyrolysis process parameters, especially temperature. The influence of temperature under 700 degrees Celsius on sludge char structural parameters is insignificant. Temperatures higher than 900 degrees Celsius enable production of materials having high specific surfaces (about 100m2/g). Condensed water from sludge raw gases comprises chlorine, nitrite, nitrate, phosphoric and sulphate ions in higher amount than raw coal gas, and pyrolysis gases from mixture of coal and biomass.
It may affect higher corrosion in industrial installations. A computer model for assessment of circulating fluidised bed gasification of sewage sludge/coal mixtures was developed to provide improved predictive behavioural modelling capabilities. For sewage sludge gasification process, developed and demonstrated in the Institute circulating fluidised bed reactor was proposed.
The modelling work comprises development of aerodynamics, particle size population balance, static and kinetic models of sewage sludge-coal mixture gasification process. The models were used to predict the calorific value of the product gas, solid to gas conversion efficiency, gas and char yields and parameters such air demand etc.
For power generation applications, the most promising options were gasification of thermally dried sludge sewage and mechanically dried sewage sludge/coal mixture (the sludge content in the fuel mixtures should not exceed 25%). For the former, the product gas LHV was predicted to be 6.1MJ/kg at an optimum temperature of 710 degrees Celsius (kinetic model).
The third option considered, gasification of dry sludge/coal mixture, was not considered to be economically viable due to the sludge drying requirements and use of the additional fuel (coal). The pre-feasibility study defined the technical and organisational conditions for the construction of a 1.7t/h sewage sludge gasification plant and confirmed the economic viability of technology.
The data of sewage sludge in Poland have been also collected. Data concern year 2000 and contain information about main sources and wastewater treatment methods, sewage sludge production, characteristic and methods of utilisation. The information by different regions in Poland ware also presented. The final product of the work is computer code for the process and design development.
Work was carried out to identify potential problems of feeding sewage sludge pellets into a gasifier. As part of this, conveying trials were undertaken to assess the handleability and degradation properties of the sewage sludge pellets. Two transport systems were investigated, namely feeding via a tapered screw and by a lean phase pneumatic conveyer. In addition, the degradation behaviour of the sludge pellets when discharged from a reception hopper and passed across a rotary valve was studied.
Conveying and degradation data for the sludge pellets was then compared to that for a reference coal. Throughout all the tests, the sewage sludge pellets showed very little degradation, even when conveyed at very high air speeds and delivery rates. The results also showed that it was possible to convey the pellets at relatively low air velocities.
Analysis was also undertaken to assess the flow characteristics of the sludge pellets using Johanson Indicizer(TM) systems to identify potential problems for feeding the sludge pellets from storage hoppers to the gasifier. The results showed that as long as the material was kept as dry as possible to prevent cohesive arching, and stored in a hopper with adequate outlet dimensions and wall angles, few problems should be encountered when handling this particular material.
To provide information on safety and design issues of storage of sewage sludge, spontaneous heating and heat explosion tests were carried out in atmospheres containing 3 to 21% oxygen. The tests indicated that the ignition temperature for the sewage sludge pellets was higher than that of the reference coal. Dust explosion tests indicated that the explosion indices were more severe for coal than for the sewage sludge pellets.
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