Evaluation of selected sewage sludge gasification technological parameters

. Evaluation of selected sewage sludge gasification technological parameters was shown in this paper. Degree of carbon conducted in combustible substance and syngas efficiency (technological readiness coefficient) in accordance with equations were calculated. Enthalpy of individual compounds formation and energy balance were calculated in accordance with rule of Hess.


Introduction
One of the main materials, that are formed during the wastewater treatment and production are sewage sludge's. Physical parameters like size of particles and composition of them depends on wastewater treatment conditions [1]. Methods of sewage sludge management should ensure sanitary safety. Disposal solutions of hazardous substances included in sludge's are available by thermal processes like gasification. Combustible gas (syngas) arises after cycle of the reactions between fuel and gasification agent is one of the main products of this process. Technological parameters like: syngas lower heating value, degree of combustible substance conversion and syngas chemical efficiency(technological readiness coefficient) for evaluation of efficiency of the process can be used. In range of temperatures over than 700°C for sewage sludge air gasification, fuel conversion degree is in the range 65-72% and range of syngas efficiency is from 30 to 50% what was shown by authors [2]. However, if temperature range increase from 600 to 900°C the chemical efficiency of gasification process will increase to 47,5%, what has been proved by [3]. The lower heating value of produced syngas achieved by authors was 4,19 MJ/m 3 . Sewage sludge may be used for co-gasification process. Then fuel used in gasification is blended from different materials like sewage sludge with wood, biomass waste or coal. Fuel mixtures changes cause increases some technological parameters like syngas efficiency. Increase of this parameter is available by increase of sludge's to wood ratio in fuel mixture what has been found by the authors of [4]. Evaluation of selected technological parameters like: degree of carbon conversion contained in combustible substance and syngas chemical efficiency during sewage sludge air gasification was the aim of this paper.

Research material
Dried sewage sludge from municipal wastewater treatment plant in Bialystok was used in the experiment. Fuel properties were prepared in accordance with actual standards and regulations: humidity was determined in accordance with [5], combustible and incombustible parts concentration with [6],volatile matter [7], carbon and hydrogen concentration [8], nitrogen concentration [9], sulphur concentration [10], chlorine [11], higher heating value [12]. Samples were prepared in accordance with [13]. Results of the tests were shown in Table 1.

Experiment
Sewage sludge was air-gasified in 5 kW countercurrent gas generator. Gas composition was measured by GAS3100 syngas analyzer. Concentrations of combustible and incombustible gases like CO, CO 2 , CH 4 , H 2 i O 2 were measured. In the range 700-800°C was the temperature of the process. The measurement of this parameter was taken using nickel type K thermocouples linked with the control panel connected to the computer. The technological parameter of air fuel ratio was calculated in accordance with equation: Where: G fuel -fuel stream (kg/h),V conv -stream of gasification agent (air) (kg/h) Description of sub processes areas of sludge air gasification in countercurrent pipeline gas generator shown in Fig.1 was prepared in accordance with [15,16] . asi i ati n pr ess nsists di erent sub pr esses where spe i i hemi al rea ti ns urs [ ] Rem ving uel humidity starts at the beginning Then in primary and se ndary rea ti ns gases like: methane, steam, arb n m n xide and als tars are rmed The last pr du t is ra ked in the ra king area in a rdan e with p st pr essing tars [ ] C n entrati ns syngas mp unds rmed in gasi i ati n were sh wn in Fig   Fig. 2. Syngas composition in percent concentrations while steady state gas generator (without starting and quenching C mbustible gases n entrati ns like: CO, H and CH were in the ranges -7%, -% and 3-% Small hanges n entrati ns testi y t stability the pr ess Des ripti ns and experimental results sewage sludge gasi i ati n were sh wn in [ ] Syngas lower heating value depends on combustible gases presence like: CO, H 2 and CH 4 Homogenous and heterogeneous reactions are fundamental in gasification process [17]. Energy balance was prepared in accordance with rule of Hess [18]. C + 2H 2  CH 4 (8) (10) Enthalpy values of individual substrates were taken from thermodynamics tables [19]. If in the syngas there is no presence of higher hydrocarbons, this energy balance is adequate. Otherwise, if there are higher hydrocarbons higher than CH 4 syngas quality analyze is necessary for preparing chemical reactions

Evaluation of selected technological parameters
Evaluation of technological parameters like: syngas lower heating value, degree of carbon conversion to combustible substances and chemical efficiency were calculated in accordance with equation [20].
The syngas lover heating value during the stable work in function of time was shown in Fig. 3. The technological parameters used for process evaluation like chemical efficiency and carbon degree conversion were calculated in accordance with [2]. Syngas chemical efficency: Degree of carbon conversion in fuel:

Results
Results of syngas chemical efficiency and carbon conversion calculations were shown in  Syngas chemical efficiency produced during municipal sewage sludge air gasification was in the range 33-35%. This value was slightly fluctuated in function of time. and mostly depends on lower heating value of fuel nd produced combustible gas. It also depends on humidity concentration in fuel. Increase of humidity cause decrease of calorific value what has been found by authors [21]. The degree of carbon conversion contained in combustible substance of fuel was in range 53-58%. Conversion changes depends on concentrations of gases in which composition one of the element was carbon. To this gases belongs CO, CH 4 and CO 2 . In correlation with carbon element transformation gasification chemical efficiency is increasing while the decrease of carbon conversion. The highest efficiency (33%) was in achieved while the value of carbon conversion was 58%. And inversely 35% syngas efficiency was achieved in 54% carbon conversion.
Transformation of coal element contained in combustible to carbon monoxide (incombustible gas) which not increase the lower heating value of syngas justify this dependence.

Conclusions
Evaluation of gasification process is possible with using selected technological parameters. Different values of parameters shown in this paper were depended on experimental conditions. Chemical efficiency was in correlation with degree of carbon conversion. Increase of chemical efficiency was increasing with syngas calorific value. One of the main gases that don't have influence on efficiency but on degree of carbon conversion was incombustible CO 2 . Combustible syngas from fuel gasification is useful for power in forms of electricity and heat production. This product can be used in combined heat and power and integrated gasification combined cycle (IGCC) technologies. High efficiency gasification in this solution is one of the most desirable parameters. It means that evaluation of gasification processes is necessary to improve and find effective solutions for energy production.