Re: Master Agreement, dated as of October 5, 2005, by and between KFx Plant II, LLC and Arch Coal, Inc. Gentlemen:
Exhibit
      10.80
    | ARCH
                COAL, INC. | C.
                ▇▇▇▇▇ ▇▇▇▇▇▇, ▇▇. Senior
                Vice President Strategic
                Development | ||
| June
                29, 2006 | VIA
                FACSIMILE AND OVERNIGHT MAIL | ||
| KFx,
                Inc. ▇▇
                ▇▇▇▇▇▇▇ ▇▇▇▇▇▇, ▇▇▇▇▇ ▇▇▇ ▇▇▇▇▇▇,
                ▇▇▇▇▇▇▇▇ ▇▇▇▇▇-▇▇▇▇ Attn:
                ▇▇▇▇▇▇▇ ▇. ▇▇▇▇▇▇▇▇ | KFx
                Plant II, LLC ▇▇
                ▇▇▇▇▇▇▇ ▇▇▇▇▇▇, ▇▇▇▇▇ ▇▇▇ ▇▇▇▇▇▇,
                ▇▇▇▇▇▇▇▇ ▇▇▇▇▇-▇▇▇▇ Attn:
                President | ||
Re:
      Master Agreement, dated as of October 5, 2005, by and between KFx Plant II,
      LLC
      and Arch Coal, Inc.
    Gentlemen:
    This
      letter will confirm our agreement with KFx Plant II, LLC, a Delaware limited
      liability company (“KFx Plant II”), and KFx, Inc., a Delaware corporation
      (“KFx”), to extend certain dates contained in the Master Agreement, dated as of
      October 5, 2005, by and between KFx Plant II and Arch Coal, Inc. (the
“Master Agreement”) and the Stock Purchase and Warrant Agreement, dated as of
      October 5, 2005, by and between KFx and Arch Coal, Inc. executed in
      connection therewith (the “Stock Purchase and Warrant Agreement”) in the manner
      set forth herein. Capitalized terms used herein but not otherwise defined shall
      have the meanings given those terms in the Master Agreement.
    By
      your acceptance of this letter, the parties agree to extend (i) the Operator
      Option Period by replacing the references to July 1, 2006 in Sections 2.1
      and 6.1(d) of the Master Agreement with October 1, 2006 and (ii) the term
      of the warrant granted by KFx to Arch pursuant to Section 2.1 of the Stock
      Purchase and Warrant Agreement by replacing (a) the reference to July 1,
      2006 in Section 3.1 of the Stock Purchase and Warrant Agreement and in each
      of
      Warrant Nos. 05-1, 05-2, 05-3 and 05-4 (collectively, the “Warrant
      Certificates”) with October 1, 2006 and (b) the references to
      August 1, 2006 in Section 3.1 of the Stock Purchase and Warrant Agreement
      and in each of the Warrant Certificates with November 1, 2006.
    If
      you are in agreement with the foregoing, please execute this letter in the
      space
      indicated below.
    Sincerely,
    /s/
      ▇▇▇▇▇ ▇▇▇▇▇▇
    ▇.
      ▇▇▇▇▇ Besten
    Senior
      Vice President - Strategic Development
    cc: ▇▇▇▇▇▇
      ▇. ▇▇▇▇
    Accepted
      and agreed to this 29th day of June, 2006:
    | KFx,
                Inc. | KFx
                Plant II, LLC | |
| By:
                /s/
                ▇▇▇▇▇ ▇. ▇▇▇▇▇▇ | By:
                /s/
                ▇▇▇▇▇ ▇. ▇▇▇▇▇▇ | |
| Name:
                  ▇▇▇▇▇
                ▇. ▇▇▇▇▇▇ | Name:
                  ▇▇▇▇▇
                ▇. ▇▇▇▇▇▇ | |
| Title:
                  SVP
                Business Development | Title:
                  SVP
                Business Development | 
| A
                  Subsidiary of | ||
| ARCH
                  COAL, INC. | ||
| ▇▇▇▇
                  ▇▇▇▇▇ ▇▇▇, ▇▇▇▇▇ ▇▇▇ | ▇▇.
                  ▇▇▇▇▇, ▇▇▇▇▇▇▇▇ ▇▇▇▇▇ | (312)
                  994-2700 |