Design of a pyrolysis oil gasifier

Abstract

In this study a design is proposed and experimentally validated for the gasification of waste plastic pyrolysis oil
at small scale for energy applications. The gasifier is designed to be implemented at an existing waste energy
recycling (WER) unit at Waste4me B.V. which processes up to 250 kg/h of waste streams in a pyrolysis reactor
which converts the waste into pyrolysis oil, pyrolysis gas and a solid residue. The pyrolysis oil investigated in
this study was produced from a DKR350 mixed plastics waste stream and was produced at a rate of 50 kg/h
with a lower heating value of 36.1 MJ/kg. Because the pyrolysis oil is currently not usable as a fuel or chemical
feedstock due to the high oxygen content and the high level of contaminants the oil should be further
converted into combustible gas which could eventually replace fossil based fuels or chemical feedstocks. To
come up with a suitable design for the gasifier and the implementation of the gasifier, the current WER unit at
the company is first reviewed to give context to the design. Then a literature study is done into the different
gasification reactions and reactors. With this knowledge the gasifier type ‘entrained flow’ is selected to be
implemented at the WER unit for the gasification of the DKR350 pyrolysis oil. Also, a plan for the
implementation of the gasifier at the WER unit is made. Then conceptual preliminary design of the gasifier is
proposed, which is a low temperature, atmospheric, air blown, non-slagging, recirculating, autothermal
entrained flow gasifier. Then an experimental setup is designed to test the potential performance of the
preliminary gasifier design. The main aspects that are studied in these experiments are the lower limit of
operation of the equivalence ratio (ER), the effects of recirculation, the composition of the product gas at
different ER and steam/fuel (S/F) ratios, the carbon conversion efficiency (CCE) and cold gas efficiency (CGE)
and the performance of the oil injector and evaporator. The oil feed is approximately 4 kg/h and the ER ranges
from 0.16 to 0.43, while the S/F ratio ranges from 0 to 1.16. The results of the experiments are compared to
the results as computed with the Gibbs equilibrium by the process simulating software COCO. From the
experiments it is concluded that the lower limit of operation of the (ER) is lower than 372 oC as the gasifier
reaches steady state at this temperature. The effects of recirculation are not investigated as the performance
of the recirculation system is not as designed. The composition of the product gas is determined with a gas
chromatograph for different ER and S/F ratios. The highest CCE and CGE are found at an ER = 0.16 and S/F = 0
with values of 97% and 64% respectively, although to ensure a robust operation of the gasifier different
settings are recommended. Generally the CCE and CGE increase with the ER, while the LHV of the dry product
gas decrease with the ER. The CGE and the LHV increase slightly due to steam addition between an ER of 0.24
and 0.3. The addition of steam generally cause an increase in CO and a decrease in CO2 concentration contradicting the results computed with the Gibbs equilibrium. At ER > 0.35 the composition, LHV, CCE and
CGE becomes less dependent on the S/F ratio. The oil injector shows no charring nor clogging while the
experiments show reasonable CCE’s and CGE’s. Therefore, it is concluded that the used oil injector is a suitable
alternative for the commonly used atomizers. With the findings from the experimental validation the
preliminary gasifier design and plan for implementation in the WER unit are evaluated and improved. Two
implementation plans are proposed, increasing the thermal efficiency of the WER unit from 52% to 58% or to
70%. Concluding from the experimental results a periodical cleaning program is recommended to clean the
system from carbonaceous deposition by combusting the deposit in a preheated reactor at T > 790 oC.