INSTITUTO COSTARRICENSE DE ELECTRICIDAD CONSEJO DIRECTIVO

8.1. Overall Conclusion

Three thermochemical processes of torrefaction, pyrolysis, and gasification were applied to convert the wastes into other forms of products. Many wastes were utilized to understand their conversion and product properties. The applied wastes used in this study included agricultural residues of rice straw and cotton stalk, aquatic waste of microalgae, and animal/human wastes of sewage sludge and dairy manure, which can be converted into clean energy sources and other value added products. Before the main thermochemical process, all the biomass wastes require a pre-process such as chopping, drying, separating or else. Sand mixed dairy manure directly from a dairy was

intensively investigated to be used for gasification process. Also, the produced bio-char and bio-oil has been gone through value-adding stages of hydrotreatment of bio-oil and activation of bio-char for adsorbent material, capacitance material, and catalyst.

Dairy manure mixed with sand was improved by removing sand to be able to use as a fuel source. First, the manure and sand particle size was understood with

pretreatment conditions of Dry_Gr, Dry_NGr, 10%_Gr, and 22%_Gr. Based on the results of the ash content and the recovered manure yield after sieving, the conditions of of Dry_Gr and 10%_Gr were suggested for a scaling-up process with a sieve mesh-40. The four stages of pilot process were designed and completed as air drying, burner

content of 72%. The mineral analysis of the processed manure predicted low potential to induce the bed agglomeration and slagging problems due to large amount of silicate. However, a longer exposure time on the bed with the dairy manure resulted a bed agglomeration. Finally, the gasification was successfully operated with the processed dairy manure with LHV of 3.3 MJ/Nm3.

Enriched-air fluidized bed gasification with the processed dairy manure were operated based on the surface methodology to investigate the effect of temperature, ERm and oxygen concentration on the syngas compositions and LHVs. The most significant condition on all of the syngas compositions and LHVs was a temperature term. The next significant term on the syngas compositions was ERm on H2, and oxygen concentration on CO and CO2. The carbon conversion efficiency was affected mainly by ERm. The highest LHV of syngas with enriched air gasification was obtained as 8.0 MJ/Nm3 _{at the} operating condition of 800 oC-0.25-40% (temperature-ERm-oxygen), while the

maximum LHV from the air gasification was 5.7 MJ/Nm3 _{at the condition of 800} o_C-0.1- 21%. A higher carbon conversion efficiency of 45 – 90% was obtained than the cold gas efficiency of 19 – 63%. The empirical equations obtained from the bench-scale

experiments were predicted the composition of syngas from a fluidized bed pilot-scale TAMU gasifier to perform the economic analysis within 10% error.

Torrefaction study was conducted with two agricultural wastes of rice straw for straw type and cotton stalks for woody type. The conditions of temperature (210 – 290 o_{C) and residence times (20 – 60 min) were applied to find the product yields and}

from rice straw at the temperature of 290 oC, which can be a substitute for brown coal and peat. The maximum solid energy densification for RS was 1.48, which was higher than that for CS at 1.37. The weight reduction by 52% for RS and 63% for CS at 290 oC with the energy recovery was over 70 – 80% can alleviate the difficulties of storage and transportation. The liquid products were mostly composed of water and majority of acids that can be used for soil amendment. The empirical equations based on the response surface methodology would help predicting the main properties of char at conditions.

Pyrolysis with three reactors (an auger, a batch, and a fluidized bed) was

performed to convert rice straw waste into three products at 500o_{C. The main purpose of} this work was done for a direct comparison of the effect of reactors on the product properties. The process of the auger and batch reactors as slow and intermediate

pyrolysis resulted in higher yields of bio-char (45 -48 %) than the yield produced from a fluidized bed reactor. On the other hand, the largest quantity of bio-oil (43%) was obtained from a fast pyrolysis. A better HHV and element ratios of H/C and O/C of bio- oil was obtained from slow pyrolysis, and their majority chemicals were composed of phenols and aliphatic compounds. The auger reactor was found as an intermediate process reactor that showed better quality of bio-oil than a fluidized bed reactor and more or similar bio-oil quantity compared to that from a batch reactor. The bio-char represented a similar characteristics regardless of the production reactor as all of the elemental ratios from different reactors was ranged in 0.02 – 0.10 for O/C and 0.56 –

The applications of the main products of bio-char and bio-oil were further

investigated. First, the rice straw bio-char from a batch reactor pyrolysis was used for the KOH chemical activation. The different activation temperatures were applied to

investigate the changes of pore sizes and volumes. At 750 o_{C, the highest surface area} (1331 m2/g) and volume (0.522 cm3/g) was obtained. The SEM, XRD, and EDX analyses on the activated carbon besides a BET analysis were implemented to

understand the improved carbons. Then, the performance of the activated carbon was evaluated for the chemical adsorption and electrical capacitance. It removed over 95% acetaminophen and ibuprofen contamination from aqueous solution in 24 hours. The RSAC (rice straw activated carbon) showed 93 F/g specific capacitance property, which is equivalent to the commercially available supercapacitors of 70 – 120 F/g. The

The microalgae bio-oil was selected to be upgraded as the bio-oil showed a higher HHV compared to other bio-oil from rice straw, sewage sludge, and Beauty Leaf Tree de-oiled cake. The first upgrading was a vacuum distillation to fractionate the bio- oil into three fractions to remove most of heavy chemicals such as paraffin gum and wax. At the same time, the bio-oil can be improved by reduction in the oxygen content and viscosity and increase in HHV. The distillates then were gone through

hydrotreatment using a commercial catalyst (Pd/C) and a developed catalyst (Ni impregnated on rice straw activated carbon). The oxygen content of upgraded bio-oil was reduced by 31% from distillate and 42% from raw bio-oil. The sulfur content was significantly reduced by 83%. A higher HHV was achieved at 41.4 MJ/kg. Both the

catalysts worked similar on the properties of bio-oil. Over 2000 times cheaper price of nickel would take an advantage over palladium.

8.2. Future Work

Based from the studies discussed above, further investigations on the following topics can be conducted as a continuing work. Especially, the author would like to finish some studies illustrated below; the development of adsorption material for

contamination removal, LCA study, the development of solar assisted pyrolysis reactor, and engine operation studies.

 Gasification

- Steam operated gasification

- CO2 and N2 removal, and H2 storing studies using different adsorption materials

- Engine operation studies and its emission level

 Torrefaction

- Pellet performance of torrefied chars

- Evaluation of moisture adsorption capacity

- Catalytic pyrolysis with a direct distillation condenser

 Bio-char utilization

- Production of activated carbon at the various conditions and biomass wastes - Adsorption removal of volatile organic compound

- Supercapacitor application with different pore characterization

 Bio-oil utilization

- The emission changes and engine or combustor efficiencies with produced raw bio-oil, upgraded bio-oil, and mixed bio-oil with petro fuels.

- The effect of the aqueous phase on soil amendment - Aqueous phase fuel upgrading

 Processing integration, simulation, modelling and economics

- Modelling and simulation studies using EES, ASPEN, and ANSYS Fluent - Economic evaluation of each process or value added products