**4. Case Study: Centralized vs. Decentralized Conversion of MSW to Methanol**

Consider the case of three cities (A, B, and C) with known biomass type in the form of refuse derived fuels (RDF) obtained from MSW at a cost of USD55/tonne. Gasification-based technology is used to convert RDF to methanol. Detailed description of this process consists of three main stages: gasification, purification, and methanol synthesis. Description of the technical, design, and operational details are given in literature [46–51]. Simulation results for the streams, operating conditions, and design specifications are taken from literature [52–54]. Based on the data provided by Iaquaniello et al. [55], key process information (on the use of electric energy, water, methanol yield, and assumed costs of electric energy, water, and methanol) are shown in Table 2.


The flowrate of RDF available in each city and the relative locations of the cities and the proposed centralized facility are show by Figure 3. A decision needs to be made on whether to build one centralized facility or multiple decentralized (smaller) facilities. The tradeoffs include cost, transportation, environmental impact, and risk.

**Figure 3.** Biomass availability and relative locations for the case study.

The maximum demands for methanol in cities A, B, and C are 0, 60,000, and 25,000 tonne/year, respectively. The transportation costs were based on data provided by Zhang et al. [56] and are USD0.011/(tonne RDF·mile) and USD0.015/(tonne methanol·mile).

## *Life Cycle Assessment for RDF to Methanol Process*

The proposed approach and optimization formulation are generally applicable for any environmental metrics. For this case study, only greenhouse gas emissions and their global warming impacts are considered to illustrate the applicability of the proposed approach and to discuss the tradeoffs. For transportation, two methods are considered for service: railroad and diesel-fueled heavy trucks. When a decentralized facility is co-located within the same city sourcing RDF, the transportation distance is assumed to be negligible.

The data of the RDF-to-methanol process described by Salladini et al. [48] are adapted to calculate the process emissions and to create the diagram shown by Figure 4 for a functional unit of one tonne of methanol produced. A linear proportionality consumption of overall inputs and outputs is assumed.

**Figure 4.** Key process inputs and outputs for a basis of one tonne methanol.

The emissions for the upstream production of electricity (Table 3) and natural gas (Table 4) are computed from emission factors provided by the Emission Factors for Greenhouse Gas Inventories (EPA, 2020). Assuming a natural gas power plant, the average emission factors for electricity in the United States are used.

**Table 3.** Non-baseload emission factors.



**Table 4.** Steam and heat emission factors assuming natural gas is entirely used at 80% efficiency.

The ton-mile factors for medium and heavy-duty trucks are used for the transportation of methanol from the centralized facility to the cities A, B, and C. A tonne-mile is one tonne of freight carried one mile, as a unit of traffic.

For the HPSI evaluation, the stream data were taken from literature [48,52–54]. The smallest plant (25,000 tonne/year) was taken as the reference.
